Electricians Notebook

The Industrial

Electrician’s Notebook

Written and Compiled by Elwood V Gilliland

Contributors Vail R Gilliland Introduction to Voltage Regulation

THIS NOTEBOOK BELONGS TO

Kilowatt Classroom

SM

“Interfacing Technology and Craftsmanship”

4PTRES

Copyright 2002 Kilowatt Classroom, LLC.

Resistance Measurements Three- and Four-Point Method

Four-Point Resistance Measurements Ohmmeter measurements are normally made with just a two-point measurement method. However, when measuring very low values of ohms, in the milli- or micro-ohm range, the two-point method is not satisfactory because test lead resistance becomes a significant factor. A similar problem occurs when making ground mat resistance tests, because long lead lengths of up to 1000 feet are used. Here also, the lead resistance, due to long lead length, will affect the measurement results. The four-point resistance measurement method eliminates lead resistance. Instruments based on the four-point measurement work on the following principle: •

Two current leads, C1 and C2, comprise a two-wire current source that circulates current through the resistance under test.

•

Two potential leads, P1 and P2, provide a two-wire voltage measurement circuit that measures the voltage drop across the resistance under test.

•

The instrument computes the value of resistance from the measured values of current and voltage.

Four-Point Measurement Diagram Leads may be any length. Instrument C1 P1 Current Source May be AC or DC.

VM

Readout in Ohms

Resistance Being Measured P2

AM

C2

The four-point method is required to measure soil resistivity. This process requires a soil cup of specific dimensions into which a representative sample of earth is placed. This process is not often employed in testing electrical ground systems although it may be part of an initial engineering study.

AN0009-1

The three-point method, a variation of the four-point method, is usually used when making ground (earth) resistance measurements. With the three-point method, the C1 and P1 terminals are tied together at the instrument and connected with a short lead to the ground system being tested. This simplifies the test in that only three leads are required instead of four. Because this common lead is kept short, when compared to the length of the C2 and P2 leads, its effect is negligible. Some ground testers are only capable of the three-point method, so are equipped with only three test terminals. The three-point method for ground system testing is considered adequate by most individuals in the electrical industry and is employed on the TPI MFT5010 and the TPI ERT1500.

Test Methods

Three-Point Resistance Measurements

GTEST1

Ground Testing Purpose / TPI Instrument Features


Purpose The purpose of electrical ground testing is to determine the effectiveness of the grounding medium with respect to true earth. Most electrical systems do not rely on the earth to carry load current (this is done by the system conductors) but the earth may provide the return path for fault currents, and for safety, all electrical equipment frames are connected to ground. The resistivity of the earth is usually negligible because there so much of it available to carry current. The limiting factor in electrical grounding systems is how well the grounding electrodes contact the earth, which is known as the soil / ground rod interface. This interface resistance component, along with the resistance of the grounding conductors and the connections, must be measured by the ground test. In general, the lower the ground resistance, the safer the system is considered to be. There are different regulations which set forth the maximum allowable ground resistance, for example: the National Electrical Code specifies 25 ohms or less; MSHA is more stringent, requiring the ground to be 4 ohms or better; electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm.

TPI Ground Test Instrument Characteristics •

To avoid errors due to galvanic currents in the earth, TPI ground test instruments use an AC current source.

•

A frequency other than 60 hertz is used to eliminate the possibility of interference with stray 60 hertz currents flowing through the earth.

•

The three-point measurement technique is utilized to eliminate the effect of lead length.

•

The test procedure, known as the Fall-of-Potential Method, is described on the following page.

Test Products International Three-Point Fall-of-Potential Ground (Earth) Resistance Testers

Ground Testing

TPI ERT1500 Earth Resistance Tester Uses 800 Hz signal at less than 50 Volts RMS for Ground (Earth) Testing.

AN0009-2

TPI MFT5010 Multi -Function Tester Uses 570 Hz signal at less than 50 Volts RMS for Ground (Earth) Testing.

GTEST2

Ground Testing Three-Point Fall-of-Potential Test Procedure


Ground Test Procedure Refer to diagram and example graph on the following page. In the Fall-of-Potential Method, two small ground rods - often referred to as ground spikes or probes - about 12 “ long are utilized. These probes are pushed or driven into the earth far enough to make good contact with the earth ( 8” - 10” is usually adequate). One of these probes, referred to as the remote current probe, is used to inject the test current into the earth and is placed some distance (often 100’ ) away from the grounding medium being tested . The second probe, known as the potential probe, is inserted at intervals within the current path and measures the voltage drop produced by the test current flowing through the resistance of the earth. In the example shown on the following page, the remote current probe C2 is located at a distance of 100 feet from the ground system being tested. The P2 potential probe is taken out toward the remote current probe C2 and driven into the earth at ten-foot increments. Based on empirical data (data determined by experiment and observation rather than being scientifically derived), the ohmic value measured at 62% of the distance from the ground-under-test to the remote current probe, is taken as the system ground resistance. The remote current probe must be placed out of the influence of the field of the ground system under test. With all but the largest ground systems, a spacing of 100 feet between the ground-under-test and the remote current electrode is adequate. When adequate spacing between electrodes exists, a plateau will be developed on the test graph. Note: A remote current probe distance of less than 100 feet may be adequate on small ground systems. When making a test where sufficient spacing exists, the instrument will read zero or very near zero when the P2 potential probe is placed near the ground-under-test. As the electrode is moved out toward the remote electrode, a plateau will be reached where a number of readings are approximately the same value (the actual ground resistance is that which is measured at 62% of the distance between the ground mat being tested and the remote current electrode). Finally as the potential probe approaches the remote current electrode, the resistance reading will rise dramatically. It is not absolutely necessary to make a number of measurements as described above and to construct a graph of the readings. However, we recommend this as it provides valuable data for future reference and, once you are setup, it takes only a few minutes to take a series of readings. The electrical fields associated with the ground grid and the remote electrode are illustrated on AN0009-5. An actual ground test is detailed on AN0009-6, and a sample Ground Test Form is provided on AN0009-7. See AN0009-8 for a simple shop-built wire reel assembly for testing large ground systems.

Short Cut Method TPI MFT5010 & TPI ERT1500

• • •

AN0009-3

•

Connect the T1 instrument jack with the 15’ green lead to the ground system being tested. Connect the T3 instrument jack with the red lead to the remote current electrode (spike) placed at distance of 65’ (full length of conductor) from the ground grid being tested. Connect the T2 instrument jack with the black lead to the potential probe placed at 40 feet (62% of the 65’ distance) from the ground grid being tested and measure the ground resistance. Move the P2 potential probe 6’ (10% of the total distance) to either side of the 40’ point and take readings at each of these points. If the readings at these two points are essentially the same as that taken at the 40’ point, a measurement plateau exists and the 40’ reading is valid. A substantial variation between readings indicates insufficient spacing.

Ground Testing

The short cut method described here determines the ground resistance value and verifies sufficient electrode spacing - and it does save time. This procedures uses the 65’ leads supplied with the TPI instruments.

GTEST3


Ground Testing Three-Point Fall-of-Potential Method

Instrument Set-Up

Yellow arrow indicates P2 potential probe @ 62 feet. Potential probe taken out at 10 foot increments.

Ground System Under Test

Keep this lead as short as possible.

Blue indicates return current path through earth. T1 T2 (C1 / P1) (P2)

T3 (C2)

Remote current probe C2 @ 100’

Ground Test Instrument Digital Display

TPI MFT5010 or TPI ERT1500

FCN

SW

Test Current Path

Select Earth ( RE )

•

A Note on Instrument Labeling Conventions

•

The TPI MFT5010 and TPI ERT1500 use the terminal designations T1 (C1/P1), T2 (P2), and T3 (C2).

•

The corresponding lead designations on the MFT5010 are E (Earth), S & H.

•

The corresponding lead designations on the ERT1500 are E (Earth), P (Potential), C (Current).

Test Current (AC ) flows from instrument T3 to remote current probe C2 on the red lead. Test Current flows from remote current probe C2 back through the earth to the ground being tested as shown by dashed blue line. Test current flows out of ground grid back to instrument T1 on the short green lead. Black potential lead P1 is connected to instrument T2 and is taken out at 10’ increments. It measures voltage drop produced by the test current flowing through the earth. (P1 to P2 potential.)

9

10

Sample Ground Resistance Plot Remote current electrode C2 @ 100 feet. Potential probe P1 taken out at 10 foot increments.

8 7 6 2

3

4 5

Ground Testing

Sufficient electrode spacing has plateau.

Ohms @ 62% of distance = 3.3 ohms

1

Resistance in Ohms

Insufficient electrode spacing has no plateau.

10

20

30

40

50 Distance in Feet

60

70

80

90

100

AN0009-4

0

GTEST4


Ground Testing Equal-Potential Planes The Existence of Equal-Potential Planes •

When current flows through the earth from a remote test electrode (in the case of a ground test) or remote fault, the voltage drop which results from the flow of current through the resistance of the earth can be illustrated by equal-potential planes. The equal-potential planes are represented in the dashed lines in drawings below where the spacing between concentric lines represents some fixed value of voltage.

•

The concentration of the voltage surrounding a grounding element is greatest immediately adjacent to that ground. This is shown by the close proximity of lines at the point where the current enters the earth and again at the point where the current leaves the earth and returns to the station ground mat.

•

In order to achieve a proper test using the Fall-of-Potential Ground Test Method, sufficient spacing must exist between the station ground mat being tested and the remote current electrode such that the equal-potential lines do not overlap. As shown by the black line in the Sample Plot on the previous page, adequate electrode spacing will result in the occurrence of a plateau on the resistance plot. This plateau must exist at 62% of the distance between the ground mat and the remote electrode for the test to be valid. Insufficient spacing results in an overlap of these equal-potential planes, as illustrated at the bottom of this page and by the red line on the Sample Plot on the previous page.

•

See the Safety Note on AN0009-6 for information on the hazards of Step and Touch-Potentials.

Station Ground Mat Current leaves the earth and returns to the source.

Remote Current Electrode or Remote Fault

Representation of Equal-Potential Planes Showing adequate spacing of electrodes

Ground Mat

Remote Current Electrode Ground Testing

AN0009-5

Representation of Equal-Potential Planes Showing inadequate spacing between the established ground and remote test electrode.

GTEST5


Ground Testing Actual Field Test

This actual ground test was conducted on a pad-mount transformer in a rural mountain area. The single-phase transformer is supplied by a 12470/7200 volt grounded wye primary and the transformer is grounded by its own ground rod as well as being tied to the system neutral which is grounded at multiple points along the line. The distribution line is overhead with just the “dip” to the transformer being underground.

Setting-Up the Ground Tester Red arrow shows location of C2 probe.

TPI MFT5010 Instrument Showing the 50 foot reading of 4.0 Ohms.

Ground Test Data

Test Procedure

Remote Current Probe C2 @ 100 Feet

Terminal T1 of the TPI MFT5010 tester was connected to the transformer case ground with the short green lead.

Instrument Reading in Ohms

10

1.83

20

3.59

30

3.85

40

3.95

50

4.0

60

4.25

62*

4.3

70

4.5

80

5.4

90

7.3

100

25.02

* Actual Ground resistance.

The remote Current Probe C2 was driven in the ground at a location 100 feet from the transformer and connected to Terminal T3 of the instrument with the red test lead. Terminal T2 of the tester was connected, using the 100’ black lead, to the P2 potential probe. This ground stake was inserted into the ground at 10’ intervals and a resistance measurement was made at each location and recorded in the table at the left. The relatively constant readings in the 4 ohm range between 40 and 70 feet is a definite plateau that indicates sufficient lead spacing. The initial readings close to the transformer are lower, and there is a pronounced “tip-up” as the P2 probe approaches the remote current electrode C2. Ground Testing

P2 Distance from Transformer in Feet

The measured ground resistance at 62 feet (62% of the distance) was 4.3 ohms and is taken as the system ground resistance. This is an excellent value for this type of an installation.

AN0009-6

Safety Note - Possible Existence of Hazardous Step and Touch Potentials It is recommended that rubber gloves be worn when driving the ground rods and connecting the instrument leads. The possibility of a system fault occurring at the time the ground test is being conducted is extremely remote. However, such a fault could result in enough current flow through the earth to cause a possible hazardous step potential between a probe and where the electrician is standing, or hazardous touch potential between the probes and the system ground. The larger the system, in terms of available fault current, the greater the possible risk.

Ohms

Ground Testing

Carrying Handle (2 Required) Do not lift assembly by reel handles.

3/4” Plywood Reel Support

Bring short length of inside conductor out from each reel for connection to instrument.

Remote Current Lead Mark at ten-foot intervals with numbered wire markers to simplify probe placement.

Outside conductor is connected to remote current ground stake.

5/16” bolt inserted in tee-nut locks center shaft in position.

Detail Center Shaft Spacers

Surplus Plastic Wire Reels (2 required)

1-1/4” PVC Spacers

Outside conductor is connected to potential ground stake.

1/2” GRC Reel Crank Handle ( 2 ) with 3/8” bolt center shaft. Fasten bolt solidly to reel but leave handle free to turn on shaft.

3/4” GRC Reel Shaft. Thread ends and use pipe cap on each end.

Remote Potential Lead Take out at ten-foot intervals toward remote current stake.

Ground Testing Reel Assembly

AN0009-8

This simple, low-cost, and easy-to-build wire reel assembly is handy for making Ground (Earth) Resistance measurements on large ground systems. The unit shown below has 500 feet of wire for testing medium-to-large ground fields typical of those found in industrial plants and substations. For testing even larger systems, such as those installed for power generating plants, wire lengths of 1000 feet can be used. Wrap-on wire markers are installed every ten feet on the current lead to simplify placement of the remote current and potential probes. Your electrical distributor will probably have empty surplus reels available for the asking - the ones shown below are about 12 inches in diameter. The conductor is standard #12 THHN. Even though the TPI ERT1500 and the MFT5010 use an AC test signal, the test results are unaffected by the inductance of any wire left on the reels.

A Shop-Built Ground Test Wire Reel Assembly

GTEST8 Copyright 2004 Kilowatt Classroom, LLC.

HVCM1

AC Current Measurement


On High Voltage Systems Page 1 of 3

Overview Quick, accurate, and safe current measurements can be made on medium and high-voltage systems at the secondary of instrument current transformers (CT’s) using the TPI A254 Low-Current Clamp-On Adapter. The low-range accuracy (down to 10 milliamps) of the adapter permits measurements on the 5-amp secondary of CT’s without placing the meter in series with the current transformer secondary, and without being in close proximity to the high-voltage side of the current transformer. WARNING! This procedure is intended for use by qualified individuals only. Work must be performed in accordance with OSHA 29 CFR 1910.269. When making AC current measurements, the TPI A254 Adapter can be used with the TPI 122, 126, 133, 153, 163, and 183 Digital Multimeters, the TPI 440 Scopemeter, and the TPI 460 Dual-Channel Oscilloscope. See additional Application Notes for specific instructions on the TPI 183, 440, and 460. Applications It is often necessary to make recordings of equipment operation, particularly of medium voltage motor starting, in order to determine the motor actual locked-rotor-amps (LRA), full-load-amps (FLA), acceleration time, and power factor. This data is often required to verify proper protective relay settings, for inclusion in commissioning reports, and to establish an operating baseline for future system troubleshooting. The TPI 440 Scopemeter, and 460 Oscilloscope both have the ability to store data and waveforms. Optional software is available to permit downloading the stored information to a computer and then printed to obtain a permanent hard-copy record.

The TPI A254 Adapter The A254 is capable of current measurements on either AC or DC systems. For this application only the AC mode is utilized. The adapter’s low-range accuracy is due in part to the small clamp-on opening diameter which insures good coupling but is large enough for metering, protective relaying, and other instrument wiring. As with all TPI current adapters, the clamp-on converts current to a proportional millivoltage (mV). The adapter is plugged into the voltage input of the multimeter, 440 Scopemeter, or 460 Oscilloscope and the instrument display is read as current. There are two switch-selectable ranges on the A254: 0-10 mV/amp and 0-100 mV/amp.

Application Note

The A254 adapter is accurate down to 10 milliamps but it has a maximum rating of 60 amps so the instrument will not be over-ranged during measurements made on the secondary of the CT which, in the case of a motor start trending, may momentarily go to six-times (or more) of the full-load current of the motor. (Maximum permissible adapter current of 60 amps would be 12 times the CT 5 amp secondary.) WARNING! - Current Transformer Safety • •

For more information on Current Transformers and CT safety see The Industrial Electrician’s Notebook Article 0016: Current Transformers - Part 1.

AN0008

This procedure is for measurement only on the secondary circuit of current transformers and may not be used within the high voltage equipment cubicle. The secondary of a current transformer must never be open-circuited. It must have a burden connected or be short-circuited. An open circuited CT can develop a dangerously high secondary voltage.

HVCM2


AC Current Measurement On High Voltage Systems

Page 2 of 3

Typical CT Current Loop High Voltage Cubicle

Low Voltage Control / Instrument Cubicle

High Voltage Bus Current Transformer

CT Shorting Block

AM 50

51 Panel Ammeter

Protective Relay in Draw-Out Case

Typical CT Secondary Loop

TPI Digital Multimeter TPI 440 Scopemeter TPI 460 Oscilloscope WARNING ! Do not enter or make measurements within this compartment.

TPI A254 Low-Current Adapter

Current may be measured anywhere in the CT secondary loop within the lowvoltage cubicle.

Typical Measurement Calculation Assume for this example the Current Transformer (CT) Ratio is 800/5. Assume that with the A254 Low-Current Adapter switch placed in the 100 mV/A position, the meter reads 246 millivolts. The CT secondary current would be 246 / 100 = 2.46 amps. The CT primary current would be 2.46 x 160 = 393.6 amps. (Note 800/5 = 160/1.)

Determining the CT Ratio The Current Transformer ratio can be determined using the System One-Line Diagram, or if an analog panel meter is used, the full-scale value of the ammeter will indicate the CT primary current. The CT ratio can also be determined from the Current Transformer nameplate, or may be painted in large numbers on the CT. Do not open or enter the high-voltage compartment without following proper de-energizing, lockout/tagout, and grounding procedures as per OSHA 29 CFR 1910.269. Application Note

WARNING! - Current Transformer Safety • •

For more information on Current Transformers and CT safety see The Industrial Electrician’s Notebook Article 0016: Current Transformers - Part 1.

AN0008

This procedure is for measurement only on the secondary circuit of current transformers and may not be used within the high voltage equipment cubicle. The secondary of a current transformer must never be open-circuited. It must have a burden connected or be short-circuited. An open circuited CT can develop a dangerously high secondary voltage.

HVCM3


AC Current Measurement On High Voltage Systems

Page 3 of 3

4500 H.P. Medium Voltage Synchronous Motor 4160 Volt Motor Control Center - Field Excitation / Relay Cubicle - Showing back of Door Current measurement using the TPI A254 Low-Current Adapter and TPI 460 Oscilloscope.

Field Current Ammeter Power Factor Meter Ammeter 0 - 5 Amp Range Scale 0 - 800 Amps

TPI 460 Dual-Channel Oscilloscope

Synchronous Motor Field Adjustment Autotransformer DC Field Ground Relay TPI A254 Low-Current Adapter Ammeter Switch

High Voltage Breaker Compartment WARNING! Do not enter or make measurements within the HV cubicle. Also see Safety Notes on Pages 1 & 2.

Schematic Diagram of Measurement Set-up

Low Voltage Cubicle

4160 Main Bus

WARNING! Measurements permitted in this switchgear section only. A254 Adapter AM

PF*

Motor Protective Relay*

52 Device Motor Breaker

Trip

AMS

*Potential transformer connections not shown. Motor

Application Note

Measurement Notes

Current Transformers (CT’s) Isolate the meter and relay circuits from the high voltage, provide for a grounded secondary, and reduce the current to a 0 - 5 amp value.

For current measurement, adapter polarity (placement around conductor) is not critical. Convenience usually dictates the point at which the current measurement is made. Any phase CT secondary would be acceptable.

AM - Panel Ammeter AMS - Ammeter Switch PF - Power Factor Meter

AN0008

460 O’Scope

When using the 460 Oscilloscope, a direct amperage reading can be obtained using Channel B.

Legend

XIST4


Transistor Test Procedure

An ohmmeter can be used to test the base-to-emitter PN junction and the base-to-collector PN junction of a bipolar junction transistor in the same way that a diode is tested. You can also identify the polarity (NPN or PNP) of an unknown device using this test. In order to do this you will need to be able to identify the emitter, base, and collector leads of the transistor. Refer to a semiconductor data reference manual if you are not sure of the lead identification. Note: While this test can be used to determine that the junctions are functional and that the transistor is not open or shorted, it will not convey any information about the common emitter current gain (amplification factor) of the device. A special transistor tester is required to measure this parameter known as the Hfe or Beta. .

PNP Test Procedure

TPI 183 Digital Multimeter •

Connect the meter leads with the polarity as shown and verify that the base-to-emitter and base-tocollector junctions read as a forward biased diode: 0.5 to 0.8 VDC.

•

Reverse the meter connections to the transistor and verify that both PN junctions do not conduct. Meter should indicate an open circuit. (Display = OUCH or OL.)

•

Finally read the resistance from emitter to collector and verify an open circuit reading in both directions. (Note: A short can exist from emitter to collector even if the individual PN junctions test properly.)

PNP Transistor Simplified Diagram COLLECTOR

Select Diode

P N

BASE

P

EMITTER

NPN Test Procedure

TPI 183 Digital Multimeter

•


•


NPN Transistor Simplified Diagram COLLECTOR

Select Diode

N BASE

P N

EMITTER

AN0007


Application Note

•

ACIMEAS


AC Current Measurement

TPI 183 Multimeter

3 Phase Supply

MCP or Circuit Breaker

Typical Across-the-Line Three-Phase Starter Select VH Z Contactor

OL Heaters

TPI A256 AC/DC Adapter Polarity does not matter .

Adapter Dual Banana Plug Insert in COM and V

2-Conductor Adapter Cable

Select Current Range 40A or 400A Based on Motor Amps

Motor

Measuring AC Amps with Clamp Adapter WARNING! Do not attempt to make a current measurement in excess of Clamp Adapter range switch setting. Note: The TPI A256 Adapter has 40 amp and 400 amp switch positions. Set the adaptor switch for the maximum expected current. For motor starting current measurement use motor Locked Rotor Amps (LRA). For motor running amps use Motor Full-Load Amps (FLA). CAUTION! Do not remove adapter dual banana plug from meter with clamp adapter around motor lead. First remove clamp adapter from motor lead then unplug adapter dual banana plug. Example: Determining Motor Amperage

• • • •

Set Meter Selector Switch on VHZ (AC Volts). Note: The TPI A256 Current Adapter converts amps to millivolts (1mV / amp). Plug Clamp Adapter dual banana plug into meter COM and V. Set the Adapter Range Switch for the maximum anticipated current (40 or 400 amps). Place adapter probe around single motor lead. Read millivolts as amps.

Application Note

•

AN0006

ACEMEAS


AC Voltage & Frequency

Simultaneous AC Frequency Measurements

TPI 183 Multimeter

The TPI 183 triple-line display permits simultaneous AC Frequency Measurements while making AC Volts Measurements. Note: See separate Instrument Application Note for making frequency measurement to 200 kHz using the Hz Selector Switch Position. •

Connect meter for AC Voltage Measurement as shown.

•

Press unmarked gray button and read frequency on third display.

L1

Select VH Z L2 FU1

FU2

L3

MCP

Measuring AC Volts CAUTION! Do not attempt to make a voltage measurement if a test lead is plugged in the A or µmA input jack. Instrument damage and/or personal injury may result. Note: Voltage is always measured across two circuit points ( in parallel with circuit element under test). On the voltage range with leads plugged-in to the meter as shown the meter has a very high input impedance and draws almost no current from the circuit under test. WARNING! Do not attempt to make a voltage measurements of more than 750 VAC or of a voltage level that is unknown. CAUTION! Always check meter test leads before use to be certain they are in good condition and use test leads with an insulating rating acceptable for the system voltage.

• • •

Application Note

Example: Measuring Control Transformer Secondary Voltage Set meter selector switch on V HZ (AC Volts) position. Plug in meter leads as shown: Black lead - Meter COM (Common), Red lead - Meter V (Volts). Apply probes to circuit test locations. Read AC Voltage on main display.

The TPI 183 is auto-ranging (selects appropriate decimal point) and will display the voltage to the greatest degree of accuracy possible. AN0005

OMEAS


Resistance Measurement

TPI 183 Multimeter

Typical Three-Phase Starter with Reduced Voltage Control

Select Ω (Ohms)

3-Phase Supply

MOTOR

120 VAC Control Circuit

1

STOP

START

OL

3

2

M Ma

Caution: Breaker must be open or fuses pulled to de-energize circuit.

Resistance (Ohms) Measurement WARNING! Do not attempt to make resistance measurements with circuit energized. Note: The ohmmeter internal battery provides power to make this measurement; therefore, ohms measurements can be made only on de-energized circuits! When practical, isolate the component from the circuit before attempting to measure its ohmic value to prevent a parallel path through other components. In the example above, the normally open START push button and normally open auxiliary contact Ma prevent a parallel path through the control transformer secondary. Example: Checking the resistance of a contactor coil. • • • •

Disconnect power from circuit to be measured. Select Meter Ω ( Ohms) function. Plug in the meter leads as shown: Black lead - Meter COM (Common), Red lead - Meter Ω (Ohms). Connect leads across coil (or any circuit component to be measured). Lead polarity does not matter. Read OHMS value on display. OUCH indicates open circuit. Application Note

AN0004

DCIMEAS


DC Current Measurement

TPI 183 Multimeter

Typical 4-20 mA Control Loop PLC Analog Input Module Select mA Other Inputs

24 VDC Loop Power Supply NEG

POS

+ POS

Ch Input COM

+

Loop-Powered 2-Wire Transmitter 4 - 20 mA output _ + _

_

Optional Test Points

Process Variable

Measuring DC Milliamps CAUTION! Do not attempt to make a current measurement with the test leads connect in parallel with the circuit to be tested. Test leads must be connected in series with the circuit. Note: Current is always measured with the meter placed in series with the circuit. On the current range with leads plugged-in to the meter as shown the meter has a very low input impedance and the current flow through the meter is limited by the circuit elements in series with the meter. WARNING! Do not attempt to make a current measurements if more than 600 volts is present. Instrument damage and/or personal injury may result. CAUTION! Always check meter test leads before use to be certain they are in good condition and use test leads with an insulating rating acceptable for the system voltage. Example: Process Control 4 -20 mA Loop Current Measurement

AN0003

• •

Set Meter Selector Switch on mA (AC or DC Milliamps). Plug in the meter leads as shown: Black lead - COM (Common), Red lead - µ mA (micro or milliamps). Open 4 - 20 milliamp loop and connect the meter in series with the loop. Note: This loop can be opened at any one of three points. Convenience usually dictates the location. Caution - Be sure loop can be opened safely without causing a system operating problem! Connect meter red lead clip to the Transmitter Negative terminal. Close the loop by connecting the meter black lead clip to the conductor which was removed from the Transmitter Negative terminal. (This results in a current flow through the meter in a positive to negative direction.)

Application Note

• • •

DCEMEAS


DC Voltage Measurements

TPI 183 Multimeter

Minus sign displayed when V input (red lead) is negative with respect to COM

Bench-Type Adjustable DC Power Supply --Select V OUTPUT ADJ POS V

COM

NEG V

Measuring DC Volts CAUTION! Do not attempt to make a voltage measurement if a test lead is plugged in the A or µmA input jack. Instrument damage and/or personal injury may result. Note: Voltage is always measured across two circuit points ( in parallel with circuit element under test). On the voltage range with leads plugged-in to the meter as shown the meter has a very high input impedance and draws almost no current from the circuit under test. WARNING! Do not attempt to make a voltage measurements of more than 1000 VDC or of a voltage level that is unknown. CAUTION! Always check meter test leads before use to be certain they are in good condition and use test leads with an insulating rating acceptable for the maximum system voltage. Application Note

Example: Checking Double-Ended DC Power Supply Output Voltage --• Set Meter Selector Switch on V (DC Volts - Steady or Pulsing) • Plug in the meter leads as shown: Black lead - Meter COM (Common), Red lead - Meter V (Volts). • Clip black test lead to power supply COM (common). • Clip red test lead to power supply POS V (PS DC Pos Out). Meter will display the positive DC voltage. • To check PS NEG Out, move red lead to power supply NEG V. Meter will indicate the negative DC voltage with negative sign on the display. AN0002

The TPI 183 is auto-ranging (selects appropriate decimal point) and will display the voltage to the greatest degree of accuracy possible.

VFD1

VFD Fundamentals


Variable Frequency Drive Fundamentals AC Motor Speed - The speed of an AC induction motor depends upon two factors: 1) The number of motor poles 2) The frequency of the applied power. 120 x Frequency AC Motor Speed Formula:

RPM = Number of Poles

Example: For example, the speed of a 4-Pole Motor operating at 60 Hz would be: Variable Frequency

120 x 60 / 4 = 7200 / 4 = 1800 RPM Inverter Drives - An inverter is an electronic power unit for generating AC power. By using an inverter-type AC drive, the speed of a conventional AC motor* can be varied through a wide speed range from zero through the base (60 Hz) speed and above (often to 90 or 120 hertz). Voltage and Frequency Relationship - When the frequency applied to an induction motor is reduced, the applied voltage must also be reduced to limit the current drawn by the motor at reduced frequencies. (The inductive reactance of an AC magnetic circuit is directly proportional to the frequency according to the formula XL = 2 f L. Where: = 3.14, f = frequency in hertz, and L= inductive reactance in Henrys.) Variable speed AC drives will maintain a constant volts/hertz relationship from 0 - 60 Hertz. For a 460 motor this ratio is 7.6 volts/Hz. To calculate this ratio divide the motor voltage by 60 Hz. At low frequencies the voltage will be low, as the frequency increases the voltage will increase. (Note: this ratio may be varied somewhat to alter the motor performance characteristics such a providing a low-end boost to improve starting torque.)

CONSTANT TORQUE

CONSTANT HP

VFD Speed Torque Characteristics

60 70

80

Blue = Horsepower Red = Torque Green = Motor Nameplate Frequency (60 Hz)

20 30

40

50

In Constant Torque Area - VFD supplies rated motor nameplate voltage and motor develops full horsepower at 60 hertz base frequency.

10

PERCENT HP AND TORQUE

90 100

Depending on the type of AC Drive, the microprocessor control adjusts the output voltage waveform, by one of several methods, to simultaneously change the voltage and frequency to maintain the constant volts/hertz ratio throughout the 0 - 60 Hz range. On most AC variable speed drives the voltage is held constant above the 60 hertz frequency. The diagram below illustrates this voltage/frequency relationship.

10 20 30 40 50 60 70 80 90 100 110 120

In Constant Horsepower Area - VFD delivers motor nameplate rated voltage from 60 Hertz to 120 hertz (or drive maximum). Motor horsepower is constant in this range but motor torque is reduced as frequency increases. Note: Motor HP = Torque x RPM

FREQUENCY HZ

Sheet 1

*Inverter Duty Motors - Initially standard AC motors were employed on inverter drives. Most motor manufacturers now offer Inverter Duty Motors which provide improved performance and reliability when used in Variable Frequency Applications. These special motors have insulation designed to withstand the steep-wave-front voltage impressed by the VFD waveform, and are redesigned to run smoother and cooler on inverter power supplies.

VFD5


Inverter Principle

Variable Frequency Drive (VFD) Output Module Shown below is a typical Medium Voltage VFD transistorized output module. One of these modules is used for each phase in a three-phase drive. Modules are a complete functional block that may include: multi-stage amplifiers, resistors, capacitors and free-wheeling diodes. Transistors are switched on and off by logic level base-toemitter signal (or gate signal in the case of IGBT’s) from the VFD microprocessor control. The length of time the transistors are turned on (duty cycle) determines the pulse width. DC Link Positive Terminal C1 Base-Emitter Signal Input Pins

Phase Output Terminal E1 C2

Variable Frequency

Size of pictured module: 4.25” wide x 2.5” deep x 1.5” high DC Link Negative Terminal E2

Module Mounting Holes Heat Sink on Module Back-Plane

Module Schematic Diagram

VFD Output Section Schematic DC Link Positive Free-Wheeling Diodes (6) Protect IGBT’s from reverse bias inductive surges due to motor field decay which results when the transistors turn off. DC Link Negative Voltage Pulses

Resultant Current

One Output Module

Three-Phase Motor

PWM Waveform Phase A to B

Inverter Principle Inverter circuitry generates an Alternating Current (AC) by sequentially switching a Direct Current (DC) in alternate directions through the load. The illustration above shows the generation of a single positive pulse (red) and a single negative pulse (green) which occurs 180 electrical degrees later. To analyze the circuit assume a conventional current flow (positive to negative direction). The black arrows on the emitter of each transistor indicate the direction of conventional current through the transistors. This is a three-phase drive, so at certain times during the cycle transistors will be turned on to cause current flow through the A - C and B - C motor windings (see next page) but for clarity this is not shown in the above illustration. For this analysis also assume that the free-wheeling diodes are non-conducting. Sheet 2

Transistors 1A and 2B are turned on and off by the microprocessor control and current flows from the DC bus positive, through the motor windings as shown by the red arrows producing the positive (red ) voltage pulse, and back to the DC bus negative. To generate the next half-cycle transistors 1B and 2A will be turned on and off and the current flow will reverse through the motor winding as shown by the green arrows which result in the negative (green) pulse.

VFD6

Output Switching Sequence


The following illustrations show the switching sequence of the output transistors, SCR’s, or GTO’s used in a VFD to produce a three-phase AC waveform. Since each these devices are functioning as solid-state switches, the circuit operation can be easily visualized by representing these devices as open or closed mechanical switches. Switches closed to the positive bus are shown in red, switches closed to the negative bus are shown in black, and open switches are shown in gray. When a particular winding is connected to the same bus potential (either positive or negative) the voltage across that winding will be zero. If a winding is connected so that the positive voltage is connected to the first letter of the winding label (for example the A in AB) the voltage produced across that winding is positive. If a winding is connected so that the positive voltage is connected to the second letter of the winding label (for example B in AB) the current flow reverses and the voltage produced across that winding will be of a negative polarity.

DC LINK POSITIVE

DC LINK

DC LINK POSITIVE

NEGATIVE

DC LINK

DC LINK POSITIVE

NEGATIVE

DC LINK

B

B

A

C

THREE-PHASE MOTOR

NEGATIVE B

A

C

THREE-PHASE MOTOR

A

C

THREE-PHASE MOTOR

0 - 60 DEG

60 - 120 DEG

120 - 180 DEG

VAB = 0

VAB = +E

VAB = +E

VBC = +E

VBC = 0

VBC = -E

VCA = -E

VCA = -E

VCA = 0

DC LINK POSITIVE

DC LINK POSITIVE

DC LINK POSITIVE

DC LINK

NEGATIVE

DC LINK

B

A

NEGATIVE

DC LINK

B

C

Variable Frequency

Below each diagram is a table listing of the number of electrical degrees through which the switches operate and the resultant phase voltage produced. Note: On a six-step drive the output devices will be closed throughout the listed operating range; on a PWM drive, pulses will be produced through this range. See next page for generated waveform.

A

NEGATIVE B

C

A

C

THREE-PHASE MOTOR

THREE-PHASE MOTOR

180 - 240 DEG

240 - 300 DEG

300 - 360 DEG

VAB = 0

VAB = -E

VAB = -E

VBC = -E

VBC = 0

VBC = +E

VCA = +E

VCA = +E

VCA = 0

Sheet 3

THREE-PHASE MOTOR

VFD7


VFD Three-Phase Waveform

Waveform Development The development of a variable frequency drive three-phase waveform is shown below. Refer to the previous page to see the switching sequences that produce a particular portion of the waveform.

VAB

Variable Frequency

VBC

VCA

0o

60o

120o

180o

240o

300o

360o

60o

120o

Sheet 4

VFD8

Pulse Width Modulation


PWM Sine Wave Synthesis High Frequency

Low Frequency Smaller pulse widths produce lower resultant voltage.

Resultant Sine Wave Current

Pulse Width

Larger pulse widths produce higher resultant voltage. Pulse Width

Variable Frequency

DC Link Voltage

One Cycle

One Cycle

PWM Drive Characteristics •

VFD drive DC link voltage is constant .

•

Pulse amplitude is constant over entire frequency range and equal to the DC link voltage.

•

Lower resultant voltage is created by more and narrower pulses.

•

Higher resultant voltage is created by fewer and wider pulses.

•

Alternating current (AC) output is created by reversing the polarity of the voltage pulses.

•

Even though the voltage consists of a series of square-wave pulses, the motor current will very closely approximate a sine wave. The inductance of the motor acts to filter the pulses into a smooth AC current waveform.

•

Voltage and frequency ratio remains constant from 0 - 60 Hertz. For a 460 motor this ratio is 7.6 volts/Hz. To calculate this ratio divide the motor voltage by 60 Hz. At low frequencies the voltage will be low, as the frequency increases the voltage will increase. (Note: this ratio may be varied somewhat to alter the motor performance characteristics such as providing a low-end boost to improve starting torque.)

•

For frequencies above 60 Hz the voltage remains constant. Some AC drives switch from a PWM waveform to a six-step waveform for 60 Hz and above.

Sheet 5

XIST1

Introduction to Transistors


Introduction Typical Transistor Switching Circuit

PLC Output Module Relay (Transistor Load) COLLECTOR

+

BASE

PLC DC Power Supply

Small Input Current From PLC logic. EMITTER

Note: The arrow on the emitter lead of the transistor shows the direction of conventional current flow (positive to negative) through the transistor.

The transistor is a semiconductor device than can function as a signal amplifier or as a solid-state switch. A typical switching circuit using a PNP transistor is shown at the left.

•

In a transistor a very small current input signal flowing emitter-to-base is able to control a much larger current which flows from the system power supply, through the transistor emitter-to-collector, through the load, and back to the power supply.

•

In this example the input control signal loop is shown in red and the larger output current loop is shown in blue. With no input the transistor will be turned OFF (cutoff) and the relay will be dropped out. When the low-level input from the PLC microprocessor turns the transistor ON (saturates) current flows from the power supply, through the transistor, and picks the relay.

Transistors

+

•

Transistor Packages There are many transistor case designs. Some conform to JEDEC Standards and are defined by Transistor Outline (TO) designations. Several case designs are illustrated below. Solid -state devices other than transistors are also housed in these same packages. In general, the larger the unit, the greater the current or power rating of the device. Small Signal Transistors Shown about twice actual size.

Power Transistor Shown about 1/2 actual size.

Collector

Emitter Emitter Base Case is Collector

TO-3 Package

Base

TO - 92 Plastic Package

Power Tab Package Shown about actual size. Used for power transistors, three-terminal voltage regulators, and SCR’s. Heat Sink Mounting Tab

TO-18 Hermetically-Sealed Case Center lead is common with heat sink tab.

JEDEC Numbering System The Joint Electronic Device Engineering Council - JEDEC - has established semiconductor interchangeability and cross-reference standards. Devices which bear the same JEDEC number can directly substituted. For example: A 2N4123 transistor is an NPN device with specific voltage and current ratings, a specified gain (amplification factor), conforms to specific temperature standards, and is housed in the TO-92 plastic package having a standardized pin configuration. A device bearing this number can be substituted regardless of the manufacturer.

Component substitution is one of the most difficult problems facing industrial electricians and technicians.

Sheet 1

However, there are thousands of semiconductors that do not conform to JEDEC standards. In order to insure device interchangeability, many manufacturers of electronic systems purchase semiconductors that meet their specific system requirements and then assign their own part numbers.

XIST2


Transistor Types

Introduction There are three main classifications of transistors each with its own symbols, characteristics, design parameters, and applications. See below and the following pages for additional details and applications on each of these transistor types. Several special-function transistor types also exist which do not fall into the categories below, such as the unijunction (UJT) transistor that is used for SCR firing and time delay applications. These specialfunction devices are described separately. Bipolar transistors are considered current driven devices and have a relatively low input impedance. They are available as NPN or PNP types. The designation describes the polarity of the semiconductor material used to fabricate the transistor.

•

Field Effect Transistors, FET’s, are referred to as voltage driven devices which have a high input impedance. Field Effect Transistors are further subdivided into two classifications: 1) Junction Field Effect Transistors, or JFET’s, and 2) Metal Oxide Semiconductor Field Effect Transistors or MOSFET’s.

•

Insulated Gate Bipolar Transistors, known as IGBT’s, are the most recent transistor development. This hybrid device combines characteristics of both the Bipolar Transistor with the capacitive coupled, high impedance input, of the MOS device.

DEVICE NAME

SYMBOL NPN

Bipolar Transistor

BASE

BASE

P-CHANNEL

DRAIN

DRAIN GATE

GATE

SOURCE

SOURCE

N-CHANNEL

P-CHANNEL

DRAIN

DRAIN SUB

GATE

SOURCE COLLECTOR

GATE

EMITTER

GATE

SUB SOURCE

Input voltage signal is applied to the gate-source junction in a reverse biased mode, resulting in a high input impedance. Input signal varies the source-to-drain internal resistance. Applications include high input impedance amplifier circuitry. Similar to the JFET above except the input voltage is capacitive coupled to the transistor. The device is easily fabricated, inexpensive, and has a low power drain, but is easily damaged by static discharge. Computer chips utilize CMOS Similar to the Bipolar NPN above except the input voltage is capacitive coupled to the transistor as with the MOSFET devices. Main application is as a switch for the output section of small and medium size Variable Frequency Drives (VFD’s).

Sheet 2

IGBT Insulated Gate Bipolar Transistor

Used as amplifiers or switches in a wide variety of equipment ranging from small signal applications to high power output devices.

EMITTER

N-CHANNEL

MOS Metal Oxide Semiconductor Field Effect Transistor

A small input current signal flowing emitter-to-base in the transistor controls the transistor emitter-tocollector internal resistance.

COLLECTOR

EMITTER

FET Junction Field Effect Transistor

CHARACTERISTICS PNP

COLLECTOR

Transistors

•

XIST3

Transistor Fundamentals Bipolar Transistors


Introduction Bipolar transistors have the following characteristics: Bipolar transistors are a three-lead device having an Emitter, a Collector, and a Base lead.

•

The Bipolar transistor is a current driven device. A very small amount of current flow emitter-to-base (base current measured in microamps - µA) can control a relatively large current flow through the device from the emitter to the collector (collector current measured in milliamps - mA). Bipolar transistors are available in complimentary polarities. The NPN transistor has an emitter and collector of N-Type semiconductor material and the base material is P-Type semiconductor material. In the PNP transistor these polarities are reversed: the emitter and collector are P-Type material and the base is N-Type material.

•

NPN and PNP transistors function in essentially the same way. The power supply polarities are simply reversed for each type. The only major difference between the two types is that the NPN transistor has a higher frequency response than does the PNP (because electron flow is faster than hole flow). Therefore high frequency applications will utilize NPN transistors.

Transistors

•

Note: Bipolar transistors are usually connected in the Common Emitter Configuration meaning that the emitter lead is common to both the input and output current circuits. The Common Collector and the Common Base configurations are sometimes used in the input or output stages of an amplifier when impedance matching is required. The following discussion is limited to the Common Emitter Configuration characteristics.

NPN Transistor Simplified Diagram COLLECTOR

N BASE

PNP Transistor Simplified Diagram

Construction •

The bipolar transistor is a three-layer semiconductor.

•

The base lead connects to the center semiconductor material of this three-layer device. The base region is dimensionally thin compared to the emitter and collector regions.

•

Two PN (diode) junctions exist within a bipolar transistor. One PN junction exists between the emitter and the base region, a second exists between the collector and the base region. (See How to Test a Bipolar Transistor on Sheet 4.)

P

COLLECTOR

P BASE

P

N

Bipolar Transistor Symbols EMITTER

•

NPN Symbol COLLECTOR

EMITTER

The arrow is always on the emitter lead and points in the direction of conventional current flow (positive-to-negative). As with the diode, the nose of the arrow points to the negative, or N-Type semiconductor material, and the tail of the arrow is toward the P-Type material.

PNP Symbol COLLECTOR

•

The arrow on the NPN points away from the base. (Remember as NPN = Not Pointing iN.)

•

The arrow on the PNP points toward the base. (Remember as PNP = Pointing iN Pointer.)

BASE

BASE

EMITTER

Sheet 3

EMITTER

N

DIODECK

Diode Test Procedure



Unlike its predecessor, the Analog Ohmmeter, Digital Ohmmeters require a special Diode Check Function because the current circulated by the normal Ohms Function of a digital meter is too low to adequately check a diode. In the Diode Check Position, the reading given by a digital meter in the forward bias direction (meter positive to diode anode and meter negative to diode cathode) is actually the voltage required to overcome the internal diode junction potential. For a silicon diode this will be about 0.5 - 0.8 volt; a germanium diode will read slightly lower, about 0.3 - 0.5 volt. Symbol Notation K (or C) = Cathode, A = Anode.

Select

K

A

A

K

Reverse Bias - Diode Blocks Correct reading: TPI Meter will read OUCH (open circuit).

Forward Bias - Diode Conducts Correct reading: Meter will read about 0.5 - 0.8 volt. Incorrect readings: If diode reads 0 in both directions, it is shorted. If it reads OUCH (open circuit) both directions, it is open.

Diode Test Procedure WARNING! Ohms and Diode Check measurements can be made only on de-energized circuits! The Ohmmeter battery provides power to make this measurement. You may need to remove the diode from the circuit to get a reliable test. See Note below. •

Plug in the meter leads as shown: Black lead - COM (Common), Red lead - Ω (Ohms).

•

Select the

•

Connect the leads to the Diode-Under-Test as shown in the drawing above and verify the readings are correct for both a forward and reverse bias. (This is sometimes referred to as checking the front-to-back ratio.)

(Diode Test) function.

Stud-Mounted Rectifiers may be either Standard Polarity (Stud Cathode - Upper Left Illustration) or Reverse Polarity (Stud Anode - Lower Right ).

Band Identifies Cathode A

K

If unmarked, you can test the diode to determine its polarity. With the meter connected as above, when the meter indicates the diode is conducting (about 0.5 - 0.8 volt) the red lead is connected to the diode anode and the black lead to the cathode.

AN0001

Typical Axial-Lead Diode

Application Note

Note: Large Stud-Mounted Diodes are bolted to a heat sink and Hockey Puck Units are compressed between the heat sinks; removing them from the circuit can be time-consuming and may be unnecessary. In these situations, test the entire assembly first, then, if the assembly tests shorted, remove and test the diodes individually. Hockey Puck Diodes must be compressed in a heat sink assembly or test fixture to be tested as they require compression to make-up the internal connections.

XIST4


Transistor Test Procedure

An ohmmeter can be used to test the base-to-emitter PN junction and the base-to-collector PN junction of a bipolar junction transistor in the same way that a diode is tested. You can also identify the polarity (NPN or PNP) of an unknown device using this test. In order to do this you will need to be able to identify the emitter, base, and collector leads of the transistor. Refer to a semiconductor data reference manual if you are not sure of the lead identification. Note: While this test can be used to determine that the junctions are functional and that the transistor is not open or shorted, it will not convey any information about the common emitter current gain (amplification factor) of the device. A special transistor tester is required to measure this parameter known as the Hfe or Beta. .

PNP Test Procedure



•


•


PNP Transistor Simplified Diagram COLLECTOR

Select Diode

P N

BASE

P

EMITTER

NPN Test Procedure



•


•


COLLECTOR

N BASE

P N

EMITTER

Sheet 4

• NPN Transistor Simplified Diagram

Select Diode

Transistors

•

XIST5

Transistor Specifications NPN Bipolar Transistor


Transistor Curves A number of performance curves are published on any particular transistor. The Collector Characteristic Curves are among the most useful. This set of curves plots the Collector-Emitter Voltage (VCE ) and the Collector Current ( IC ) in milliamps for various values of Base Current ( Ib ) in microamps. In the drawing below each curve represents a base current step of 5 microamps beginning with the bottom curve and progressing upward.

Curve Interpretation

Load Line

Saturation

30µa Base Current

The following design consideration refers to the schematic diagram on the following page. In this example a power supply voltage of 30 volts DC was selected and the maximum collector current established at 20 milliamps. •

Before the Collector Characteristic Curves can be utilized a load line must be established which shows the circuit operation of the specific application. Here the maximum applied voltage VCE is shown by the red dot and the Maximum Collector Current IC is shown by the green dot. A load line has been constructed between these two points.

•

To evaluate the circuit operation, select a specific base current and follow it to the intersection of the base current line and the load line (shown by the yellow dot). From intersection of the selected curve and the load line, project straight down to determine the VCE (the voltage which will appear across the transistor from emitter to collector as a result of the 30 microamp base current) and project straight across to determine IC (the current which will flow in the collector as a result of the specified base current.

5 µa Curve

Cutoff

VCE

Region

• Saturation Region The transistor is fully turned ON and the value of collector current IC is determined by the value of the load resistance RL . The voltage drop across the transistor VCE is near zero. • Cut Off Region The transistor is fully turned OFF and the value of the collector current IC is near zero. Full power supply voltage appears across the transistor. Because there is no current flow through the transistor, there is no voltage drop across the load resistor RL .

Transistors

Region

2N4123

In this example for a base current Ib of 30 microamps: the transistor collector voltage VCE across the transistor will be 15 volts, and the collector current IC is 10 milliamps. The voltage across the amplifier load resistor RL will be the difference between the power supply voltage of 30 VDC and the 15 volts dropped across the transistor.

• Active Region (Linear Amplification Area) Is the region to the left of the load line. Linear amplifiers operate in this area of the curves.

The Common Emitter Configuration The emitter lead is common to both the input and output current loops. This is the most common circuit configuration because it provides both a current gain and a voltage gain. The common base and common collector configurations are generally used for impedance matching only.

There is a 180 degree phase shift between the input and output signals in the common emitter configuration.

Sheet 5

The common emitter current gain is defined as the BETA or Hfe (which stands for: H parameters, forward current transfer ratio, common emitter configuration).

XIST6


Transistor Amplifiers NPN Bipolar Transistor Amplification

An amplifier is a circuit that uses a small input variable to control a larger output quantity. Amplifiers may be electronic, electrical, hydraulic, pneumatic or mechanical. In the case of a bipolar transistor amplifier a small base current in microamps changes the transistor internal resistance and controls a larger amount of current in milliamps or amps which flows through the transistor emitter to collector. The emitter to collector current is sourced by the system power supply. The transistor load is normally placed in the collector circuit of the transistor. Transistor amplifiers can amplify either AC or DC signals. A single transistor circuit will have a specific circuit gain, or amplification factor. Where additional gain is required multiple stages of amplification are employed. Single-stage NPN Transistor Amplifier

Power Supply 30 VDC + Note 1

_

mA

Amplifiers

RL Load Resistor Output Coupling Capacitor Input Coupling Capacitor

Small AC Input Signal

Note 1: Note 2: Note 3: Note 4:

RB2

RB1 C µA

2N4123

Note 2

E

Amplified AC Signal Phase-Shifted 180o

Placement of milliammeter for measurement of transistor collector current. Placement of microammeter for measurement of transistor base current. See previous page for the transistor collector characteristic curves and operating parameters for this amplifier. This is a common emitter amplifier; the emitter lead is common to both the input and output signal loops.

Circuit Analysis Biasing - The two rules for biasing a common emitter amplifier (either NPN or PNP) are: 1) The emitter-to-base junction is always forward biased. In this example because the transistor is an NPN the base is P-Type material. The voltage divider consisting of RB1 and RB2 provides this forward bias as the base will be positive with respect to the emitter. Resistors are sized to set the quiescent or steady state operating point at the middle of the load line (shown by the yellow dot on load line). 2) The collector is always reverse biased. Because this is an NPN the collector is N-Type material so the collector is connected to the power supply positive. Load Resistor - Is sized to limit the collector current to 20 milliamps (shown by the green dot on the curves) when the transistor is fully turned on (saturated). Use Ohms Law to calculate this: Power supply voltage divided by IC. Supply Voltage - Any voltage can be used as long as it is below the maximum allowable collector voltage for the transistor (which for the 2N4123 is 40 VDC). 30 VDC has been chosen for this example and is shown by the red dot. Input Signal - This is the AC signal to be amplified. For example: a microvolt radio signal off of an antenna. This signal passes through the input coupling capacitor and adds to the base bias during the positive half-cycle and subtracts from the base bias during the negative half-cycle. It is said that the signal “swings around the base bias”. Sheet 6

Output Signal - The AC input signal applied to the transistor base causes the DC collector current to vary from its quiescent steady-state value upward and downward at an AC rate. The AC component of the signal then passes through the output coupling capacitor for further amplification or detection. To prevent output waveform distortion (amplitude limiting or clipping) the output signal should not hit the cutoff or saturation levels of the transistor.

Transformer Polarity


The Importance of Polarity An understanding of polarity is essential to correctly construct three-phase transformer banks and to properly parallel single or three-phase transformers with existing electrical systems. A knowledge of polarity is also required to connect potential and current transformers to power metering devices and protective relays. The basic theory of additive and subtractive polarity is the underlying principle used in step voltage regulators where the series winding of an autotransformer is connected to either buck or boost the applied line voltage. Transformers

Transformer Polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to the way the leads are brought out of the transformer. Bushing Arrangement The position of the High Voltage Bushings is standardized on all power and instrument transformers. The rule is this: when facing the low voltage bushings, the Primary Bushing H1 is always on the left-hand side and the Primary Bushing H2 is on the right-hand side (if the transformer is a three-phase unit, H3 will be to the right of H2). Distribution Transformers are Additive Polarity and the H1 and X1 bushings are physically placed diagonally opposite each other. Since H1 is always on the left, X1 will be on the right-hand side of a distribution transformer. This standard was developed very early in the development of electrical distribution systems and has been adhered to in order to prevent confusion in the field when transformers need to be replaced or paralleled with existing equipment. Instrument Transformers (PT’s and CT’s) and large substation transformers are Subtractive Polarity, so the H1 and X1 Bushings will be on the same side of the transformer. This standard was later adopted to make it easier to read electrical schematics and construct phasor diagrams. Additive Polarity Primary Bushing H1

Primary Bushing H2

H1 and X1 bushings are located diagonally opposite. Secondary Bushing X3

Secondary Bushing X1

Secondary Bushing X2 (Neutral)

A Typical Distribution Transformer Two-bushing primary and center-tapped 120 / 240 volt three-bushing secondary. Subtractive Polarity . Primary H1Terminal

Primary H2 Terminal

H1 and X1 on same side of transformer.

Secondary X2 Terminal 24 kV Potential Transformer

Sheet 1

Secondary X1 Terminal

Transformer Polarity Test


Polarity Test In situations where the secondary bushing identification is not available or when a transformer has been rewound, it may be necessary to determine the transformer polarity by test. The following procedure can be used.

Transformers

The H1 (left-hand) primary bushing and the left-hand secondary bushing are temporarily jumpered together and a test voltage is applied to the transformer primary. The resultant voltage is measured between the right-hand bushings. If the measured voltage is greater than the applied voltage, the transformer is Additive Polarity because the polarity is such that the secondary voltage is being added to the applied primary voltage. If, however, the measured voltage across the right-hand bushings is less than the applied primary voltage, the transformer is Subtractive Polarity. Note: For safety and to avoid the possibility of damaging the secondary insulation, the test voltage applied to the primary should be at a reduced voltage and should not exceed the rated secondary voltage. In the example below, if the transformer is actually rated 480 - 120 volts, the transformer ratio is 4:1 (480 / 120 = 4). Applying a test voltage of 120 volts to the primary will result in a secondary voltage of 30 volts (120 / 4 = 30). If transformer is subtractive polarity, the voltmeter will read 90 volts (120 - 30 = 90). If the voltmeter reads 150 volts, the transformer is additive polarity (120 + 30 = 150). The red arrows indicate the relative magnitude and direction of the primary and secondary voltages.

120 VAC

120 VAC Ind = 90

Temporary Jumper

V

30 VAC

Ind = 150 Temporary Jumper

V

30 VAC

Sheet 2

Instrument Transformers


Instrument Transformers Current Transformers and Potential (voltage) Transformers are used to supply a reduced value of current or voltage to instrument circuits. They provided isolation from the high voltage system, permit grounding of the secondary circuit for safety, and step-down the magnitude of the measured quantity to a value that can be safely handled by the instruments. Transformers

Burden The load on an instrument transformer is referred to as a “burden”. Polarity All instrument transformers are subtractive polarity.

Potential Transformers (PT’s) are voltage transformers which are used to supply a proportional voltage to the voltage input of metering and relaying equipment. The standard PT secondary voltage is 120 VAC to match the standard full-scale value of switchboard indicating instruments, power metering equipment, and protective relays. The transformer high-side voltage rating will be the same as the nominal system voltage. PT ratios are expressed as the ratio of the high voltage divided by the secondary voltage. The Potential Transformer pictured at the left has a 24000 volt (24 kV) primary and a standard 120 volt secondary, so the ratio is 200:1 (24000 / 120 = 200).

200:1 Two-Bushing PT

Where three-phase Delta systems are metered, 2 two-bushing PT’s would normally be connected open-delta. Three-phase Wye systems would normally be metered with 3 single-bushing PT’s connected phase-to-ground, or with 3 two-bushing PT’s, rated for the phase-to-ground voltage, having one primary bushing on each transformer connected to ground.

Current Transformers (CT’s) are used to supply a proportional current to the current input of metering and relaying equipment. The standard CT secondary current is 5 amps to match the standard full-scale current rating of switchboard indicating devices, power metering equipment, and protective relays. CT ratios are expressed as a ratio of the rated primary current to the 5 amp secondary. The 300:5 CT pictured below will produce 5 amps of secondary current when 300 amps flows through the primary. As the primary current changes the secondary current will vary accordingly. For example, with 150 amps through the 300 amp rated primary, the secondary current will be 2.5 amps ( 150 : 300 = 2.5 : 5 ). On the Window or Donut-type CT’s, such a pictured below, the conductor, bus bar, or bushing which passes through the center of the transformer constitutes one primary turn. On Window-type units with low primary current ratings, where the primary conductor size is small, the ratio of the transformer can be changed by taking multiple wraps of the primary conductor through the window. If, for example, a window CT has a ratio of 100:5, placing two primary conductor wraps (two primary turns) through the window will change the ratio to 50:5. Some types of equipment employ this method to calibrate the equipment or to permit a single ratio CT to be utilized for several different sizes (ampacities) of equipment. Caution: the secondary of a Current Transformer must always have a burden (load) connected; an open-circuited secondary can result in the development of a dangerously-high secondary voltage. Draw-out type meter and relay cases incorporate shorting contacts which short-circuit the CT secondary when the instrument is removed from the circuit for calibration. X2

X1 Secondary (X1) Polarity Mark

Polarity marks identify (are adjacent to) the H1 and X1 terminals.

300:5 Window-Type CT

Sheet 3

Primary (H1) Polarity Mark

Polarity Marking


Transformers

Polarity Marks To insure correct wiring, polarity marks are shown on Instrument Connection Diagrams, Control Schematics, and Three-Line Power Diagrams. The polarity mark is usually shown as a round dot, on or adjacent to, the H1 and X1 terminals of PT’s and CT’s. Sometimes alternate marking, in the form of a square dot, slash mark ( / ), or plus/minus sign ( + ) will be used to identify the polarity terminals on electrical drawings. Instrument transformers may also have the terminals identified with polarity marks as shown in the illustration of the 300:5 CT on Sheet 3. If instrument transformers do not have polarity marks on them, it is understood that the H1 (primary) and the X1 (secondary) terminals are polarity. Meters, relays, and other equipment which require proper polarity connections may also have polarity marks, but usually this information must be obtained from the Instrument Connection Diagram.

CT Primary Polarity Mark PT Primary Polarity Mark PT Secondary Polarity Mark

Transformer Secondary Circuit Grounds

CT Secondary Polarity Mark

Instrument Voltage Coil Instrument Current Coil

Typical Current and Potential Transformer Connection Diagram The PT, CT, and instrument polarity marks are shown by the red dots on the above drawing. (Red dots were used in this example only for clarity.) Current elements of the instruments are connected in series, voltage inputs are connected in parallel. Polarity is not a consideration on single-element devices such as an ammeter or voltmeter, but is essential for proper operation of power measuring devices, and for directional or differential protective relays.

Sheet 4

TAB6

Manufacturers’ Literature File


Don’t throw those instructions away ! File them in Section 6 of your Industrial Electrician’s Notebook. Section 6 of the Industrial Electrician’s Notebook has been reserved for filing Manufacturers’ Data Sheets. Next time you open a box containing an electrical component, save the literature by inserting it in your own copy of the Electrician’s Notebook . There is a wealth of information on those little, folded-up sheets supplied with every electrical component. So, whether it be a push button, contact block, relay, lighting ballast, control transformer, fuse link, overload heater, or other small component, read the information and then save it for future reference. Inclusion of the data sheet in your notebook will, during the course of your electrical career, result in the compilation of valuable reference library; you’ll be surprised how little time it takes and how often you will refer to it.

Save those manufacturers’ data sheets.

Instructions

To begin your collection of data sheets, use your browser back button to return to the Electrician’s Notebook Page and click the Typical Data Sheet link to download a two-page data sheet on auxiliary contacts provided by General Electric.

Sheet 1

SYNCMTR1

Synchronous Motor Characteristics


Synchronous Motors are three-phase AC motors which run at synchronous speed, without slip.

Sync Motors

(In an induction motor the rotor must have some “slip”. The rotor speed must be less than, or lag behind, that of the rotating stator flux in order for current to be induced into the rotor. If an induction motor rotor were to achieve synchronous speed, no lines of force would cut through the rotor, so no current would be induced in the rotor and no torque would be developed.) Synchronous motors have the following characteristics: •

A three-phase stator similar to that of an induction motor. Medium voltage stators are often used.

•

A wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC field current to the rotor. The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the rotor to “lock-in-step” with the rotating stator flux.

•

Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an Amortisseur winding, which produces torque for motor starting.

•

Synchronous motors will run at synchronous speed in accordance with the formula: 120 x Frequency Synchronous RPM = Number of Poles Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be: 120 x 60 / 24 = 7200 / 24 = 300 RPM

Synchronous Motor Operation •

The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to bring the synchronous motor up to speed.

•

When the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the rotor producing Pull-in Torque and the rotor will pull-in -step and “synchronize” with the rotating flux field in the stator. The motor will run at synchronous speed and produce Synchronous Torque.

•

After synchronization, the Pull-out Torque cannot be exceeded or the motor will pull out-of-step. Occasionally, if the overload is momentary, the motor will “slip-a-pole” and resynchronize. Pull-out protection must be provided otherwise the motor will run as an induction motor drawing high current with the possibility of severe motor damage. Advantages of Synchronous Motors

The initial cost of a synchronous motor is more than that of a conventional AC induction motor due to the expense of the wound rotor and synchronizing circuitry. These initial costs are often off-set by: Precise speed regulation makes the synchronous motor an ideal choice for certain industrial processes and as a prime mover for generators.

•

Synchronous motors have speed / torque characteristics which are ideally suited for direct drive of large horsepower, low-rpm loads such as reciprocating compressors.

•

Synchronous motors operate at an improved power factor, thereby improving overall system power factor and eliminating or reducing utility power factor penalties. An improved power factor also reduces the system voltage drop and the voltage drop at the motor terminals.

Sheet 1

•

SYNCMTR2


Synchronous Motor Construction

2000 Horsepower Synchronous Motor In Refinery Service

Characteristics and Features The rotation of a synchronous motor is established by the phase sequence of the three-phase AC applied to the motor stator. As with a three-phase induction motor, synchronous motor rotation is changed by reversing any two stator leads. Rotor polarity has no effect on rotation.

•

Synchronous motors are often direct-coupled to the load and may share a common shaft and bearings with the load.

•

Large synchronous motors are usually started acrossthe-line. Occasionally, reduced voltage starting methods, such as autotransformer or part-winding starting, may be employed.

Electric Machinery Photo

Sync Motors

•

DC field leads (2) attached to shaft.

Synchronous Motor Rotors Bearing Retainer •

The Salient-Pole unit shown at the right is a brush-type rotor that uses slip rings for application of the DC field current.

•

Low voltage DC is used for the rotating field. 120 VDC and 250 VDC are typical.

•

Slip ring polarity is not critical and should be periodically reversed to equalize the wear on the slip rings. The negative polarity ring will sustain more wear than the positive ring due to electrolysis.

•

Slip rings are usually made of steel for extended life.

Bearing DC Slip Rings ( 2 ) Wound Field Poles Amortisseur (Squirrel Cage) Winding Electric Machinery Photo

Detail of Amortisseur Winding Synchronous motors start as an induction motor utilizing the Amortisseur winding which is a squirrel-cage-type winding with short-circuited rotor bars. Wound Field Pole - Energized by separate source of DC for synchronous operation. Squirrel-Cage Rotor Bars


Sheet 2

Shorting Ring - One on each end of rotor.

SYNCMTR3


Synchronous Motor Brush-Type Excitation Systems Excitation Methods

Two methods are commonly utilized for the application of the direct current (DC) field current to the rotor of a synchronous motor. Brush-type systems apply the output of a separate DC generator (exciter) to the slip rings of the rotor.

•

Brushless excitation systems utilize an integral exciter and rotating rectifier assembly that eliminates the need for brushes and slip rings.

Sync Motors

•

Brush-Type Excitation System Three-Phase AC High-Voltage Stator Power

Field Control Single -Phase Control Power Field Excitation Control

Static Field Control 52a 52

Negative DC Brushes

Breaker (or Running Contactor)

56

Stator Windings (AC)

Exciter Drive Commutator Positive DC Brushes Exciter Stationary Field Pole

Output Shaft to Driven Load Rotating Field (DC)

56 Exciter Field Application Relay Slip Rings

Synchronous Motor Kilowatt Classroom Drawing

System Analysis In this excitation method the DC field current for the synchronous motor is provided by a separate DC generator known as an exciter. The exciter is a shunt-or compound-wound DC machine that is driven either by the synchronous motor itself (dashed line) or by a separate drive motor. Excavators, for example, often have an “exciter line” consisting of a number of exciters which are driven by an single AC induction motor. The shunt field of the exciter is separately excited by the solid state control. Some excitation controls provide for manual adjustment of the field strength. Other systems automatically regulate the synchronous motor field in a closed-loop configuration designed to maintain adequate field strength for varying loads or to maintain a constant power factor. The exciter shunt field is energized when the 52a auxiliary contact in the main breaker closes.. In the above illustrated system, the exciter shunt-field strength controls the DC output of the exciter which is picked off by the commutator brushes, bused to the motor slip-ring brushes, and applied via the slip rings to the main rotating field of the synchronous motor. Sheet 3

The synchronous motor starts as a induction motor. When the rotor achieves near-synchronous speed, the motor field current is applied by the closure of the Field Application Relay (Standard Device Designation #56).

SYNCMTR4


Synchronous Motor Brushless Exciters Brushless Excitation System Three-Phase AC High-Voltage Stator Power

Field Control

Field Excitation Control

Static Field Control

Sync Motors

Single -Phase Control Power

52a 52

Rotating Rectifier Assy

Breaker (or Running Contactor)

Common Shaft

Exciter Stationary Field Poles

Stator Windings (AC) Output Shaft to Load

Exciter Rotor

Rotating Field (DC) Exciter Synchronous Motor Field Application SCR Kilowatt Classroom Drawing

Brushless Machine Rotor

Exciter Rotor

Motor Rotor (Field) Rotating Rectifier Assembly

Cage Winding Shorting-Rings


System Analysis This excitation method eliminates the need for brushes, both on the exciter and the motor. When the motor is started the machine breaker (Std Device #52) closes and applies three-phase AC to the motor stator windings. The motor starts as an induction motor using the Amortisseur winding in the rotor.

Sheet 4

The Machine Breaker 52a auxiliary contact also closes and applies the DC output of the solid-state Field Control to the exciter stationary winding. A three-phase alternating current is induced in the exciter rotor windings and this induced voltage is rectified by the rotating rectifier assembly. When the rotor achieves near synchronous speed the Field Application SCR is fired by the Synchronizing Control Package and the rectified DC is applied to the synchronous motor rotating field. See schematic on the next page for additional details.

SYNCMTR5


Synchronous Motor Synchronizing Principle

Three-Phase AC

Schematic Diagram Synchronous Motor Brushless Excitation System

Field Monitor Relay 78 Device

Exciter Control Trip

Exciter Stationary Field

Positive Bus

PT Input Sync Motors

Field Monitor Relay monitors the power factor of the system and trips the motor and exciter field off if synchronism is not achieved within a specific length of time or if the motor pulls out-of-step.

Breaker Trip

CT Input

SCR1

FDR

Exciter Armature 3-Phase AC Out

Field Application Circuit

Rotating Field

Stator

SCR2

Sync. Motor Negative Bus Kilowatt Classroom Drawing

Rotating Components Field Application System

The Field Application Circuit in a synchronous motor excitation system must perform three functions: •

Provide a discharge path for the current which is induced into the wound rotor during start and open this circuit when excitation is applied. During start the motor is operating as an induction motor with the torque being produced by the squirrel cage winding. The wound rotor is also being cut by the rotating stator flux and has a voltage induced in it. During this phase of the start-up SCR2 in the above diagram is gated “on” by the Field Application Circuit and provides a discharge path for the induced rotor current through the Field Discharge Resistor (FDR) as shown by the dashed red arrows. The frequency of this induced rotor current “tells” the application circuit the speed at which the rotor is running. See oscilloscope waveform below.

•

When the rotor speed reaches about 97% of synchronous and the rotor polarity is correct to achieve synchronism, SCR2 will turn “off ” and SCR1is gated “on” allowing the rectified DC current from the rotating threephase bridge rectifier to flow through the rotating field, as shown by the green dashed arrows, producing the necessary Synchronizing Torque for the rotor to pull-in step with the rotating stator flux.

•

The Field Application Circuit must remove excitation immediately if the motor pulls out-of-step.

60 Hz

50% Percent Motor Speed

90%

Motor Synchronized 3 Hz

95%

Waveform of Induced Field Current During Start

Sheet 5

0%

Frequency of Field Discharge Current 30 Hz 6 Hz

SYNCMTR6

Synchronous Motor Power Factor


Synchronous Motor Power Factor

Sync Motors

An important advantage of a synchronous motor is that the motor power factor can be controlled by adjusting the excitation of the rotating DC field. Unlike AC induction motors which always run at a lagging power factor, synchronous motors can run at unity or even at a leading power factor. This will improve the over-all electrical system power factor and voltage drop and also improve the voltage drop at the terminals of the motor. (See The Electrician’s Notebook article Principles of Voltage Regulation for a description of how improving the system power factor also improves the system voltage drop.)

Typical “V” Curves

Interpreting “V” Curves The synchronous motor “V Curves” shown above illustrate the effect of excitation (field amps) on the armature (stator) amps and on system power factor. There are separate “V” Curves for No-Load and Full-Load and sometimes the motor manufacturer publishes curves for 25%, 50%, and 75% load. Note that the Armature Amperage and Power Factor “V” Curves are actually inverted “V’s”. Assume it is desired to determine the field excitation which will produce unity power factor operation at full motor load. Project across from the unity power factor (100%) operating point on the Y-Axis to the peak of the inverted Power Factor “V” Curve (blue line). From this intersection, project down (red line) from the full-load unity power factor (100%) operating point to determine the required field current on the X-Axis. In this example the required DC field current is shown to be just over 10 amps. Note at unity power factor operation the armature (stator) full-load amps is at the minimum value. Increasing the field amps above the value required for unity power factor operation will cause the machine to run with a leading power factor, while field weakening caused the motor power factor to become lagging. When the motor runs either leading or lagging, the armature (stator) amps increases above the unity power factor value. Sheet 6

SCR1

SCR Silicon Controlled Rectifier


This article, written by Elwood Gilliland, was first published in the June, 1982 Issue of Electrical Contractor Magazine. A portion of the original material is reproduced here with permission of the author.

Definition The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member of a family of control devices known as Thyristors. The SCR has become the workhorse of the industrial control industry. Its evolution over the years has yielded a device that is less expensive, more reliable, and smaller in size than ever before. Typical applications include : DC motor control, generator field regulation, Variable Frequency Drive (VFD) DC Bus voltage control, Solid State Relays and lighting system control. The SCR is a three-lead device with an anode and a cathode (as with a standard diode) plus a third control lead or gate. As the name implies, it is a rectifier which can be controlled - or more correctly - one that can be triggered to the “ON” state by applying a small positive voltage ( VTM ) to the gate lead.

•

Once gated ON, the trigger signal may be removed and the SCR will remain conducting as long as current flows through the device.

•

The load to be controlled by the SCR is normally placed in the anode circuit. See drawing below.

Gate Lead (White)

Cathode Lead

Thyristors

•

Auxiliary Cathode Lead (Red) Extends cathode potential to the control circuit.

Commutation For the SCR to turn OFF the current flow through the device must be interrupted, or drop below the Minimum Holding Current ( IH ) , for a short period of time (typically 10 -20 microseconds) which is known as the Commutated-Turn-OffTime ( tq ). •

•

When applied to Alternating Current circuits or pulsating DC systems, the device will self-commutate at the end of every half -cycle when the current goes through zero. When applied to pure DC circuits, in applications such as alarm or trip circuit latching, the SCR can be reset manually by interrupting the current with a push button. When used in VFD’s or inverters, SCRs are electronically forced OFF using additional commutating circuitry, such as smaller SCRs and capacitors, which momentarily apply an opposing reverse-bias voltage across the SCR. (This is complicated - everything has to be exactly right.)

Stud Anode

Stud- Mounted SCR 110 Amp RMS Rating

The GTO

Anode AC SOURCE

Another member of the Thyristor Family is the GTO, or Gate-Turn-Off Device. While this component has been around for many years, it has just recently evolved to the point where it is capable of carrying the high currents required for motor control circuitry.

Cathode

SCR Connection Diagram Showing load placement in the anode circuit. This arrangement would provide control of onehalf of the sine wave. For full-wave control, the SCRs would be arranged in a bridge configuration.

Sheet 1

Unlike the SCR, the GTO can be turned ON and OFF with a signal applied to the gate. The turn-on signal is a small positive voltage; the turn-off signal is a negative current pulse. The GTO is now finding applications in the output stage of medium-voltage, high horsepower, Variable Frequency Drives.

GATE

SCR2

SCR Theory of Operation


Volt-Ampere Characteristics Figure One below illustrates the volt-ampere characteristics curve of an SCR. The vertical axis + I represents the device current, and the horizontal axis +V is the voltage applied across the device anode to cathode. The parameter IF defines the RMS forward current that the SCR can carry in the ON state, while VR defines the amount of voltage the unit can block in the OFF state. Biasing The application of an external voltage to a semiconductor is referred to as a bias. Forward Bias Operation A forward bias, shown below as +V, will result when a positive potential is applied to the anode and negative to the cathode.

•

Even after the application of a forward bias, the device remains non-conducting until the positive gate trigger voltage is applied.

•

After the device is triggered ON it reverts to a low impedance state and current flows through the unit. The unit will remain conducting after the gate voltage has been removed. In the ON state ( represented by +I), the current must be limited by the load, or damage to the SCR will result.

Thyristors

•

Reverse Bias Operation •

The reverse bias condition is represented by -V. A reverse bias exists when the potential applied across the SCR results in the cathode being more positive than the anode.

•

In this condition the SCR is non-conducting and the application of a trigger voltage will have no effect on the device. In the reverse bias mode, the knee of the curve is known as the Peak Inverse Voltage PIV (or Peak Reverse Voltage - PRV) and this value cannot be exceeded or the device will break-down and be destroyed. A good Rule-of -Thumb is to select a device with a PIV of at least three times the RMS value of the applied voltage.

SCR Volt-Amp Characteristics

REVERSE LEAKAGE CURRENT

NOTE: In the drawing that a small amount of leakage current through the device exists even when it is in the OFF state.

Sheet 2

CAUTION: When working on solid-state equipment, the equipment must be disconnected with a separate disconnecting means to insure that the equipment is deenergized; simply stopping the equipment may still result in the existence of a hazardous potential.

SCR3

SCR Phase Control


In SCR Phase Control, the firing angle, or point during the half-cycle at which the SCR is triggered, determines the amount of current which flows through the device. It acts as a high-speed switch which is open for the first part of the cycle, and then closes to allow power flow after the trigger pulse is applied. Figure Two below shows an AC waveform being applied with a gating pulse at 45 degrees. There are 360 electrical degrees in a cycle; 180 degrees in a half-cycle. The number of degrees from the beginning of the cycle until the SCR is gated ON is referred to as the firing angle, and the number of degrees that the SCR remains conducting is known as the conduction angle. The earlier in the cycle the SCR is gated ON, the greater will be the voltage applied to the load. Figure Three shows a comparison between the average output voltage for an SCR being gated on at 30 degrees as compared with one which has a firing angle of 90 degrees. Note that the earlier the SCR is fired, the higher the output voltage applied to the load. Thyristors

The voltage actually applied to the load is no longer sinusoidal, rather it is pulsating DC having a steep wavefront which is high in harmonics. This waveform does not usually cause any problems on the driven equipment itself; in the case of motor loads, the waveform is smoothed by the circuit inductance. However, radio or television interference can occur. Often times the manufacturer of the SCR equipment will include an Electro-MagneticInterference (EMI) filter network in the control to eliminate such problems.

Sheet 3

SCR4

SCR Protection / Firing Circuits / Testing


SCR Protection The SCR, like a conventional diode, has a very high one-cycle surge rating. Typically, the device will carry from eight to ten time its continuous current rating for a period of one electrical cycle. It is extremely important that the proper high-speed, current-limiting, rectifier fuses recommended by the manufacturer be employed - never substitute with another type fuse. Current limiting fuses are designed to sense a fault in a quarter-cycle and clear the fault in one-half of a cycle, thereby protecting the SCR from damage due to short circuits. Switching spikes and transients, which may exceed the device PIV rating, are also an enemy of any semiconductor. Surge suppressors, such as the GE Metal-Oxide-Varistor (MOV), are extremely effective in absorbing these shortterm transients. High voltage capacitors are also often employed as a means of absorbing these destructive spikes and provide a degree of electrical noise suppression as well.

Computing the Required Firing Angle Thyristors

For accurate SCR gating, the Firing Circuit must be synchronized with the AC line voltage being applied anodeto- cathode across the device. Without synchronization, the SCR firing would be random in nature and the system response erratic. In closed-loop systems, such as motor control, an Error Detector Circuit computes the required firing angle based on the system setpoint and the actual system output. The firing circuit is able to sense the start of the cycle, and, based on an input from the Error Detector, delay the firing pulse until the proper time in the cycle to provide the desired output voltage. An analogy of a firing circuit would be an automobile distributor which advances or retards the spark plug firing based on the action of the vacuum advance mechanism. In analog control systems the error detector circuit is usually an integrated circuit operational amplifier which takes reference and system feedback inputs and computes the amount of error (difference) between the actual output voltage and the desired setpoint value. Even though the SCR is an analog device, many new control systems now use a microprocessor based, digital, firing circuit to sense the AC line zero -crossing, measure feedback and compare it with the setpoint, and generate the required firing angle to hold the system in-balance.

Testing the SCR Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail shorted rather than open). Measure the anode-to-cathode resistance in both the forward and reverse direction; a good SCR should measure near infinity in both directions. Small and medium-size SCRs can also be gated ON with an ohmmeter (on a digital meter use the Diode Check Function). Forward bias the SCR with the ohmmeter by connecting the red ( + ) lead to the anode and the black ( - ) lead to the cathode. Momentarily touch the gate lead to the anode; this will provide a small positive turn-on voltage to the gate and the cathode-to-anode resistance reading will drop to a low value. Even after removing the gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the anode or cathode will cause the SCR to revert to its non-conducting state. When conducting the above test, the meter impedance acts as the SCR load. On larger SCRs, the unit may not latch ON because the test current is not above the SCR holding current. Special testers are required for larger SCRs in order to provide an adequate value of gate voltage and load the SCR sufficiently to latch ON.

Some equipment manufacturers provide tabulated ohmmeter check-data for testing SCR assemblies.

Sheet 4

Hockey puck SCRs must be compressed in a heat sink (to make-up the internal connections to the semiconductor) before they can be tested or operated.

Dedication During the forty-plus years that I’ve worked in the electrical field, more people than I can possibly mention have helped me understand the many facets of this special trade. Some were experts in electrical theory, while others stressed the importance of craftsmanship, the necessity for teamwork, or fostered the development of safe work practices. Often a fellow worker shared a technique to accomplish a task better, faster, or easier. By far the most influential of my mentors has been my father, Vail Gilliland, who always has time for questions. He continues to enthusiastically encourage me in my writing, teaching, and experimentation. His outstanding knowledge and skill, coupled with infinite patience and a willingness to share, are characteristics I greatly admire and appreciate. I dedicate this work to him. Preface The harnessing of electrical power to lighten mankind’s work-load is only a little over a century old. During my own career, I have witnessed many changes in equipment, work methods, industry organization, and company philosophies. In today’s fast-paced work environment, one of the greatest challenges electricians face is that of keeping abreast with technological change. Interestingly, part of the solution may be technology itself - we now have powerful, inexpensive computers and the Internet. These tools greatly simplify the tasks of writing, illustration, and dissemination of information. This medium seems the logical avenue to communicate the specialized knowledge that we as electricians need so acutely. I’ve long thought about compiling a book containing information that would help apprentices and electricians to better understand and perform their jobs. I’m planning to write and publish this information in seria l form on my web site www.kilowattclassroom.com. I envision a book that will provide, in addition to electrical theory and reference data, a section for manufacturers’ literature, a place to keep personal notes and records from various jobs, and a section to file other related technical articles and information. It’s also important, I believe, to remember the contributions of some of the electrical industry’s early pioneers – it helps us to keep our own knowledge in perspective - so the beginning of each chapter will contain some historical information as well. It will be in loose-leaf form so it can be easily updated with new information and permit removal of the data that is obsolete or no longer needed. You can, of course, download only the material which is of interest to you. Eventually, perhaps, these articles can be made available as an e-book with hyperlinks that can quickly jump the reader to related articles, definitions, or required reference data. I hope you find The Electrician’s Notebook interesting and helpful. Elwood V. Gilliland

The Electrician’s Notebook © 2002 Kilowatt Classroom, LLC. Published by Kilowatt Classroom, LLC. Electric Power Training for Industry 9267 Red Creek Road Casper, Wyoming 82601 www.kilowattclassroom.com The information made available free-of-charge at www.kilowattclassroom.com is for personal use only. All rights reserved. Reproduction or use without express permission, of editorial or pictorial content in any manner, is prohibited. While every precaution has been taken in the preparation of this book, the publisher assumes no responsibility for errors or omissions. Neither is any liability assumed for damages resulting from the use of the information contained herein.

PAM1

Phase Angle Measurement


Requirements for Phase Angle Measurement The phase angle meter is a valuable tool for verifying the proper installation of medium- and high-voltage primary metering equipment and sophisticated protective relays that receive input from Potential and Current Transformers (PTs & CTs).

•

Phase angle meters are also used to verify the correct connection of three-phase transformer banks which must be paralleled with an existing electrical bus or high voltage line. The process of making these measurements is known as “phasing-out” and is performed before the tie-in is made.

•

This equipment is also used for conducting electrical system load and power factor studies. The system power factor is equal to the cosine of the phase angle (expressed as a percent) that exists between the system voltage and current. Once the system power factor is determined, the system power triangle (true power in watts, apparent power in volt-amperes, and reactive power in vars) can be developed and analyzed.

•

Phase angle measurement is also employed to analyze the operation of AC synchronous generators and synchronous motors to verify the proper operation of field regulators and synchronizing equipment.

Measurements

•

Types of Meters Numerous manufacturers offer phase angle meters, either as a separate metering device, or as an integral part of AC power measurement and recording equipment. The display readout is generally digital but may also be analog viewed in quadrants, analog with a circular 360o scale, or as a phasor diagram displayed on a laptop computer. The ATS-100 Phase Angle Meter described in this article is a low-cost, easy to operate unit developed by Kilowatt Classroom LLC. It is unique in that it can measure the phase angle directly on distribution power lines to 34.5 kV using an insulated fiber optic link. Operation of the ATS-100 is similar to other stand-alone instruments and is featured in this article to illustrate the measurement procedure. For background information on this subject see The Industrial Electrician’s NotebookTM articles: Understanding Transformer Polarity, and Power in AC Circuits on the web @ www.kilowattclassroom.com

ATS-100 Phase Angle Meter Used for voltage-voltage or voltage-current phase angle measurements.

Analog Scale 360o Electrical Degrees Displayed in four quadrants.

Circuit One Reference Voltage 120 / 208-240 / 480 VAC

Circuit Two Accepts voltage or current input.

Scale Selection Switch

Switch / Label Side is Polarity Place this side toward current source or CT polarity mark.

Sheet 1

TPI A256 Current Adapter Permits direct voltage-current phase angle measurement on circuits to 400 amps and 600 VAC. (See Sheet 2 for listing of additional adapters.)

PAM2


ATS-100 Phase Angle Meter Instrument Arrangement

Front Panel Layout Measurements

3

1

CIRCUIT TWO LEADS CIRCUIT ONE

2

NORMAL

NULL

DELTA

4

5

See Sheet 3 for Scale Interpretation Instructions and Sheet 4 for Condensed Operating Instructions.

1.

CIRCUIT ONE - Reference Voltage Input. Input ranges 120 / 208-240 / 480 with respect to Common (COM).

2.

CIRCUIT TWO - Adapter Input Receptacle which accepts the following adapters: Voltage Adapter - 120, 208 - 240, and 480 VAC adapters are available. Low Current Adapter - TPI Model 254 (10 mA to 60 amps). Recommended for current measurements on the secondary of 5 amp Instrument Current Transformers (CT’s). High Current Adapter - TPI Model 256 (0 to 400 amps). For direct phase angle measurement on motors and other loads to 400 amperes. Fiber Optic Adapter - ATS Model 110 Receiver. For use with the ATS Model 111 Fiber Optic Transmitter which permits direct phase angle measurement on distribution power lines to 34.5 kV up to 400 amps. ANALOG METER - Displays the number of electrical degrees which CIRCUIT TWO leads CIRCUIT ONE. Two scales, 0 - 360 degrees in four quadrants, five degrees / division.

4.

SCALE SWITCH - Selects the UPPER (90o - 360o / 0 - 270o ) meter scale, or the LOWER (270o - 180o - 90o ) meter scale.

5.

DELTA NULL SWITCH - Used to simplify voltage-current phase angle measurement on Delta Systems. On a Delta System, at unity power factor, there is a 30o phase shift between the phase voltage and the line current. Holding this momentary-action switch in the NULL position will automatically compensate for this phase shift. (Switch is spring return to the NORMAL position.)

Sheet 2

3.

PAM3


ATS-100 Phase Angle Meter Scale Interpretation Measurement Standards Phase Angle Meters are manufactured using two different standards:

Measurements

1) Showing the number of degrees that Circuit Two leads Circuit One. 2) Showing the number of degrees that Circuit Two lags Circuit One. Because Phasors (electrical vectors) are always analyzed with a Counter Clockwise Rotation (CCW), the first standard, showing the number of degrees Circuit Two leads Circuit One, is more consistent with this theory and is employed on the ATS-100 instrument. The second standard, showing degrees of lag, is sometimes preferred when power factor measurements only are being made as the angle of lag for single-phase analysis will always be in the fourth quadrant. The ATS-100 instrument displays the 360o electrical degree measurement two quadrants at a time as determined by the position of the SCALE SWITCH (see previous page). The UPPER SCALE switch position is for Quadrants 1 & 4 with the number of degrees being read from the Upper 90o - 360o / 0o - 270o meter scale. The LOWER SCALE switch position is for Quadrants 2 & 3 and the number of degrees is read from the lower 270o - 180o - 90o meter scale. When making measurements, place the SCALE SWITCH in the position that gives an upscale reading.


Upper scale meter reading corresponds to red phasor position illustrated below.

90o Lower Meter Scale

Upper Meter Scale Circuit Two Voltage or Current Angle of Lead

Quadrant2 2 Quadrant

Quadrant 1 30o 0o

180o

360o Quadrant Quadrant 3 3

Circuit One Reference Voltage Zero Degrees

Quadrant 4

Sheet 3

270o

PAM4

ATS-100 Phase Angle Meter Condensed Operating Instructions


Refer to Illustration on Sheet 2

1)

Apply the reference potential to the appropriate CIRCUIT ONE input banana jacks. Use the red lead for the polarity (+) connection and the black lead for the common (COM) connection. (With voltage applied, the meter hand will move upscale from the position indicated by the dashed blue line to the 90o / 270o mark shown by the solid blue line.)

2)

Connect the appropriate voltage adapter with potential leads to the CIRCUIT TWO adapter input. Use the red lead for the polarity connection and the black lead for the non-polarity connection.

3)

Place the SCALE SWITCH in the position that provides an upscale reading and read the indicated phase angle from the appropriate scale. For example: if an upscale reading is obtained with the SCALE SWITCH in the UPPER position the reading indicated by the red hand would be read as 30 degrees; if an upscale reading is obtained with the SCALE SWITCH in the LOWER position, the reading would be taken from the LOWER scale, which for this example, would be read as 210 degrees. The meter scale indicates the number of electrical degrees that the potential applied to CIRCUIT TWO leads the reference voltage applied to CIRCUIT ONE.1)

Measurements

Making Voltage-Voltage Phase Angle Measurements

Note: All voltage-voltage phase angle measurements are made with the momentary-action DELTA NULL SWITCH in the NORMAL position. See Sheet 6 for information on determining the system phase rotation and Sheets 7 & 8 for details on making measurements and constructing a system phasor diagram.

Making Voltage-Current Phase Angle Measurements 1) Apply the reference potential to the appropriate CIRCUIT ONE input banana jacks. Use the red lead for the polarity (+) connection and the black lead for the common (COM) connection. 2)

Use the TPI A254 Low Current Adapter when making measurements on the 5 amp secondary side of instrument Current Transformers (CT’s), on small loads under 60 amperes (600 volts or less), or for analyzing small motor starting where the Locked Rotor Amps (LRA) is less than 60 amps. The TPI A256 Clamp Adapter should be used (600 volts or less) on for loads up to 400 amps, motor loads where the Full Load Amps (FLA) does not exceed 400 amps, or for analyzing motor starts where the LRA does not exceed 400 amps. Connect the appropriate current adapter to the CIRCUIT TWO adapter input. A special identification circuit in the adapter plug “tells” the instrument which adapter is being used. Place the Current Adapter Selector Switch in the AC position and place the adapter around the currentcarrying conductor . The side of the adapter with the writing and switch is the polarity side (+) and must be placed toward the power source or toward the current transformer polarity mark when measuring on the secondary side of a CT. Place the SCALE SWITCH in the position that provides an upscale reading. The meter scale indicates the number of electrical degrees that the current applied to CIRCUIT TWO leads the reference voltage applied to CIRCUIT ONE.

4)

The DELTA NULL SWITCH can be placed in the NULL position to compensate for the 30o phase shift that exists between the phase voltage and the line current in a Delta System. Leave the NULL switch in the NORMAL position for making Voltage-Current phase angle measurements on a WYE System. See Sheet 6 for information on determining the system phase rotation and Sheets 7 & 8 for details on making measurements and constructing a system phasor diagram.

Sheet 4

3)

PAM6


Transformer Bank Connections and Phasor Diagrams Paralleling Considerations

Measurements

The following connection diagrams illustrate how a change in the connection of three single-phase transformers in a three-phase bank will change the phasor diagram for the bank. This condition holds true for all bank configurations; only the Delta-Delta connection with additive polarity transformers is shown here. In order for the transformer banks to be paralleled, the banks must be connected so that the same phasor diagrams results. Prior to interconnection of a three-phase bank with an existing bank or an existing system (bus or line) the proper connection is verified by a process known as “phasing-out”. This can be accomplished on Low Voltage Systems (below 600 volts) using a voltmeter. On Medium Voltage Systems a Phase Angle Meter can be used to compare the 120 volt secondaries of Instrument Potential Transformers (PT’s), or insulated Phase Sticks, which incorporate a meter and voltage dropping resistors, can be employed. Lamp -type high voltage testers should not be used for phasing-out because a small angular difference between the systems, such as exists with a 30o phase shift, may not produce enough voltage to illuminate the lamp.

Angular Displacement 0 o H1

H2

Angular Displacement 180o H1

H3

H3

H2

H1 H2

H1 H2

H1 H2

H1 H2

H1 H2

H1 H2

X2

X2

X2

X2

X2

X2

X1

X1

X1

X1

X3

X2

The connection above produces the standard phase relationship illustrated below. This connection would be used internally on a three-phase transformer but would not normally be used for connecting three single-phase transformers where a standard phasor relationship is not required.

X1

H3

X1

X2

X1

X3

H2

X3

X1

The connection above produces the non-standard phase relationship illustrated below. This configuration is most common on distribution lines because of its simplicity (no crossed conductors) and will parallel with other banks provided they are wired exactly the same. This bank will not parallel with the one shown at the left.

X2

H2

H1

X1

H1

X1

X3

H3

X2

The dashed lines in the symbols above indicate the phase relationship between the primary and secondary of a particular connection configuration. For the connection shown above left, which has a 0o angular displacement between the primary and the secondary, the position of the dashed reference is identical. In the configuration shown above right, the position of the dashed lines indicate a 180o phase shift between the primary and the secondary. Sheet 5

PAM6


System Rotation

Determining Rotation

Summit SPD480 Phase Rotation Indicator

Measurements

The rotation of a three-phase system or motor can be changed by reversing any two of the three leads. An easy way to determine the phase rotation of a three-phase system is to use a phase rotation indicator such as the one shown at the left. The unit pictured is an induction–disk instrument that is essentially a three-phase induction motor. The lead identification is: A Phase - Red, B-Phase - Blue, C Phase White. When connected to a system with ABC rotation the disk rotates clockwise. If the system rotation is ACB the disk rotates counter-clockwise. Phase rotation indicators which use lamps to show the phase sequence are also available. A motor rotation indicator is also made which can be used to determine the phase sequence required to produce the desired motor rotation. The instrument is connected to the motor leads prior to wiring and then the motor shaft is turned in the desired direction of rotation; the meter will show the necessary phase sequence to be applied. (When the motor shaft is turned, the motor acts as an induction generator.)

Standard Phasor Rotation While the rotation of electric motors is referred to as either clockwise or counter-clockwise, for the purpose of analyzing three-phase systems, the rotation of electrical phasors is always shown as counter-clockwise (CCW) with the reference phasor drawn horizontally pointing to the right. (Phasors are electrical vectors which show magnitude and direction.) If the observer stands at Point X in the drawing below, the phasors can be imagined as turning past the observer in a CCW direction. •

To illustrate an ABC system rotation the phasors are labeled so as to appear in an ABC sequence.

•

To show reversed system rotation the phasors are labeled so that the phase labels appear in a ACB sequence.

Assigning the labels A, B, and C is actually somewhat arbitrary; all one really knows is the rotation sequence. The A, B, C labels are usually initially applied at the source transformer or facility main disconnect and then follow the conductors through the system. The National Electrical Code reference on the subject is shown below.

Wye Phasor Diagram

BC Axis of Rotation

CCW Phasor Rotation

AA

X

Reference @ Zero Degrees Observer

CB

Black letters show ABC rotation. Blue letters show ACB rotation Imagine standing at point X and watching the phasors rotate past. Wait until A goes past and then note the following order.

Sheet 6

2002 NEC Code Reference: ARTICLE 408 Switchboards and Panelboards. 408.3 Support and Arrangement of Busbars and Conductors. 408.3(E) Phase Arrangement. The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard or panelboard. The B phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta connected systems. Other busbar arrangements shall be permitted for additions the existing installations and shall be marked.

PAM7


Phasor Measurement

Sample Phase Angle Measurement Problem

Bank Connection Diagram

Phasor Diagram Primary Diagram is Known

H3

H2

H1

High Voltage

Construct Secondary Diagram

H2 H1 H2

H1 H2

X1

X2

H3

X1

X2

X1

V8o

X2

V7

X3

X2

Open Delta Metering PT Secondary

480 VAC X1

X1

X3

H1 H2 H1

X2

Measurements

Assume it is desired to measure the phase angles that exist on the secondary of the transformer bank connected in the manner shown below and then to construct the secondary phasor diagram from the measurements. (In this case we already know the answer based on the phasor diagram with the angular displacement of 180o shown on the Sheet 5.) Assume, also, that high-side bus potential transformers (not shown) are available for supplying a reference potential for the phase angle meter, and that the primary phasor diagram and the secondary lead designations (X1, X2, X3) are known.

V9

Phase Angle Meter Connections The reference phasor is always drawn horizontally to the right; selecting the primary H1 to H3 voltage for a meter reference will result in an applied voltage that is in the same direction. Because the primary is high voltage, and the meter Circuit One input voltage is 120 VAC, the high voltage bus potential transformers (PTs) are used to supply the 120 VAC for the meter reference input. The PT secondary V7 to V9 voltage will be in the same direction as the primary H1 to H3 voltage. •

Connect the PT secondary V7 to V9 voltage to Circuit One of the phase angle meter. The PT secondary V7 is connected to the phase angle meter Circuit One red + lead. The PT V9 is connected to the Circuit One black lead.

•

Using the 480 Volt Circuit Two Adapter, sequentially connect the transformer bank secondary potentials X1, X2, and X3 to the phase angle meter Circuit Two input leads as shown in the following table. The phase angle degree measurement that would result for each input is recorded in the Measured Angle Degrees column of the table.

Measurement Results Circuit Two Lead Connections

Measured Angle Degrees

Black Lead

X1

X2

240

X2

X3

120

X3

X1

0

See Sheet 8 for phasor diagram construction based on the above measurements.

Sheet 7

Red Lead +

PAM8


Phasor Diagram Construction

Plot of the phase angle measurements made on Sheet 7 Measurements

90o Lower Meter Scale

Upper Meter Scale Circuit Two Voltage Angle of Lead

120o Quadrant 1 Quadrant 2

30o 0o

o

0o

180

360o

Quadrant 3

Circuit One Reference Voltage V7 + to V9

Quadrant 4 240o

270o Construct the phasor diagram in the space below in accordance with the following rules: •

Move each of the phasors to the space below without changing its orientation.

•

Label each phasor with the X designation used for the measurement. The tail of each arrow is the polarity + end of the phasor.

•

Connect the phasors together: X1 to X1, X2 to X2, and X3 to X3.

Move

Label

X3

Connect X3

X1 X1

X3 X2

X3

X1

X1 X2

X2

X2

Sheet 8

This matches the diagram on the right of Sheet 5

PAM9


Phase Angle Measurement Voltage-Current Purpose

There are two reasons for making electrical system voltage-current phase angle measurements. Measurements

1. To determine the system power factor for system load studies and power factor correction studies. 2. To verify that power metering equipment and protective relays are properly connected.

EC

IC Lags EC by 15o

Phasor rotation ( blue arrow) is always shown CCW. This system rotation is shown to be ABC by the labeling of the phasors. (A is first, followed by B, followed by C)

Wye System Vectors Showing different angle of current lag on each phase. Black - Voltage Red - Current

EA IA o

IB Lags EB by 20

EB

Reference Drawn @ Zero Degrees

On A-Phase IA lags EA by 30o . Shown by violet dashed arrow. It can also be said that: IA leads EA by 330o . ( 360o - 30o = 330o ) Shown by green dashed arrow.

1. Phase Angle Measurement for Power Factor Determination In the ideal AC electrical system the voltage and current are in phase. This condition only occurs on systems where all of the load is resistive, such as electric heat, incandescent lighting, or fluorescent lighting with power factor corrected ballasts. Electrical utilization equipment such as motors and welders have a considerable amount of inductance and the inductive reactance (XL which is measured in ohms) causes the circuit current to lag the applied voltage. The actual amount, or number of degrees of lag, depends on the ratio of the Inductive Reactance ( XL ) in ohms to the ohmic value of Resistance ( R ) of the system. The system power factor is the cosine of the phase angle between the system voltage and the system current expressed as a percent. For example, if the current is determined by measurement to lag the applied voltage by 30 degrees, as shown for A-Phase in the example above, the power factor of the system would be 86.6 percent. This is determined by finding the cosine of 30 degrees which is 0.866 (you can use either a Trigonometry Table or an Engineering Calculator for this) and multiplying the cosine of the angle by 100 to obtain the percent power factor. Once the system power factor is known, power factor correction, if desired, can be applied to the system using power factor correction capacitors or by using synchronous motors, either of which can supply leading Volt Amperes Reactive (VARs) to the system to compensate for the lagging power factor. Most electric utilities charge a penalty for poor system power factor, so keeping the power factor above the required minimum value will result in a lower utility bill and will also improve the voltage drop on the system.

Sheet 9

When using the ATS-100 Phase Angle Meter, or similar instrument, the power factor is measured one-phase-at-atime. On a three-phase system the load will rarely be perfectly balanced, so the power factor on each phase may differ because of the unbalance of the single-phase loads. If all of the load was due to three-phase motors the power factor on each phase would be the same, at least in theory. However, in practice, there is always some voltage imbalance between phases which will result in an even greater percentage of current imbalance.

PAM10


Phase Angle Measurement Wye System

Instrument Connections EC

Measurements

C-PHASE CONDUCTOR NAMEPLATE SIDE OF CLAMP -ON IS POLARITY AND MUST FACE SOURCE

480 VOLT GROUNDED WYE SOURCE (277 VOLTS TO GROUND)

EA A-PHASE CONDUCTOR

THREE-PHASE MOTOR

VOLTAGE POLARITY RED LEAD

B-PHASE CONDUCTOR

EB VOLTAGE NON-POLARITY BLACK LEAD

2-CONDUCTOR CABLE


NORMAL

NULL

DELTA

Measurement Analysis Voltage-Current phase angle measurement is easily accomplished on a WYE system because, on any given phase, the phase current and the phase-to-ground voltage are in-phase at unity power factor. The voltage-current phase angle measurement may be taken directly from the phase angle meter. See the following page for a description of the requirements for making voltage-current phase angle measurements on a DELTA system. With all phase angle measurements, whether they be voltage-voltage or voltage-current, lead polarity is critical. Polarity for the voltage leads and current probe for the ATS-100 are shown above. Check the instruction manual for the instrument you are using. Sheet 10

Some phase angle meters, including the ATS-100, measure the angle of lead between the reference voltage which is applied to CIRCUIT ONE of the meter and the current which is applied to CIRCUIT TWO of the meter. When a meter using this standard is employed, the measurement reading is subtracted from 360o to give the angle of lag. In the example on the preceding page, the 30o angle of lag would be read on the upper scale of this meter as 330o lead.

PAM11


Phase Angle Measurement Ungrounded Delta System - Sheet 1 of 2

Instrument Connections B Measurements

B-PHASE CONDUCTOR 480 VOLT UNGROUNDED DELTA SOURCE

C

A

C-PHASE CONDUCTOR

THREE-PHASE MOTOR

NAMEPLATE SIDE OF CLAMP -ON IS POLARITY AND MUST FACE SOURCE

A-PHASE CONDUCTOR VOLTAGE POLARITY RED LEAD

VOLTAGE NON-POLARITY BLACK LEAD 2-CONDUCTOR CABLE


NORMAL

NULL

DELTA

Measurement Analysis On a DELTA system, there is an inherent 30o phase shift (at unity power factor) between the line (phase) voltage and the line current which must be accounted for. This is because the line current on a DELTA system is the vector sum of two separate phase currents (see the DELTA system phasor diagram on Sheet 1of the AC Systems Article). In order to obtain a correct reading, voltage and current of the proper phase and polarity must be applied to the instrument. See the following page, Sheet 12, for information on phase identification and meter connections.

Sheet 11

The diagram above shows the proper connections for measuring the phase angle between the A-Phase Current and the A-Phase Voltage (Line EC-A ). Assume the motor is running at a 30o lag (86% Power Factor). Because the ATS100 meter indicates the number of degrees that Circuit Two leads Circuit One, the 30o lag will be read as 330o lead. However, because of the inherent 30o lag on the DELTA system, the meter will actually read 300o lead as shown by the solid red hand. The 30o difference must be added to the 300o to obtain the correct 330o reading. On the ATS100, holding the momentary action DELTA switch in the NULL position will automatically adjust the reading by 30o and the meter hand will register the 330o reading as shown by the dashed red line.

PAM12

Phase Angle Measurement Ungrounded Delta System - Sheet 2 of 2


Phase Identification Measurements

Phase identification for correct instrument connection on a DELTA system is most easily accomplished using a phase rotation meter (See Sheet 6). Simply connect a rotation meter to the phase conductors so that a clockwise ABC rotation is indicated on the meter, then label the phases to match the A, B, C, labeling on the rotation meter leads. (Remember, even though the rotation meter shows a clockwise rotation, for the purpose of system analysis, all phasors are assumed to have a Counter-Clockwise Rotation.) The table below shows the voltage and current connections required for making phase angle measurements on a DELTA system.

Current and Voltage Polarity for Delta System Phase Angle Measurement Current Probe on Phase (Polarity Toward Source)

Potential Lead Connections Red ( + ) Polarity Black (COM)

A

A

C

B

B

A

C

C

B

Sheet 12

The Industrial Electrician’s Notebook SM

Kilowatt Classroom, LLC. Industrial Electrical Training www.kilowattclassroom.com

“Interfacing Technology and Craftsmanship”

IND1

Inductors


Schematic Symbols

Adjustable Inductor

17 mh power supply smoothing reactor. One-half actual size.

Iron Core Inductor

Above: Large DC link reactors used in 4160 volt 5000 hp VFD.

Inductors

Inductor

Powdered Iron Core

Radio frequency “choke” coil wound on ceramic powdered iron core. Shown actual size.

Left: Reactor used in conjunction with capacitors for harmonic filtering.

Inductor Characteristics •

An inductor is created when a conductor is wound into a coil.

•

The unit of inductance is the Henry - named after the American inventor Joseph Henry. By definition: an inductor has an inductance of one (1) henry if an electromotive force of one (1) volt is induced in the inductor when the current through the inductor changes at the rate of one (1) ampere per second. The abbreviation for the Henry is h and mh stands for millihenry.

•

The inductance of a coil is affected by a number of factors including: the type and size of the core material, the size of the conductor, and the way in which the coil is wound.

•

In an electrical circuit, an inductor opposes a change in current. This characteristic has resulted in the term “choke coil”, particularly in radio work.

•

Adjustable inductors are made by changing the amount of core material within the coil. The drawing below left illustrates a common method of achieving “slope control” in a welder by raising or lowering the iron core within the coil. The the AM broadcast band antenna coil pictured below right is tuned by moving the position of the powdered iron core within the coil form; a non-magnetic “tuning wand” is required for this adjustment.

Tuning Wand Adjustment Slot Coils (3)

Adjustable Powdered Iron Core Non-magnetic Threaded Rod Threaded clip fits into bottom of coil form. Sheet 1

Arc Welder Slope Control

Loopstick Antenna Coil Shown with core slug removed from coil form. Shown one-half actual size.

XL1

AC Theory Inductive Reactance - Page 1


Inductive Reactance is the opposition to the flow of current in an electrical circuit due to inductance and is measured in ohms.

•

The symbol for reactance is X; inductive reactance is represented by the symbol XL .

•

The formula for inductive reactance is: XL = 2

AC Theory

•

fL

Where: XL = Inductive Reactance in ohms, f = Frequency in hertz, L = Inductance in henrys, 2

= 6.28.

•

As illustrated by the formula above, inductive reactance is directly proportional to frequency. When an alternating current is applied to an inductor, the inductive reactance will increase as the frequency increases.

•

The opposition offered to the flow of steady-state Direct Current (DC) by an inductor is equal to the resistance of the inductor only (the ohmic value of the conductor with which the coil is wound ). During the application of DC to an inductor, during any fluctuations or ripple, or during de-energization of the coil, inductive reactance becomes a factor.

•

An inductor opposes a change in current. The mechanical analogy of inductance is inertia. Voltage of Self Inductance

When a changing current is applied to an inductor, a counter electromotive force (cemf) is generated. This generated voltage is termed a “counter” or “back” emf because it is in a direction which opposes the applied voltage. Figure A of the drawing at the right illustrates how this counter emf is generated. As current is applied to a coil and flows through the conductors of that coil, an expanding magnetic field will be established that surrounds each of the conductors. This expanding flux cuts through the adjacent conductors and induces a voltage in these adjacent conductors. Using the Left Hand Rule, it can be seen that the direction of this induced voltage is in a direction that opposes the applied DC voltage. When the switch is opened (or the level of the applied voltage is reduced), the reverse effect takes place. The magnetic field will collapse and effectively cut through the adjacent conductors in the opposite direction than was previously described. The counter emf will reverse and will oppose the reduction on the applied voltage. This directional change is illustrated in Figure B, on the right. Remember - To Generate a Voltage: Lenz’s Law

Sheet 2

The induced EMF in any circuit is always in a direction to oppose the effect that produced it.

A conductor can be moved so as to cut the lines of force of a magnetic field . Or An expanding or collapsing magnetic field can cut through a stationary conductor.

XL2

AC Theory Inductive Reactance - Page 2


Continued from Page 1 In a purely inductive circuit, the circuit current will lag the applied voltage by 90o . This is a theoretical condition, since any circuit will have some value of resistance or capacitive reactance in addition to the inductance. AC Theory

In this circuit the current is all reactive and no work will be done. Single-phase power in watts in an AC circuit is: P =E x I x Cos 0. The phase angle in this case is 90o . Since Cos 90o = 0, the circuit power therefore equals zero. Remember: • There are 360 degrees in a sine wave. • Electrical Phasors rotate counter-clockwise (CCW). • Phasors (electrical vectors) show two things: (1) magnitude, and (2) direction.

(Reference Voltage @ 0o )

Phasor Axis of Rotation

E REF

X

Observer

o

0 = 90 Angle of lag (Circuit Current) I CCW Phasor Rotation

AC

L Phasor Diagram

Time Increasing 90o

180o

T0

0o

Circuit Diagram

If the observer stands a point X above and watches the phasors rotate CCW, the voltage phasor will appear first, followed 90o later by the current phasor.

Degrees shown for voltage waveform

Positive 1/2 Cycle Zero Amplitude Negative 1/2 Cycle

0

Phase Angle 0 = 900 Lagging (Voltage is Reference) Sine Wave Relationship Red - Current Black - Voltage Sheet 3

In the above drawing, the voltage crosses zero and goes positive 90o before the current crosses zero and goes positive.

4PTRES


Resistance Measurements Three- and Four-Point Method

Four-Point Resistance Measurements Ohmmeter measurements are normally made with just a two-point measurement method. Test Methods

However, when measuring very low values of ohms, in the milli- or micro-ohm range, the two-point method is not satisfactory because test lead resistance becomes a significant factor. A similar problem occurs when making ground mat resistance tests, because long lead lengths of up to 1000 feet, are used. Here also, the lead resistance, due to long lead length, will affect the measurement results. The four-point resistance measurement method eliminates lead resistance. Instruments based on the four-point measurement work on the following principle: •

Two current leads, C1 and C2, comprise a two-wire current source that circulates current through the resistance under test.

•

Two potential leads, P1 and P2, provide a two-wire voltage measurement circuit that measures the voltage drop across the resistance under test.

•

The instrument computes the value of resistance from the measured values of current and voltage.

Four-Point Measurement Diagram Leads may be any length. Instrument C1 P1 Current Source May be AC or DC.

VM

Readout in Ohms

Resistance Being Measured P2

AM

C2

Three-Point Resistance Measurements The three-point method, a variation of the four-point method, is usually used when making ground (earth) resistance measurements. With the three-point method, the C1 and P1 terminals are tied together at the instrument and connected with a short lead to the ground system being tested. This simplifies the test in that only three leads are required instead of four. Because this common lead is kept short, when compared to the length of the C2 and P2 leads, its effect is negligible. Some ground testers are only capable of the three-point method, so are equipped with only three test terminals. The three-point method for ground system testing is considered adequate by most individuals in the electrical industry. Sheet 1

The four-point method is required to measure soil resistivity. This process requires a soil cup of specific dimensions into which a representative sample of earth is placed. This process is not often employed in testing electrical ground systems although it may be part of an initial engineering study.

GTEST1

Ground Testing Methods


Purpose

Ground Testing

The purpose of electrical ground testing is to determine the effectiveness of the grounding medium with respect to true earth. Most electrical systems do not rely on the earth to carry load current (this is done by the system conductors) but the earth may provide the return path for fault currents, and for safety, all electrical equipment frames are connected to ground. The resistivity of the earth is usually negligible because there so much of it available to carry current. The limiting factor in electrical grounding systems is how well the grounding electrodes contact the earth, which is known as the soil / ground rod interface. This interface resistance component, along with the resistance of the grounding conductors and the connections, must be measured by the ground test. In general, the lower the ground resistance, the safer the system is considered to be. There are different regulations which set forth the maximum allowable ground resistance, for example: the National Electrical Code specifies 25 ohms or less; MSHA is more stringent, requiring the ground to be 4 ohms or better; electric utilities construct their ground systems so that the resistance at a large station will be no more than a few tenths of one ohm. Grounding methods and techniques for ground system improvement will be covered in a future article.

Fall-of-Potential Instrument Characteristics •

To avoid errors due to galvanic currents in the earth, most ground test instruments use an AC current source.

•

A frequency other than 60 hertz is used to eliminate the possibility of interference with stray 60 hertz currents flowing through the earth. The TPI instrument pictured at left uses 575 Hz @ less than 50 volts.

•

A three- or four-point measurement technique is utilized to eliminate the effect of lead length.

•

The test procedure, known as the Fall-of-Potential Method, is described on the following page.

TPI MFT5010 Multi -Function Tester A Three-Point Fall-of Potential Instrument

Clamp-On Instrument Characteristics The clamp -on ground test instrument is a relatively new concept which is particularly well suited for testing the effectiveness of individual equipment grounding conductors that are connected to an existing ground grid. Clamp -on type ground testers are simple and easy-to-use. The instrument injects a current pulse into the ground conductor and calculates the value of the ground conductor resistance from the current pulse amplitude.

•

Some instruments can store the result of a number of readings which simplifies field record keeping.

•

Calibration loop is included with instrument.

Clamp-On Type Ground Tester Shown with calibration loop

Sheet 2

•

GTEST2

Ground Testing Three-Point Fall-of-Potential Test Procedure


Ground Test Procedure Refer to Diagram and Example Graph on the Following Page. Ground Testing

The instrument connections shown on the following page are for a three-point instrument, so C1 and P1 are common on the instrument and only three test leads are used. To use a four-point instrument, simply tie the C1and P1 leads together (most four-point instruments have a removable shorting link between the C1 and P1 terminals for this purpose). AC current of a non-standard frequency is usually used for ground testing to minimize the effect of galvanic (DC) currents as well as 60 Hz fundamental and harmonic currents which are present in the earth. The TPI 5010 Multifunction tester detailed in this article produces a 50 volt, 575 Hz test signal. In the Fall-of-Potential Method, two small ground rods - often referred to as ground spikes or probes - about 16 “ long are utilized. These probes are pushed or driven into the earth far enough to make good contact with the earth ( 8” - 12” is usually adequate). One of these probes, referred to as the remote current probe, is used to inject the test current into the earth and is placed some distance (often 100’ ) away from the grounding medium being tested . The second probe, known as the potential probe, is inserted at intervals within the current path and measures the voltage drop produced by the test current flowing through the resistance of the earth. In the example shown on the following page, the remote current probe C2 is located at a distance of 100 feet from the ground system being tested. The P2 potential probe is taken out toward the remote current probe C2 and driven into the earth at ten-foot increments. Based on empirical data (data determined by experiment and observation rather than being scientifically derived), the ohmic value measured at 62% of the distance from the ground-under-test to the remote current probe, is taken as the system ground resistance. The remote current probe must be placed out of the influence of the field of the ground system under test. With all but the largest ground systems, a spacing of 100 feet between the ground-under-test and the remote current electrode is adequate. With adequate spacing between electrodes exists, a plateau will be developed on the test graph. Note: A remote current probe distance of less than 100 feet may be adequate on small ground systems. When making a test where sufficient spacing exists, the instrument will read zero or very near zero when the P2 potential probe is placed near the ground-under-test. As the electrode is moved out toward the remote electrode, a plateau will be reached where a number of readings are approximately the same value (the actual ground resistance is that which is measured at 62% of the distance between the ground mat being tested and the remote current electrode). Finally as the potential probe approaches the remote current electrode, the resistance reading will rise dramatically. The electrical fields associated with the ground grid and the remote electrode are illustrated on Sheet 5. An actual ground test is detailed on Sheet 6 and a sample Ground Test Form is provided on Sheet 7.

Short Cut Method It is not absolutely necessary to make a number of measurements as described above and to construct a graph of the readings. We recommend this as it provides valuable data for future reference and, once you are set-up, it takes only a few minutes to take a series of readings. However, the short cut method described here determines the ground resistance value and verifies sufficient electrode spacing - and it does save time. Connect the instrument P1/C1 lead to the ground system being tested with a short conductor.

•

Locate the remote current electrode C2 at distance of 100 feet from the ground grid being tested.

•

Place the P2 potential probe at 62 feet from the ground grid being tested and measure the ground resistance.

•

Move the P2 potential probe 10’ to either side of the 62’ point (this would be at 52’ and 72’ from the ground grid) and take readings at each of these points. If the readings at these two points are essentially the same as that taken at the 62’ point, a measurement plateau exists and the 62’ reading is valid.

Sheet 3

•

GTEST3


Ground Testing Three-Point Fall-of-Potential Method

Instrument Set-Up

Keep this lead as short as possible.

Blue indicates return current path through earth. T1 T2 (C1 / P1) (P2)

T3 (C2)

FCN

Remote current probe C2 @ 100’

Ground Tester

Digital Display

TPI 5010 Multifunction Tester

Ground Testing

Yellow arrow indicates P2 potential probe @ 62 feet. Potential probe taken out at 10 foot increments.

Ground Mat Under Test

SW

Test Current Path

Select Earth ( RE )

•

A Note on Instrument Labeling Conventions

•

Most Ground Testers are single-function units and the test terminals are referred to as C1/P1, P2 & C2, as shown in parenthesis in the diagram above. The test leads carry the same designations.

• •

The TPI tester is a multifunction tester and uses the terminal designations T1, T2, & T3. The corresponding lead designations are E (Earth), S & H.

Test Current (575 Hz ) flows from instrument T3 to remote current probe C2 on the red lead. Test Current flows from remote current probe C2 back through the earth to the ground being tested as shown by dashed blue line. Test current flows out of ground grid back to instrument T1 on the short green lead. Black potential lead P1 is connected to instrument T2 and is taken out at 10’ increments. It measures voltage drop produced by the test current flowing through the earth. (P1 to P2 potential.)

9

10

Sample Ground Resistance Plot Remote current electrode C2 @ 100 feet. Potential probe P1 taken out at 10 foot increments.

8 7 6 4 5 3 2

Sufficient electrode spacing has plateau.

Ohms @ 62% of distance = 3.3 ohms

1

Resistance in Ohms

Insufficient electrode spacing has no plateau.

10

20

30

40

50 Distance in Feet

60

70

80

90

100

Sheet 4

0

GTEST4


Ground Testing Equal-Potential Planes The Existence of Equal-Potential Planes

When current flows through the earth from a remote test electrode (in the case of a ground test) or remote fault, the voltage drop which results from the flow of current through the resistance of the earth can be illustrated by equal-potential planes. The equal-potential planes are represented in the dashed lines in drawings below where the spacing between concentric lines represents some fixed value of voltage.

•

The concentration of the voltage surrounding a grounding element is greatest immediately adjacent to that ground. This is shown by the close proximity of lines at the point where the current enters the earth and again at the point where the current leaves the earth and returns to the station ground mat.

•

In order to achieve a proper test using the Fall-of-Potential Ground Test Method, sufficient spacing must exist between the station ground mat being tested and the remote current electrode such that the equal-potential lines do not overlap. As shown by the black line in the Sample Plot on the previous page, adequate electrode spacing will result in the occurrence of a plateau on the resistance plot. This plateau must exist at 62% of the distance between the ground mat and the remote electrode for the test to be valid. Insufficient spacing results in an overlap of these equal-potential planes, as illustrated at the bottom of this page and by the red line on the Sample Plot on the previous page.

•

See the Safety Note on Sheet 6 for information on the hazards of Step and Touch-Potentials.

Station Ground Mat Current leaves the earth and returns to the source.

Ground Testing

•

Remote Current Electrode or Remote Fault

Representation of Equal-Potential Planes Showing adequate spacing of electrodes

Ground Mat

Remote Current Electrode

Sheet 5

Representation of Equal-Potential Planes Showing inadequate spacing between the established ground and remote test electrode.

GTEST5


Ground Testing Actual Field Test

Setting-Up the Ground Tester Red arrow shows location of C2 probe. Ground Test Data

Ground Testing

This actual ground test was conducted on a pad-mount transformer in a rural mountain area. The single-phase transformer is supplied by a 12470/7200 volt grounded wye primary and the transformer is grounded by its own ground rod as well as being tied to the system neutral which is grounded at multiple points along the line. The distribution line is overhead with just the “dip” to the transformer being underground.

TPI MFT5010 Instrument Showing the 50 foot reading of 4.0 Ohms.

Test Procedure

Remote Current Probe C2 @ 100 Feet P2 Distance from Transformer in Feet

Instrument Reading in Ohms

10

1.83

20

3.59

30

3.85

40

3.95

50

4.0

60

4.25

62*

4.3

70

4.5

80

5.4

90

7.3

100

25.02

* Actual Ground resistance.

Terminal T1 of the TPI 5010 tester was connected to the transformer case ground with a short green lead. The remote Current Probe C2 was driven in the ground at a location 100 feet from the transformer and connected to Terminal T3 of the instrument with the red test lead. Terminal T2 of the tester was connected, using the 100’ black lead, to the P2 potential probe. This ground stake was inserted into the ground at 10’ intervals and a resistance measurement was made at each location and recorded in the table at the left. The relatively constant readings in the 4 ohm range between 40 and 70 feet is a definite plateau that indicates sufficient lead spacing. The initial readings close to the transformer are lower, and there is a pronounced “tip-up” as the P2 probe approaches the remote current electrode C2. The measured ground resistance at 62 feet (62% of the distance) was 4.3 ohms and is taken as the system ground resistance. This is an excellent value for this type of an installation.

Sheet 6

Safety Note - Possible Existence of Hazardous Step and Touch Potentials It is recommended that rubber gloves be worn when driving the ground rods and connecting the instrument leads. The possibility of a system fault occurring at the time the ground test is being conducted is extremely remote. However, such a fault could result in enough current flow through the earth to cause a possible hazardous step potential between a probe and where the electrician is standing, or hazardous touch potential between the probes and the system ground. The larger the system, in terms of available fault current, the greater the possible risk.

Ohms

FM1

Field Monitor Relay


Purpose

Field Monitor Relay Detail

Sync Motors

Pictured below is the Dresser Rand (EEMIC) Field Monitor II relay used to provide pull-out protection for large synchronous motors (IEEE Standard Device Designation 78) . The Field Monitor Relay measures the power factor of the motor and trips the motor stator and DC exciter field if synchronism is not achieved within a specific length of time or if the motor pulls out-of-step while running. Connection of the field monitor relay in the synchronous motor control scheme is shown in simplified form on Sheet 5 and a detailed connection diagram of the current and potential inputs is provided on Sheet 8. Note: This article provides general installation and operation information only. If troubleshooting or installing a similar system, be sure to use the exact relay connection diagram and system prints for the specific switchgear.

Sync Motor Field Excitation Cubicle Component Arrangement

Kilowatt Classroom Photo


Exciter Field Rectifier

CR2 Reset Push Button

Transformer AC Supply Circuit Breaker

Field Monitor Relay Restart Timer Field Supply Transformer Exciter Field Fuses See connection diagram on Sheet 8.

DC leads to exciter stationary field. See diagram on Sheet 4. Relay Connections

For correct connection of the relay, the rotation of the system must be known and a single-phase voltage and current of the correct phase relationship and polarity must be supplied. To assist the user in this regard, the manufacturer provides the Connection Table shown on the following page. The basic field monitor connection criteria are as follows: • • •

Sheet 7

•

The voltage connection is line-to-line and the required matching current in derived from the other phase. For the correct connection, the applied field monitor current will lead the applied voltage by 90o when the synchronous motor is running at unity power factor. If the polarity of the voltage is reversed, the correct connection can be maintained by reversing the polarity of the corresponding current. To verify correct connection of the current and potential, the analog output terminals 8 (-) and 9 (+) of the Field Monitor II Relay can measured. This voltage will be about 4 VDC with the system at unity power factor. The DC analog output voltage will increase toward a maximum of 8 VDC as the system goes leading and will decrease below 4 VDC as the motor runs lagging.

FM2


Field Monitor Relay Connections Connection Diagram Dashed lines show example connection.

Medium Voltage Bus Sync Motors

A Potential Transformer

L1 L2 L3

B

52 or 42 V7

V80

C3 C2 C1

5 4

Field Monitor Relay 78 Device See Note 1

See Note 3

CR1

1

2

3

CO

See Note 2

4

5

6

7

8

CR2

9 10 11

PT Primary Circuit - Blue CT Secondary Circuit - Red

12

STATOR See Note 4

52a or 42a Run

Note 1: Note 2: Note 3: Note 4:

CR1 - Excitation (Field On) Interlock. CR2 - Cage Winding Protection Interlock. To 52 circuit breaker trip or 42 contactor hold coil. Analog Output - 0 to 8 VDC proportional to motor power factor. When motor starts, auxiliary contact applies voltage to field monitor relay to initiate timer .

Field Monitor Connection Table (Highlighted areas show connection example.) If A & B Voltage is connected as shown

And the phase sequence is: 1- 2 - 3

And the phase sequence is: 3-2-1

A

L1

L3

L2

L1

L3

L2

L1

L2

L2

L3

L3

L1

B

L3

L1

L1

L2

L2

L3

L2

L1

L3

L2

L1

L3

Connect 4 & 5 Current as shown 4

C2 CO C3

CO C1

CO

C3 CO C1 CO C2

CO

5

CO C2 CO C3 CO

C1

CO C3

C2

CO C1 CO

Determination of PT and CT Connections

Sheet 8

Assume when measuring the system phase rotation at the bus potential transformer secondary fuses (see photo on Sheet 9) it is determined that the system rotation is L1, L2, L3. Assume also the Field Monitor voltage input is connected with L1 to Terminal 6 (follow Line 1 through the transformer to V7 ), and L2 to Terminal 7 (follow Line 2 through the transformer to V80 ), For these conditions, the current transformer input must be connected with Terminal 4 to CO and Terminal 5 to C3. The correct phase relationship can verified by measuring the analog output of the Field Monitor as described on the preceding page.

FM3


Field Monitor Relay Verifying System Phasing & Rotation System Phasor Diagram

Sync Motors

In this example, assume two Bus Potential Transformers (PT’s) are used and they are connected in an opendelta configuration. The 120 volt secondary phase designations are V7 , V80 , V9 , with B-Phase (Line 2) being grounded. This ground is indicated by the V80 labeling where the suffix “0” indicates a grounded connection. ( If Line 2 were not grounded, it would be referred to a V8 ). Assume also that the system rotation was shown to be L1, L2, L3 using the phase rotation indicator. The delta would be labeled as shown in the diagram below left. Because phasors are always rotated counter-clockwise for analysis, a reversed system rotation would be shown by re-labeling the delta as L1, L3, L2, or L3, L2, L1. (On a three-phase system, reversing any two leads changes the rotation.) The phasor diagram shown below right has been constructed to illustrate the 90 degree phase relationship between the current phasor C3 and the L1-L2 voltage phasor and shows the current leading the voltage by 90o as required for proper operation of the Field Monitor II Relay. Using a similar process the other voltage and current combinations in the Connection Table can be analyzed.

To construct the phasor diagram shown on the right, keep L1-L2 phasor in its original orientation and move the tail of the C3 phasor over to tail of L1-L2 phasor without changing the angular relationship of C3. The tail of the C3 phasor is CO. L2 (V80 )

L2 (V80 )

C2

Phasors rotate CCW (counter-clockwise) for analysis.

Axis of Rotation C3

C1 C3 L3 (V9 )

L1 (V7 )

C3’ L1 (V7 )

PT & CT Secondary Phasors

Move C3 over to C3’ position.

Relationship of L1-L2 Phasor to C3 Phasor Showing C3 leading L1-L2 by 90 degrees.

Phase Rotation Measurement Example Switchgear Bus PT Cubicle

TPI Phase Sequence Indicator

PT Secondary Fuse Block Phase rotation check was made at this location. Potential Transformers (PT’s) and high voltage fuses located behind swing-out panel.

The sequence indicator leads are connected: Red - Line 1, White - Line 2, Blue - Line 3. The disk turns clockwise for a L1, L2, L3 system rotation, and runs counter-clockwise if the rotation is reversed.

Sheet 9

Phase Sequence Indicator Used to establish system rotation as required for correct Field Monitor II connection. See table on Sheet 8.

DIODE1


Semiconductor Diodes

The Semiconductor Diode The semiconductor diode is a device that will conduct current in one direction only. It is the electrical equivalent of a hydraulic check valve. The semiconductor diode has the following characteristics: •

A diode is a two-layer semiconductor consisting of an Anode comprised of P-Type semiconductor material and a Cathode which is made of N -Type semiconductor material.

•

The P-Type material contains charge carriers which are of a positive polarity and are known as holes. In the N-Type material the charge carriers are electrons which are negative in polarity.

•

When a semiconductor diode is manufactured, the P-Type and N-Type materials are adjacent to one another creating a P-N Junction. Biasing Diodes

A bias refers to the application of an external voltage to a semiconductor. There are two ways a P-N junction can be biased. •

A forward bias results in current flow through the diode (diode conducts). To forward bias a diode, a positive voltage is applied to the Anode lead ( which connects to P-Type material) and the negative voltage is applied to the Cathode lead ( which connects to N-Type material).

•

A reverse bias results in no current flow through the diode (diode blocks). A diode is reverse biased when the Anode lead is made negative and the Cathode lead is made positive. P-N Junction Characteristics

The P-N Junction region has three important characteristics: 1) The junction is region itself has no charge carriers and is known as a depletion region. 2) The junction (depletion) region has a physical thickness that varies with the applied voltage. A forward bias decreases the thickness of the depletion region; a reverse bias increases the thickness of the depletion region. 3) There is a voltage, or potential hill, associated with the junction. Approximately 0.3 of a volt is required to forward bias a germanium diode; 0.5 to 0.7 of a volt is required to forward bias a silicon diode.

Symbol

+I Anode

Axial Lead Diode Cathode

A

C (K)

Current Flow PIV -V

+V

P

VF

N +

-

Forward Bias

N

Diode X-Y Characteristic Curve

-

+

Reverse Bias (No Current Flow)

Sheet 1

P -I

DIODE2

Silicon Diodes


Ratings Three characteristics must be defined for proper application or replacement of a semiconductor diode: Voltage Rating is the maximum voltage which the diode will block in the reverse-biased mode. •

This is expressed as the Peak-Reverse-Voltage (PRV) or Peak-Inverse-Voltage (PIV).

•

It is important to remember that this is a peak value of voltage not the root-mean-square (RMS) value. As a “Rule -of-Thumb, to provide a margin of safety, the PIV rating of a diode should be at least 3 times the RMS voltage of the circuit.

Current Rating is the maximum current the device can carry in the forward biased direction. Package Configuration Small, low current diodes are available in an axial lead configuration. The band end is the cathode.

•

High current diodes come in a press-fit, stud- mounted, or hockey puck package. Stud mounted diodes are available in Standard Polarity (stud cathode) and Reverse Polarity (stud anode).

Diodes

•

Thermal Limits •

It is essential that semiconductors operate within the device temperature ratings.

•

Semiconductor charge carriers are released thermally as well as electrically. Heat-sinking may be required during soldering and when the device is in operation to prevent thermal damage.

•

The forward resistance of a diode decreases with temperature; this results in an increase in current, which in turn produces more heat. As a result, thermal run-away can occur and destroy the semiconductor.

Sheet 2

DIODE3

Diode Test Procedure



Unlike its predecessor, the Analog Ohmmeter, Digital Ohmmeters require a special Diode Check Function because the current circulated by the normal Ohms Function of a digital meter is too low to adequately check a diode. In the Diode Check Position, the reading given by a digital meter in the forward bias direction (meter positive to diode anode and meter negative to diode cathode) is actually the voltage required to overcome the internal diode junction potential. For a silicon diode this will be about 0.5 0.8 volt; a germanium diode will read slightly lower, about 0.3 - 0.5 volt. Symbol Notation K (or C) = Cathode, A = Anode.

Select

A

A

K

Reverse Bias - Diode Blocks Correct reading: TPI Meter will read OUCH for open circuit indication. (Some meters read OL.)

Diodes

K

Forward Bias - Diode Conducts. Correct reading: Meter will read about 0.5 - 0.8 volt. Incorrect Readings: If meter reads 0 both directions, it is shorted. If it reads OUCH (open circuit) both directions, it is open.

Diode Test Procedure Caution: Ohms and Diode Check measurements can be made only on de-energized circuits! The Ohmmeter battery provides power to make this measurement. You may need to remove the diode from the circuit to get a reliable test. See Note below. •

Connect leads to meter as shown - Black COM, Red Ω .

•

Select the

•

Connect the leads to the Diode-Under-Test as shown in the drawing above and verify the readings are correct for both a forward and reverse bias. (This is sometimes referred to as checking the front-to-back ratio.)

(Diode Test) function.

Note: Large Stud-Mounted Diodes are bolted to a heat sink and Hockey Puck Units are compressed between the heat sinks; removing them from the circuit can be time-consuming and may be unnecessary. In these situations, test the entire assembly first, then, if the assembly tests shorted, remove and test the diodes individually. Hockey Puck Diodes must be compressed in a heat sink assembly or test fixture to be tested as they require compression to make-up the internal connections.

Sheet 3


Rectifier Circuits

DIODE4

Rectification Rectification is the process of converting an Alternating Current (AC) to a Direct Current (DC).

•

In the circuits below, the DC output voltage is defined as pulsating DC because it has the same waveform as one-half cycle of the applied alternating current. It is DC because it always has the same polarity with respect to zero volts. On single-phase rectifiers, the output DC voltage goes to zero after each rectified half cycle.

•

To convert a pulsating DC to a pure DC, such as that produced by a battery or DC generator, the DC output voltage must be filtered.

•

The diode symbol points in the direction of conventional current flow (positive to negative).

•

To analyze the operation of a rectifier circuit supplied by an AC circuit, arbitrarily assign a polarity to the transformer winding and analyze the diode operation, then reverse the polarity assignment and again analyze the operation of the diode. When the anode of the diode is made positive with respect to the cathode the diode will conduct. When the anode of the diode is made negative with respect to the cathode the diode will block the flow of current.

•

When the diode is conducting, current flows through the diode and the voltage drop across the diode is very small (typically 0.5 - 0.7 volts for a silicon diode). The current flow through the load resistor produces a voltage drop across the load resistor.

•

When the diode is non-conducting, no current flows through the diode and the applied voltage appears across the diode. Because there is no current flow, there will be no voltage drop across the resistor. AC Input Voltage +

120 VAC

RL

24 VAC

Rectifiers

•

DC Output Voltage

24 VAC RMS

0V

0V

_

DCOUT = VRMS / 2 = 12 VDC

Half-Wave Rectifier

24 VAC RMS

_ 120 VAC

+

24

0V

0V

RL

DCOUT = VRMS = 24 VDC

Bridge Circuit

12VAC

_

12 VAC RMS

+

120 VAC

0V

Center-Tap Circuit

0V DCOUT = VRMS = 12 VDC

Sheet 4

12VAC

RL

DIODE5

A

B


Three-Phase Rectifiers

Delta-Wye Rectifier Transformer

+ RL

Output Waveform

C

Rectifiers

Three-Phase Half-Wave Rectifier On three-phase rectifiers, the pulsations do not return to zero as with a single phase rectifier. This reduces the amount of ripple and simplifies filtering. A diode is forward biased when the anode is made more positive with respect to the cathode. Each of the diodes is forward biased when the voltage of the phase leading it becomes lower than the diode anode voltage and the diode is reverse biased when the voltage of the phase lagging it becomes higher that diode anode voltage.

Positive Bus

+ RL

Negative Bus

Three-Phase Full-Wave Rectifier Showing rectifier transformer delta secondary only. When the diodes are replaced with SCR’s, the output voltage of the rectifier can be controlled by phase-firing of the SCR’s. This arrangement is referred to as a six-pulse system.

Six-Phase Systems

Sheet 5

Some special medium-voltage rectifier transformers have dual secondary windings one delta, the other wye - which are 30 degrees out-of-phase. The phase-to-phase voltage of the wye matches the phase voltage of the delta. The outputs are individually rectified and the rectifiers are connected in series, resulting in a six-phase system with very low ripple, that has an output voltage which is double the voltage of the individual windings. The dashed line in the corner of the delta shows the phase shift between the two windings.

DELTA


Three-Phase AC Delta System

AC Systems

The Delta is a 3-wire system which is primarily used to provide power for three-phase motor loads. The system is normally ungrounded and has only one three-phase voltage available. The lack of a system ground makes it difficult to protect for ground faults. Often, a ground detection scheme, employing ground lamps, is used to provide an indication or alarm in the event of a system ground. The Delta System is sometimes corner grounded to protect for ground faults on the other two phases. In a delta system the line voltage is equal to the phase voltage i.e. (Line Voltage E Line 1 - 3 = E A Phase ) and the line current is the vector sum of two individual phase currents i.e. (Line Current I1 = IA + IC’ ). For balanced loads: Line Current I1 = IA x 1.732. On 240 volt Delta Systems, where single -phase lighting is desired, a 4- wire system can be configured by grounding the center-tap of one 240 volt transformer to provide 120 volts single-phase for lighting. The corner of the delta which is opposite the lighting circuit ground is referred to as the “high leg” or “wild leg” and cannot be used for lighting as the voltage to ground is 1.732 times the voltage of the single-phase center-tapped transformer. When this 4-wire scheme is utilized, the lighting transformer will usually have a larger kVA rating because it must carry both the single-phase lighting load and the three-phase motor or other loads. Center-tapped lighting transformer (4-wire system). Note: Only one intentional system ground point can be utilized, otherwise a short circuit would exist.

AF

“wild leg” or “high leg” (opposite center-tapped ground)

BF

CF

Corner Ground (if used)

IC’ Phasors rotate CCW Reference Phasor @ zero degrees X = Observer

Sheet 1

2002 NEC Code Reference: Article 110.15 High-Leg Marking. On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded to supply lighting and similar loads, the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present. See also ARTICLE 408. 3 (E) Phase Arrangement.


Three-Phase AC Wye System

WYE

AC Systems

The Wye (also know as Star - especially in the motor rewind industry) is a 4-wire system which provides two different supply voltages. The center-point of the Wye is the system neutral and is usually solidly grounded. Where it is desirable to limit the phase-to-ground fault magnitude the center-point of the Wye may be connected to ground through and neutral grounding resistor or a current limiting reactor. Because the system is tied to ground it is easy to provide system ground fault protection. Three-phase loads can be connected phase-to-phase and singlephase loads can be connected from any phase to the system neutral. On a wye system, the phase unbalance current is carried by the system neutral. On a Wye system the line current is equal to the phase current i.e. ( ILine 1 = IPhase A ) and the line-to-line voltage is equal to the vector sum of two individual phase voltages i.e. (E Line1 -2 = E PhaseA + E PhaseB’ ). In a Wye system the phase-to-phase voltage is 1.732 x the phase-to-ground voltage. Some typical Wye system voltages are: 120/208Y, 277/480Y, 2400/4160Y, 4160/7200Y, 7200/12470Y, 7620/13200Y,and 19920/34500Y.

Ground Resistor If Used

Three-Phase Load

A Neutral Ground B

C Single -Phase Loads System Neutral

EB’

IB’

Sheet 2

CT1

Current Transformers Ratio / Polarity / Types


Application

Transformers

Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to meters, protective relays, and other instruments. CT’s provide isolation from the high voltage primary, permit grounding of the secondary for safety, and step-down the magnitude of the measured current to a value that can be safely handled by the instruments. Ratio The most common CT secondary full-load current is 5 amps which matches the standard 5 amp full-scale current rating of switchboard indicating devices, power metering equipment, and protective relays. CT’s with a 1 amp full-load value and matching instruments with a 1 amp full-range value are also available. Many new protective relays are programmable for either value. CT ratios are expressed as a ratio of the rated primary current to the rated secondary current. For example, a 300:5 CT will produce 5 amps of secondary current when 300 amps flows through the primary. As the primary current changes the secondary current will vary accordingly. With 150 amps through the 300 amp rated primary, the secondary current will be 2.5 amps ( 150 : 300 = 2.5 : 5 ). When the rated primary amps is exceeded, which is usually the case when a fault occurs on the system, the amount of secondary current will increase but, depending on the magnetic saturation in the CT, the output may not be exactly proportional. Polarity All current transformers are subtractive polarity. Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case. On subtractive polarity transformers the H1 primary lead and the X1 secondary lead will be on the same side of the transformer (the left side when facing the low-side bushings). See the article Understanding Transformer Polarity in the Archive Catalog of the Kilowatt Classroom Web Site for more information on polarity.

Donut or Window-Type CT

White Lead is Secondary Polarity Primary Polarity Mark Startco Engineering Ltd Photo

Bar-Type CT’s have primary connections that bolt-up directly to the substation bus bars. Outdoor-rated versions of this equipment are used in pole-mounted primary metering installations.

Bar -Type CT

Primary Polarity Mark H1 Terminal

X1 Terminal X1 Polarity Mark

X2 Terminal Kilowatt Classroom Photo

H2 Terminal

Sheet 1

This type of CT often has compensating windings which improve the accuracy across the full-load range of the transformer.

On the Window or Donut-type CT’s, such as pictured on the left, the conductor, bus bar, or bushing which passes through the center of the transformer constitutes one primary turn. On Window-type units with low primary current ratings, where the primary conductor size is small, the ratio of the transformer can be changed by taking multiple wraps of the primary conductor through the window. If, for example, a window CT has a ratio of 100:5, placing two primary conductor wraps (two primary turns) through the window will change the ratio to 50:5. Some types of equipment employ this method to calibrate the equipment or to permit a single ratio CT to be utilized for several different ampacities of equipment.

CT2


Current Transformers Symbols

Current Flow Analysis In analyzing the current flow in a system utilizing CT’s the following observation can be made: Transformers

When current flows in the CT primary from the H1 lead (polarity + ) to the non- polarity H2 lead, current will be forced out the secondary X1 (polarity + ) lead, through the burden (load), and return to the secondary X2 non-polarity lead. The next half-cycle the current will reverse, but for the purpose of analysis and for constructing phasor diagrams, only the above indicated one-half cycle is analyzed. Electrical Drawing Conventions The polarity marking on electrical drawings may be made in several different ways. The three most common schematic conventions are shown below. The drawing symbol for meters and relays installed in a draw-out case that automatically short the CT secondary is shown in the drawing at the lower right.

CT One-Line Diagram Symbol Secondary Winding

One-Turn Primary Secondary Conductors to Relays or Instruments

Polarity Marks Shown as Dots Source

Polarity Marks Shown as Squares Source

Current Elements in Meters or Relays

H1


X1 X2 H2

Secondary Safety Ground Load


Polarity Marks Shown with Slash Source

Draw-Out Meter or Relay Case Source

Symbol for draw-out case with CT Shorting



Secondary Safety Ground Load Sheet 2

CT3


Current Transformers Shorting Methods

Caution: The secondary of a Current Transformer must always have a burden (load) connected; an opencircuited secondary can result in the development of a dangerously-high secondary voltage. Energized but unused CT’s must be kept short-circuited. Transformers

Startco MPU-16 Motor Protective Relay

Draw-Out Instrument Cases Meters and protective relays are available in draw-out cases that automatically short-circuit the CT when the instrument is removed for testing and calibration. Voltage and trip-circuit contacts will be opened. See symbol for draw-out case on Sheet 2.

Retrofit installation in draw-out case. Startco Engineering Ltd Photo

CT Shorting Terminal Strips The illustration below shows the termination of a multi-ratio CT on a special shorting terminal strip. Insertion of shorting screw through shorting bar ties isolated terminal strip points together. Any shorted winding effectively shorts the entire CT.

X1 X2 X3

Multi-Ratio CT

X4

Shorting Bar Shorting screw in any other locations shorts CT. Relay connected to CT tap which provides the desired ratio. Lead X3 becomes polarity.

X5

Shorting screw ties X5 CT lead to ground.

Spare Shorting Screw Stored for future shorting requirement.

Safety Ground Shorting screw ties shorting bar to ground. Terminal Strip Mounting Hole

Auxiliary Current Transformers Startco Engineering Ltd has developed a system utilizing an Auxiliary CT (pictured at left) which permits safe removal of hard-wire protective relays from the system. The current transformers are permanently wired to the input of the Auxiliary CT and the output of the Auxiliary unit is wired to the protective relay current inputs. This arrangements keeps a burden on the CT secondary circuits and permits the protective relays to be removed for repair, calibration, or replacement. The Auxiliary CT is installed as close as possible to the current transformers. This reduces the CT burden by reducing the length of the CT secondary current conductors.

Sheet 3

Startco Engineering Ltd Photo

CT4

Current Transformers CT Accuracy Classes


ANSI Accuracy Classes Current Transformers are defined by Accuracy Classes depending on the application. Metering Accuracy CT’s are used where a high degree of accuracy is required from low-load values up to full-load of a system. An example of this application would be the current transformers utilized by utility companies for large capacity revenue billing.

•

Relaying Accuracy CT’s are used for supplying current to protective relays. In this application, the relays do not normally operate in the normal load range, but they must perform with a reasonable degree of accuracy at very high overload and fault-current levels which may reach twenty times the full-load amplitude.

Transformers

•

Notes: 1) Instrument Transformers (PT’s & CT’s) are defined in ANSI C57.13-1978. 2) The load on an instrument transformer (PT or CT) is referred to as the “burden”.

Metering Accuracy Classifications Available in Maximum Ratio Error Classes of: + 0.3% , + 0.6% , + 1.2%, +2.4%. For Burdens (Loads) of: 0.1, 0.2, 0.5, 0.9, 1.8 ohms. Which equals 2.5, 5.0, 12, 22-1/2, 45 volt-amperes ( va ). Since Power = I2 xR, use 5 amp secondary for I, and burden value for R. Typical Number 0.3 B 0.2

Max Ratio Error + %

Burden

Ohms (Burden)

Relaying Accuracy CT’s Class C (C for Calculated) is low leakage reactance type - typical of donut units - Formerly Class L ( L for Low Leakage). Class T (T for Tested) is high leakage reactance type - typical of bar-type units - Formerly Class H ( H for High Leakage). Typical Number 10 C 800

10% Max Ratio Error at 20 times Rated Current Low Leakage Unit Max secondary voltage developed at 20 times rated current without exceeding the +10% ratio error.

Will support burdens of: 0.1, 0.2, 1.0, 2.0, 4.0, 8.0 ohms.

Sheet 4

Available secondary voltages: 10, 50, 100, 200, 400, 800.

CT5

Current Transformers Multi- Ratio CT’s

Westinghouse Multi-Ratio Bushing-Type CT For External Installation


Installation Considerations

Transformers

This bushing CT is designed for use on existing circuit breakers and power transformers and is installed externally (See Sheet 11). It is housed in an aluminum case which provides electrostatic shielding. Care must be taken with the installation to insure that the mounting clamp bolts do not contact the case resulting in a one-turn primary short circuit. Also because the case is metal and is installed externally it can decrease the bushing strike distance. The circuit breaker or transformer manufacturer should be consulted to verify acceptability of the installation.

Secondary Turns Diagram

600/5 CT

Diagram at the left shows the number of turns for each winding on a 600/5 multi-ratio CT. The full number of 120 turns, from X1 - X5, is used to obtain the 600/5 ratio. (Since there is one primary turn, 120:1= 600:5).

Polarity Mark

Another example: X1 - X2 has 20 turns, so 20:1= 100:5. Any combination of adjacent turns can be utilized. The lowest lead number of the combination will be the polarity lead. See the CT shorting strip diagram on Sheet 7 for a typical termination arrangement.

Selection Guide Shows ANSI Accuracy Classes, Dimensions, and Mfg Style Number

Sheet 5

CT6

Current Transformers Typical Excitation Characteristics


Excitation Curves

Transformers

The family of curves below describe the excitation characteristics for the 600/5 multi-ratio bushing current transformer shown on the previous sheet. This is a plot of the CT secondary current against secondary voltage. These curves illustrate how high the secondary voltage will in rise in order to force the rated secondary current through the burden. The effect of magnetic saturation is also illustrated by the knee of the curve. Next month’s article will show how to perform a CT Saturation Curve Test.

Sheet 6

CT7


Current Transformers Typical Installations Load-Side CT’s

Line-Side CT’s Bushing CT’s

Transformers

Bushing CT’s may be mounted externally or internally on circuit breakers and transformers. Multi-ratio units are often used. Where single-ratio CT’s are employed, the CT primary rating may match either the full-load ampacity of the circuit breaker or of the feeder. In the latter case, upgrading the feeder ampacity will require replacement of the CT’s. The CT secondary leads land on a termination strip in the breaker or transformer control cubicle. Siemens SF6 Circuit Breaker With externally mounted bushing CT’s Kilowatt Classroom Photo

External Portion of Bushing Note “Petticoats” which shed moisture and increase creepage distance. Multi-Ratio CT Bushing Internal Porcelain This section is submerged in oil.

Westinghouse Oil-Filled Vacuum Recloser With tank dropped showing internally mounted CT’s on line-side bushings. In this configuration the protective relays fed by the CT’s are said to “look through” the breaker. Kilowatt Classroom Photo

Control Circuit Contact Block Mates with breaker control block.

Bus Bars Breaker Bus-Side Stabs With single donut CT on B-Phase Breaker Machine-Side Stabs With donut CT on A and C Phases


Sheet 7

General Electric 480 Volt Metal-Clad Switchgear Cubicle with generator breaker racked out.

CT8


Current Transformers Types Hall Effect CT’s Hall-Effect CT’s are not current transformers in the conventional sense, rather they are electronic circuit transducers which can be applied in the measurement of either AC or DC circuit currents. These devices have many applications; they are commonly used in Variable Frequency Drives (VFD’s) to measure the DC link current and are also employed in AC/DC instrument probes such as the TPI-A254 Current Adapter shown at the right.

TPI AC/DC Current Probe Transformers

Hall-Effect devices contain an null-balance type amplifier circuit. The magnetic flux (field) produced by the current flow through the primary (usually one-turn) results in an output voltage which is balanced by an equal and opposite output from the control or measuring circuit. Because the circuit is an amplifier, it requires external operating power which is supplied by the control circuit power supply, or in the case of a portable instrument probe, batteries are used. As with conventional current transformers, Hall-Effect devices provide isolation from the high voltage circuit and reduce the measured current to a proportional value which can be safely measured by the control or instrument circuit. Hall-Effect devices do not pose the same danger as conventional bus-bar or donuttype CT’s with regard to an open circuited secondary. (Note: some instrument current probes are conventional CT’s; these usually have a burden resistor within the probe or may be protected from an open circuit with back-to-back zener diodes.) However, good practice dictates that instrument current probes should not be disconnected from the meter while current is passing through the device primary.

Probe employs the HallEffect principle and produces a millivoltage output that is applied to a TPI Digital Multimeter. The meter interprets the probe voltage as a current value. Batteries are used in the probe to power the amplifier circuitry. Test Products International Photo

Hall-Effect CT Used on the DC Link of a 5000 H.P. VFD. Control Circuit Connections Heat Sink

Tubular High Voltage Bus Bar Passes through Hall-Effect CT Power Transistors


Typical VFD Block Diagram Showing Hall-Effect CT (HCT) Connections 3-Phase AC Motor

DC Link

Single or Three-Phase AC Input

Rectifier

Inverter HCT

Speed Reference

Regulator

HCT Power

Frequency Control Signal Motor Voltage Feedback

Sheet 8

HCT Output Signal

CT9

Current Transformers CT Saturation Curve Testing


CT Saturation Curve Tester Designed by Vail Gilliland

Sheet 1 of 2

Purpose Test Methods

This circuit is used to plot the Saturation Curve of an Instrument Current Transformer. The test results are compared with the manufacturer’s published data (see sample curves on Sheet 6 of the Current Transformer article published last month). A transformer with shorted secondary turns or a one-turn primary short due to improper mounting will result in a test plot which varies from the published curve. This test is performed only on de-energized, out-of-service equipment. The CT under test need not be removed from the equipment provided the primary is first de-energized and isolated and the secondary is then disconnected. See Circuit Description and Test Procedure on Sheet 10.

WARNING ! This test set-up develops high voltage. The test procedure is intended for use by experienced electrical personnel only and requires the use of established safety procedures and proper Personal Protective Equipment (PPE). This test is performed only on de-energized, out-of-service equipment, and requires that the CT primary be de-energized and isolated, and then the CT secondary must be disconnected from its burden (load). Be certain to properly reconnect the current transformer at the conclusion of the test - an open-circuited CT can develop a dangerously-high voltage; an incorrectly connected CT may not trip the protective relay!

AUTOTRANSFORMER SWITCH 120 VAC HOT AUTOTRANSFORMER FUSE

ADJUSTABLE AUTOTRANSFORMER 2000 VA, 0 - 135 VAC

A DIGITAL AMMETER 0 - 10 AMPS SEE CIRCUIT DESCRIPTION ON THE FOLLOWING SHEET.

120 VAC NEUTRAL

Test Set Schematic

Sheet 9

SAFETY GROUND ON TRANSFORMER CASES

CT10

Current Transformers CT Saturation Curve Testing CT Saturation Curve Tester


Sheet 2 of 2

Test Method Overview

Test Methods

The circuit on the preceding page (Sheet 9) is used to provide an adjustable 0 - 1000 VAC which is injected on the secondary winding of the current transformer being tested. Using the adjustable autotransformer, the secondary excitation voltage and current applied to the CT are gradually increased from zero while incremental voltage and current readings are taken. A plot of the CT secondary voltage and current is made on log - log (logarithmic) scale engineering graph paper at each step of the test. The constructed plot is then compared with the manufacturers published curves (see Sheet 6 ); a deviation from these curves indicates either a primary one-turn short circuit due to improper mounting or shorted secondary turns. Circuit Description Two 480 - 120 volt control transformers are back-fed with the 120 volt windings connected in parallel and the 480 volt windings connected in series. (The kVA rating of these transformers must be large enough to supply 5 amps of current on a momentary basis to get above the “knee” of the saturation curve.) An adjustable 0 - 135 VAC is supplied by the autotransformer which feeds the parallel connected 120 volt transformer windings. To achieve accurate test results, both the CT secondary excitation voltage and excitation current need to be accurately measured. The voltmeter must have a 1000 VAC range. One method of measuring the excitation current is to series a Digital Multi-Meter (DMM) with the test circuit output and measure the current directly using the AC amps function. Care needs to be taken not to exceed the internal current rating of the instrument (10 amps on most DMM’s). An alternate approach, for making the current measurement, is to use a low-current clamp -on adapter such as the TPI A254 (see picture on Sheet 8) which has the ability to read AC currents as low as 10 milliamps. Several wraps of the conductor through this current probe will extend the low-end accuracy; the meter reading is then divided by the number of turns used. Test Procedure WARNING! This test set-up develops high voltage - see precautions on previous sheet. Verify that the adjustable autotransformer is un-plugged, turned off, and set at zero.

•

Connect the test equipment as shown in the diagram on the preceding page and connect the output leads to the secondary leads of the current-transformer-under-test.

•

Apply the 120 VAC power to the autotransformer input.

•

Gradually increase the autotransformer setting until a small output current is measured. Ten milliamps (0.010 amps) is a good first step. Read the voltage at this step and plot the voltage and current readings on the log-log graph paper.

•

Continue to increase the autotransformer setting in a series of small steps, taking voltage and current readings at each step and plotting the results on the graph paper. Watch for the development of the “knee” of the curve and make very careful adjustments in this voltage and current range. The current will increase in much larger increments at this point for a given amount of voltage increase, so use care to prevent blowing a meter fuse or autotransformer fuse.

•

At the conclusion of the test, reduce the autotransformer output voltage to zero and remove power from the system.

Sheet 10

•

HCT1

Current Injection Testing


Purpose

Test Methods

Electrical equipment such as circuit breakers, protective relays, and meters are routinely tested to verify proper operation of current sensing elements. This testing is performed using high-current, low-voltage test equipment that provides a means of adjusting the value of current and also of measuring the operating time of the device under test. The output waveform of the test current is critical and must be sinusoidal; testing with equipment that produces a non-sinusoidal waveform - such as SCR’s - will not produce accurate results. CAUTION! Current injection testing is performed on de-energized, out-of-service equipment only! Types of High-Current Testing •

Primary Injection Testing is used to test the overall operation of a current circuit. In this type of test, a high current is injected in the Current Transformer (CT) primary winding and the resulting secondary current is measured in each of the CT secondary devices such as meters and relays. This test is primarily conducted during commissioning of new equipment or after a major circuit modification to insure that the equipment is correctly connected. The polarity of the current may also be critical and other equipment, such as a Phase Angle Meter, may be used in conjunction with the high-current test source.

•

Secondary Injection Testing is periodically performed on the individual devices such as relays and meters to verify the accuracy and proper operation of the equipment. These devices receive their input current from the CT secondary winding so these tests will be at a much lower level of current than that used for primary injection. Proper operation of the current-sensing protective equipment can be verified by comparing the device operating characteristics with the manufacturers published time-current characteristic curves. Frequency of Tests

The frequency of these preventive maintenance current tests depend up the importance of the protection: high voltage equipment will often be tested annually; medium-voltage equipment is often tested and calibrated every-other year, and a three- or four-year interval for 480 volt equipment may be considered adequate. Testing Thermal Devices •

Thermal overload relays for 480 volt and lower voltage equipment - either bimetallic or melting alloy type are not usually tested as the test current can damage the element. (Critical applications should be protected by a more reliable device such as an electromechanical or electronic relay.)

•

Large thermal circuit breakers are sometimes periodically tested for proper operation by current injection. If successive tests are made the device must have time to cool-down between tests for accurate results to be obtained.

•

Electromechanical thermal relays must be tested within the instrument case for proper results. As with thermal breakers, the device must have time to cool down between successive tests. Testing Instantaneous Elements Because of the high current involved when testing magnetic trip elements in circuit breakers or relays, the current should be adjusted as quickly as possible using the test set MOMENTARY FUNCTION to prevent damage to the equipment-under-test.

•

The maximum trip point setting of the instantaneous magnetic trip element of a thermal/magnetic circuit breaker is usually 10 times the thermal element value. Testing the magnetic element may result in damage to the thermal element (which is in series with the magnetic trip coil) if the test current is prolonged. Motor Circuit Protectors (MCP’s) have a magnetic trip element only and can be safely tested.

•

Protective relay instantaneous elements are tested at either the engineered setting or the “as -found” setting.

Sheet 11

•

HCT2

High Current Testing CT Primary Injection


Test Set-Up

Test Methods

In the photo below a high current test set is used to check the trip and reclose timing on a three-phase 35.4 kV substation vacuum recloser. The relaying scheme is tested one-phase-at-a-time. The high current leads are attached to the phase being tested. The test set timer start/stop leads are attached to a different phase using this pole as a “dry” set of contacts. See Sheet 7 for a photo of the recloser internal bushing current transformers. This recloser uses 130 VDC station battery for trip and close power. CAUTION ! This procedure cannot be used on electronic controlled reclosers that have a high-voltage closing coil or on hydraulic reclosers that have a high-voltage series trip coil and a high-voltage closing coil.

Recloser is out-of-service. Note open disconnect ( 6 total ). Test set high current leads attached to A -Phase bushings. Timer start/stop leads attached to C-Phase bushings.

Vacuum Recloser

Control Cabinet

High Current Test Set

Field Test on 34.5 kV Recloser Kilowatt Classroom Photo

Test Set-Up Schematic Recloser shown top view. Load-Side Bushings

Line-Side Bushings

Blue = High Current Leads High Current Test Set T = Timer A = AC Ammeter ADJ = Amps Adjust Control

A

T

ADJ

Red Circles = Internal Bushing CT’s

Green = Timer Start / Stop Leads Recloser Main Contacts

Sheet 12

Red Dashed Lines = CT secondary leads to multi-function protective relay. Relay “looks” through the breaker. When the relay trips the breaker, the fault current is interrupted and the protective relay will reset.

Electrical Fundamentals –Tab 1 Electrical Basics –Electrical terms, electrical symbols for formulas and prints, electrical calculations, measurement methods, sources of electrical energy, resistance in electrical circuits DC Circuits – Ohms Law; electrical power; series, parallel and series-parallel circuits AC Circuits – Magnetism, Sine Wave fundamentals, inductance, capacitance, reactance, impedance, single-phase AC systems Three-Phase AC Systems – Delta, Wye, three-phase power measurements and calculations, power factor correction

Electrical Equipment – Tab 2 Introduction to Electrical Equipment – Switches, relays Transformers Electrical Protective Devices – Fuses, Overload Relays, Circuit Breakers, MCP’s DC Motors & Generators AC Motors & Generators

Electrical Control Systems – Tab 3 Fundamentals of Motor Control Introduction to Solid State Components – Diodes, transistors, SCR’s, FET’s, IGBT’s, GTO’s; introduction to integrated circuits Introduction to Analog and Digital Systems – Analog Systems: op amps. Digital Systems: logic circuits, counters, shift registers. Programmable Control Systems – Programmable Relays, Programmable Logic Controllers (PLC’s) Motor Drives – DC Motor Drives. AC Motor Drives: Soft Start Systems, Variable Frequency Drives

Medium Voltage Systems – Tab 4 Electric Power Substations Distribution Systems –Underground, Overhead, Surface Mine Radial Feeder Analysis - System Impedances, Available Fault Current, Motor Starting Voltage Drop Introduction to Protective Device Coordination Principles of Voltage Regulation

Reference Data – Tab 5 Symbols Formulas Standard Device Designation Numbers Types of Electrical Prints – Block Diagrams, Schematics, Wiring Diagrams, Flow Charts, One-Line Diagrams

Manufacturer’s Data Sheets – Tab 6 Insert manufacturers’ data sheets and instructions in this section.

My Notes – Tab 7 A handy place to keep your personal field notes from various jobs.

Appendix – Tab 8 Technical Article File – Insert trade publication articles, stories, and other literature you want to keep in this section.

See back for Details

Electrical Fundamentals

Electrical Equipment

Electrical Control Systems

Medium Voltage Systems

Reference Data

Manufacturers’ Data Sheets

My Notes

Appendix

CAP1

Capacitors


Schematic Symbols +

Oil-filled AC Capacitor 17.5 mfd @ 280 volts

Non-polarized Mica 0.1 mfd @ 100 volts

Polarized DC Capacitor Plus sign indicates proper connection polarity

Adjustable Capacitor Tuning and trimming very small values only

Polarized Electrolytic Note lead polarity mark 470 mfd @ 50 VDC

Adjustable Capacitor 10 - 30 picofarads

Capacitors

Capacitor

Capacitor Characteristics •

A capacitor consists of two plates separated by an insulating medium known as a dielectric. (A dielectric is similar to an insulator but is more electrically “flexible”. All dielectrics are insulators, but not all insulators make good dielectric material.)

•

A capacitor is a device which stores an electrostatic charge. CAUTION: All power capacitors must be fully discharged before working on the equipment!

•

Capacitors are rated in Farads - named after the scientist Michael Faraday. By definition: a one (1) Farad capacitor will store a one (1) Coulomb charge when connected across a one (1) Volt potential. The farad is a very large quantity, so capacitors are rated in picofarads (10-12 farads), nanofarads (10-9 farads ), or microfarads (10-6 farads). The abbreviations pf for picofarads, nf for nanofarads, and mf or mfd for microfarads, are commonly used.

•

Electrolytic capacitors can be applied to DC circuits only and must be connected in the circuit with the correct polarity in order for the dielectric material to properly form. The capacitor case will indicate the required lead polarity. With a electrolytic capacitor it is possible to manufacture capacitors of large microfarad ratings (up to several thousand microfarads) in a relatively small case. CAUTION: Improperly connected electrolytic capacitors may explode! Capacitors used on AC systems must be of the non-polarized type.

•

All capacitors have a “working voltage” voltage which cannot be exceeded.

•

In an electrical circuit a capacitor opposes a change in voltage.

•

In an electrical circuit a capacitor will block Direct Current (DC) and will pass Alternating Current (AC).

•

Many electrical components, other than capacitors, exhibit a certain amount of capacitance. For example: high voltage cable which has an inner conductor and an outer shield can act as a capacitor and will store a considerable charge. (The cable conductor acts as one capacitor plate, the shield becomes the second capacitor plate, and the cable insulation constitutes the capacitor dielectric.)

CAUTION: All cables, motor windings, and other components which can exhibit capacitance must be discharged before working on the components or associated circuitry!

Sheet 1

•

CAP2

Capacitors Factors Determining Capacitance


Capacitors

The formula for determining the capacitance of a capacitor is given below left. A table of some typical capacitor dielectric materials along with the approximate dielectric constant K is included below right. As the dielectric constant K is in the numerator of the formula, the capacitance C of the capacitor is directly proportional to this value. An increase in the value of K will result in an increase in capacitance. Dielectric Constants Kind of Dielectric

Approx K Value

Capacitance Parallel Plate Capacitor Air (at atmospheric Pressure)

1.0

Bakelite

5.0

Cambric

4.0

Fiber

5.0

Glass

8.0

Mica

6.0

Paraffin Coated Paper

3.5

S = Area of one plate in square inches

Porcelain

6.0

N = Number of plates

Pyrex

4.5

Quartz

5.0

Rubber

3.0

Wood

5.0

K S (N - 1) C = 0.224 d Where C = Capacitance in picofarads K = Dielectric Constant

d = Distance between plates in inches

These values are approximate since true values depend on grade of material used , moisture content, temperature, and frequency characteristics.

The distance d between the plates is in the denominator of the capacitance formula, so the capacitance C will be inversely proportional to this value. The adjustable trimmer capacitor pictured below left has a mica dielectric material and the capacitance is varied by adjusting the screws which change the distance d between the plates. Tightening the screws brings the plates closer together, causing the capacitance to increase; loosening the screws allows the plates to separate, resulting in a decrease in capacitance. A non-metallic tuning wand must be used to prevent affecting the adjustment.

Trimmer Capacitor

Tuning Capacitor Sheet 2

The capacitance of a capacitor is directly proportional to the effective area S of the plates. The tuning capacitor pictured above right utilizes this formula parameter for achieving a change in capacitance. As the shaft is rotated, a change in the plate mesh between the rotor and stator plates will result in a change in the plate area.

CAP3


Capacitors RC Time Constant

Capacitors

The length of time it takes for a capacitor to charge to 63.2% of the supply voltage is the RC Time Constant. (The 63.2% figure is used because the charging curve, as shown below, is logarithmic and it is difficult to tell exactly when the capacitor is fully charged - the 63.2% value can be more readily determined.) The Time Constant Formula is: T = RC Where: T = the time is seconds to reach 63.2% charged, R = the resistance in Ohms, and C = the capacitance in farads. For electronic work, a more usable set of values (making a decimal point change) is: T = time in milliseconds, R = resistance in k ohms, and C=capacitance in microfarads. It takes approximately five (5) time constants for a capacitor to become fully charged. RC Charge and Discharge Circuit The circuit at the right can be used to illustrate the RC Time Constant. When the switch is moved to the Charge position, the capacitor C begins to charge through the resistor R. The larger the ohmic value of R, the longer it will take the capacitor to charge.

EC

With the capacitor fully charged (EC = 10 Volts ), when the switch is moved to the Discharge position the capacitor will discharge through R.

IC = 10 Amperes

Voltage Charge / Discharge Curve Charge

Discharge

100 80 EC Volts

As shown at the right, a capacitor opposes a change in voltage. When the switch is closed current begins to flow immediately to the capacitor, but the voltage across the capacitor builds at a logarithmic rate and the length of time for the capacitor to charge is determined by the rating of the capacitor and the size of the series resistor in accordance with the formula T=RC.

63.2% @ 1RC

60 40

36.8% on Discharge

20 Circuit Applications

0

RC Time Constant Circuits have many applicatons. Analog time delay relays, for example, rely on this principle. The relay time delay is set by an adjustable resistor which is in series with the timing capacitor. When the firing voltage of a transistor or SCR is reached, an output relay is energized.

10

1RC 2RC 3RC 4RC 5RC Charge

Discharge

8

Capacitor Charge / Discharge Current

IC Amperes

6 Free-running oscillators based on this principle generate a characteristic “sawtooth” waveform.

4 2 0

The graph at the right shows the immediate rise in charging current when the switch is placed in the Charge position. The current then decays to zero amps as the capacitor becomes fully charged.

-1

When the switch is moved to the Discharge position, the current instantaneously reverses in direction and then decays to zero as the capacitor become fully discharged.

-4

-2 -3 Sheet 3

-5 T0

T1

T2

T3

T4

T5

XC1


AC Theory Capacitive Reactance - Page 1

Capacitive Reactance is the opposition to the flow of current in an electrical circuit due to capacitance and is measured in Ohms.

•

The symbol for reactance is X; capacitive reactance is represented by XC

•

The formula for capacitive reactance is:

AC Theory

•

1 XC = 2

fC

Where: XC = Capacitive reactance in ohms f = Frequency in hertz C = Capacitance in farads 2

= 6.28

[Note: the value of pi (

) is 3.1416]

•

As illustrated by the formula above, capacitive reactance is inversely proportional to frequency.

•

Direct Current (DC) will not flow through a capacitor because the frequency of pure DC (having no ripple or changes in amplitude) is zero hertz , therefore the value of capacitive reactance in ohms is, theoretically, infinite (there is always some small amount of leakage current through the capacitor dielectric). A capacitor is said to “block” direct current. Even though direct current will not flow through a capacitor, the impressed voltage will cause an electrostatic charge to accumulate on the plates and the capacitor will store an electrical charge according to the formula: Q = CE Where: Q = Quantity stored in coulombs E = Potential across the capacitor in volts C = Capacitance in farads

•

The drawing below illustrates how an electrostatic charge accumulates on the plates of a capacitor.

Figure (A) No charge on capacitor.

Orbital Electrons in Capacitor Dielectric Material Position of dielectric electrons undistorted without presence of electrostatic field.

Capacitor Plate Capacitor Plate Figure (B) Charged capacitor.

Continued on Sheet 5

Negative Plate

Sheet 4

Positive Plate

Position of dielectric electrons is distorted with presence of electrostatic field and an electrical charge accumulates on the plates. In this illustration, the left plate is positive and the right is negative.

XC2

AC Theory Capacitive Reactance - Page 2


Continued from Sheet 4 In a purely capacitive circuit, the circuit current will lead the applied voltage by 90o . This is a theoretical condition, since any circuit will have some value of resistance or inductive reactance in addition to the capacitance. AC Theory

In this circuit the current is all reactive and no work will be done. Single-phase power in watts in an AC circuit is: P = E x I x Cos The phase angle in this case is 90o . Since Cos 90o = 0, the circuit power therefore equals zero. Remember: • There are 360 degrees in a sine wave. • Electrical Phasors rotate counter-clockwise (CCW). • Phasors (electrical vectors) show two things: (1) magnitude, and (2) direction. • The symbol Theta is used to represent phase angle.

I

(Circuit Current) CCW Phasor Rotation

C AC

= 90 o Angle of lead

Phasor Axis of Rotation

Circuit Diagram

Phasor Diagram

E REF

X

Observer

(Reference Voltage @ 0o )

Time Increasing 90 o

180 o

T0

0o

If the observer stands a point X above and watches the phasors rotate CCW, the current phasor will appear first, followed 90o later by the voltage phasor.

Degrees are shown for current waveform.

Positive 1/2 Cycle Zero Amplitude Negative 1/2 Cycle

Phase Angle = 900 Leading (Voltage is Reference) Sine Wave Relationship Red - Current Black - Voltage Sheet 5

In the above drawing, the current crosses zero and goes positive 90o before the voltage crosses zero and goes positive.

BE1

Brushless Excitation EM Synchro-Pac®


Rotating Assembly Components Refer to “Rotating Components” section of Schematic Diagram on Sheet 5.

Sync Motors

In this brushless excitation system, the exciter stationary DC field induces three-phase AC into the rotating exciter armature. The armature leads (3) are attached to the motor shaft and connect to the rotating three-phase bridge rectifier. The DC output of the rotating rectifier is applied to the DC rotating field of the synchronous motor, with these leads also being fastened to the motor shaft, so that no bushes, commutator, or slip rings are required The rotating assembly, often referred to as the Diode Wheel, in addition to the rotating rectifiers, also carries the field discharge resistors, the Syncrite® Field Application Module, the Syncrite® Filter, and the Silicon Controlled Rectifiers (SCR’s) used for the application of the DC field for synchronous operation (SCR1) and to discharge the voltage induced in the rotating field (SCR2) during motor start-up.

Inspecting Rotating Field Assembly Components Electric Machinery 4500 HP Synchronous Motor

Inspection cover removed.

Rotating Assembly (Diode Wheel) Opposite Drive End (ODE) Pedestal Bearing Motor Shaft


Rotating Assembly Close-Up View Syncrite® Filter Mounted on back (inboard) side of diode wheel.

Syncrite Filter Mounting bolts and insulator.

Insulated stand-off bushings (3) for connection of exciter armature leads to three-phase diode assembly.

Positive Bus Diodes (3) Stud Cathode Units

DC Positive Heat Sink Heat Sink Insulator Top portion of Rotating Assembly (Diode Wheel)


See next page for layout of components on the Diode Wheel. Synchro-Pac® and Synchrite® are registered trademarks of the Electric Machinery Division of Dresser-Rand.

Sheet 10

Leads (3) from exciter armature. Shown disconnected for insulation resistance test of armature winding and to use the Diode Wheel Tester (See description on Sheet 13).

BE2



Rotating Assembly Component Arrangement Viewed from inspection access opening. Shaft must be rotated for all components to be visible. Refer to “Rotating Components” section of Schematic Diagram on Sheet 5. Sync Motors

Inspection access opening. Shown by dashed red line.

Field Discharge Resistors (6) Termination Access Hole Field Lead Terminals

Negative Bus Diodes See Notes 2 & 7

SCR 2 FWD D3 See Note 5

Syncrite

Field Application Module

Exciter Armature Lead Terminals

Syncrite Filter

See Note 6

Termination Access Hole Positive Bus Diodes

SCR 1 See Note 4

See Note 1

Field Discharge Resistors (6) Kilowatt Classroom Drawing

See Note 3 Notes 1. 2. 3. 4. 5.

7.

Synchro-Pac® and Syncrite® are registered trademarks of the Electric Machinery Division of Dresser-Rand.

Sheet 11

6.

Positive Bus heatsink with three (3) stud cathode diodes. Negative Bus heatsink with three (3) stud anode diodes. Field Discharge Resistors (12 units) are 24 ohms each and are connected in series/parallel for 4 ohms total. (Depending on the size of the rotating field, different wattage and ohmic values are used - consult instructions.) SCR 1 applies rectified DC to the rotating field of the synchronous motor when fired by the Syncrite® Field Application Module. SCR 2 connects synchronous motor rotating field to the field discharge resistors during start as induction motor. Free-Wheeling Diode D3 protects SCR 2 against high counter-emf voltage produced by collapsing motor field. Syncrite® modules, six field discharge resistors, exciter and field termination insulators/studs, and interconnecting wiring harnesses are mounted on back (inboard) side of wheel. Blue rectangles are heatsinks for stud-mounted diodes and SCR’s. Heatsinks are insulated from diode wheel.

BE3


Brushless Excitation EM Synchro-Pac® Component Testing

Sync Motors

While brushless excitation systems eliminate the need for brushes, a commutator, and slip rings, testing and adjustment of the systems can be difficult because the components cannot be checked or adjusted with the unit in service. Because all the excitation components are rotating, the equipment must be shut-down for inspection and maintenance. To assist the user in making the necessary tests, EM developed two specialized testers. The Syncrite® Module Tester (shown below) is used to test the Syncrite® Field Application Module and the Syncrite® Filter Module. A Diode Wheel Tester is used to test the diodes and SCRs mounted on the rotating assembly (see following page). The only adjustment that is necessary is the synchronizing speed adjustment on the Syncrite® Field Application Module (shown below). Module tests include: Zener Voltage Test, Zero Slip Test, Slip Trigger Test - Low Slip (99% of speed or 1% slip), Slip Trigger Test - High Slip (95% of speed or 5% slip), Out-of-Step Inhibit Test, and Positive Hold Test. The Syncrite Filter Module is tested for proper Zener operation. Syncrite® Module Tester Tests Field Application and Filter Modules. Syncrite® Field Application Module to be tested is plugged into mating connector on top of tester. If desired, unit can be tested without removal from diode wheel by using the appropriate interconnecting cable.

Syncrite® Filter Modules are tested by connecting the permanently-attached module leads to the tester banana posts. Patch cords are also available to permit in-place filter tests. Kilowatt Classroom Photo

Rear View

Syncrite® Field Application Module Size: 4-1/2” x 2” x 1-3/4”

Front View

Module Status Indicating Lights 2 Red - FDR in Circuit 2 Green - Field Off

9-pin Cable Connector

Synchronizing Speed Adjustment Potentiometer sets rotor slip rate at which the DC voltage is applied to the motor rotating field.


Sheet 12

1/4 - 28 Mounting Studs Attach unit to diode wheel.

Kilowatt Classroom Photos

BE4



Exciter Diode Wheel Tester A companion test set in the same style case as the Syncrite® Module Tester shown on the preceding page is available for testing the diodes and SCRs used in the Synchro-Pac® System. The semiconductors do not need to be removed from the diode wheel for testing. A drawing to the test set control panel is shown below. Sync Motors

The Exciter Diode Wheel Test Set will indicate shorted or open power diodes, silicon controlled rectifiers (SCRs), and an open field discharge resistor (FDR). It also checks for proper SCR firing and checks for grounded components (inadvertent connection to the motor shaft). The tester is easy-to-operate with the connection steps and appropriate light sequences shown on the nameplate.

Multimeter Tests The primary advantages of the Diode Wheel Tester are its ease-of-operation and the fact that it tests the components at their rated voltage. If this tester in not available, the semiconductor components, field discharge resistors, and wiring harnesses can also be checked with a conventional multimeter. The SCRs and diodes need to be checked using the meters DIODE CHECK function. A multimeter does not draw enough current through large SCRs to provide a “seal-in” of the device, but they can be checked for shorts and opens. See the Electrician’s Notebook articles Semiconductor Diodes and Rectifier Circuits and The Silicon Controlled Rectifier for further details on semiconductor testing.

Ohmmeter Testing of Field Discharge Resistors



Sheet 13

The photo at the left shows ohmmeter checks being made on the FDRs of a Synchro-Pac® system. The wattage and ohmic value of the resistors will vary depending on the size of the rotating field of the machine. Various series/parallel combinations are utilized and the instruction manual for the exact piece of equipment needs to be consulted for the value employed in a particular system.

Electricians Notebook

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