Speed control Methods of Induction motors : -

As we know that the speed of the induction motor is given by,

$$N = \frac{120f}{P}$$

From the above equation, we can conclude that 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:

  • By changing the supply frequency

  • By changing number of stator poles

  • By changing the supply voltage

• Control From Rotor Side:

  • By inserting resistance in rotor circuit

  • By various ways of cascade connection

  • 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.

• 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. $T ∝ V^2$

  • This is the cheapest and easiest method, but it is rarely used because of the below reasons.

  • A small change in speed requires a large change in voltage.

  • 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.