“STARTING OF 3-PHASE SLIP RING INDUCTION MOTOR USING PROGRAMMABLE LOGIC CONTROLLER (PLC)” Project report submitted in partial fulfillment of the requirements For the award of the degree of
BACHELOR OF TECHNOLOGY IN ELECTRICAL AND ELECTRONICS ENGINEERING By S.SHRI KRISHNA (07241A0250) S. SRINIVAS (07241A0253) T.JEEVAN KISHORE(08245A0202) Under the guidance of
Mr. E.Venkateshwarlu Associate Professor
Department of Electrical and Electronics Engineering GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING & TECHNOLOGY, BACHUPALLY, HYDERABAD-72 2011 1
GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND TECHNOLOGY Hyderabad, Andhra Pradesh.
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
CERTIFICATE
This is to certify that the project report entitled “STARTING OF 3-PHASE SLIP RING INDUCTION MOTOR USING PROGRAMMABLE LOGIC CONTROLLER (PLC)” that is being submitted by Mr. S.SRINIVAS in partial fulfillment for the award of the Degree of
Bachelor of Technology in
Electrical and Electronics Engineering to the Jawaharlal Nehru Technological University is a record of bonafide work carried out by him under my guidance and supervision. The results embodied in this project report have not been submitted to any other University or Institute for the award of any graduation degree. Mr.P.M.Sharma
Mr.E.Venkateshwarlu
Mr. S.N.Saxena
HOD, EEE
Assistant Professor, EEE Dept. Professor, Coordinator,
GRIET, Hyderabad
GRIET, Hyderabad
EEE Dept.
(Project Guide)
G.R.I.E.T, Hyderabad (Internal Guide)
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ACKNOWLEDGEMENT
This is to place on record my appreciation and deep gratitude to the persons without whose support this project would never seen the light of day. I wish to express my propound sense of gratitude to Mr. P. S. Raju, Director, G.R.I.E.T for his guidance, encouragement, and for all facilities to complete this project. I have immense pleasure in expressing my thanks and deep sense of gratitude to my guide Mr.E.Venkateshwarlu, Assoc. Professor, Department of Electrical Engineering, G.R.I.E.T for his guidance throughout this project. I am also thankful to Mr.Chakravarthi, Assoc. Professor, Department of Electrical Engineering, G.R.I.E.T who helped us a large wit his excellent guidance. I also express my sincere thanks to Mr.P.M.Sharma, Head of the Department, G.R.I.E.T for extending his help. I express my gratitude to Mr. S.N. Saxena, Professor, Department of Electrical and Electronics Engineering, Coordinator, Project Review Committee, G.R.I.E.T for his valuable recommendations and for accepting this project report. Finally I express my sincere gratitude to all the members of faculty and my friends who contributed their valuable advice and helped to complete the project successfully.
S.SHRI KRISHNA (07241A0250) S. SRINIVAS (07241A0253) T.JEEVAN KISHORE(08245A0202)
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ABSTRACT
The aim of this project is to limit the starting current and increase the starting torque. High Starting torque is a desired feature in some special industrial applications which use 3-Ph Slip Ring Induction motor. There are different methods for starting of the Slip Ring induction Motor. But we have used the Rotor Resistance Control method for Starting the Induction Motor. We are using a Programmable Logic Controller which can be programmed as per our requirement. We have designed a control panel and programmed the PLC according to our requirements. The motor will Start with high rotor resistance and as time passes the rotor resistance is been shorted and the motor will run at rated speed. The resistance is been cut from the rotor in two parts in two different time intervals. So this process of time management and controlling the relays in been done by the PLC. Depending upon the outputs of PLC the relays gets shorted and the resistance is been cut from the rotor.
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ABBREVIATIONS
SMPS
-
Switching Mode Power Supply
PLC
-
Programmable Logic Controller
FBD
-
Functional Block Diagram
SFC
-
Sequential Flow Chart
IL
-
Instruction List
LD
-
Ladder Diagram
NS
-
Synchronous speed
Rr
-
Rotor Resistance
Rs
-
Stator Resistance
Xr
-
Rotor Reactance
Xs
-
Stator Reactance
SCADA
-
Supervisory Control And Data Acquisition
DOL
-
Direct Online Starter
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CONTENTS Chapter No.
Name Of The Chapter
Page No.
1
Introduction
6
2
3-Phase Slip Ring Induction Motor
7
2.1- Construction Of Slip-Ring Induction Motor 2.2- Equivalent Circuit Of Slip-Ring Induction Motor 2.3- Principle Of Operation 2.4- Rotor Resistance In Slip-Ring Induction Motor
3
Starting And Speed Control Of 3-Phase Slip Ring Induction Motor 3.1. Starting Methods 3.1.1. Direct Online Starting 3.1.2 Star-Delta Starting 3.1.3series Reactor Connection 3.1.4 Variable Frequency Drive 3.1.5 Rotor Resistance Starting 3.2. Speed Control Methods 3.2.1. Changing Applied Voltage 3.2.2. Changing Applied Frequency 3.2.3. Changing The Number Of Stator Poles
6
17
3.2.4. Changing The Rotor Resistance 4
Programmable Logic Controller
24
4.1 Block Diagram Of PLC 4.2 ABB AC-31 50 Series PLC 4.3 Binary Extensions 4.4 Addressing The Inputs And Outputs Of PLC 4.4.1addressing Of Binary Extensions 4.4.2 Addressing Of Analog Extensions
5
Programming Of PLC & Communication With PLC 5.1 Different Programming Languages 5.1.1 Ladder Diagram &Quick Ladder Diagram Languages 5.1.2 Functional Block Diagram Language 5.1.3 Sequential Function Chart Language 5.1.4 Instruction List Language 5.2 Communication With PLC
7
32
6
Design Of The Panel Board & Working Of The Panel, Components Used In The Panel 6.1 Panel Board Design
37
6.2 Panel Board Circuit 6.3 Working Of The Panel Board 6.4 Components Used In The Panel Board 6.4.1 Relays 6.4.2 Contactors
7
Program Used In The PLC
44
7.1 Working Of The Program 7.2 Functions Used In The Program 7.2.1 Binary Function 7.2.2 Timer Functions 7.3 Program
8
Conclusions And Scope For Future Expansion
50
APPENDIX
51
REFERENCES
53
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CHAPTER-1 INTRODUCTION What happens if the motor is started as a normal induction motor? If the slip ring induction motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as low as 60%. So, it is not advised to start a slip ring induction motor with its rotor terminals shorted. So, there are different methods to control the starting current and speed of 3-ph Slip Ring Induction Motor. 1) Frequency Control Method 2) V/f Control Method 3) Rotor Emf Injection Method 4) Rotor Resistance Control Method. In the 1st and 2nd methods stator side power electronic hardware is required. In case of rotor Emf injection method, Generating AC voltage at slip frequency is a difficult task. By using Rotor resistance control, Stator side hardware is not required. The disadvantages of the first 3 methods can be over come in the rotor resistance control method.
At the time of normal
running of induction motor, for a constant torque load, Slip is proportional to rotor resistance. Therefore we can also control the speed of the Induction motor for a given load. This can be achieved using the ABB AC-31 Programmable logic controller. PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. PLC programs are typically written in a special application on a personal computer, and then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. So, using the outputs of the PLC we have designed a control panel in which we have used the contactors to give the supply to the motor and relays to short or cut the resistance from the rotor. 9
CHAPTER-2 3-PHASE SLIP RING INDUCTION MOTOR
An induction motor or asynchronous motor is a 3 phase 4 pole induction motor. This is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction. The 3 phase 4 pole induction motor electric motor turns because of magnetic force is exert between the stationary electromagnet called the stator and a rotating. This 3 phase 4 pole inductions electric motor turns because of magnetic force exert between a motionless electromagnet called the stator and a rotating electromagnet called the rotor.
2.1 CONSTRUCTION OF SLIP-RING INDUCTION MOTOR:-
Fig.2.1 10
STATOR: The stator consists of 3-ph winding forms wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator (a salient pole) for each pole, with windings around it; in fact, to optimize the distribution of the magnetic field, the windings are distributed in many slots located around the stator, but the magnetic field still has the same number of north-south alternations. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).
SLIP RING ROTOR: The slip ring induction motors usually have “Phase-Wound” rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which has slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart.
Fig.2.2 The rotor is wound for as many poles as the number of poles in the stator and is always 3-phase, even though the stator is wound for 2-phase.
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These three windings are “starred” internally and other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of spring assembly. These three carbon brushes are further connected externally to a 3-phase star connected rheostat. Thus these slip ring and external rheostat makes the slip ring induction motors possible to add external resistance to the rotor circuit, thus enabling them to have a higher resistance during starting and thus higher starting torque.
2.2 EQUIVALENT CIRCUIT OF SLIP-RING INDUCTION MOTOR: To understand the behavior of an induction motor when the rotational speed and supply frequency varies, it is helpful to look at the equivalent circuit. The equivalent circuit shows an electrically equivalent circuit to the motor's construction, where the two leftmost terminals would be connected to a power supply.
Fig.2.3
On the left side of the circuit, the equivalent resistance of the stator, which consists of the copper resistance and core resistance in series, is shown as Rs.
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During asynchronous operation, the stator also induces some reactance, which is represented by the inductor Xs. The next inductor Xr represents the effect of the rotor passing through the stator's magnetic field. The effective resistance of the rotor (again with rotating in a magnetic field), Rr, is composed of:
The equivalent value of the machine's real power (which changes with the torque and the load on the machine)
The ohmic resistance of the stator windings and the squirrel cage of shorted rotor windings.
At idle, the induction motor equivalent circuit is essentially just Rs and Xs, which is why this machine only takes up mostly reactive power. The idle current draw is often near the rated current, due to the copper and core losses which exist even at no load. In these conditions, this is usually more than half the power loss at rated load. If the torque against the motor spindle is increased, the active current increases by Rr, and thus in the rotor. Due to the construction of the induction motor, the two resistances both induce a magnetic field, in contrast to the three-phase synchronous machine, where the magnetic flux is induced only by the reactive current in the stator windings. The current produces a voltage drop in the cage portion of the Rr, but only a slightly higher voltage drop in the stator windings. Consequently, the losses increase with increasing load in the rotor faster than they do in the stator. The copper resistance Rs and the "copper" resistance from the cage portion of Rr both cause I2R losses, and therefore the efficiency of the machine improves with increasing load. The efficiency of the machine reduces with temperature. In contrast with a smaller frequency of the reactance Xs also getting smaller. In compliance with the rated current must shrink by the drive voltage delivered. Thus, the ratio of the voltage divider Rs to Xs and Rs and this increases engine power losses. In continuous operation this can only be an approximation because a nominal torque is generated because the cooling of rotor and stator is not included in the calculation. At higher than the rated speed or rated frequency induction motor can, however - in the context of isolation - are working on higher voltages and is more effective.
13
Frequently today, Rs / Rr are measure automatically and are thus in a position for any motor connected to automatically configure itself and thus to be protected from overload. A holding torque or speed close to zero can be achieved with a vector control. Here, too though, there can be problems with cooling since the fan is usually mounted on the rotor.
2.3 PRINCIPLE OF OPERATION: The principle of operation of the induction machine is based on the generation of a rotating magnetic field.
Production of Rotating Magnetic Field: A symmetric rotating magnetic field can be produced with as few as three coils. The three coils will have to be driven by a symmetric 3-phase AC sine current system, thus each phase will be shifted 120 degrees in phase from the others. For the purpose of this example, the magnetic field is taken to be the linear function of the coil's current.
Since the flux is proportional to magnetizing current drawn by the three phase winding three magnatic fluxes occurs as shown in the fig.
Mathematically: Let us consider waveforms as
Fig.2.4
14
As shown in figure2.4 angle of the resultant flux varies linearly and magnitude of the resultant flux will be maintained constant, therefore produces the rotating magnetic field.
Fig.2.5
The above figure 2.5 shows the direction of the three phase flux at various instants.
The induction motor does not have any permanent magnets on the rotor; instead, a current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energized with a poly-phase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. 15
According to Lenz’s law these currents will induce in a direction such that it opposes the cause of production of that currents, as a result rotor also rotates in the same direction as the rotating magnetic field.
However, for these currents to be induced the speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator (the synchronous speed Ns) or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is rere induced and then the rotor continues as be before. fore. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip.. It is unit less and is the ratio between the relative speed of the magnetic field as seen by the rotor (the slip speed) speed to the speed of the rotating stator field. Due to this, an induction motor is sometimes referred to as an asynchronous machine
Synchronous speed The Synchronous speed of the Induction motor is the speed at which stator Rotating magnetic field rotates in the air gap of the machine. It can be shown that the synchronous speed of a motor is determined by the following formula:
Where ns is the synchronous speed of the machine (in rpm), f
is the frequency of the AC supply (in Hz)
p
is the number of magnetic poles per phase.
In this Project, a 4-pole pole motor operating on 50 Hz power would have a speed of:
݊௦ ൌ
ଵଶൈହ ସ
=1500rpm
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Slip
Fig.2.6 Fig.2.6 represents the typical torque curve as a function of slip slip. The slip is defined as a ratio of relative to the synchronous speed and is calculated using:
Where s
is the slip, usually between 0 and 1
nr is rotor rotation speed in rpm ns is synchronous speed in rpm
2.4 ROTOR RESISTANCE IN SLIP SLIP-RING RING INDUCTION MOTOR: A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series serie with the rotor windings while starting. This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart.
Fig.2.7
17
Fig.2.8
Why put resistance in series with the rotor? Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting. While this is not a severe problem for small motors, it is for large (10's of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, but also increases the starting torque. Figure below shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting.
Fig.2.9
Breakdown torque peak is shifted to zero speed by increasing rotor resistance.
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The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is started. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor. This motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its' synchronous speed
In this project we are going to introduce the rotor circuit external resistance up to three ohms, and these resistances can be cut down in three steps, so that we can achieve three different speeds.
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CHAPTER-3 STARTING AND SPEED CONTROL OF 3-PHASE SLIPRING INDUCTION MOTOR 3.1. STARTING METHODS: Normally in any electrical machine whenever the electrical supply given the EMF will be induced in the machine which opposes the main supply. In rotating machines like induction motor or DC motor this induced emf is called Back EMF. The Back EMF induced in the Induction motor or any rotating machine is directly proportional to the Speed of the motor at which it is running. At the time of starting, since the motor is at rest, the Back EMF is Zero. Therefore there will not be any opposition to the main supply. As a result huge amount of current will be drawn by the motor. Therefore, 3-phase induction motors employ a starting method not to provide a starting torque at the rotor, but because of the following reasons; 1) Reduce heavy starting currents and prevent motor from overheating. 2) Provide overload and no-voltage protection. There are many methods in use to start 3-phase induction motors. Some of the common methods are;
Direct on-line starter Star-Delta starter Series Reactance method Variable frequency drive Rotor Resistance method of starting 20
3.1.1. DIRECT ONLINE STARTING: Direct online starting also known as across the line starting and full voltage starting, involves connecting each terminal of a three-phase induction motor to a separate line of a device. In this arrangement, the motor current is the same as the line current and the terminal voltage of the induction motor equals the line voltage. A disadvantage of the direct online starting method is that the stator draws a high current that can damage the windings of the three-phase induction motor. The direct online starting method can also cause a voltage drop or fluctuation that can affect devices along the line. The DOL starter attracts high current. Therefore, it may be inconvenient to the other users of the supply line since whenever a motor with a DOL starter is turned on, they will experience a voltage drop. DOL starter also provides a very high starting torque. This can be a strain on the driven load. The high starting torque may cause mechanical wear on the components connected to the load. Because of these reasons, the DOL starter can be used for only low power or more specifically stated, motors with a rating of less than 5KW.
3.1.2 STAR-DELTA STARTING: In a star connection, the windings of the induction motor connect from the supply phases to the neutral. In a delta or mesh connection, the windings connect between the supply phases. A star connection creates higher voltage to the windings of the three-phase induction motor than a delta connection. A starter with the ability to utilize both star and delta connections, also known as a star start delta run connection, initializes the three-phase motor using a star connection then transfers to a delta connection when the motor reaches a set speed. A disadvantage of the stardelta connection is the reduction in voltage and the low starting torque that can have an adverse effect on devices or pumps that have a high breakaway torque. A star-delta connection is more complex than a direct online connection because it utilizes a speed switch and timers. The induction motor requires a shock to start and another shock when the star-delta connection transfers from star to delta. The star-delta method is often convenient for partial acceleration.
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The star-delta starting method achieves low starting current by first connecting the stator windings in star configuration, and then after the motor reaches a certain speed, a double throw switch changes the winding arrangements from star to delta configuration. This method provides slow starting torque which can cause the motor to stall.
3.1.3SERIES REACTOR CONECTION: A reactor in series with the terminals of the motor decreases the terminal voltage of the induction motor, decreasing the initial current. The impedance decreases as the induction motor accelerates until a bypass method makes the motor run at full speed and full voltage.
3.1.4 VARIABLE FREQUENCY DRIVE: A variable-frequency drive starts a three-phase induction motor at a frequency low enough to initialize a full-rated torque without an inrush of current. The low frequency increases the torque because it increases the impedance of the rotor circuit with slip frequency.
3.1.5 ROTOR RESISTANCE STARTING: This method allows external resistances to be connected to the rotor through slip rings and brushes. Initially, the rotor resistance is set to maximum and is then gradually decreased as the motor speed increases, until the resistance becomes zero. The rotor resistance starting mechanism is usually very simple when compared with other methods. It also has no maintenance costs. A considerable amount of heat is generated through the resistors when current runs through them. However, the rotor impedance method is known to be the smoothest and least stressful method of accelerating an induction motor. The mail advantage is that it allows the motor to be started while the motor is on load
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3.2. SPEED CONTROL METHODS: Unlike D.C. Motors, A.C. Induction Motors are not suitable for variable speeds. Their speed control and regulation is comparatively difficult when compared with D.C. Motors. These are some of the methods which are commonly used for the speed control of squirrel cage induction motors: 1. Changing Applied Voltage 2. Changing Applied Frequency 3. Changing Number Of Stator Poles 4. Changing the rotor circuit resistance Of the above four methods first three can be used for both squirrel cage and slip ring induction motors, where as forth method is only applicable for slip ring induction motor. 3.2.1. CHANGING APPLIED VOLTAGE: As we know the Electromagnetic torque developed by the motor is given by the equation is
Load Torque ሺܶሻ
௦ாమమ ோమ
ൌ ܭ. ோమ ାሺ௦ మ
Where S = Slip of the motor, ܧଶ = Rotor induced EMF at standstill condition, ܴଶ = Rotor resistance, ܺଶ = Rotor winding reactance at standstill condition
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మሻ
మ
At normal working conditions the Slip of the induction motor is very low and for constant torque load, Therefore equation can be written as
Torque ሺܶ ሻ ൌ
Therefore,
௦ாమమ ோమ
sܧଶଶ = constant.
Since the Rotor induced EMF is directly proportional to the applied voltage to the Stator,
ܸଶ =
ேೄ ேೄ ିேೝ
Since the synchronous speed (ܰௌ ) is constant, By changing the applied voltage ‘V’, it is possible to vary the Rotor running speed (ܰ ). This method, even though easiest, it is rarely used. The reasons are (a) For a small change in speed, there must be a large variation in voltage. (b) This large change in voltage will result in large change in flux density, thereby seriously disturbing the magnetic distribution/condition of the motor. (c) This method also requires a large power electronic circuit (AC voltage controller). As the Slip is inversely proportional to the square of the voltage, to increase the speed above synchronous speed, voltage has to be increased more than the rated, therefore ‘v/f’ ratio greatly increases, Thereby the flux density increases and causes some abnormal conditions.
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3.2.2. CHANGING APPLIED FREQUENCY: We all know that the synchronous speed of the induction motor is given by Ns = 120f/P. So from this relation, it is evident that the synchronous speed and thus the speed of the induction motor can by varied by the supply frequency. Limitations of these methods are: (a) The motor speed can be reduced by reducing the frequency, if the induction motor happens to be the only load on the generators. (b) If supply is taken from the GRID, It requires a Cyclo converter circuits at the stator side which is very complex. Even then the range over which the speed can be varied is very less. This method is famous in some electrically driven ships although not common in shore.
‘v/f’ control: Fig.3.1 Characteristics of ‘v/f’ control
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For the speeds below rated speed for large variation of voltage, small change in speed occurs. Therefore normally ‘v/f’ control is used. In this method, voltage and frequency are varied with respect to each other, so that the ratio is maintained constant. Therefore the flux density will be maintained constant. This method combines the advantages of both above two methods. But this method requires A Converter- Inverter circuit at the stator side.
3.2.3. CHANGING THE NUMBER OF STATOR POLES: As we know the relation between the synchronous speed and the number of poles, Ns = 120f/P. So the number of poles is inversely proportional to the speed of the motor. This change of number of poles can be achieved by having two or more entirely independent stator windings in the same slots. Each winding gives a different number of poles and hence different synchronous speed. Since the Induction motors are normally designed for a Specific number of poles, By changing the number of poles it works with less efficiency. And by using this method only two sets of speeds can be achieved.
3.2.4. CHANGING THE ROTOR RESISTANCE: As we discussed in the voltage control session The Load torque ሺܶ ሻ
ൌ
௦ாమమ ோమ
For a constant torque and constant applied voltage, the slip to rotor resistance ratio is constant. Therefore S = k ܴଶ By increasing the rotor resistance, it is possible to increase the slip; thereby we can control the speed of the induction motor.
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This method of speed control of is also useful for starting of the induction motor. Since rotor is short circuited, at the time of starting motor will draw large currents into the rotor. So to reduce the starting current this method is used. This method not only reduces the starting current but also increases the starting current. As we know the torque equation of induction motor is
௦ா మ ோ
మ మ Torque ሺܶሻ ൌ ܭ. మ ோ ାሺ௦ ሻమ మ
మ
And the starting torque is Starting torque ሺܶሻ
ாమோ
మ మ ൌ ܭ. ோమ ାሺ ሻమ మ
మ
And the slip corresponding to maximum/Breakdown torque is S = ܴଶ /ܺଶ By considering all the above points Torque-slip or Torque speed characteristics are given as below.
ܴଶ > ܴଵ > ܴ
Fig.3.2
Torque-speed characteristics for different rotor resistances
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CHAPTER- 4 PROGRAMMABLE LOGIC CONTROLLER
A PROGRAMMABLE LOGIC CONTROLLER (PLC) is a digital computer used for automation of electromechanical processes Before the PLC, control sequencing, and safety interlock logic for manufacturing automobiles was accomplished using hundreds or thousands of relays, cam timers and drum sequencers and dedicated closed-loop controllers. Early PLCs were designed to replace relay logic systems. These PLCs were programmed in “ladder logic”, which strongly resembles a schematic diagram of relay logic. The computer is connected to the PLC through Ethernet, RS-232, RS-485 or RS-422cabling A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. PLC programs are typically written in a special application on a personal computer, and then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. The PLC’s have many applications in the day to day life. They are easily programmable and they can be operated using the cables, modems etc. All the automation processes are been done but using the PLC’s, as they are more reliable.
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4.1 BLOCK DIAGRAM OF PLC: Fig.4.1
The central unit, with a local inputs / outputs extensions interface and a CS 31 bus interface A SRAM memory where the user program and data is loaded. A Flash EPROM memory which contains: a backup of the user program with the program constants, the user program is a set of universal functions conceived to cover all applications After being translated into instructions understandable by the central unit it is loaded in RUN or STOP mode into the SRAM and then saved from the SRAM to the Flash EPROM. Thereby, at each program launch the user program, saved in the Flash EPROM, is copied to the SRAM for processing by the microprocessor
29
4.2 ABB AC-31 50 Series PLC: Fig.4.2
This is the central unit of the ABB AC 31 Programmable Logic Controller. The AC 31 brings accessibility to automation users alike, for any application with 14 to 1000 inputs / outputs and more, using the same set of basic components. Extensible central unit with CS31 bus with 8 isolated inputs 24 V D.C. and 6 incorporated relay outputs 250 V A.C. / 2 A, RS232 or RS485 interface for programming or ASCII or MODBUS_ communication ,24 V D.C. power supply Each central unit incorporates a specific number of binary inputs / outputs and occasionally Analog. It is possible, depending on the central unit, to increase the number of inputs / outputs, to add input / output extensions connected directly to the central units or remote input / output units via the CS 31 twisted pair. The 50 series central unit, with a local inputs / outputs extensions interface and a CS 31 bus interface. In the 50 series it’s possible to increase the number of inputs / outputs of the basic central unit by adding remote units. 30
The central unit controlling the system is called the MASTER central unit. The maximum bus length is 500 m without an amplifier and 2000 m with 3 amplifiers (1 NCB or NCBR unit enables bus amplification for 500 m). The master central unit can manage up to 31 connection points called SLAVES, such as: A remote unit with extension possibilities: a maximum of 6 extension units comprising of a maximum of 8 analog input channels and 8 analog output channels. A simple remote unit (without extension) with analog or binary inputs / outputs The maximum number of remote ANALOG units depends on the MASTER central unit: 50 series: - a maximum of 31 remote analog input units - a maximum of 31 remote analog output units - a maximum of 15 extensible remote units (ICMK14F1) with analog input / output extensions + 1 remote analog input / output unit (15 x 2 + 1 = 31) - or a mixed binary / analog configuration within the previous limits
The 50 series central unit memory is composed of two distinct areas: - A SRAM memory where the user program and data is loaded. - A Flash EPROM memory which contains: a backup of the user program with the program constants, the configuration data, and the system program protected against access from the user program. An incorporated battery, which is available only in the 50 series, also enables the backup of internal variables. The user program is a set of universal functions conceived by the constructor to cover all applications and ensure all the basic PLC functions. It is developed with the AC31GRAF software. After being translated into instructions understandable by the central unit it is loaded in RUN or STOP mode into the SRAM and then saved from the SRAM to the Flash EPROM. Thereby, at each program launch the user program, saved in the Flash EPROM, is copied to the SRAM for processing by the microprocessor
31
4.3 BINARY EXTENSIONS EXTENSIONS:
Fig.4.3
We are using a binary extension XC08L1, it consists 8 user configurable channels which can be used as Inputs as well as outputs. It works with 24VDC, 0.5A as an input, and also gives 24DC as an output.
4.4 ADDRESSING THE INPUTS AND OUTPUTS OF PLC:
Fig.4.4
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The Central unit is automatically addressed as 62; the remaining extensions will be addressed as 63, 64, and so on. The binary INPUTS of the central unit are addressed as %I62.00, %I62.01, %I62.02......%I62.07 ,The The binary OUTPUTS are given as %O62.00 ,%O62.01 ,%O62.02......%O62.05
In case of analog inputs it is %IW62.00, %IW62.01.....%IW62.07 and analog outputs are given as %OW62.00, %OW62.01....%OW62.05
4.4.1ADDRESSING ADDRESSING OF BINARY EXTENSIONS:
Fig.4.5
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4.4.2 ADDRESSING OF ANALOG EXTENSIONS:
Fig.4.6
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CHAPTER-5 PROGRAMMING OF PLC & COMMUNICATION WITH PLC The AC31GRAF software is used with all of the AC 31 central units. This software runs under Windows® (3.1, NT or 95). 12 Megabytes of free disk space are required for the installation. Execute the setup.exe for an automatic installation. The software allows you to create, send, test, recover and print user programs as well as initializing, starting and stopping the central unit.
5.1 DIFFERENT PROGRAMMING LANGUAGES: Ladder Diagram &Quick Ladder Diagram Functional Block Diagrams Sequential Flow Chart Instruction List
5.1.1 LADDER DIAGRAM &QUICK LADDER DIAGRAM Languages: Ladder Diagram (LD) is a graphic representation of Boolean equations, combining contacts (input arguments) with coils (output results). The LD language enables the description of tests and modifications of Boolean data by placing graphic symbols into the program chart. Using the Quick LD editor, you connect function boxes to Boolean lines.
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Fig.5.1
Example of Ladder Diagram
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5.1.2 FUNCTIONAL BLOCK DIAGRAM Language: FBD diagram describes a function between input variables and output variables. A function is described as a set of elementary function blocks. An entire function operated by an FBD program is built with standard elementary function blocks from the AC31GRAF library.
Fig.5.2
Example of Functional Block
5.1.3 SEQUENTIAL FUNCTION CHART Language:
Fig.5.3 37
Sequential Function Chart (SFC) is a graphic language used to describe sequential operations. The process is represented as a set of well defined steps, linked by transitions. The basic graphic rules of the SFC are: A step cannot be followed by another step A transition cannot be followed by another transition.
Fig.5.4
Example of SFC
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5.1.4 INSTRUCTION LIST Language: IL is a low level language. It is highly effective for smaller applications or for optimizing parts of an application. Example:Start: LD BUTTON1 (* push button *) ANDN%I62.02 (* command is not forbidden *) ST START-MOTOR (* start motor *)
5.2 COMMUNTICATION WITH PLC:
Fig.5.5
PROJECT MANAGER 39
We use our personal computer to communicate with the device. A software which acts as an interface between the user and the device .The software is required to be installed in the PC. An RS232 cable acts as in interface between PC and the PLC. The AC31GRAF software allows you to create, send, test, recover and print user programs as well as initializing, starting and stopping the central unit. There will be a project manager in this software which provides programming as per the requirement of the application of the user.
With the use of this project manager we can write the programs in different languages and store it in the PLC. This project manager consists of many features, and has many predefined functional blocks in it which can be used for the programming. All the changes in the program can be done in the PC and then it can be transferred in to the PLC by using an RS-232 cable. The PLC is very fast, and it responds immediately to any change in the programs. The AC-31 GRAF software provides a wide range of functions. We can divide the program into different parts and write each part in different languages and then arrange them as per our requirement. We can call the programs written in different projects.
Fig.5.6 40
CHAPTER-6 DESIGN OF THE PANEL BOARD & WORKING OF THE PANEL, COMPONENTS USED IN THE PANEL
The PLC we have used is ABB AC-31, it takes DC 24volts input signals and gives out DC 24volts output signals. But here the aim is to start and run the three phase 415 volts slip ring Induction motor. Therefore we need to use the Relays and Contactors to interface the Induction motor with PLC.
6.1 PANEL BOARD DESIGN:
Fig.6.1 41
All the outputs of the PLC are brought on to the Panel Board. This Panel is consisting of all the Relays and Contactors and main line supply terminals. So, this Panel board interfaces the three phase induction motor stator to the main line terminals. And it also interfaces the short circuited terminals to the rotor resistance terminals. The circuit diagram of the Panel board is shown in figure 6.1.
6.2 PANEL BOARD CIRUIT:
Fig.6.2
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6.3 WORKING OF THE PANEL BOARD: As per the Program written in FBD language, first output signal comes on the terminal %o62.00, this output will be given to Contactor1, then it closes its three terminals, then R-Y-B phase sequence supply will goes to the induction motor terminals then the motor starts running in one direction. Then after 10 seconds of time delay the output signal comes on the %o62.01 terminal and this output will be connected to Relay2 in the Panel board. As Relay2 shorts the first part of the resistance Speed of the induction motor will be gradually increased. Then after 10 seconds the output comes on the terminal %o62.02, and this output will be connected to Ralay1 on the Panel board. Therefore this relay will short the second part of the rotor resistance, then motor is having only its internal resistance so it reaches to its high speed. As per the FBD program all the outputs will come to zero state after 10 seconds. Therefore supply to the motor will be cut-off and two resistance circuits will be opened.
Fig.6.3
43
Then after 18 second when output comes on the terminal %o62.03, as it is connected to the three phase supply in reverse phase sequence (B-Y-R). Reverse phase sequence supply will be connected to the stator, so the motor starts running in reverse direction, then after 10 second PLC gives the output on the terminal %o62.01 the corresponding relay that is Relay2 will cut-off the first part of the resistance, so the motor speed gradually increases, then after 10 seconds the output will come on the terminal %o62.02, then corresponding relay will cut-down the total resistance then motor attains its full speed, then after sometime all the outputs will become low, then all the contactors and relays become inactive. Then motor comes to rest.
6.4 COMPONENTS USED IN THE PANEL BOARD: 6.4.1 RELAYS : Relays are basically switches which operate with respect to electrical signals. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. Solid-state relays control power circuits with non moving, instead using a semiconductor device to perform switching.
Fig.6.4 44
A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is deenergized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. Since the rotor three sets of resistors, to cut down the three resistors at a time In this project we are using the relays which are having three NO’s. When the output comes from the PLC the relay will be activated three NO’s will be closed at a time.
6.4.2 CONTACTORS: When a relay is used to switch a large amount of electrical power through its contacts, it is designated by a special name: contactor. Contactors typically have multiple contacts, and those contacts are usually (but not always) normally-open, so that power to the load is shut off when the coil is de-energized. Perhaps the most common industrial use for contactors is the control of electric motors.
Fig.6.5 The top three contacts switch the respective phases of the incoming 3-phase AC power, typically at least 480 Volts for motors 1 horsepower or greater. The lowest contact is an "auxiliary" contact which has a current rating much lower than that of the large motor power contacts, but is actuated by the same armature as the power contacts. 45
Fig6.6
The auxiliary contact is often used in a relay logic circuit, or for some other part of the motor control scheme, typically switching 230 Volt AC power instead of the motor voltage. One contactor may have several auxiliary contacts, either normally-open or normally-closed, if required. • •
Contactor is a large relay, usually used to switch current to an electric motor or other high-power load. Large electric motors can be protected from over current damage through the use of overload heaters and overload contacts. If the series-connected heaters get too hot from excessive current, the normally-closed overload contact will open, de-energizing the contactor sending power to the motor.
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CHAPTER -7 PROGRAM USED IN THE PLC We have written the program in the FUNCTIONAL BLOCK DIAGARM LANGUAGE. As we have to use the timers in the program, we have chosen the FBD language so that we can’t make use of the timer functions which are predefined in the library of the Project Manager. We had made use of the T-ON and T-OFF functions and designed the program as per required time delay.
7.1 WORKING OF THE PROGRAM: According to our program the motor will start with a delay of 2seconds after pushing the input switch, as the motor starts exactly after 10seconds from the time of starting the 1st relay will be closed and the part of resistance gets shorted, after another 10seconds the 2nd relay will be closed and the total external resistance will be cut off. Then the motor runs with the rated speed for 15seconds and the motor comes to halt and the relay gets opened. Exactly after 18seconds the contactor which has been given the reverse supply phase sequence will get energized and the motor starts rotating in the anti clockwise direction, as in case of the clockwise direction again the 1st relay will be shorted after 10seconds and after another 10seconds the other relay will also gets shorted, and the total resistance is cut off, the motor rotates with rated speed in the anti clock wise direction for 15seconds and turns off.
7.2 FUNCTIONS USED IN THE PROGRAM: We have used a Binary function (OR) and 2 Timer functions (T-ON AND T-OFF).
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7.2.1 BINARY FUNCTION (OR):
Fig.7.1 PARAMETERS: E1 BINARY %I, %M, %O, %S, %K E2 BINARY %I, %M, %O, %S, %K A1 BINARY %M, %O, %S
Operand 1 Operand 2 Result of the OR combination
DESCRIPTION: This connection element realizes a logical OR combination of the operands at the inputs. The result is allocated to the operand at the output. Truth table:
E1
E2
A1
0
0
0
0 1 1
1 0 1
1 1 1
7.2.2 TIMER FUNCTIONS:
T-ON DELAY:
Fig.7.2
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PARAMETERS: IN PT Q ET
BINARY WORD, DW BINARY WORD,DW
%I, %M, %O, %S, %K Input signal %KW , %KW+1, %MW , %MW+1,%MD,%KD Preset time %M, %O Delayed signal %MW & %MW+1,%MD Time visualization
DESCRIPTION:The 0-1 edge of the input IN is delayed by the time PT at the output Q. The output Q retains 0 level if the input IN returns to 0 level before the time PT has elapsed. The time elapsed can be consulted at the output ET and the preset time value at the Input PT can be modified when the timer is running. The preset time is specified in Milliseconds. The time range which can be specified is: 1 m ... 24.8 days. Maximum time offset at the output : < 1 cycle time Meaningful range for PT: > 1 cycle time
Fig.7.3 Note: If the time is less than 65s, a word can be used for the preset time PT. Then the PT Input can be used: - With all the other word functions - From the central unit potentiometer - For MODBUS communication (double word are not allowed in MODBUS) directly without double word to word conversion. If word variables (%MW or %KW) are used for the parameter PT, two consecutive addresses are necessary. Never use %MW+1 or %KW+1 in your program in this case. Started timers are processed by the PLC operating system and are therefore completely independent of processing of the PLC program. An appropriate message of the operating system is not issued to the affiliated timer block in the PLC program until the timer has elapsed. Processing of a timer in the PLC operating system is influenced by the following commands. All running timers are stopped and initialized when one of the following actions occurs: Abort PLC program RUN/STOP switch from RUN -> STOP Warm or cold start 49
T-OFF DELAY:
Fig.7.4
PARAMETERS IN PT Q ET
BINARY WORD,DW BINARY WORD.DW
%I, %M, %O, %S, %K Input signal %KW , %KW+1,%MW , %MW+1 ,%MD, %KD Preset time %M, %O Delayed signal %MW & %MW+1,%MD Time visualization
DESCRIPTION The 1-0 edge of the input IN is delayed by the time PT at the output Q. If the input IN returns to 1 level before expiry of the time PT, the output Q retains 1 level. The time elapsed can be consulted at the output ET and the preset time value at the input PT can be modified when the timer is running. The preset time is specified in milliseconds. The time range which can be specified is : 1 ms ... 24.8 days. Maximum time offset at the output : < 1 cycle time Meaningful range for PT : > 1 cycle time.
Fig.7.5
50
7.3 PROGRAM:The figure below is the program written in the FBD language .. As shown in the figure the input is given at %I62.00 The output of PLC %O62.00 is connected to RYB Supply Contactor . The output of PLC %O62.01 is connected to 1st Resistance Relay. The output of PLC %O62.02 is connected to 2nd Resistance Relay. The output of PLC %O62.03 is connected to BYR Supply Contactor
Fig.7.6
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During the working condition :-
Fig.7.7 This program is been sent into the PLC using an RS-232 cable .This program is written as per the requirement of the panel design
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CONCLUSIONS: This PLC based system is highly reliable. Without changing in any hardware connection just by simply changing the program in the PLC, The motor can be made to run in any of the two directions and for any duration of time. It is also possible to change the speed at any instant of time and to run at any one of three speeds for any required duration without altering any hardware. This system also used for one of the starting method of three phase slip ring Induction motor this system not only reduces the starting current to a limit, but also develops High starting torque which is required in many of the induction motor applications. This can be applicable to run the lift, by changing the logic in a program and it can also be used for any industrial applications. This PLC based system requires less hardware compared to any microcontroller or microprocessor based system. Programmable Logic Controllers (PLC) are widely used in industrial control because they are inexpensive, easy to install and very flexible in applications. A PLC interacts with the external world through its inputs and outputs.
SCOPE FOR FUTURE EXPANSION:
By connecting analog extension module Analog quantities like Speed of the motor and Voltage can be read and a closed-loop control systems can be implemented. By using analog extension modules it is possible to get the characteristics like Speed-torque, speed-frequency, torque-current etc. By connecting required number of electrical devices, it can extend to develop the SCADA system and those types of systems are more reliable. .
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Appendix
ABB AC-31 50SERIES PLC: 1 CS31 twisted-pair remote unit 8 opto-isolated inputs 24Vdc type PNP or NPN 6 relay outputs 250V A.C. / 2A or 6 transistor outputs : 2 outputs 24 VDC, 1A 4 outputs 24 VDC , 0.5 A The variables used by the AC 31 central units are of different types: - Bit variables (status 0 or 1) - Word variables (range -32768 to 32767) - Double word variables (range -2147483648 to 2147483647) - Texts (ASCII characters) Time values for the timer functions: KD01.00...KD07.15 double word constants MD00.00...MD07.15 internal double words.
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References BOOKS: 1.) Dale R.Patrick, “Rotating Electrical machines and Power Systems” 2.) J.D. Edwards, “Electrical Machines”. 3.) V.U. Bakshi & U.A. Bakshi, “Electrical Machines”. 4.) www.wekipedia.com, PLC, http://en.wikipedia.org/wiki/PLC 5.) AC-31 GRAF software Programming Manual.
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