Mechanical Epu&t 17&18 Speedcontrolof Threephase Inductionmotor

8/11/2019 Mechanical Epu&t 17&18 Speedcontrolof Threephase Inductionmotor

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PARUL INSTITUE OF ENGG AND

TECH

SUBJECT:ELECTRIC POWER UTILIZATIONAND TRACTION

TOPIC:SPEED CONTROL OF 3 PHASE

INDUCTION MOTOR

GUIDED BY:SIR PRATIK PATEL

CREATED BY:ANKIT R

NAGAR(18)&PARMAR NIJANAND(17).

MECHANICAL 5THSEM A


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Induction Motors


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Construction

An induction motor has two main parts- a stationary stator

consisting of a steel frame that supports a hollow, cylindrical core

core, constructed from stacked laminations (why?), having a

number of evenly spaced slots, providing the space for the statorwinding

Stator of IM


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Construction

Squirrel cage rotor

Wound rotor

Notice the

slip rings


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Introduction

Three-phase induction motors are the mostcommon and frequently encountered machines in

industry

- simple design, rugged, low-price, easy maintenance

- wide range of power ratings: fractional horsepower to

10 MW

- run essentially as constant speed from no-load to full

load

- Its speed depends on the frequency of the power source

not easy to have variable speed control

requires a variable-frequency power-electronic drive for

optimal speed control


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Construction

- a revolving rotor composed of punched laminations, stacked to create a series of rotor

slots, providing space for the rotor winding

one of two types of rotor windings

conventional 3-phase windings made of insulated wire (wound-rotor)

similar to the winding on the stator aluminum bus bars shorted together at the ends by two aluminum rings,

forming a squirrel-cage shaped circuit (squirrel-cage)

Two basic design types depending on the rotor design

- squirrel-cage: conducting bars laid into slots and shorted at both

ends by shorting rings.- wound-rotor: complete set of three-phase windings exactly as the

stator. Usually Y-connected, the ends of the three rotor wires are

connected to 3 slip rings on the rotor shaft. In this way, the rotor

circuit is accessible.


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Construction

Cutaway in a

typical wound-

rotor IM.Notice the

brushes and the

slip rings

Brushes

Slip rings


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Principle of operation

This rotating magnetic field cuts the rotor windings andproduces an induced voltage in the rotor windings

Due to the fact that the rotor windings are short circuited, for

both squirrel cage and wound-rotor, and induced current

flows in the rotor windings The rotor current produces another magnetic field

A torque is produced as a result of the interaction of those

two magnetic fields

Where indis the induced torque andBRandBSare the magnetic

flux densities of the rotor and the stator respectively

ind R skB B


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Induction motor speed

At what speed will the IM run?- Can the IM run at the synchronous speed, why?

- If rotor runs at the synchronous speed, which is the

same speed of the rotating magnetic field, then the rotor

will appear stationary to the rotating magnetic field andthe rotating magnetic field will not cut the rotor. So, no

induced current will flow in the rotor and no rotor

magnetic flux will be produced so no torque is

generated and the rotor speed will fall below thesynchronous speed

- When the speed falls, the rotating magnetic field will

cut the rotor windings and a torque is produced


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Induction motor speed

So, the IM will always run at a speed lowerthanthe synchronous speed

The difference between the motor speed and the

synchronous speed is called the Slip

Where nslip

= slip speed

nsync= speed of the magnetic field

nm = mechanical shaft speed of the motor

slip sync mn n n


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METHOD OF SPEED CONTROL

Speed control from stator side

Control from rotor side

S CO O O S A O


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SPEED CONTROL FROM STATOR

SIDE

A) Changing applied voltage:This method is most easiest and cheapest. In this

method speed of the motor is controlled by

changing the applied voltage across the motor

terminals.

Decreasing applied voltage will decrease the speed

of the motor and increasing voltage will increase

the speed. But this method is not used widely forfollowing two reasons

(i) Large change in voltage is required for

relatively small change in motor speed


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ii) This large change in voltage may disturb themagnetic conditions of the motor, as it changes the

flux density.

B) Changing the applied frequency:

We know that the synchronous speed of

induction motor, Ns = 120f/p(where f = supply

frequency, p = total no. of stator poles).

Thus speed can be varied by changing supplyfrequency. As changing in the supply frequency is

a difficult task, this method is used where motor is

directly powered from a generator.


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We can change the supply frequency by generatorby changing speed of prime mover of the

generator.

As we increase supply frequency, speed of the

motor also increases and vice versa.

This method is being used to some extent in

electrically driven ships.


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) Changing the number of stator poles:As said above, Ns = 120f/p. Thus by changing in

number of stator poles we can change the speed

of induction motor.

This method is easily applicable for squirrel cage

type induction motors, as rotor of these motors

adopts itself for any number of poles. To use this

method, stator is wound for two or more differentwinding with different poles. Only one winding

will be in circuit at a time other being

disconnected.


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E.g. stator can be wound with two differentwindings having no. of poles 2 and 4 respectively.

In this case if supplied frequency is 50 Hz,

(i) Ns = 120 * 50 / 2 = 3000 rpm (for p = 2) and

(ii) Ns = 120 * 50 / 4 = 1500 rpm (for p = 4).

this method is being used in elevator and traction

motors.


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CONTROL FROM ROTOR SIDE

A) Rotor rheostat control:This method is applicable for slip ring motors. It is

same as that of armature control method for dc

series motor.

A star connected, 3 phase rheostat is joined in

series with the rotor circuit via slip rings. Here slip

rings are not short circuited as they are when

rheostat is only used for starting of a inductionmotor. Slip for a given torque can be varied by

varying the rotor resistance.
http://www.indiastudychannel.com/resources/151884-Starting-methods-3-phase-induction-motor.aspxhttp://www.indiastudychannel.com/resources/151884-Starting-methods-3-phase-induction-motor.aspxhttp://www.indiastudychannel.com/resources/151884-Starting-methods-3-phase-induction-motor.aspxhttp://www.indiastudychannel.com/resources/151884-Starting-methods-3-phase-induction-motor.aspx


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B) Cascading two motors:In this method, two motors are mounted on a single

shaft, say motor A and motor B. The motor A is

directly fed from the 3 phase supply. Supply for

motor B is taken out from the rotor of motor A viaslip rings. Thus motor B is supplied through motor

A. In such way, four speeds can be obtained in

following cases.

(i) Motor A may be run separately from the supply

giving synchronous speed Ns = 120f / Pa, (Pa is

no. of stator poles for motor A).


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(ii) Motor B may be run separately givingsynchronous speed Ns = 120f / Pb, (Pb is no. of

stator poles for motor B).

(iii) Two motors may be connected in cumulative

cascade giving Ns = 120f / (Pa + Pb).(iv) Two motors may be connected in differential

cascade giving Ns = 120f / (Pa - Pb).

C) Injecting emf in rotor circuit:An emf of same frequency as that of slip of the

motor is injected in rotor circuit via slip rings.


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When we insert voltage which is in phase withinduced rotor emf, it is equivalent to decreasing

resistance of rotor.

Whereas when we insert voltage which is opposite

in phase with induced emf in rotor, its like

increasing resistance of rotor circuit.

Hence by injecting emf in rotor circuit we can

control the speed of a induction motor.


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THANKYOU

Mechanical Epu&t 17&18 Speedcontrolof Threephase Inductionmotor

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