Polyphase Induction Motors

1. Insertion of resistance in the stator of an induction motor

  1. increases the load torque
  2. decreases the starting torque
  3. increases the starting torque
  4. none of the above

Correct answer: (B)
decreases the starting torque

2. The starting torque of a three phase induction motor can be increased by

  1. increasing slip
  2. increasing current
  3. both (a) and (b)
  4. none of the above

Correct answer: (C)
both (a) and (b)

3. 5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest?

  1. 730 r.p.m.
  2. 960 r.p.m.
  3. 1440 r.p.m.
  4. 2880 r.p.m.

Correct answer: (A)
730 r.p.m.

4. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be

  1. 200 Hz
  2. 50 Hz
  3. 2 Hz
  4. 0.2 Hz

Correct answer: (C)
2 Hz

5. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor

  1. will continue running burning its one phase
  2. will continue running burning its two phase
  3. will stop and carry heavy current causing permanent damage to its winding
  4. will continue running without any harm to the winding

Correct answer: (C)
will stop and carry heavy current causing permanent damage to its winding

6. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor

  1. will continue running burning its one phase
  2. will continue running burning its two phases
  3. will stop and carry heavy current causing permanent damage to its winding
  4. will continue running without any harm to the winding

Correct answer: (A)
will continue running burning its one phase

7. A 3-phase slip ring motor has

  1. double cage rotor
  2. wound rotor
  3. short-circuited rotor
  4. any of the above

Correct answer: (B)
wound rotor

8. A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be

  1. 0.01
  2. 0.02
  3. 0.03
  4. 0.04

Correct answer: (D)
0.04

9. A change of 4% of supply voltage to an induction motor will produce a change of appromimately

  1. 4% in the rotor torque
  2. 8% in the rotor torque
  3. 12% in the rotor torque
  4. 16% in the rotor torque

Correct answer: (D)
16% in the rotor torque

10. A double squirrel-cage induction motor has

  1. two rotors moving in oppsite direction
  2. two parallel windings in stator
  3. two parallel windings in rotor
  4. two series windings in stator

Correct answer: (C)
two parallel windings in rotor

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