Speed Control of Induction Motors

SPEED CONTROL OF INDUCTION MOTORS

We can control the speed of the 3ϕ induction motor, by the following methods:

I. Control from stator side

1. By changing the applied voltage

2. By changing the supply frequency

3. By changing the number of stator poles

1. Changing applied voltage

This method, though cheapest and easiest, is rarely used because a large change in voltage is required for a relatively small change in speed. This large change in voltage will result in a large change in the flux density thereby seriously disturbing the magnetic conditions of the applied frequency.

2. Changing the supply frequency

This method is also used rarely. We have seen that the synchronous speed of an induction motor is given by,

 N_S = 120f  / P

Clearly, the synchronous speed of an induction motor can be changed by changing the supply frequency 'f '. This method has been used to some extent on electrically‒driven ships.

3. By changing the number of Stator poles

This méthod is easily applicable to squirrel cage motors because the squirrel cage rotor adapts itself to any reasonable number of stator poles. From the synchronous speed equation, N_S = 120ƒ / p, it is evident that the speed of an induction motor could also be changed by changing the number of stator poles. This change of number of poles is achieved by having two or more entirely independent stator windings in the same slot. Each winding gives a different number of poles and hence different synchronous speed.

II Control from rotor side

1. Rotor rheostat control

2. By operating two motors in cascade (Concatenation Method)

1. Rotor Rheostat Control

In this method, which is applicable only for slipring induction motor, the motor speed is reduced by introducing an external resistance in the rotor circuit. [Ref Fig. 5.21(a)].

2. Cascade / Concatenation / Tandem operation

This method requires 2 motors, the first of which must have a wound rotor. It should also have a one‒to‒one voltage ratio. So that in addition to cascading, each motor may be run from the supply mains separately. That is, at standstill with rotor circuit open, the voltage across the slip‒rings should be equal to that across the stator terminals. The stators of both motors should be wound for the same voltage. The second motor may be of the squirrel cage type or have a wound rotor with external resistance. The rotor shafts are directly coupled, so that both run at the same speed.

The stator of the first motor, A is connected to the 3‒ϕ supply. Then the rotor of the first motor is connected to the stator of the second motor B. The starting resistance is connected to the rotor circuit of the second motor

In cascade method, there are 4 ways to obtain different speeds by the combination of motors.

Motor 1 may be run separately from the supply,

Synchronous speed, N_SA = 120f / P_A [for motor A]

Where,

 f = supply frequency

P_A = no. of stator poles of the motor A

Motor 2 may be allowed to run separately from the supply

Synchronous speed of motor B, N_SB =  120f / P_B [for motor B]

Where, P_B = No. of stator poles of the motor B.

Cumulative Cascade

Motor A and motor B are allowed to operate in cumulative cascade. In this, the state fields of the motor A and B are having the phase rotation in the same direction. The synchronous speed of the cascaded set can be derived as follows.

N_SA= 120f / P_A [synchronous speed of motor A]

Running speed, N = (1‒S_A) N_SA = (1 ‒ S_A) 120f / P_A → (A)

On no load S_B=0 and the above eqn becomes

Differential Cascade

In this case, the rotating magnetic fields of motors A and B are in opposite directions. i.e, the phase rotation of stator field of A and B are opposing. This reversal of phase rotation is obtained by inter‒changing any of its 2 leads.

In this case, the synchronous speed obtained is

N_SC = 120f / [P_A‒P_B]