# 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