Induction Motor Speed Control Methods

Speed Control Methods of Induction Motor:

In this post let us see what are the various methods to control the speed of Induction Motor...We know that the speed of the induction motor is given by,
N  = \frac{120f}{P} \

From the above equation, the speed of the motor can be controlled
by changing supply frequency and
by changing number of poles in the stator.

Control From Stator Side:

  1. By changing the supply frequency
  2. By changing number of stator poles
  3. By changing the supply voltage

Control From Rotor Side:

  1. By inserting resistance in rotor circuit
  2. By various ways of cascade connection
  3. By injecting EMFs in the rotor circuit.

Speed Control by frequency variation:

  • By varying supply frequency (on small amount), we can vary the speed.
  • But a decrease in supply frequency decreases the speed and increases the flux, core losses which leads heating and low efficiency.
  • Increase in frequency increases the speed and reduces the torque.
  • A separate costlier auxiliary equipment is required to provide a variable frequency.
  • So this method is not used in practical.

Speed Control By Pole Changing:

  • The change of number of poles is done by having two or more entirely independent stator windings in the same slots.
  • Each winding gives a different number of poles, so we will get different speeds.
  • Due to cost and complex switching arrangements, it is not practical to provide more than two arrangements of poles (ie, two normal speeds).
  • This method is applicable to squirrel cage induction motor only.
  • It is not practically applicable with wound rotor.

Speed control by varying Supply voltage:

  • The speed of induction motor can be varied by changing supply voltage.
  • The torque developed in this method is proportional to the square of the supply voltage.
    T ∝ V2
  • This is the cheapest and easiest method, but it is rarely used because of the below reasons.
  1. A small change in speed requires a large change in voltage.
  2. This large change in voltage will result in a large change in the flux density.

Speed control by varying Rotor Resistance:

  • This method is applicable to three-phase slip-ring induction motor only.
  • By introducing external resistance in the rotor circuit, the speed of the motor can be reduced.
  • The change in speed depends upon both rotor circuit resistance and load.
  • Due to power loss in the resistance ,this method is used where speed changes are required for short period only.
  • This method is similar to armature rheostat control method of DC shunt motors.

Speed control by injected EMF:

  • Instead of applying the resistance into the rotor circuit of the motor, the speed can be varied by applying EMFs into the circuit.
  • These EMFs are applied at the rotor by a suitable source whose frequency should be same as slip frequency.
  • Inserting the EMF in phase with the rotor induced EMF is equivalent to decreasing the rotor resistance .
  • Inserting the EMF in phase opposition to the induced rotor EMF is equivalent to increasing its resistance.
  • Thus by injecting EMF into the rotor the speed can be controlled.

Speed control by Cascade Connection:

  • This method needs two motor, one of them is wound motor.
  • The two motors are mechanically coupled together to drive a common load.
  • The starter of slip ring induction motor is connected to three-phase supply and its rotor is connected to stator of the other machine.
  • There are four possible speeds can be obtained by this arrangement.
    N  = \frac{120f}{P1}\
    N  = \frac{120f}{P2} \
    N  = \frac{120f}{P1+P2} \
    N  = \frac{120f}{P1-P2} \
    Where f = supply frequency

    P1 = No of poles in slip ring motor
    P2 = No of poles in other motor

Read More:
How to Control the Speed of DC Shunt Motors?  
How to control Speed of DC series Motor?  
How fluorescent lamp works?  
Double Cage Induction Motor – Introduction  
Difference Between Mechanical and Electronic Commutator  

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2 Responses

  1. gebremichael negash says:

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  2. mukim says:

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