Question

Q5. (a) Give five names of different traditional speed-control methods of induction motor.

(b) For an induction motor, derive the operating condition that would provide efficient speed control over a wide range?

(c) What is the concept of “constant volts per hertz (Va/a) control” of induction motor speed control?     [5+5+5=15 Points]

Answer 1

Induction Motor Speed Control From Stator Side

1. By Changing The Applied Voltage

2. By Changing The Applied Frequency

3. Constant V/F Control Of Induction Motor

4. Changing The Number Of Stator Poles

Speed Control From Rotor Side:

1. Rotor Rheostat Control

2. Cascade Operation

3. By Injecting EMF In Rotor Circuit

(c) The speed of an induction motor can be easily controlled by varying the frequency of the 3-phase supply; however, to maintain a constant (rated) flux density, the applied voltage must also be changed in the same proportion as the frequency (as dictated by Faraday’s law).This speed control method is known as Volts per Hz. Above rated speed, the applied voltage is usually kept constant at rated value; this operation is referred to as constant HP. At low frequencies (i.e. speeds), the voltage must be boosted in order to compensate for the effects of the stator resistance.

sorry for part (b)