Three Phase Induction Motors – Question and Answer

1. What are the advantages of three-phase induction motor?
  1. Simple construction
  2. High reliability
  3. Rugged construction
  4. High efficiency
  5. Low cost
  6. Less maintenance
  7. Self starting torque
  8. Good power factor.
2. What are the disadvantages of three phase induction motor?
  1. The speed is not constant with load.
  2. Reduction in efficiency when speed is varied.
  3. Low starting torque compared to d.c shunt motor.
3. What are the two main parts of an induction motor?
  1. Stator
  2. Rotor
4. What are the two types of rotors of an induction motor?
  1. Squirrel cage rotor
  2. Slip ring or wound rotor
5. What is an end ring?

In squirrel cage rotor, the copper bars are placed in the slots. These bars are short circuited at each end with the help of conduction copper ring called end ring.

6. The squirrel cage rotor is also known as short circuited rotor. Why?

The copper bars are shorted to each other through end ring. The entire rotor resistance is very small. Hence this rotor is called short circuited rotor.

7. Define slip speed in an induction motor.

The slip speed is defined as the difference in speed between the rotating magnetic field produced by stator (Ns) and rotor speed (N).

8. What is the speed of rotor field in space?

The speed of rotor field in space is speed of stator rotating field.

9. What is the condition for maximum starting torque?

The starting torque is maximum when rotor resistance equals rotor reactance.
R2 = X2

10. What are the characteristics of maximum torque?
  1. The maximum torque is independent of rotor resistance R2.
  2. It is directly proportional to the square of the applied voltage.
  3. It is inversely proportional to standstill reactance and hence the reactance (rotor) should be kept as small as possible.
11. What is meant by stator voltage control?

The speed of an induction motor can be controlled by varying the stator voltage. This method of speed control is known as stator voltage control. Here, the supply frequency is constant. The stator voltage can be controlled by two methods.

12. What are the different methods of stator voltage control?
  1. Using auto transformer
  2. Primary resistors connected in series with stator winding.
13. What are the disadvantages of cascade control?
  1. This method requires two motors.
  2. More expensive.
  3. Wide range of speed control is not possible.
  4. It cannot be operated when P1 = P2 or P1 < P2
14. What is meant by slip power recovery scheme?

Slip power can be returned to the supply source and can be used to supply an additional motor which is mechanically coupled to the main motor. This type of drive is known as slip power recovery system and improves the overall efficiency of the system.

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Santhakumar Raja

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