Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor?
(A) By changing the number of stator poles
(B) Rotor rheostat control
(C) By operating two motors in cascade
(D) By injecting e.m.f. in the rotor circuit
Answers
Answer:
Speed Control of Induction Motors:
These are some of the methods which are commonly used for the speed control of squirrel cage induction motors: Changing Applied Voltage.
Answer:
The correct answer is (A) By changing the number of stator poles.
Explanation:
The three methods listed below are the most regularly used to control the speed of squirrel cage induction motors.
Changing the Voltage Applied: Even though it is the simplest way, it is rarely employed. The reasons for this are that (a) a minor change in speed necessitates a substantial change in voltage. (b) The large change in voltage will cause a large change in flux density, causing the magnetic distribution/condition of the motor to be seriously disrupted.
Changing the Applied Frequency: We all know that the induction motor's synchronous speed is given by Ns = 120f/P. As a result of this relationship, it is clear that the supply frequency can affect the synchronous speed and consequently the speed of the induction motor. This approach has its own set of drawbacks. If the induction motor is the only load on the generators, the motor speed can be lowered by lowering the frequency. Even yet, the speed can only be changed across a very small range. This technology is well-known aboard some electrically powered ships, although it is not widely used on land.
Changing The Number Of Stator Poles: As a result, the number of poles is inversely related to the motor's speed. Two or more fully independent stator windings in the same slots can be used to vary the number of poles. Each winding produces a variable amount of poles, and hence synchronous speed.