Speed Control Methods For A 3 Phase Induction Motor

Speed of 3 phase induction motor needs to varied according to requirement. This article explains different methods for controlling the speed of 3 phase induction motor. Speed controls from stator side as well as rotor side are also explained in this article.

Speed control of 3 phase induction motor

A 3 phase induction motor is practically constant speed motor provided that applied parameters (supply, frequency etc.) remains unchanged. The speed can be varied according to requirement. The speed of induction motor depends on various factors, like supplied voltage, frequency, number of poles. There are different methods for controlling the speed from stator side and rotor side. These speed control methods are explained below.

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 for following two reasons
(i) Large change in voltage is required for relatively small change in motor speed
(ii) This large change in voltage may disturb the magnetic 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 supply frequency. As changing in the supply frequency is a difficult task, this method is used where motor is directly powered from a generator. We can change the supply frequency by generator by 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.

C) 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 different winding with different poles. Only one winding will be in circuit at a time other being disconnected. E.g. stator can be wound with two different windings 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.

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 induction motor. Slip for a given torque can be varied by varying the rotor resistance. But the main disadvantage of this method is I^2.R (Cu) losses are also increased with increase in rotor resistance. Because of these increased losses this method is used where speed control is required for a short time.

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 via slip 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).
(ii) Motor B may be run separately giving synchronous 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. When we insert voltage which is in phase with induced 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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