Speed Control of Induction Motors:
Unlike D.C. Motors, A.C. Induction Motors are not suitable for variable speeds. Their speed control and regulation is comparatively difficult when compared with D.C. Motors. These are some of the methods which are commonly used for the speed control of squirrel cage induction motors:
1. Changing Applied Voltage
2. Changing Applied Frequency
3. Changing Number Of Stator Poles
The above three methods are most commonly used for the speed control of squirrel cage Induction motors.
1. Changing Applied Voltage: This method, even though easiest, it is rarely used. The reasons are (a) for a small change in speed, there must be a large variation in voltage. (b) This large change in voltage will result in large change in flux density, thereby seriously disturbing the magnetic distribution/condition of the motor.
2. Changing Applied Frequency: We all know that the synchronous speed of the induction motor is given by Ns = 120f/P. So from this relation, it is evident that the synchronous speed and thus the speed of the induction motor can by varied by the supply frequency. This method has its own limitations. The motor speed can be reduced by reducing the frequency, if the induction motor happens to be the only load on the generators. Even then the range over which the speed can be varied is very less. This method is famous in some electrically driven ships although not common in shore.
3. Changing The Number Of Stator Poles: As we know the relation between the synchronous speed and the number of poles, i.e. Ns = 120f/P. So the number of poles is inversely proportional to the speed of the motor. This change of number of poles can be achieved by having two or more entirely independent stator windings in the same slots. Each winding gives a different number of poles and hence different synchronous speed. For example, for the same motor, if no. of poles = 2 , 4 or 6, which can be changed as per speed requirement, and lets say the supply frequency f = 50 Hz,
No. of Poles P = 2, then Ns = 120 * 50/2: So Ns = 3000 r.p.m
No. of Poles P = 4, then Ns = 120 * 50/4: So Ns = 1500 r.p.m
No. of Poles P = 6, then Ns = 120 * 50/6: So Ns = 1000 r.p.m.
Thus the speed control of squirrel cage induction motor can be done easily, but as steps of reduced speed. This method is used for elevator motors, traction motors and also for machine tools.