Starting of Induction Motor Using PLC

7/28/2019 Starting of Induction Motor Using PLC

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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


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GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND TECHNOLOGY

Hyderabad, Andhra Pradesh.

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

C E R T I F I C A T EC E R T I F I C A T EC E R T I F I C A T EC E R T I F I C A T E

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.Twho 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.Tfor 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

2

3

Introduction

3-Phase Slip Ring Induction Motor

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

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

7

17


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4

5

3.2.4. Changing The Rotor Resistance

Programmable Logic Controller

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

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

24

32


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6

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Design Of The Panel Board & Working Of The Panel,

Components Used In The Panel

6.1 Panel Board Design

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

Program Used In The PLC

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

Conclusions And Scope For Future Expansion

APPENDIX

REFERENCES

37

44

50

51

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 thefirst 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. PLCis 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.


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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


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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 asRs.


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During asynchronous operation, the stator also induces some reactance, which is represented by

the inductorXs. The next inductorXrrepresents 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 justRs andXs, 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 byRr, 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 theRr, 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 ofRrboth causeI2R 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 reactanceXs also getting smaller. In

compliance with the rated current must shrink by the drive voltage delivered. Thus, the ratio

of the voltage dividerRs toXs andRs and this increases engine power losses. In continuous

operation this can only be an approximation because a nominal torque is generated becausethe 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.


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Frequently today,Rs /Rrare 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


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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.


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According to Lenzs law these c

of production of that currents, as

magnetic field.

However, for these curren

the speed of the rotating magneti

magnetic field will not be movin

induced. If by some chance this

induced and then the rotor conti

and speed of the rotating magnet

between the relative speed of the

of the rotating stator field. Due t

asynchronous machine

Synchronous speed

The Synchronous speed of

magnetic field rotates in the air

It can be shown that the sy

formula:

Where ns is the synchro

f is the frequency

p is the number o

In this Project, a 4-pole motor

=1500rpm

16

rrents will induce in a direction such that it op

a result rotor also rotates in the same direction

ts to be induced the speed of the physical rotor

c field in the stator (the synchronous speed Ns)

g relative to the rotor conductors and no current

appens, the rotor typically slows slightly until

ues as before. This difference between the spee

ic field in the stator is called slip. It is unit less

magnetic field as seen by the rotor (the slip spe

this, an induction motor is sometimes referred

he Induction motor is the speed at which stator

ap of the machine.

chronous speed of a motor is determined by th

ous speed of the machine (in rpm),

of the AC supply (in Hz)

magnetic poles per phase.

perating on 50 Hz power would have a speed o

oses the cause

s the rotating

must be less than

or else the

s will be

current is re-

d of the rotor

nd is the ratio

ed) to the speed

to as an

Rotating

following

:


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Slip

Fig.2.6

Fig.2.6 represents the typical tor

Theslip is defined as a ratio of

Where

s is the slip, usually bet

nr is rotor rotation spee

ns is synchronous speed

2.4 ROTOR RESISTANC

A wound rotor induction

rotor with insulated windings brapplied to the slip rings. Their so

rotor windings while starting. Th

rotor look electrically like the sq

Fig.2.7

17

ue curve as a function of slip.

elative to the synchronous speed and is calculat

ween 0 and 1

in rpm

in rpm

IN SLIP-RING INDUCTION MOT

otor has a stator like the squirrel cage inductio

ught out via slip rings and brushes. However, nle purpose is to allow resistance to be placed in

is resistance is shorted out once the motor is sta

irrel cage counterpart.

ed using:

R:

motor, but a

o power isseries with the

rted to make the


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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 SLIP-

RING 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


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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 tothe 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 star-

delta 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 theinduction motor, decreasing the initial current. The impedance decreases as the induction motoraccelerates 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 torquebecause 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 .

Where

S = Slip of the motor,

= Rotor induced EMF at standstill condition,

= Rotor resistance,

= Rotor winding reactance at standstill condition


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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

Therefore, s = constant.

Since the Rotor induced EMF is directly proportional to the applied voltage to the Stator,

=

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 APPLI

We all know that the s

Ns = 120f/P.

So from this relation, it is

induction motor can by varied b

Limitations of these methods are

(a)The motor speed can be

happens to be the only l

(b)If supply is taken from t

which is very complex.

Even then the range o

famous in some electrically

v/f control:

Fig.3.1 Characteristics of v/

25

D FREQUENCY:

nchronous speed of the induction motor is giv

evident that the synchronous speed and thus th

the supply frequency.

:

reduced by reducing the frequency, if the induc

ad on the generators.

he GRID, It requires a Cyclo converter circuits

er which the speed can be varied is very less.

driven ships although not common in shore.

f control

n by

speed of the

tion motor

at the stator side

his method is


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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 ofnumber 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 synchronousspeed.

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 ofspeeds can be achieved.

3.2.4. CHANGING THE ROTOR RESISTANCE:

As we discussed in the voltage control session

The Load torque

For a constant torque and constant applied voltage, the slip to rotor resistance ratio isconstant. Therefore

S = k

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

Torque .

And the starting torque is

Starting torque .

And the slip corresponding to maximum/Breakdown torque is

S = /

By considering all the above points Torque-slip or Torque speed characteristics are given as

below.

> >

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 PLCs 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 PLCs, 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


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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 its possible to increase the

number of inputs / outputs of the basic central unit by adding remote units.


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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 of31 remote analog input units

- a maximum of31 remote analog output units

- a maximum of15 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


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4.3 BINARY EXTENSIO

Fig.4.3

We are using a binary extens

be used as Inputs as well as

24DC as an output.

4.4 ADDRESSING TH

Fig.4.4

32

S:

ion XC08L1, it consists 8 user configurable ch

utputs. It works with 24VDC, 0.5A as an input,

INPUTS AND OUTPUTS OF PLC:

nnels which can

and also gives


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The Central unit is automatically

63, 64, and so on.

The binary INPUTS of the centr

,The binary OUTPUTS are give

In case of analog inputs it is %I

as %OW62.00, %OW62.01....%

4.4.1ADDRESSING OF B

33

addressed as 62; the remaining extensions will

l unit are addressed as %I62.00, %I62.01, %I6

n as %O62.00 ,%O62.01 ,%O62.02......%O62.0

62.00, %IW62.01.....%IW62.07 and analog ou

W62.05

NARY EXTENSIONS:

Fig.4.5

be addressed as

.02......%I62.07

5

tputs are given


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4.4.2 ADDRESSING OF

34

NALOG 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 isdescribed 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


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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


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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


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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 ringInduction 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


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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 circuitdiagram 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


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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 usean 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


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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 de-

energized 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 NOs. When the output comes from the PLC the

relay will be activated three NOs 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 isdesignated 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.


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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, ifrequired.

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 hotfrom excessive current, the normally-closed overload contact will open, de-energizingthe 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 cant 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, asin 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 Operand 1

E2 BINARY %I, %M, %O, %S, %K Operand 2

A1 BINARY %M, %O, %S Result of the OR combination

DESCRIPTION:

This connection element realizes a logical OR combination of the operands at theinputs. The result is allocated to the operand at the output.

Truth table:

7.2.2 TIMER FUNCTIONS:

T-ON DELAY:

Fig.7.2

E1 E2 A1

0 0 0

0 1 1

1 0 1

1 1 1


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PARAMETERS:

IN BINARY %I, %M, %O, %S, %K Input signal

PT WORD, DW %KW , %KW+1, %MW , %MW+1,%MD,%KD Preset time

Q BINARY %M, %O Delayed signal

ET WORD,DW %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 Qretains 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 timeMeaningful range for PT: > 1 cycle time

Fig.7.3Note: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 arenecessary. 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 completelyindependent 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. Allrunning 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


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T-OFF DELAY:

Fig.7.4

PARAMETERS

IN BINARY %I, %M, %O, %S, %K Input signal

PT WORD,DW %KW , %KW+1,%MW , %MW+1 ,%MD, %KD Preset timeQ BINARY %M, %O Delayed signal

ET WORD.DW %MW & %MW+1,%MD Time visualization

DESCRIPTIONThe 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 theinput 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 timeMeaningful range for PT : > 1 cycle time.

Fig.7.5


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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 therequirement 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.

Starting of Induction Motor Using PLC

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