Adjustable Speed Drive

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$15.95 CDN

Adjustable Speed Drive REFERENCE GUIDE

4th Edition

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First Edition, November 1987 Second Edition, March 1991 Third Edition, February 1995 Fourth Edition, August 1997

Revised by: Richard Okrasa, P.Eng. Ontario Hydro

Neither Ontario Hydro, nor any person acting on its behalf, assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, equipment, product, method or process disclosed in this guide.

In-House Energy Efficiency Energy Savings are Good Business

Printed in Canada Copyright © 1997 Ontario Hydro

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ADJUSTABLE SPEED DRIVE Reference Guide

4th Edition

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TA B L E

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CONTENTS

INTRODUCTION .............................................................................................. 1 Latest Improvements .................................................................................2 CHAPTER 1: CLASSIFICATIONS ......................................................................... 3 Classification of Motors .......................................................................... 3 Classification of Drives ............................................................................ 3 CHAPTER 2: PHYSICAL APPEARANCE ................................................................. 5 CHAPTER 3: PRINCIPLES OF OPERATION ............................................................ 7 Conventional Fixed-speed AC Systems .................................................. 7 DC Drives ................................................................................................ 8 AC Drives ................................................................................................ 8 Eddy Current Clutches ............................................................................. 8 Switched Reluctance Drives ...................................................................... 9 Vector Drive .......................................................................................... 10 Wound-rotor Motor Controllers ............................................................... 10 Variable Voltage Controllers .................................................................... 11 Variable Frequency Drives ..................................................................... 11 Components .......................................................................................... 12 Types of Inverters .................................................................................. 13 Waveforms ............................................................................................ 14 Switching Devices (Power Electronics) ........................................................14 Medium Voltage Drives...........................................................................14 Recommended Specifications .....................................................................15 CHAPTER 4: COMPARISON OF ASDS ............................................................. 17 AC Drives .............................................................................................. 17 i

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Variable Voltage Inverter (VVI) ............................................................... 17 Current Source Inverter (CSI) ................................................................. 18 Pulse Width Modulator (PWM) .............................................................. 20 Power Factor Comparison ....................................................................... 22 DC Drives .............................................................................................. 23 Eddy Current Coupling ......................................................................... 25 Cycloconverter.........................................................................................26 CHAPTER 5: STANDARD AND OPTIONAL FEATURES ......................................... 33 CHAPTER 6: ADVANTAGES ............................................................................. 35 Speed Control ........................................................................................ 35 Position Control ..................................................................................... 36 Torque Control ...................................................................................... 36 High Energy Savings Potential ................................................................ 36 Soft Start/Regenerative Braking .............................................................. 36 Equipment Life Improvement .................................................................. 37 Multiple Motor Capability ..................................................................... 37 Bypass Capability ................................................................................. 37 Safe Operation in Harsh Environments .................................................... 37 Temporary or Back-up Operation .............................................................37 Reduction in Vibration and Noise Level .................................................... 38 Re-acceleration Capability ...................................................................... 38 Tips and Cautions .................................................................................. 38 CHAPTER 7: APPLICATION CONSIDERATIONS .................................................. 39 How to Select an ASD ........................................................................... 39 Software ...................................................................................................42 ii

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Financial Evaluation ................................................................................42 Load Characteristics ............................................................................... 42 Application Types by Load ..................................................................... 43 Tips and Cautions .................................................................................. 46 Motor/Drive System .............................................................................. 49 Thermal Considerations ......................................................................... 54 Other Considerations ............................................................................ 56 Efficiency .............................................................................................. 57 Reliability of ASDs ................................................................................ 58 Applications .......................................................................................... 59 Performance Required ............................................................................ 60 Starting and Stopping Characteristics ...................................................... 62 Torque .................................................................................................. 62 Environment .......................................................................................... 63 Weight and Space ................................................................................. 63 Accessories ............................................................................................ 64 Safety .................................................................................................. 65 Service and Maintenance ....................................................................... 65 Tips and Cautions .................................................................................. 67 CHAPTER 8: ECONOMICS .............................................................................. 69 Economic Factors ................................................................................... 72 Capital Costs ........................................................................................ 72 Capital Savings .................................................................................... 73 Operating Costs and Savings ................................................................. 73 Tips and Cautions .................................................................................. 75

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CHAPTER 9: HARMONIC DISTORTION ........................................................... 77 Harmonics .............................................................................................. 77 What Harmonic Distortion Can Do ...................................................... 78 Production and Transmission ................................................................ 79 Isolation Transformers ............................................................................ 80 Other Guidelines (IEEE 519-1992) ........................................................ 81 APPENDIX A: FORMULAS FOR CALCULATING APPLICATIONS ............................. 83 APPENDIX B: CONVERSION FACTORS ............................................................. 93 ABBREVIATIONS ............................................................................................ 95 BIBLIOGRAPHY .............................................................................................. 97 INDEX .......................................................................................................... 99 ASD SUPPLIERS IN ONTARIO ....................................................................... 101

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LIST

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FIGURES

OF

1. Comparison of Range Process Speed Control ......................................1 2. Physical Appearance of Variable Frequency Drive/Motor System ............................................................................ 5 3. 8/6 Pole Switched Reluctance Motor .................................................. 9 4. Vector Drive .........................................................................................10 5. Closed Loop (Feedback) Adjustable Frequency Inverter System .................................................................................. 12 6. VVI – Variable Voltage Inverter .......................................................... 17 7. VVI – Waveforms ............................................................................... 18 8. CSI – Current Source Inverter ............................................................ 19 9. CSI – Waveforms ............................................................................... 19 10. Block Diagram for a Typical CSI Drive ............................................. 19 11. PWM – Pulse Width Modulated Inverter .......................................... 21 12. PWM – Waveforms ............................................................................ 21 13. Block Diagram for a Typical PWM Drive .......................................... 21 14. Power Factor Comparison ................................................................. 22 15. DC Drive ............................................................................................ 23 16. ECC – Eddy Current Coupling .......................................................... 26 17. Cycloconverter Circuit.........................................................................27 18. Duty Cycles ....................................................................................... 43 19. Variable Torque Load ......................................................................... 45 20. Constant Torque Load ....................................................................... 45 21. Constant Horsepower Load .............................................................. 45 22. Power Required is Proportional to RPM3 Centrifugal Fan/Blower, Pump .............................................................................. 46 23. Power Savings in Fans and Pumps Using ASDs ............................... 48

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FIGURES

24. Motor Derating Curves vs. Speed Range When Applied to Adjustable Frequency AC Drives (6-Step Waveform or PWM) ............................................................. 53 25. Watts Loss (Efficiency) Comparison ................................................ 57 26. Typical AC Drive Efficiency ............................................................. 57 27. Motor Performance, Typical 60 Hz ................................................. 63 28. Ideal Torque-Speed Curves .............................................................. 64 29. NEMA Design B Motor Torque-Speed Curve ................................. 64 30. Capital Cost Comparison of Motor/Drive Systems Medium HP, Voltages ........................................................ 76 31. Harmonic Distortion ........................................................................ 78 A-1. Calculating Hollow Shafts ............................................................... 88 A-2. Calculating the Inertia of Complex, Concentric Rotating Parts ................................................................ 89

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LIST

1. 2. 3. 4. 5.

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Comparison of Adjustable Speed Drives ............................................. 29 ASD and Electronic Motor Features .................................................... 34 Suitability of Inverters for NEMA Motor Designs ............................... 55 ASD Checklist of Costs/Savings .......................................................... 70 ASD Investment Decision Technique .................................................. 71

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INTRODUCTION

An adjustable speed drive (ASD) is a device used to provide continuous range process speed control (as compared to discrete speed control as in gearboxes or multi-speed motors). An ASD is capable of adjusting both speed and torque from an induction or synchronous motor. An electric ASD is an electrical system used to control motor speed. ASDs may be referred to by a variety of names, such as variable speed drives, adjustable frequency drives or variable frequency inverters. The latter two terms will only be used to refer to certain AC systems, as is often the practice, although some DC drives are also based on the principle of adjustable frequency.

Continuous

Speed

Discrete

Operation

FIGURE 1. Comparison of Range Process Speed Control

Introduction

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In this guide, “drive” refers to the electric ASD. Application concerns in connecting electric or mechanical ASDs have similar effects on the driven load, and these are covered in this guide.

L ATEST I MPROVEMENTS • Microprocessor-based controllers eliminate analogue, potentiometer-based adjustments. • Digital control capability. • Built-in Power Factor correction. • Radio Frequency Interference (RFI) filters. • Short Circuit Protection (automatic shutdown). • Advanced circuitry to detect motor rotor position by sampling power at terminals, ASD and motor circuitry combined to keep power waveforms sinusoidal, minimizing power losses. • Motor Control Centers (MCC) coupled with the ASD using real-time monitors to trace motor-drive system performance. • Higher starting torques at low speeds (up to 150% running torque) up to 500 MP, in voltage source drives. • Load-commutated Inverters coupled with synchronous motors. (precise speed control in constant torque applications.

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CHAPTER 1

CLASSIFICATIONS

C LASSIFICATION

OF

M OTORS

• There are two main types of motors, AC (alternating current) and DC (direct current). • AC motors can be sub-classified as induction (squirrel-cage and wound-rotor) and synchronous. • Induction motors are often classified as either high efficiency or standard.

C LASSIFICATION

OF

D RIVES

• Adjustable speed drives are the most efficient (98% at full load) types of drives. They are used to control the speeds of both AC and DC motors. They include variable frequency/voltage AC motor controllers for squirrel-cage motors, DC motor controllers for DC motors, eddy current clutches for AC motors (less efficient), wound-rotor motor controllers for wound-rotor AC motors (less efficient) and cycloconverters (less efficient).

Chapter 1: Classifications

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• Other types of drives include mechanical and hydraulic controllers. Examples of mechanical drives are adjustable belts and pulleys, gears, throttling valves, fan dampers and magnetic clutches. Examples of hydraulic drives are hydraulic clutches and fluid couplings. • In this guide, emphasis is on AC variable frequency drives, or inverters, which are used to control industry’s workhorse, the standard AC induction motor. This is because this motor is replacing the DC motor for many applications. In addition, some information is provided on the DC motor/drive system, since it remains the most suitable choice for certain applications. • Drives may be classified according to size ranges (horsepower, voltage) for which increasing specifications are required in designing an ASD driven system: - Less than 500 HP. - Medium sized (up to 2000 HP). - Motors rated 4kV and up. • An output transformer between the drive and motor, common mode voltage is isolated from the motor and put on the drive side transformer winding.

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CHAPTER 2

PHYSICAL APPEARANCE

• Variable frequency AC drives are comprised of many electrical circuits and components usually arranged within a cabinet that provides heat dissipation and shielding.

ASD + transformer (if required)

LOAD

Feedback Loop (Optional)

Tachometer

Can be hundreds of metres away

Motor

FIGURE 2. Physical Appearance of Variable Frequency Drive/Motor System Chapter 2: Physical Appearance

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• Drives vary greatly in size, depending upon their horsepower and voltage rating and type. • Electrical cables connect the motor to the drive, which might involve a considerable distance. • Small AC drives may be built on to their associated motors.

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CHAPTER 3

PRINCIPLES OF OPERATION

• Both AC and DC drives are used to convert AC plant power to an adjustable output for controlling motor operation. • DC drives control DC motors, and AC drives control AC induction and synchronous motors.

C ONVENTIONAL F IXED - SPEED AC S YSTEMS (AC M OTOR W ITHOUT D RIVE ) • Standard squirrel-cage induction motors are usually considered to be constant speed motors. • These systems require some means of throttling (via valves, dampers, etc.) to meet process changes. • If a reduction in demand occurs, excess energy is wasted in the control device (dampers, throttling valves, recirculation loops) since the power delivered does not decrease in proportion to the reduction in demand.

Chapter 3: Principles of Operation

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DC D RIVES • The DC motor is the simplest to which electronic speed control can be applied because its speed is proportional to the armature voltage. • The DC voltage can be controlled through a phase-controlled rectifier or by a DC-DC converter if the input power is DC. This is usually accomplished by a separate motor-generator set producing a DC output. • The speed of a DC motor can be adjusted over a very wide range by control of the armature current and/or field currents (brushless DC drives, vector controlled DC drives).

AC D RIVES E DDY C URRENT C LUTCHES • Eddy current clutches can be used to control standard AC squirrel-cage induction motors. However, they are low efficiency compared to ASDs and have limited applications. • An eddy current clutch has essentially three major components: a steel drum directly driven by an AC motor, a rotor with poles and a wound coil that provides the variable flux required for speed control. • Efficiency is significantly lower than ASDs. • A voltage is applied to the coil of wire, which is normally mounted on the rotor of the clutch to establish a flux, and thus relative motion occurs between the drum and its output rotor.

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• By varying the applied voltage, the amount of torque transmitted, and therefore the speed, can be varied. S WITCHED R ELUCTANCE D RIVES • Switched reluctance (SR) drives have a high power to weight ratio. • In closed-loop control, they are well suited for speed and torque control.

FIGURE 3. 8/6 Pole Switched Reluctance Motor (one phase winding shown)

• The rotor has salient poles with no windings or electric connections. • A pair of opposite stator poles magnetically pulls rotor poles in-line. • Rotor position sensor controls switch each pole pair in sequence, giving continuous rotation. Chapter 3: Principles of Operation

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V ECTOR D RIVE • Vector drive control of AC motors is similar to DC drive performance in speed, torque and horsepower. • It can produce full torque from start to full speed. (The motor needs to control heat at full torque and low speed.) • It requires complex electronics (digital signal processors, or DSPs) to calculate servomotor phase currents. • Magnitude and direction of armature current together are a vector quantity which must be regulated to adjust torque. • Slip speed and motor speed are tracked by an encoder. • Synchronous motors can be controlled by vector drives by eliminating magnetizing current and slip values. Speed Regulator 2 Phase to 3 Phase

Encoder Current Regulator Motor

Flux Command Controller

Position Signal

FIGURE 4. Vector Drive

W OUND - ROTOR M OTOR C ONTROLLERS • Wound-rotor motor controllers are used to control the speed of wound-rotor induction motors. 10


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• By changing the amount of external resistance connected to the rotor circuit through the slip rings, the motor speed can be varied. • The slip energy of the motor is either wasted in external resistance controllers (in the form of heat) or recovered and converted to useful electrical or mechanical energy. For conversion to useful electrical energy, the system would be known as a wound-rotor slip energy recovery drive. VARIABLE V OLTAGE C ONTROLLERS • Variable voltage controllers can be used with induction motors. • Motor speed is controlled directly by varying the voltage. • These controllers require high slip motors and so are inefficient at high speed. • Only applications with narrow speed ranges are suitable.

VARIABLE F REQUENCY D RIVES • A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor. • The drive also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio (V/Hz) of voltage to frequency, as required by the characteristics of the AC motor to produce adequate torque. •

In closed-loop control, a change in demand is compensated by a change in the power and frequency supplied to the motor, and thus a change in motor speed (within regulation capability).


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Feedback Signal

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Speed Reference from Process

TACHOMETER

REGULATOR (Controls)

Motor RECTIFIER

Constant Frequency Constant Voltage AC Power Supply

LOAD

INVERTER (Switching Section)

Fixed or Variable DC Voltage

Variable Frequency Variable Voltage AC Power Output

FIGURE 5. Closed Loop (Feedback) Adjustable Frequency Inverter System

C OMPONENTS • A variable frequency drive has two stages of power conversion, a rectifier and an inverter. (“Inverter” is also used to refer to the entire drive.) • The system functions this way: - 60 Hz power, usually 3-phase, is supplied to the rectifier. The input voltage level is usually standard 208V, 230V, 460V, 600V, 4,160V, etc. (Higher than 600V requires step-down transformers.) - The rectifier is a circuit which converts fixed voltage AC power to either fixed or adjustable voltage DC.

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- The inverter is composed of electronic switches (thyristors or transistors) that switch the DC power on and off to produce a controllable AC power output at the desired frequency and voltage. - A regulator modifies the inverter switching characteristics so that the output frequency can be controlled. It may include sensors to measure the control variables. T YPES

OF I NVERTERS

• There are three basic types of inverters commonly employed in adjustable AC drives: - The variable voltage inverter (VVI), or square-wave six-step voltage source inverter (VSI), receives DC power from an adjustable voltage source and adjusts the frequency and voltage. - The current source inverter (CSI) receives DC power from an adjustable current source and adjusts the frequency and current. - The pulse width modulated (PWM) inverter is the most commonly chosen. It receives DC power from a fixed voltage source and adjusts the frequency and voltage. (PWM types cause the least harmonic noise.) • AC/AC adjustable frequency drives are used only for large horsepower applications (1000 hp and above). They include cycloconverters (AC/AC) and load-commutated inverters (LCIs). Both can be used with induction or synchronous motors. (Since these drives are usually custom-designed for each application, they will not be fully discussed in this guide.)


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W AVEFORMS • The voltage and current waveforms produced by inverter systems approximate, to varying degrees, the pure sine wave. •

Of the three most common inverter systems, the pulse width modulated inverter produces output current waveforms that have the least amount of distortion.

S WITCHING D EVICES • Advances in Power Electronic technology have greatly enhanced performance range and reliability of ASDs. • New switching devices are faster, produce less heat, and less harmonics into the motor circuit. Some types are: - SCR (silicon - controlled rectifier). - Diode. - GTO (gate turnoff thyristor). - IGBT (insulated gate bi-thermal thyristor).

M EDIUM V OLTAGE D RIVES • Voltages above 2300V, and controlling induction motors between 1,000 HP to 15,000HP are becoming increasingly available. - Input line isolation transformer. - Internal cooling (liquid or air). - Input circuit breaker, output contactor with isolation switches.

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- Motor harmonics filter to supply maximum 5% current total harmanic distortion. - DC link reactor to prevent saturation at faulted conditions. R ECOMMENDED S PECIFICATIONS •

Nominal power at +- 10% voltage, 3 phase, 60 Hz ( +- 2%).

• Capable of operation during temporary voltage drop of 70% to 90% lasting up to 6 voltage wave cycles. • Bus voltage restored within 5 seconds, drive automatically restarts, if not, drive automatically trips and shuts down. Manual reset required to start. • Uninterruptible Power Source (UPS) recommended to provide control circuit power during supply power disturbances, from 5 seconds up to 15 minutes UPS supply recommended. - Ambient Indoor Conditions: -

0°C to 40°C.

-

Relative humidity up to 95% non condensing.

- Overload capability: 15% rated current for 60 seconds. - Class H insulation, class B temperature rise. - ANSI C57.12.01 construction materials. - NEMA Std. TR-27 for noise.


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CHAPTER 4

COMPARISON OF ASDS

AC D RIVES VARIABLE V OLTAGE I NVERTER (VVI) •

A controlled rectifier transforms supply AC to variable voltage DC. The converter can be an SCR (silicon-controlled rectifier) bridge or a diode bridge rectifier with a DC chopper. The voltage regulator presets DC bus voltage to motor requirements. AC to DC Rectifier

DC Link

DC to AC Inverter

M

Constant Voltage

Voltage Smoothing

Variable Voltage/ Frequency Control

FIGURE 6. VVI – Variable Voltage Inverter

•

Output frequency is controlled by switching transistors or thyristors in six steps. Chapter 4: Comparison of ASDs

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Current (Line)

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6 Step

Time

FIGURE 7. VVI – Waveforms

•

VVI inverters control voltage in a separate section from the frequency generation output.

•

Approximate sine current waveform follows voltage.

•

VVI is the simplest adjustable frequency drive and most economical; however, it has the poorest output waveform. It requires the most filtering to the inverter.

•

Ranges available are typically up to 500 horsepower but can be up to 1000 horsepower.

•

Voltage source inverters use a constant DC link voltage.

C URRENT S OURCE I NVERTER (CSI)

18

•

AC current transformers are used to adjust the controlled rectifier. Input converter is similar to the VVI drive. A current regulator presets DC bus current.

•

The inverter delivers six step current frequency pulse, which the voltage waveform follows. Switches in the inverter can be transistors, SCR thyristors or gate turnoff thyristors (GTOs). Adjustable Speed Drive Reference Guide

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AC to DC Rectifier

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DC to AC Inverter

DC Link

M

Variable Voltage Control

Current Smoothing

Variable Frequency Control

FIGURE 8. CSI – Current Source Inverter Voltage (Line to Neutral)

Current (Line)

0

0 Time

FIGURE 9. CSI – Waveforms AC Line

AC/DC Converter

Filter

Inverter Motor

Current Regulator

Frequency Control

Speed Speed or Voltage Control

FIGURE 10. Block Diagram for a Typical CSI Drive Chapter 4: Comparison of ASDs

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•

The capacitor in the inverter is matched to motor size.

•

Voltage exhibits commutation spikes when the thyristors fire.

•

Because it is difficult to control the motor by current only, the CSI requires a large filter inductor and complex regulator.

•

CSI drives are short circuit proof because of a constant circuit with the motor.

•

They are not suitable for parallel motor operation.

•

Braking power is returned to the distribution system.

•

The CSI drive’s main advantage is in its ability to control current and, therefore, control torque. This applies in variable torque applications.

•

CSI-type drives have a higher horsepower range than VVI and PWM (typically up to 5000 horsepower).

P ULSE W IDTH M ODULATOR (PWM) •

Diode rectifiers provide constant DC voltage. Since the inverter receives a fixed voltage, the amplitude of output waveform is fixed. The inverter adjusts the width of output voltage pulses as well as frequency so that voltage is approximately sinusoidal.

•

The better waveforms require less filtering; however, PWM inverters are the most complex type and switching losses can be high.

•

The range of PWM inverters is typically up to 3000 horsepower, but each manufacturer may list larger sizes (usually customengineered).

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AC to DC Converter

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DC Link

DC to AC Inverter

M

Variable Voltage Control

Voltage Smoothing

Variable Frequency Control

FIGURE 11. PWM – Pulse Width Modulated

Voltage (Line to Neutral) 0

Current (Line) 0

FIGURE 12. PWM – Waveforms

AC Line

Diode Bridge Rectifier

Filter

Inverter Motor

Speed Reference Voltage & Frequency Control

FIGURE 13. Block Diagram for a Typical PWM Drive Chapter 4: Comparison of ASDs

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•

Motors run smoothly at high and low speed (no cogging); however, they are current limited.

•

PWM drives can run multiple parallel motors with acceleration rate matched to total motor load.

•

At low speeds, PWM drives may require a voltage boost to generate required torque.

• A vector drive can control similar to a DC drive. •

PWM is the most costly of the three main AC ASD types.

•

Pulse amplitude modulation (PAM) drives are a variation of PWM drives.

Power Factor

1.0 PWM & Vector Drive

.75

VVI .50

CSI

.25 0 450

900

1350

1800

Speed (RPM)

FIGURE 14. Power Factor Comparison

P OWER FACTOR C OMPARISON • The power factor of VVI and CSI drives declines with speed as the thyristor firing angle varies in the controlled rectifier.

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• PWM drives have near unity power factor throughout the speed range, due to the diode rectifier and constant voltage DC bus. • Note that true Root-Mean-Square (RMS) meters will determine the real power factor on three-phase systems. It may be less than the displacement power factor (kW/kVA) which appears on single-phase meters.

DC D RIVES • DC drives are a simpler, more mature technology than AC drives, and they continue to have applications where larger horsepower is required due to high voltage capacity. • Armature voltage-controlled DC drives are constant torque drives capable of rated motor torque at any speed up to rated motor base speed.

% of Rated Power

100

0

0

Armature Voltage Control

Field Current Control

Constant Field Current

Constant Armature Voltage

Constant Torque

Constant Power

100 % of Base Speed

FIGURE 15. DC Drive Chapter 4: Comparison of ASDs

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• Field voltage-controlled DC drives provide constant horsepower and variable torque. A variable voltage field regulator can provide alternate armature and field voltage control. • Motor speed is directly proportional to voltage applied to the armature by the ASD. A phase-controlled bridge rectifier with logic circuits is used. Tachometer feedback achieves speed regulation. • DC drives have good efficiency throughout the speed range and are larger than AC for the same horsepower. However, with DC drives, the power factor decreases with speed, it is not possible to bypass the drive to run the motor and maintenance costs are high due to armature connections through a brush and commutator ring. • Regenerative DC drives can invert the DC electrical energy produced by the generator/motor rotational mechanical energy. • Cranes and hoists use DC regenerative drives to hold back “overhauling loads,” such as a raised weight or a machine’s flywheel. • Non-regenerative DC drives are those where the DC motor rotates in only one direction, supplying torque in high friction loads such as mixers or extruders. The load exerts a strong natural brake. If desired, the drive’s deceleration time can affect speed regulation. • Flywheel applications such as stamping presses have overhauling load; hence, braking torque or “dynamic braking” is applied. All DC motors are DC generators as well. • Regenerative drives are better speed control devices than nonregenerative but are more expensive and complicated. 24


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• Armature voltage control DC drives have constant torque features, capable of rated torque across the motor speed range. These drives must be oversized to handle constant horsepower applications. • Field voltage control of shunt wound DC motors with a voltage regulator coordinate armature and field voltage for extending speed range in constant horsepower applications. • Table 1 compares the electric variable speed drives that may be used to control the speed of standard squirrel-cage induction motors. For comparison, information on DC systems is also provided. Note that this table covers products representative of the types available. Actual product lines may differ. In addition, special order equipment may not conform to these guidelines. Voltage ranges depend on the manufacturer as well as the need for auxiliary equipment, such as step-down transformers, line filters and chokes. E DDY C URRENT C OUPLING • The eddy current coupling (ECC) is similar in principle to a friction-type clutch. It provides electromechanical coupling with torque transmitted by eddy currents. The eddy currents are generated by rotation. • The ECC has electrically energized magnetic coil windings on the rotor via slip rings. The magnetic fields in the drum are caused by eddy currents. • Horsepower Slip Loss =

motor hp x slip speed RPM motor RPM

Chapter 4: Comparison of ASDs

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Drum Motor

Load

SR TR

TD SD

Magnetic Rotor TD = Drum Torque SD = Drum Speed

TR = Rotor Torque SR = Rotor Speed

FIGURE 16. ECC – Eddy Current Coupling

C YCLOCONVERTER • Mainly used in large synchronous motor drives in low frequency applications: - Steel rolling mill end tables. - Cement mill furnaces. - Mine hoists. - Ship propulsion drives. • Limitation: wave forms become distorted above 40% of input frequency (i.e., 20Hz from 50Hz supply). • Advantage: high power factor using synchronous motors.

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A.C. Supply

Bridge A

Load

Bridge B

A.C. Supply

FIGURE 17. Cycloconvertor Circuit


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TABLE 1. Comparison of Adjustable Speed Drives Type of Electric Drive

Variable Voltage Inverter (VVI)

Current Source Inverter (CSI)

Pulse Width Modulated Inverter (PWM)

MOTOR COMPATIBILITY

• Squirrel-cage induction or synchronous • Can handle motors smaller than inverter rating

• Squirrel-cage induction or synchronous • Can handle motors smaller than inverter rating (at reduced rating)

• Squirrel-cage induction or synchronous • Can handle motors smaller than inverter rating

TYPICAL POWER RANGE (hp)

1 – 1,000

50 – 5,000

SPEED REDUCTION (typical) = Maximum Speed

10:1

10:1

CONTROL OPEN LOOP CAPABILITY (no feedback) (Note: Can be improved with feedback controls)

5%

5%

ADAPTABILITY OF MOTOR TO HOSTILE ENVIRONMENTS

Good

Good

EFFICIENCY RANGE • for system: drive & motor

88 - 93%

88 - 93%

Yes Yes

Yes Yes

DC Drive

Wound Rotor with Slip Energy Recovery

Eddy Current Coupling (ECC)

Commutated DC

Wound rotor induction

Squirrel-cage induction

0 – 10,000

400 – 20,000

1 – 1,000

20:1 open loop 200:1 with tachometer

5:1

34:1 but may be difficult to control above 2:1

0.1 - 5% depending upon feedback methods

2 - 5%

3 - 5%

Poor due to high maintenance of motor

Medium

Good

90 - 94%

92 - 96%

0 - 70%

Rotor current

Field winding

5 – 5,000

30:1

Minimum Speed 5%

Good

85 - 95%

TORQUE hp • Constant • Variable • Control Method

Yes Yes

Yes Field voltage, armature voltage or both

600 VOLTAGE RANGE


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TABLE 1. Comparison of Adjustable Speed Drives (cont’d) Type of Electric Drive




DC Drive

MULTIPLE MOTOR CAPABILITY (e.g., two 200 hp motors on a single 400 hp drive)

Yes, unlimited within inverter rating

No

Yes, unlimited within inverter rating

SOFT STARTING

Yes

Yes

Power Factor to Motor (PF)

Better than CSI (*2) Drops with speed

(*2)

Drops with speed

Worst

OUTPUT SYSTEMS HARMONICS (dependent on leakage reactance) COMPLEXITY OF: • POWER CIRCUIT • CONTROL CIRCUIT PRINCIPLE



Yes, with manufacturer’s engineering for load sharing

No

No

Yes

Yes

Yes, if starting resistors used

Yes

Near unity (excellent)

(*2)

Relatively low (can be improved with capacitors)

Good

Better than VVI

Least

Yes

Yes

No

Simple Simple

Simple Semi-complex

Simple Complex

Simple Simple

N/A Simple

Simple

The inverter receives DC power from an adjustable voltage source and adjusts the frequency.

The inverter receives DC power from an adjustable current source and adjusts the frequency and voltage. The DC current regulator is controlled by a closed loop speed regulator.

The inverter receives DC power from a fixed voltage source (diode rectifier) and controls voltage and frequency. The RMS voltage amplitude is fixed, but the width of voltage intervals is varied.

Speed is adjusted by changing field voltage and/or armature voltage.

Changes current in rotor circuit by means of a rectifier and converter connected to rotor winding. Energy recovered is usually fed back into power supply.

30 Adjustable Speed Drive Reference Guide

The output speed is varied by controlling the magnetic coupling between two rotating members. This is done by means of a field winding which controls the clip between them.

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DC Drive



CIRCUIT PROTECTION • Inverter Open Circuit

Inherent voltage limit

Requires careful design



N/A

N/A

• Inverter Short Circuit

Must be carefully designed to handle DC bus capacitor discharge

Inherent current limit

Same as for VVI, except PWM circuit is very fast acting

Inherent current limit

N/A

N/A

CONTROL VARIABLE

Motor voltage, frequency

Motor voltage, frequency and current

Motor voltage and frequency

Motor armature voltage, current and/or field voltage (not common)

Rotor current

Field between rotating member

REGENERATIVE BRAKING

Option with added circuitry

Standard

Option

Option

No

No

REVERSE CAPABILITY

Yes

Yes

Yes

Yes

No

Poor

RIDE-THROUGH CAPABILITY

Difficult

Difficult

Yes, using battery or capacitive storage

Special applications only

No

No

SIZE & WEIGHT

Intermediate

Large

Small

Intermediate

Small

Small controller; large rotating element

MAIN ADVANTAGES

• High output frequencies (higher than 60 Hz if necessary) • Can be retrofitted to existing fixed speed motor • Soft start

• Short circuit and overload protection due to current control of regulator • Soft start

• Excellent power factor; harmonics are minimal • Can be retrofitted to existing fixed speed motor • Soft start

• Simple system • Wide speed range • Soft start

• Costs are relatively low for narrow variable speed ranges • Simple circuitry • Adaptable to existing wound rotor motors

• Low costs • Simple compact control • Wide constant torque speed range

MAIN DISADVANTAGES

• Harmonics increase losses in motor • Standard inverter cannot operate in a regenerative mode

• Instability may result under partial loading • Harmonics increase losses in motor • Difficult to retrofit to existing fixed speed motor drive

• Motor is subject to voltage stresses • Complex logic circuits

• Brush and commutator maintenance is high • Limited to medium and lower speed applications; special motor enclosures may be specified if higher speed capability is required (TENV, TEAO)

• Maintenance of brushes is high • May pose problems in hazardous environments • Relatively low power factor • Limited speed range • Regenerative braking n/a

• Efficiency low at low speeds • Lack of reversing capability • Limited speed range • Maintenance of brushed is required

Chapter 4: Comparison of ASDs 31

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MAIN DISADVANTAGES (cont’d)

• Lower horsepower ranges typically

• Only single motor control

• High initial cost

• Not suitable for hazardous environments where explosive gases may exist • Expensive, large motor • Power factor always poor at low speed

• General purpose lowmedium horsepower (<500 horsepower), multiple motor control

• General purpose when regenerative braking wanted (hoists)

• Best reliability AC type, at added cost • Also suitable for most applications

• • • •

• • • •

• • • • •

APPLICATIONS • General

• Specific

(*1) (*2)

Conveyors Machine tools Pumps Fans

Pumps Fans Compressors Blowers

Slow speed ranges Conveyors Pumps Fans Packaging equipment



• For applications with a wide range of speed adjustment and a lowmoderate starting torque • Used for medium and low speed applications • General purpose

• Used if speed range is narrow (70%-100%) and reversing not required

• General purpose for equipment normally operating at full speed

• • • • • • • • • • •

• Large pumps & fans with limited speed range • Compressors • Kilns • Conveyors • Mixers

• • • • •

DC Drive

Extruders Machine tools Mine hoists Cranes Elevators Rotary kilns Rubber mills Printing presses Shakers (foundry or car) Winches Public transportation

A totally enclosed motor is usually required because the ECC is normally used in close proximity to the driven machine (e.g., machine tools). The VVI, CSI and DC drives have power factors that decrease with speed. For the AC inverters, this can be corrected by implementing a diode and chopper control. This will slightly increase acoustical noise and slightly reduce efficiency.

N/A Not Applicable 32 Adjustable Speed Drive Reference Guide

Fans Pumps Blowers Fluid propulsion systems Driving extruders

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CHAPTER 5

STANDARD AND OPTIONAL FEATURES • See Table 2 on the following page for a general guideline list of standard and optional features for AC variable frequency drives and new power electronic devices. Note, however, that manufacturers may differ on some factors.

Chapter 5: Standard and Optional Features

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TABLE 2. ASD and Electronic Motor Features ASD Standard Protection Features

ASD Optional Features

Overvoltage

Soft start

Undervoltage

Overload protection

Overcurrent

Torque limit

Loss of control power

Power outage ride-through

Across-the-line start Line-to-line shorts on output Line-to-ground shorts on output

Brake stop Coast stop Bypass

Continuous overload

Motor slip compensation

Locked rotor

Electronic reversing

Motor single phasing

Voltage boost (at start) Accel/decel Regenerative power protection Low speed jog IR compensation

34


New Power Electronic Devices Metal oxide semiconductor (MOS) controlled thyristors (inverter switches) Insulated-gate bi thyristors (IGBT are more capable of rapid energizing)

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CHAPTER 6

ADVANTAGES

• Electronic AC or DC adjustable speed drives have a number of advantages over mechanical, hydraulic and fixed speed drives. They include a continuous speed range from 0 to full speed, improved process control, improved efficiency and potential energy savings, enhanced product quality and uniformity, soft starting/regenerative braking, wider speed, torque and power ranges, short response time, equipment life improvement, multiple motor capability (except CSI), easy to retrofit (except CSI), bypass capability, increased productivity, safe operation in hazardous environments, reduction in vibration and noise level, re-acceleration capability, reduced maintenance and downtime and operation above full load speeds. • Motor diagnostics are available in feedback controls.

S PEED C ONTROL • ASDs are used to control production speed in conveyor systems in the food, paper, automotive, and consumer goods industries. In mining, ASDs are used in crushers, grinding mills, rotary kilns, presses, rolling mills, and textile machinery.

Chapter 6: Advantages

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P OSITION C ONTROL • ASDs are used for machine tools. T ORQUE C ONTROL • ASDs are used for tensioning (winders). H IGH E NERGY S AVINGS P OTENTIAL • Applications with highest energy savings potential are centrifugal pumps and fans (power is proportional to speed cubed), pumping applications (municipal water systems, centrifugal chillers, chemical/petrochemical industries, pulp and paper plants and food industries) and replacing damper controls in air handling and ventilation applications. S OFT S TARTING /R EGENERATIVE B RAKING • When a constant speed drive starts up, the surge of inrush current that moves the motor out of its stationary position is about six times the ordinary current, thus producing much stress on the equipment, especially the windings. • With adjustable frequency drives, acceleration times can be adjusted from instantaneous up to several minutes, thus providing soft starting capabilities. • Regenerative braking is used when the rapid reduction of motor speed in a controlled manner is needed for production or safety reasons. It is a form of dynamic braking in which the kinetic energy of the motor and driven machinery is returned to the power supply system. The motor becomes a generator when the driven load is applying torque in the reverse direction.

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E QUIPMENT L IFE I MPROVEMENT • The soft starting feature reduces water hammer and cavitation situations for fluid systems to prolong equipment life. • Operation of motors, transformers, cables, pump seals, pipes, valves and impellers may be prolonged. • Soft starting reduces inrush current and voltage drop during starting and therefore also reduces stresses on windings, starting currents and heating. M ULTIPLE M OTOR C APABILITY • One multiple motor ASD (except CSI) can control a number of synchronized motors at the same speed (e.g., in the textile industry). B YPASS C APABILITY • The adjustable frequency drive can be for service, without need to shut down the driven equipment (with additional circuitry optional). S AFE O PERATION

IN

H ARSH E NVIRONMENTS

• Adjustable frequency drives offer safe operation in harsh environments since the drive can be housed in a remote location. T EMPORARY

OR

B ACK - UP O PERATION

• Instead of operating a second pump or fan for temporary service when extra pressure or flow is required, use a larger capacity single pump or fan under ASD control to meet the EXACT requirements at ALL times.

Chapter 6: Advantages

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R EDUCTION

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N OISE L EVEL

• Vibration and noise level are reduced when the operating speed of the equipment is lowered and because valves or vanes are eliminated. R E - ACCELERATION C APABILITY • Some adjustable frequency drives continue to have power supply during power losses of short duration, whereas fixed speed devices would trip out.

T IPS

AND

C AUTIONS

• If using multiple motors, each one must be protected by its own overload relay. The total current drawn by all the attached motors must be equal to or less than the current rating of the controller. • Equipment life will be prolonged only if the proper precautions are taken for power conditioning. Poor quality power can cause overheating, insulation damage and even equipment destruction. • Consider torsional harmonics. Avoid operating at speeds coincident with rotating equipment natural frequencies (resonance).

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CHAPTER 7

APPLICATION CONSIDERATIONS H OW

TO

S ELECT

AN

ASD

• Use this section as a general guide. The information provided does not address differences in types of driven equipment. • Essentially, selecting an ASD involves matching the performance of the ASD to the needs of the motor and load. - Determine the need for speed or process flow control. Without varying speed requirements, equipment may simply be oversized for the needs of the process, if present throttling devices are frequently on. - Describe the range of speed control. An ASD offers a continuous range from 0 to full speed. If only a few select operating points are required, a multi-speed motor may be a better choice. - Estimate the process duty cycle (see Figure 18). Duty cycle is a listing of the process operating points (for example, fan pressure and flow) and the duration each point occurs. This is perhaps the most important part of assessing the need for an ASD in a particular application. The duty cycle characterizes the process being served by the motor. Chapter 7: Application Considerations

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- Gather equipment performance data. Performance curves supplied by the equipment manufacturer describe the power requirements of the driven equipment at selected operating points. It is necessary, however, to check that the “as installed” performance matches that of the performance curves. Otherwise, improper performance selection of the ASD may result. Also note that performance ratings and field ratings may differ. Consider getting the help of a qualified installation and set-up contractor to verify field performance. - Operating points are the intersection of the particular process system curve and the equipment’s characteristic performance curve. - System curve is the set of points that describes the volume of flow and resistance to flow as defined by the application. - Throttling, or dampers, change the system curve by increasing the resistance to flow. - Performance curve is the set of points of flow vs. pressure that the particular fan, pump or blower must follow at a particular speed and fluid density. Manufacturers usually supply performance curves that give the selected design point. - Brake horsepower and efficiency vs. flow are also supplied by the manufacturer. They determine the motor and any gearbox or belt sheave reduction necessary to achieve the correct speed. - Calculate constant and ASD power requirements. Using the formulas in Appendix A, calculate the power required for each operating point in the duty cycle for constant speed (throttling flow control) and adjustable speed cases.

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- Calculate energy consumption. Multiply the power required at each operating point by the annual hours at the point from the duty cycle, then sum the total for constant and adjustable speed. - Select a drive type and features and estimate costs. Based on the load type (constant vs. adjustable torque, horsepower, starting time, speed regulation, speed, torque range, regeneration, shielding, transformers, installation, control logic and other specific features listed in this guide), select the type of drive for the application. Obtain manufacturers’ quotes. Prices will depend greatly on whether you need a custom-designed ASD or an off-the-shelf model. - Calculate simple payback (based on energy savings alone). Total the cost to install a drive. Multiply the estimated annual energy savings (adjustable vs. constant speed) by the utility energy rate charge. Divide the total installed cost by annual energy savings. The result is simple payback in years. - Consider other ASD savings, such as reduced wear due to soft start, lower maintenance costs and less material wastage resulting from more accurate speed adjustment. These savings are difficult to estimate and can usually be determined only through ASD operating experience. - Note: Measure power in kW, not kVA. Use power meters, not ammeters. Power factor must be measured. kW = kVA x p.f. Check that phases are balanced in a three-phase system. (Do not assume three phase = 1.73 x single phase.)

Chapter 7: Application Considerations

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S OFTWARE F INANCIAL E VALUATION • Software is available from several ASD suppliers, including some utilities. Be careful to include lower part-load efficiencies when inputting performance data. L OAD C HARACTERISTICS Varying Duty Cycle • The load profile or duty cycle will also indicate the potential suitability of an ASD for an application. The duty cycle shows the typical speeds and corresponding time intervals for which a motor operates annually. From an energy standpoint, the ingredients of a good ASD application are high percent throttling (changing load) and high annual operating hours.

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% Flow

100

Good Application

0

Time

% Flow

100

Poor Application

0

Time

FIGURE 18. Duty Cycles

A PPLICATION T YPES

BY

L OAD

• There are three main types of adjustable speed loads: variable torque/variable horsepower (hp = torque x RPM) (centrifugal pumps, fans), constant torque and constant horsepower, (constant tension winders, machine tools).


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• The behaviour of the horsepower and torque as a function of percent speed partially determines the requirements of the motor/controller system. • For an induction motor, the speed-torque relationship depends on the voltage and frequency of the supplied voltage as well as the characteristics of the rotor conductors. • Constant torque drives are often supplied as “standard” drives. To make a variable torque drive, the manufacturer usually adds a jumper and chopper to the standard model. • Examples of variable torque loads are centrifugal loads, where torque is proportional to RPM2, where horsepower is proportional to RPM3 such as fans, pumps and blowers (dynamic). • Examples of constant torque loads are agitators, positive displacement compressors, conveyors (belt, batching, chain, screw), crushers, drill presses, extruders, hoists, kilns, mixers, packaging machines, positive displacement pumps, screwfeeders, roll out tables and winders-surface. Note that some may not be constant torque loads but require constant torque drives due to shock overloading, overload or high inertia load conditions. • Examples of constant horsepower loads are drilling machines, lathes, machine tools, milling machines and centre-driven winders. Note that torque is inversely proportional to speed.

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Percent hp and Torque 100 80 60 40

Torque

hp

20 0

50 Percent Speed

100

FIGURE 19. Variable Torque Load Percent hp and Torque Torque

100 80 60

hp

40 20 0

50 Percent Speed

100

FIGURE 20. Constant Torque Load Percent hp and Torque hp 100 80 60

Torque

40 20 0

50 Percent Speed

100

FIGURE 21. Constant Horsepower Load Chapter 7: Application Considerations

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C AUTIONS

• The variable torque controller is designed to provide 100% rated torque continuously with no overload capability. This should be used only for applications where the load torque varies proportionally with speed, such as fans and centrifugal pumps. The current rating of the motor must be checked with Fan/Blower (incompressible flow) Outlet Damper Control

ASD Control

Pressure

Unstable Area

Performance System Inlet Guide Vane Control

Flow

Flow

Pump Valve Control

ASD Control System

Perf

nce

Performance System

Static

Pressure Dynamic

orma

Flow

Flow

FIGURE 22: Power Required is Proportional to RPM3 Centrifugal Fan/Blower, Pump 46


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the current rating of the controller to ensure that the controller can provide the full horsepower capability of the motor. • Low speed motor cooling does not limit the speed range with a variable torque load since the load requires less torque at lower speeds. For this type of load, it is important to choose a horsepower rating for the highest speed attained. • The minimum allowable motor speed for continuous constant torque or constant horsepower operation is determined by the motor cooling requirements at low speeds. These methods can be used to increase the motor’s constant torque speed range: - Use a separate blower for motor cooling. - Use an oversized motor, and operate it at less than its nameplate rating. This provides additional mass for heat dissipation. However, this may result in oversizing the drive to compensate for the increased magnetizing current. - Use a motor with a high service factor. Specify class F or H insulation. - Use a high efficiency motor. • Also, see “Thermal Considerations.” • Torsional harmonics may occur if resonant frequencies coincide with reduced speeds. These can be programmed out by the ASD. • Low speed operation can cause mechanical instability if it results in operating too far up the fan/pump performance curve (the unstable region before peak pressure).


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• Multiple fan/pump systems will run at the same pressure if in parallel operation. So, do not put an ASD on only one of parallel pumps or fans. • Sizing the drive means matching torque, speed, voltage, current and horsepower to the load and motor requirements. • The cost for custom-engineered applications (mostly DC, synchronous or wound-rotor motors with slip energy recovery, load-commutated inverters) will be higher. • ASDs are generally selected for their speed control capability, not specifically for energy savings. Energy savings are achieved, however, when process control dampers or throttling valves or recirculation lines are replaced by higher efficiency ASDs. • ASDs offer the best potential for energy savings when controlling the speed of centrifugal fans, pumps and blowers. The power required is proportional to RPM3. Therefore, a 10% drop in speed results in a 27% drop in power consumption (1.0-0.93).

Power Required

Damper Control

Saving ASD Control

Speed

FIGURE 23. Power Savings in Fans and Pumps Using ASDs 48


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• Demand savings are not attributable to ASD control, however, since achieving better speed control does not usually result in downsizing absolute power requirements. There may be “time of use” demand savings (taking advantage of reduced speed operation during utility peak demand periods). • In-rush current is about 600% rated current when started at full voltage and frequency. If the motor is started at low voltage and frequency through an ASD, it will never need more than 150% of rated current (started at 2 Hz). This soft start reduces stresses on the motor, extending its life. M OTOR /D RIVE S YSTEM • If, after examining the load characteristics and process requirements of an application, it appears that an ASD may be an asset, investigate motor/drive compatibility. • If a drive is to be retrofitted to an existing motor, get this information from the motor: nameplate voltage and horsepower, current and torque data, insulation class and NEMA design characteristic. • Manufacturers’ curves should be consulted to aid in motor selection for new systems. • When considering the information here, also look at Table 1, because the table lists typical applications for each of the drives and may help you narrow the choices available for a particular application. It should be used when conducting the remainder of the selection process.


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Motor Type • Your choice of available drives depends to a large extent on the motor used. Although DC systems were largely used in the past, AC motors are much more popular now due to their relatively low cost, low maintenance requirements and better reliability. For most low- and medium-speed applications, squirrel-cage AC induction motors are now used. • Variable Speed Brushless DC “Electronically Commutated” motors are available in ≤600 horsepower sizes. Horsepower Rating • Induction motors are best suited for power levels up to approximately 500 horsepower (325 kW), although they can be used for higher power levels. Above 1,000 horsepower, synchronous motors are often used and are usually driven by current source inverters or by load-commutated inverters or cycloconverters. These high-powered systems are very expensive to purchase for use in the lower end of their operating ranges. Medium Voltage AC induction motors are now available under ASD control. • It is important to determine the maximum horsepower requirements of the driven load and how the required power varies with speed.

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Voltage Requirements • These are the size ranges usually available for AC variable frequency drives: Horsepower Range

Voltages Available

<50 50-200 200-1,000

208V to 600V three-phase 460V to 600V three-phase low voltage (460V, 600V) and medium voltage (2,300V, 4,160V) mostly medium voltage (2,300V, 4,160V) medium voltage (4,160V, 6,900V, 13,800V) 13,800V

1,000-2,500 *2,500-10,000 *>10,000 (usually DC, or wound rotor)

• Note that suitably rated transformers may be used to match the drive voltage rating to that of available power supply voltages. • The system voltage should be within the deviation permitted by the specifications for the ASD. This is usually +10% and -5% per NEMA standards. Specific values can be obtained from the manufacturer. Torque and Current • After checking the horsepower requirements, ensure that the starting torque and full load torque are within the motor’s rating. • Continuous permissible running torque decreases with motor speed.


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• It is important to ensure that the drive can supply the required current. Inverters are current-limited and may only allow a relatively high output current for short time periods. An estimate of the motor torque to current ratio can be made by referring to the motor speed, torque and current characteristics. • The drive must have a maximum continuous current rating that is greater than or equal to the motor’s full-load current rating. Speed and Speed Range • Consider the minimum and maximum speed requirements. • The speed range depends on the motor used. A standard efficiency, class F insulated motor is applicable only to a 2:1 constant torque speed ratio. A high efficiency motor can provide a 3:1 ratio. To obtain wider speed ranges, the motor can be oversized. • Below 6 Hz, however, significant motor cogging may occur as the motor tries to follow the waveshape. A practical speed range of 10:1 below 60 Hz is suggested for VVIs and CCIs. (This is not a concern for PWMs.) • If precise speed control is needed, a synchronous or synchronous reluctance motor can be used for an AC system. Otherwise, a DC system could be used.

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Speed Range Percent (%) of 60 Hz Torque Rating

100 z 0-60 H 2:1 3 z 0-6 0 H 3:1 2 0 Hz 6 5 1 4:1 0 Hz 1 0-6 6:1 0 Hz 6 5 . 7 8:1 0 -6 H z 1 0:1 6

90 80 70 60

Induction Motor: Constant Torque Load, USEM 4-P TEFC 460 V 30 60 Hz Motor With Boost At Low Frequency.

50 2 0 : 1 3-6 0

40

Hz

Source Data: EIC Program Based on Constant Temp. Method

30 0 1

20

40 50

75

100 125 150 200 Motor Horsepower – 60 Hz Rated

250

FIGURE 24: Motor Derating Curves vs. Speed Range When Applied to Adjustable Frequency AC Drives (6-Step Waveform or PWM)

• The speed range of an AC motor can be extended in using a drive above 60 Hz, provided the V/Hz ratio is maintained. The motor is rated at V/Hz; as speed increases at constant rated torque, the horsepower output increases. The drive must be sized to accommodate the horsepower rating as well as motor current and voltage. Speed Regulation • Mechanical loads cause a drop in motor speed (according to its speed/torque curve). • Tachometers can monitor motor shaft speed through a feedback loop to the drive controller, which sends a compensation speed increase signal to the ASD.


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• NEMA design B is the most common standard duty AC motor. Speed can be held within 3% of setpoint (which is motor slip). • Thyristors are limited in their switching speed, which determines ASD speed regulation capability. Time required to accelerate the load: T(sec) = WK2 (lb-ft2) x change in RPM 308 x torque (lb-ft) (load inertia: WK2 total = sum of WK2 components, W is weight, K is radium of gyration.) Torque (lb-ft) = HP x 5250 RPM T HERMAL C ONSIDERATIONS • If variable frequency controllers are used, there are a number of important factors to consider to ensure that the motor/drive system is compatible from a thermal standpoint. •

The main concern when retrofitting existing motors with variable frequency drives is to ensure that the controller can provide the current required for the load torque to prevent motor overheating.

• Since the cooling systems of most motors are designed for a fixed speed, the cooling action will be reduced when operating at reduced speeds (since cooling fan speed decreases with motor speed). This is especially true for constant torque applications and applications in which CSI drives are used. For these situations it is important to provide additional cooling or overframe or derate the motor. An overframed motor may also require a larger controller. See “Tips and Cautions.”

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• NEMA type 1 vented enclosure to dissipate ASD heat within. Ambient limits as specified. • It is also important to ensure that the motor will not overheat because of the harmonics in the AC waveform supplied by the inverter. This is especially true for standard motors. (See Chapter 9, “Harmonic Distortion.”) • Harmonic losses are affected by the design type of NEMA speed torque characteristics as well as the characteristics of the motor under consideration. The motor leakage reactance, which limits harmonics, varies with each NEMA design. The compatibility of variously rated motors with inverters is useful to know. See Table 3 for the most suitable motor design/inverter combination to use. TABLE 3. Suitability of Inverters for NEMA Motor Designs Motor NEMA Design

VVI

CSI

PWM

High Efficiency Motor A B C* D*

X X X

X

X X

* These motors are very undesirable for adjustable frequency control, due to high harmonic losses. • NEMA design B squirrel-cage induction motors are commonly used in industry. • Energy efficient motors have lower losses than standard motors and therefore provide wider torque capability when used with variable frequency drives. Chapter 7: Application Considerations

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O THER C ONSIDERATIONS • The next step in the decision process is to evaluate the relative importance of each of the remaining factors to be considered. One of these factors may exclude one drive system. For example, if the system is to be used in an explosive environment, commutators and brushes cannot be used because of the sparks that would be generated. • These are some other selection considerations: economics, process requirements and load characteristics, performance required (speed regulation/control accuracy, efficiency and reliability), starting and stopping characteristics (load inertia), torque (breakaway torque, accelerating time and torque and decelerating time and torque), environment, weight and space, maintenance, programmability needed, lead time for delivery, line power factor and mechanical considerations. • Process requirements and load characteristics were discussed at the beginning of this chapter. Although initially used as indicators, the importance of these factors should now be compared with all other considerations.

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E FFICIENCY • At full speed and full load, VVI, CSI and PWM drives are all about 95% efficient. Efficiency drops at approximately a square rate with speed, as commutation losses (thyristor closing) vary with torque and current.

Driver Losses (kW)

4.0 3.5 PWM

3.0 2.5 2.0

VVI

1.5

CSI

1.0 .5 450

900

1350

1800

Speed (RPM)

FIGURE 25. Watts Loss (Efficiency) Comparison

100% Percent Efficiency

75% Load

100% Load

94% 90% 50% Load 86%

25% Load

82% 78% 74% 0

20

40

60

80

100

Percent Speed

FIGURE 26. Typical AC Drive Efficiency (PWM) Chapter 7: Application Considerations

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• CSI drives tend to be more efficient than VVI and PWM as speed is reduced. • Higher horsepower sizes, as well as drives operating close to their maximum design rating, tend to be at higher efficiency. • Information about efficiency of drives is generally not easily obtained from manufacturers since so many factors affect it. • Motor efficiency at reduced speed needs to be recalculated. R ELIABILITY

OF

ASD S

• Reliability of ASDs has improved as power electronics technology has advanced. Thyristors convert to AC to DC power and GTO designs improved reliability. Metal oxide semi-conductor controlled thyristors, surface mount technology and specific integrated circuits are reducing drive sizes. • Voltage drop temporary “ride-through” (see Harmonics section). • Current rise or drop limits are features specified. •

Sizing the controller to handle required load currents is important.

• Motor heating at low speeds will not be a problem with centrifugal loads due to the drop in motor current and I2R losses. • CSI drives use the motor as part of the circuit, so selecting the motor and drive together will minimize risk of mismatching. • Transistors can be made for high current and voltage and faster response than thyristors.

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• Constant voltage/frequency ratio means the motor will not stall when overloaded, maintaining constant speed regardless of load. • The motor may trip out when decelerating rapidly. With large inertia loads, regeneration of power back through the drive may trip the voltage protection bus. Elevators and lowering conveyors are examples. Sizing the protective bus to suit the application should prevent it, (see recommended technical specifications for medium voltage drive). A PPLICATIONS • Constant torque (hoists, presses and conveyors) operation up to 120 Hz can be provided by applying constant V/Hz to the motor. This requires an AC drive with twice the voltage output capability than the supply voltage to the motor (@ 60 Hz). Since a motor is rated at V/Hz, it can be operated at rated torque and twice the speed if voltage and frequency are both doubled. Operation at twice the motor-rated horsepower requires sizing the AC drive at that horsepower and considering stresses and balancing on the motor. • Position control is important in materials handling, machining and robotics. • Multiple motor operation in parallel by a single voltage inverter AC drive can be done by sizing the drive to the sum of the maximum continuous running currents of each motor. All motors start and stop together. If motors are coupled together through the load, load sharing must be considered. High-slip NEMA design D motors may be required. Also, individual motor overload protection is necessary. • Cogging refers to torque pulsation at below 6 Hz frequency. If smooth operation is needed at low speed, it may be necessary Chapter 7: Application Considerations

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to use a six- or eight-pole motor with a 90 Hz or 120 Hz maximum frequency, (eliminated with vector drives). • IR compensation is a circuit that senses changing motor load and reduces voltage boost when the motor is lightly loaded. This improves starting torque and low speed overload capability. • Regenerative braking occurs when the motor acts as a generator when driven by the load. The energy is returned to the power lines through the drive. The drive must be sized to handle the energy absorbed. Hoists, flywheels and other constant torque applications make use of regenerative braking. Centrifugal loads, such as fans, pumps and blowers, do not. P ERFORMANCE R EQUIRED Speed Regulation/Control Accuracy • The importance of the drive’s sensitivity to changes in load, temperature, humidity, drift and line voltage fluctuations should be determined. • If there is to be no speed deviation, a synchronous motor is used. • Vector drives can smoothly hold position and speed and torque over a full range from 0% to 100% of scale.

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Efficiency • System efficiency = mechanical power output from motor shaft electrical power input to drive • The efficiency of a motor/drive system depends on characteristics of the connected motor (power factor, efficiency), speed range and duty cycle, load, measurement method and instrument accuracy, inverter size and horsepower rating, input power tie voltage variation and manufacturing variations. (Sometimes, it’s better to use a high efficiency motor.) • The motor design and specific operating points are the largest contributors to efficiency differences. • High efficiency motors are more susceptible to tripping due to heat, voltage or current drops. • Multi-speed motors (i.e., pole changing motors) offer fixed speed combinations (two to four is typical) that are a much cheaper alternative to ASDs if continuous speed adjustment is not needed. • A more important consideration is: Energy Lost = Output Power – Input Power • Higher horsepower drives tend to have higher efficiencies.


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• The CSI controller tends to maintain better efficiency than other inverters as operating speed is reduced. S TARTING

AND

S TOPPING C HARACTERISTICS

• Are soft starts or controlled acceleration needed for the driven machine? • Does the power supply system need reduced voltage starting or controlled acceleration? • Does the driven machine require accurate positioning, controlled deceleration or regenerative braking? T ORQUE • The ability of the drive to reach the torque required at various points in the process cycle should be considered. • Breakaway or locked rotor torque is needed to start the load from rest to overcome static friction. An ASD can provide a voltage boost that will permit this torque to be higher than normal. The inverter components may have to be sized larger and the current limit set higher if this is the case. • Accelerating time and torque is needed to increase the speed of the machine. An ASD permits short or long accelerating times. For high inertia loads, such as machines with flywheels or large blowers, care must be taken to ensure that the system can provide enough accelerating torque.

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• Decelerating time and torque is needed for high inertia loads. An ASD permits long or short deceleration times. E NVIRONMENT • Abrasive, moisture laden, explosive, dusty or otherwise difficult environments may affect the ability of the drive/motor system to function and the ability to provide adequate maintenance. The effects can be eliminated by careful design or locating the drive in a clean, cool room. Providing an adequate cooling air supply for air-cooled converters is another important consideration. W EIGHT

AND

S PACE

Motor Torque 7 hp (Continuous) % Motor Full Load Rating

• There may be space and economic considerations involved in decisions concerning large drives due to their size. Standard Operating Speed Range

Extended Speed Range

150 High Eff. Motors 100 80

Torque

hp

hp

1.15 SF Motors 50

Torque

hp

0 6 15

30

60 Frequency (Hertz)

90

120

FIGURE 27. Motor Performance, Typical 60 Hz


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Percent Torque

300

200

100

2

10

20 30 40 Frequency (Hertz)

50

60

FIGURE 28. Ideal Torque-Speed Curves Pull Out or Breakdown Torque 200 Locked Rotor or Stall Torque 100

NEMA Design B Motor Torque vs. Speed Operating Point Load Torque Slip

Percent Torque 0 10 20 30 40 50 60 70 80 90 100 Percent Speed Synchronous Speed Operating Speed

FIGURE 29. NEMA Design B Motor Torque-Speed Curve

A CCESSORIES • Accessories include auto transformers (for voltage overload protection), regenerative braking circuits (overhauling loads in constant torque such as cranes), bypass loop (for operating the motor directly bypassing the drive), filters and the line chokes 64


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(to reduce electric harmonics), cooling fans and programmable logic controllers (PLC) for speed control through feedback process monitoring. S AFETY • Follow NEMA recommended enclosure design and installation specifications. High voltage and current are present. DC power is more dangerous than AC; DC is found in both AC and DC drives. The DC bus can be more than 600V in AC drives if input power is not checked in harmonic spikes. All circuit boards should be covered in metal for shielding and cooling. Access interlocks should shut down and disconnect the drive input power before the cabinet can be opened. Manual control panels are restricted to 120V. • It is important to separate control wiring from power wiring. Use separate metal conduits to reduce electronic “noise” from power to control circuits (see Chapter 9). S ERVICE

AND

M AINTENANCE

• AC and DC drives can be located several hundred feet away from the motor where heat, humidity and contaminants can be controlled. • DC motors require commutator and brush replacement periodically, (except brushless DC types). • Installation is usually a simple matter of three-phase electrical connection to the motor and power lines. • Solid-state electronics are relatively maintenance free. Most drive manufacturers supply built-in diagnostics as well as protection relays and fuses. As proper drive performance depends on matching motor and load requirements, expert trouble-shooting


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may be necessary. For this reason, consider purchasing a service contract until you have experience with the particular drive. • The frequency and degree of complexity of maintenance requirements for a particular system can be a significant factor. Are company personnel restricted to the type of equipment they service? Are self-diagnostics supplied with the control module? Is the supplier willing to service the drive after purchase? Does the supplier have representatives located close to you? Purchasing equipment from a supplier who has no representatives close to the installation may result in the supplier losing interest in installation and maintenance after the purchase is made. • Mechanical devices that provide adjustable speed need more maintenance than their electrical counterparts. • Servicing a PWM inverter requires a complex diagnostic aid equivalent to a logic analyzer. CSIs and VVIs, on the other hand, are easy to service since each part of the system can be operated independently to isolate problems. Programmability Needed • Will it be necessary to frequently change the operating characteristics of the drive, as offered by a PC or a drive equipped with a microprocessor? Lead Time for Delivery • Lead time for delivery depends on whether the application requires a small horsepower drive (the equipment may be purchased off the shelf) or if increased horsepower or custom-engineered features, such as line filters, chokes and autotransformers, are needed. Drive sophistication or special options, such as cooling requirements, unique process control interface or packaging, tend to lengthen lead time. Many applications require a custom-engineered drive system. 66


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Line Power Factor • If a specific power factor is specified or limitations are imposed for an installation, this should be considered. Normally, the power factor is satisfactory. Mechanical Considerations • If adjustable speed is being considered, the natural torsional frequencies of the connected mechanical loads should be checked to ensure that they do not correspond to the frequencies produced at lower operating speeds. This may still be a concern even if the motor is mounted to a massive support pad.

T IPS

AND

C AUTIONS

• When a few discrete speeds are needed, a multiple speed motor may be satisfactory. It would be significantly cheaper than purchasing a variable frequency drive. This can be accomplished by using a special stator winding arrangement. This method is often used for applications involving pumps, fans, blowers, conveyors and printing presses. For example, a 2:1 speed ratio is easily obtained from a single stator winding by reconnection. • On new installations, an ASD can replace the standard motor starter. All that is needed is a feeder breaker to protect the cables to the controller. • If retrofitted, the existing motor is usually retained, but it must be derated. • It is important to ensure that the electrical supply line uses the correct voltage. This is particularly important in view of the


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large number of electrical drives that are manufactured outside Canada and where different standards may be used. • Circuit breakers, transformers, fuses and disconnect switches may or may not be included in an ASD system. If this apparatus is to be mounted separately, its current rating should be based on the input current of the ASD, not the motor full load current. This is due to harmonic effects which cause the ASD input current to be greater than the motor full load current for a given power level. • If a motor drives a load through a gearbox at reduced speed under ASD control, it may not deliver enough running torque. Check minimum torque times speed requirements. • Induction motors cause supply current to lag behind supply voltage. The ratio of kW to kVA (true power to apparent power) is the “displacement power factor.” This is the cosine of the phase angle between current and voltage, when current and voltage are assumed to be clean sine waveforms. • Harmonic currents from inverter switching may increase apparent power and decrease power factor. This “true power factor” will be less than displacement power factor cosine of current to voltage phase angle. • A true RMS meter is required to measure non-linear loads and filter harmonic currents (usually 5th and 7th are most significant.)

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CHAPTER 8

ECONOMICS

• Economics is usually one of the most important factors involved in selecting industrial equipment, but it is not straightforward. Many economic aspects are often ignored in ASD evaluations. • Use the Table 4 ASD checklist of costs and savings to avoid overlooking certain economic aspects. • The simple payback method is frequently used to determine how long it would take for a piece of equipment to “pay for itself” in terms of savings: Number of Years =Total Initial Capital Cost (including any service) Total Annual Savings • This method should only be used as a risk indicator, however, since it is very inaccurate and neglects the impact of inflation. ASD quotes of two- to three-year paybacks often underestimate the true period until breakeven.

Chapter 8: Economics

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• The net present value method is a good technique that can be used to appraise the profitability of an investment. By using the discounted cash flow technique, it takes into account the time value of money. A summary of this approach appears in Table 5. TABLE 4. ASD Checklist of Costs/Savings

Capital Costs

Capital Savings

Drive

Control valves

Motor

Gear box

Power conditioning equipment

Fluid coupling/ mechanical speed changing equipment

Installation Electrical system upgrade

Reducedvoltage starters

Torsional analysis Space requirements Cooling

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Operational Costs and Savings Energy (total energy consumed, peak demand charge) Maintenance/ useful life/ downtime Overspeed capability

Other Salvage value Tax implications

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TABLE 5. ASD Investment Decision Technique For detailed examples of this procedure and relevant software, contact your local utility. 1.

Evaluate the cost/savings of the factors in Table 4 for each option you are considering (for example, purchasing an ASD, purchasing a mechanical drive system, not purchasing a variable speed drive). Capital costs will be expressed in total dollars; operating expenses will be expressed in terms of time.

2.

Determine the real discount rate that should be used for each timedependent and future-valued factor. For example, for energy savings calculations: x% per annum = nominal discount rate y% per annum = rate at which electricity rates will rise i% = {x/y – 1}% As another example, a salvage value n years from the present should be discounted using the rate at which the interest rate is expected to rise between now and n years.

3.

All factors for each option should be discounted to their present values, using the appropriate discount rate. The number of years used for timedependent factors should be chosen as a reasonable payback period. Present value tables and annuity tables are useful for the discounting process.

4.

The net present value (NPV) of each option is found by summing the costs and savings that have been calculated in present value terms for each factor.

5.

For any option, if

6.

The option with the greatest positive value of NPV is the most profitable.

7.

The procedure could be repeated assuming different total time periods.

8.

A comparison between two options could also be made by using the relative difference between the option for each factor and finding one NPV.

NPV >0, NPV <0, NPV = 0,

there is a net gain there is a net loss breakeven occurs at the time under consideration.


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E CONOMIC FACTORS C APITAL C OSTS Drive • The cost of this major item will vary greatly, depending on the options required. The cost should include speed controls, start/stop controls, engineering, cable, conduit, foundations, spare parts and any related modifications. For example, a battery back-up for the controls may be provided for auto restart or shut-down sequences. Motor • The cost of a motor must be considered for a new system. Power Conditioning Equipment • The cost of any power conditioning equipment, such as harmonic filters, should be included. This includes filters for incoming power to the motor as well as power conditioners for harmonic voltages and currents sent back to the power supply from the drive. Installation • Installation, labour and commissioning charges for the drive and motor and power conditioning apparatus should be determined. Electrical System Upgrade • Upgrading of the electrical system may be necessary if higher reliability is required than the present system can offer. Potential upgrades include relay protective systems, supply transformer redundancy, transfer switching/alternate feeders, maintenance and emergency staff training and preventive maintenance programs. 72


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Torsional Analysis • A torsional analysis will define the vibration effects of inverter harmonics in the drive train. It should be conducted for large drive applications. Space Requirements • This includes the cost of any indoor space requirements for the drive and filters, as well as any outdoor space costs, such as those associated with transformers, filters or reactors. Cooling • Additional cooling may be required for drive installation. For large applications, although HVAC equipment is often used, water cooling may be a much more economical alternative. C APITAL S AVINGS • Use of an ASD may avoid certain capital investments. Examples are gear boxes, control valves, fluid coupling/mechanical speed changing equipment and reduced voltage starters. O PERATING C OSTS

AND

S AVINGS

Energy • There may be savings in terms of both energy consumed and peak demand charge. The extent of these savings depends on the local utility’s rate schedule. If an ASD is installed, the total energy consumed will likely be reduced. • The other element of electrical power cost is the demand charge, measured in kVA, which compensates the utility for the peak current it must deliver during the month. The most significant factor affecting KVA demand is the power required by the load,


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which varies with the cube of the speed. Because of this, adjustable speed drives may provide significant savings. • It is important to keep in mind that the kilowatt-hours of energy saved are the last ones that would have otherwise been purchased. The use of average energy cost can be very misleading. Maintenance/Useful Life/Downtime • The reduction of maintenance and downtime may be quite substantial if an AC variable frequency drive is employed. Contributing factors are elimination of control valves, current-limit feature (prevents motor burnouts caused by multiple restarts) and protection of the motor insulation (so it is shielded from voltage problems). • Useful life of equipment, such as bearings, can be extended by operating at reduced speeds. Stresses and metal fatigue in the drive train shafts will be lowered. • Repairs to variable frequency drive systems do not usually take much time. Overspeed Capability • The overspeed capability of adjustable frequency drives can save considerable operating costs, as well as investment, if increases in production levels occur. For example, the airflow through an existing fan can be increased by retrofitting a variable speed drive to its motor so that the motor is supplied with a frequency higher than an existing 60 Hz rating.

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C AUTIONS

• An adjustable frequency drive is the most cost effective choice if the duty cycle is more evenly distributed over the entire range of flow rates. • Relative energy savings improve if the performance and system resistance curves are steep. • Many potentially good ASD applications are passed up because benefits other than energy savings are overlooked. Frequently, however, process control and reliability far outweigh efficiencyrelated benefits to the user. • By using the average cost of energy in savings analyses, the savings can be significantly overstated for variable frequency applications. Instead, both the energy and demand charges of the local utility’s rate schedule should be used. • For variable torque loads, the variable frequency drive is very energy conscious, since the horsepower varies proportionally to the cube of the speed. • Coupling systems (eddy current and hydraulic) have the quickest economic payback. Electronic drive systems have the highest dollar return. • Cost savings through reduced energy consumption often result in ASD payback periods of five to six years, rather than the two to three years normally required by industry. • Induction motors tend to be cheaper than DC motors for similar horsepower ratings.


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• See Figure 27 for a relative purchase price comparison for various motor/drive systems. For explosion-proof environments, the relative cost of the DC motor would increase substantially. • For horsepower applications above 50, installation costs are usually comparable to the total capital cost for the drive. Below this power rating, installation costs may be as much as double the drive cost. • Software packages which evaluate the economic aspects of adjustable speed drives are available. It is important to keep in mind that these programs require part-load efficiency within their analyses.

Controller Valve

Eddy Current Coupling

Motor

Motor

Valve Control

Slip Control

Motor

Controller

Motor

DC AC Solid-state Control

FIGURE 30. Capital Cost Comparison of Motor/Drive Systems Medium HP, Voltages

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CHAPTER 9

HARMONIC DISTORTION

• Adjustable Speed Drives have defined capability to withstand voltage and current waveform distortion. • The latest IEEE standard 519 defines harmonic distortion limits acceptable on the input side to the AC power system. • For a full discussion of harmonic distortion and mitigation techniques, see Ontario Hydro’s Power Quality Reference Guide and Power Quality Mitigation Reference Guide.

H ARMONICS • There are two types or harmonics: electrical and mechanical. • The inverter (switching) section of an ASD generates harmonics. • Electrical harmonics cause waveform distortion. They are currents or voltages that oscillate at integer multiples of the fundamental 60 Hz frequency, which is the main power frequency. For example, a frequency five times the fundamental frequency is called the fifth harmonic. Chapter 9: Harmonic Distortion

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Bus Voltage

Line Current

FIGURE 31. Harmonic Distortion

• If large, electrical harmonics may cause power system waveforms to deviate from perfect sinusoids, eg.: capacitor switching, large induction, motors start-up. • Any static power converter that converts AC to DC or DC to AC, or any solid-state switch, generates harmonics (e.g., thyristors or SCRs). • All adjustable frequency drives with power switching devices generate harmonics. • The odd harmonic amplitudes usually decrease with increasing frequency, so the lowest order harmonics are the most significant. Even harmonics are normally not generated by three-phase converters.

W HAT H ARMONIC D ISTORTION C AN D O • As with many other forms of pollution, harmonic distortion affects the whole electrical environment. It propagates through 78


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the power system and may even show up at distant points outside the plant, thus causing problems for other equipment connected to the power supply. • Typical effects of harmonics on the motor/drive system are reduced motor efficiency due to increased losses, increased heating of motors, cables and transformers, excessive voltage stress on insulation of motor windings and torque pulsations, (“torsional harmonics”). • General problems caused by harmonics are general degradation of power quality, voltage dips or voltage ripple, premature equipment failure, improper operation of important control and protection equipment, interference with telecommunication or computer systems, amplification of harmonic levels resulting from resonance, incorrect readings on mechanical timing relays and watt-hour meters and blown fuses. • All capacitors, including those used for power factor correction, tend to be very susceptible to harmonic damage. Disastrous consequences can occur if capacitors are exposed to excessive harmonic voltages or currents. • The harmonics produced by a converter may increase motor losses by 5% to 10%.

P RODUCTION

AND

T RANSMISSION

• Harmonics are produced in utility or industrial electrical systems by equipment that switches repetitively in less than a cycle, such as variable frequency AC drives, cycloconverters, DC drives, rectifiers, UPS systems, arc furnaces and static VAR generators. Fluorescent and gaseous discharge lamps can also produce harmonics. Chapter 9: Harmonic Distortion

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• Harmonics occur as long as the harmonic generating equipment is in operation. They tend to be of a steady magnitude. • Harmonic currents flow through the impedance of the transmission and distribution network and generate harmonic voltages, which distort the electrical user’s input voltage. • Harmonic currents often flow in the neutral line. • The order and magnitude of the harmonics generated by a drive depend on the drive configuration and the system impedance. • Harmonics may be greatly magnified by power factor correction capacitors. Supply system inductance can resonate with the capacitors at some harmonic frequency, causing large currents and voltages to develop. This may damage equipment. In addition, since the impedance of a capacitor decreases with increasing frequency, capacitors tend to act as sinks for higherorder harmonics. I SOLATION T RANSFORMERS • Isolation transformers are frequently used to protect the drive as well as the AC line from distortion. They may also decrease the available short circuit current in a fault situation and prevent drive shutdown and possible damage in the event of a motor line ground fault. If their use is not properly planned, however, they may cause electrical difficulties elsewhere in the system. • The description of the power system used when ordering equipment should include fault level at the service entrance, rating and impedance of transformers between the service entrance and the input to the power conditioning equipment and details of all capacitor banks on the same utility substation.

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O THER G UIDELINES • There are no current CSA standards specifically relating to ASDs. CSA approval may be granted to many different drive designs, many of which are imported. • Minimum guaranteed full load, full speed ASD efficiency of 95%, (including any supplied equipment: isolation transformers and filters). • Harmonic Distortion: Latest recommended specification: IEEE 519-1992: total voltage harmonic distortion shall not exceed 5% at common coupling ASD to motor.

Chapter 9: Harmonic Distortion

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APPENDIX A

APPENDIX A Formulas for Calculating Applications

C ALCULATING H ORSEPOWER Once the machine BHP (speed x torque) requirement is determined, horsepower can be calculated using the formula: rated motor hp = motor efficiency (%) 100 = available hp BHP =

TxN 5,250

(required hp)

Where, hp = horsepower, supplied by the motor T = torque (lb-ft), force x radius N = base speed of motor (rpm) If the calculated horsepower falls between standard available motor ratings, select the higher available horsepower rating. It is good practice to allow some margin when selecting the motor horsepower. Appendix A

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For many applications, it is possible to calculate the horsepower required without actually measuring the torque required. The following will help: BHP = brakehorsepower, the mechanical load required by the driven equipment F OR C ONVEYORS hp (vertical) =

weight (lb) x velocity (FPM) 33,000 x efficiency

hp (horizontal) =

weight (lb) x velocity (FPM) x coef. of friction 33,000 x efficiency

F OR W EB T RANSPORT S YSTEMS

AND

S URFACE W INDERS

Note that the tension value used in this calculation is the actual web tension for surface winder applications, but it is the tension differential (downstream tension – upstream tension) for sectional drives. C ENTRE W INDERS (A RMATURE C ONTROL O NLY ) hp =

tension (lb) x line speed (FPM) x buildup 33,000 x taper

C ENTRE W INDERS (F IELD C ONTROL ) If taper x field range ³ buildup, then hp =

84

tension (lb) x line speed (FPM) 33,000



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If taper x field range ² buildup, then tension (lb) x line speed (FPM) x buildup

hp =

33,000 x taper x field range

Note that these formulas for calculating horsepower do not include any allowance for machine function windage or other factors. These factors must be considered when selecting a drive for a machine application. F OR FANS

AND

B LOWERS

Effect of speed on horsepower hp = k1 (RPM)3 – horsepower varies as the 3rd power of speed T = k1 (RPM)2 – torque varies as the 2nd power speed Flow = k3 (RPM) – flow varies directly as the speed hp

=

hp

=

CFM x pressure (lb/in2) 229 x (eff. of fan) CFM x (inches of water gauge total pressure) 6,362 x (eff. of fan)

Total pressure = static pressure + velocity pressure Velocity pressure = *(velocity in fpm)

2

x air density (velocity* 1,096 )

Appendix A

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F OR P UMPS hp =

GPM x head (ft) x (specific gravity) 3,960 x (% eff. of pump)

Specific gravity of water = 1.0 1 ft3 per sec = 448 GPM 1 PSI = A head of 2.309 ft for water weighing 62.36 lb/ft3 at 62°F C ONSTANT D ISPLACEMENT P UMPS Effect of speed on horsepower hp = k(RPM) – horsepower and capacity vary directly as the speed. Displacement pumps under constant heat require approximately constant torque at all speeds. C ENTRIFUGAL P UMPS Effect of speed on horsepower = k1 (RPM)3 – horsepower varies as the 3rd power of speed T = k1 (RPM)2 – torque varies as the 2nd power of speed Flow = k3 (RPM) – flow varies directly as the speed hp

P UMP E FFICIENCY ( TYPICAL ) 500 to 1,000 gal/min = 70% to 75% 1,000 to 1,500 gal/min = 75% to 80% Larger than 1,500 gal/min = 80% to 85% Displacement pumps may vary between 50% to 80% efficiency, depending on size of pumps. 86



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H ORSEPOWER R EQUIRED hp = torque (lb-ft) x speed (RPM) 5,250 hp = torque (lb-in) x speed (RPM) 63,000 Torque (lb-ft) =

hp x 5,250 speed (RPM)

Accelerating torque (lb-ft) =

WK2 (lb-ft2) x RPM 308 x t (sec)

Where, WK2 = inertia (lb-ft2) reflected to motor shaft ÆRPM = change in speed t = time (seconds) to accelerate t =

WK2 (lb-ft2) x ÆRPM = time to accelerate (sec) 308 x t (lb-ft)

RPM =

FPM .262 x diameter (inches)

Inertia reflected to motor = load inertia

2

Load RPM ( Motor RPM )

I NERTIA (WK 2) The factor WK2 is the weight (lb) of an object multiplied by the square of the radius of gyration (k). The unit measurement of the radius of gyration is expressed in feet.

Appendix A

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For solid or hollow cylinders, inertia may be calculated by using the equations given here. To calculate hollow shafts, take the difference between the inertia values for the OD and ID (see Figure A-1). The inertia of complex, concentric rotating parts may be calculated by breaking the part up into simple rotating cylinders, calculating their inertias and summing their values, as shown in Figure A-2.

L

Hollow D1

Solid

D2 D

L

WK2 = .000681 r LD4

WK2 = .000681 r L(D24 – D14) WK2 = lb.ft.2 D1D2, D1 and L = in. r = lb.in.3 r (aluminum) r (bronze) r (cast iron) r (steel) r (paper)

FIGURE A-1. Calculating Hollow Shafts

88


= = = = =

.0924 .320 .260 .282 .0289


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+

=

2

+

2

2

WK 2tot = WK 1 = WK 2 = WK 3 FIGURE A-2. Calculating the Inertia of Complex, Concentric Rotating Parts

WK 2

OF

R OTATING E LEMENTS

In practical mechanical systems, all the rotating parts do not operate at the same speed. The WK2 of all moving parts operating at each speed must be reduced to an equivalent WK2 at the motor shaft, so that they can all be added together and treated as a unit, as follows: Equivalent WK2 = WK2

( NN )

2

m

Where, WK2 = inertia of the moving part N = speed of the moving part (RPM) Nm = speed of the driving motor (RPM) When using speed reducers, and the machine inertia is reflected back to the motor shaft, the equivalent inertia is equal to the machine inertia divided by the square of the drive reduction ratio.

Appendix A

89


WK 2

OF

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L INEAR M OTION

Not all driven systems involve rotating motion. The equivalent WK2 of linearly moving parts can also be reduced to the motor shaft speed as follows: Equivalent WK2 =

W (V)2 39.5 (Nm)2

Where, W = weight of load (lb) V = linear velocity of rack and load or conveyor and load (FPM) Nm = speed of the driving motor (RPM) This equation can only be used where the linear speed bears a continuous fixed relationship to the motor speed, such as a conveyor. Synchronous (RPM) motor speed =

% Slip =

synchronous RPM

Amperes =

volts ohms volts

amperes

Volts = amperes x ohms 90

no. of poles

synchronous (RPM – full load RPM) x 100

O HMS L AW

Ohms =

Hz x 120



P OWER

IN

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10:09 AM

DC C IRCUITS volts x amperes

Horsepower

=

Watts

= volts x amperes

Kilowatts

=

Kilowatt-hours

=

P OWER

IN

Page 91

746

volts x amperes 1,000 volts x amperes x hours 1,000

AC C IRCUITS

Kilovolt-amperes (kVA) kVA (single-phase) =

kVA (three-phase) =

volts x amperes 1,000 volts x amperes x 1.73 1,000

Kilowatts (kW) kW (single-phase) =

kW (three-phase)

=

Power factor

=

volts x amperes x power factor 1,000 volts x amperes x power factor x 1.73 1,000 kilowatts kilovolts x amperes Appendix A

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T HREE - PHASE AC C IRCUITS HP = E x I x

3 x EFF x PF 746

Motor amps

=

Motor amps

=

Motor amps

Power factor

hp x 746 E x 3 x EFF x PF

kVA x 1,000 3xE kW x 1,000 = 3 x E x PF =

Kilowatt-hours =

kW x 1,000 ExIx 3 E x I x hours x 1,000

3 x PF

PF = displacement power factor = cos q =

kW kVa

Power (watts) = E x 1 x 3 x PF EFF = mechanical efficiency E = volts I = amps kVA 1 kW = 1 Ton= 1 hp = = = = 92

56.88 BTU/min 200 BTU/min 0.7457 kW 550 lb-ft per sec 33,000 lb-ft per min 2,545 BTU per hour

f


kW

kVARi

kVARc

I N D U C T I V E

C A P A C I T I V E

(AC Added motors) (to correct KVARi) to improve PF


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APPENDIX B

APPENDIX B Conversion Factors

Length

Torque

Rotation

Multiply

By

To Obtain

Metres

3.281

Feet

Metres

39.37

Inches

Inches

.0254

Metres

Feet

.3048

Metres

Millimetres

.0394

Inches

Newton-Metres

.7376

lb/ft

lb-ft

1.3558

Newton-Metre

lb-in

.0833

lb-ft

lb-ft

12.00

lb-in

RPM

6.00

Degrees/sec

RPM

.1047

Rad/sec

Degrees/sec

.1667

RPM

Rad/sec

9.549

RPC

Appendix B

93


Moment of Inertia

Power

Temperature

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Page 94

Multiply

By

To Obtain

Newton-Metres2

2.42

lb-ft2

oz-in2

.000434

lb-ft2

lb-in2

.00694

lb-ft2

Slug-ft2

32.17

lb-ft2

oz-in-sec2

.1675

lb-ft2

lb-in-sec2

2.68

lb-ft2

Watts

.00134

HP

lb-ft/min

.0000303

HP

hp

746.

Watts

hp

33000.

lb-ft/min

Degree C = (Degree F-32) x 5/9 Degree F = (Degree C x 9/5) + 32

94



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Page 95

A B B R E V I AT I O N S

ABBREVIATIONS AC ANSI ASD BHP CSA CSI DC DSP ECC GTO HDF IGBT IEEE LCI NEMA NPV PAM PLC PWM SCR SR V VSI VVI

= = = = = = = = = = = = = = = = = = = = = = = =

alternating current American National Standards Institute adjustable speed drive brakehorsepower Canadian Standards Association current source inverter direct current digital signal processor eddy current coupling gate turnoff (thyristor) harmonic distortion factor insulated gate bi-thermal thyristor Institute of Electrical and Electronics Engineers load-commutated inverter National Electrical Manufacturers Association net present value pulse amplitude modulation programmable logic controller pulse width modulated (inverter) silicon-controlled rectifier switched reluctance voltage variable source inverter variable voltage inverter Abbreviations

95


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Page 97

BIBLIOGRAPHY

ANSI/IEEE Standard 446-1987 (February 1987): IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications. Hanna, R, Dr. and Prabhu, S., Study of Medium Voltage Drives. Ontario Hydro, Technology Services Department. 1995. Jarc, Dennis, and John Robochuck. Reliance Electric: Static Motor Drive Capabilities for Petro. Ind. New York: IEEE Press, 1981. Mohan, N., et al. Power Electronics: Converters Applications & Design. John Wiley & Sons: 1989. Persson, E. “Energy Savings and Pay-Back of Adjustable Speed Drives in Flow Control,” Pulp and Paper Canada, 88:6 (1987). Pollack, J.J. “Some Guidelines for the Application of AdjustableSpeed AC Drives,” Adjustable Speed Drive Systems. New York: IEEE Press, 1981. Proceedings of the Symposium on Electric Variable Speed Drives, Ontario Hydro/Ministry of Energy/CCE, 1987. Radovanovic, V. “Variable Speed Drives,” Electrical Business, June 1987. Reason, J. “Special Report: AC Motor Control,” Power Magazine, February 1981, Vol. 125, No. 2.

Bibliography

97


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Page 98

Reason, J. “Special Report: Powerplant Motors,” Power Magazine, March 1986. Stevenson, A.C. “Fundamentals and Applications of Static Power Conversion,” IEEE 1984 Conference Record of Pulp and Paper Industry Technical Conference. Wennerstrom, C.H., et al. “Motor Application Considerations on Adjustable Frequency Power,” IEEE 1984 Conference Record of Pulp and Paper Industry Technical Conference.

98



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INDEX

Adjustable speed drive (ASD) cost, 70 definition, 1 Advantages comparison, 31 general, 35 Applications comparison, 29 energy savings, 36, 43, 73 speed control/process requirements, 35 Constant horsepower, 44 Constant torque, 44 Control accuracy, 60 Current-source inverter (CSI), 13, 18, 29 DC drive general, 23 principle of operation, 8 Delivery time, 66 Economics cost/saving factors, 69 general, 72 methods, 69 Eddy current clutch general, 29 principle of operation, 8

Efficiency comparison, 29 discussion, 57 Environment, 63 Harmonics comparison, 29 definition, 77 effects, 78 guidelines, 81 losses, 55 production and transmission, 79 Horsepower rating comparison, 29 general, 50 Inverter (see also variable frequency drive), 12 Maintenance, 65 Motors classification, 3 motor/drive requirements, 49 NEMA motor designs, 55 Net present value, 70 Power conditioning equipment, 72 Pulse width modulated inverter (PWM), 13, 20, 25

Index

99


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Rectifier, 12 Regenerative braking comparison, 31 description, 36 simple payback, 69 Regulator, 13 Softstarting comparison, 30 description, 36 Speed regulation, 53, 60 requirements, 29 Thermal considerations, 54 Torque considerations, 62 requirements, 51

100


Page 100

Variable frequency drive (see also adjustable speed drive) comparison, 29 principle of operation, 11 types, 13 Variable torque, 44 Variable voltage controllers, 11 Variable voltage inverter (VVI), 13, 17, 29 Voltage requirements, 51 Voltage source inverter (VSI) - (see variable voltage inverter) Waveforms, 14 Wound rotor motor controllers general, 29 principle of operation, 10


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ASD SUPPLIERS

IN

Page 101

O N TA R I O *

ABB 4410 Paletta Court Burlington, Ontario L7L 5R2 Contact: Steve Seppanen (905) 577-1986 Fax: (905) 681-2810

ITT Fluid Products Canada 55 Royal Road Guelph, Ontario N1H 1T1 Contact: Phil Searle (519) 821-1900 fax: (519) 821-5316

Canadian Drives Inc. 40 Claireville Drive Etobicoke, Ontario M9W 5T9 Contact: Andrew J. Houston (416) 213-1022 Fax: (416) 213-0821

Rockwell Automation/Allen-Bradley 135 Dundas Street Cambridge, Ontario N1R 5X1 Contact: (519) 623-1810

Cegelec Automation 5112 Timberlea Boulevard Mississauga, Ontario L4W 2S5 Contact: Roger D. Coote (905) 624-2026 Fax: (905) 629-8203 G.E. Canada Inc. 2300 Meadowvale Boulevard Mississauga, Ontario L5N 5P9 Contact: Mike Marshall (905) 858-5128 fax: (905) 858-5132

Siemens Electric Limited Energy and Automation Division 2185 Derry Road West Mississauga, Ontario L5N 7A5 Contact:: Drives Sales Representative (905) 819-5800 ext. 6414 Fax: (905) 819-5802 Toshont-Toshiba 2295 Dunwin Drive Unit #4 Mississauga, Ontario L5L 3S4 Contact: Tom Johnson (905) 607-9200 Fax: (905) 607-9203

*at time of printing

Suppliers

101

ASD cover

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Page 4

OTHER IN-HOUSE REFERENCE GUIDES: • • • • • • •

Energy Monitoring & Control Systems Fans Lighting Motors Power Quality Power Quality Mitigation Pumps

COMMENTS: For any changes, additions and/or comments call or write to: Scott Rouse Account Executive Ontario Hydro 700 University Avenue, H10-F18 Toronto, Ontario M5G 1X6 Telephone (416) 592-8044 Fax (416) 592-4841 E-Mail [email protected]

ASD cover

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Page 1

In-House Energy Efficiency Energy Savings are Good Business "The sun represents sustained life while the lightning bolt depicts energy. The integration represents the perfect partnership of energy utilization and the environment that encourages wise use and respect for all natural resources. The roof represents the in-house aspect of energy efficiency throughout Hydro." Marcel Gauthier Georgian Bay Region - Retail

Printed on recycled papers

Ontario Hydro

Adjustable Speed Drive

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