mechanical epu&t 17&18 speedcontrolof threephase inductionmotor
TRANSCRIPT
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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