Induction Motor MCQs Part 19

by

Induction Motor MCQs Part 19

If air gap in induction motor is increased, its

If air gap in induction motor is increased, its:

  1. Efficiency will improve
  2. Speed will reduce
  3. Power factor will decrease
  4. Speed will increase

Correct answer: 3. Power factor will decrease

A 3-phase induction motor is connected to a normal supply voltage

A 3-phase induction motor is connected to a normal supply voltage. EMF induced between slip rings at standstill is 50. The resistance and standstill reactance per phase are 0.7 Ω and 3.5 Ω respectively. Calculate the rotor’s power factor provided that the rotor of motor is star-connected.

  1. 0.1 lag
  2. 0.2 lag
  3. 0.3 lag
  4. 0.4 lag

Correct answer: 2. 0.2 lag

Solution:

Rotor power factor = R2/Z2

= 0.7/3.5 = 0.2 lag

The stator of 6 pole, 3-phase induction motor is fed from 3-phase, 60 Hz supply which contains a pronounced fifth harmonic

The stator of 6 pole, 3-phase induction motor is fed from 3-phase, 60 Hz supply which contains a pronounced fifth harmonic. The speed of rotating file produced by 5th harmonic will be:

  1. 100 RPM
  2. 200 RPM
  3. 240 RPM
  4. 580 RM

Correct answer: 3. 240 RPM

Solution:

Synchronous speed = 120f/P = 120 * 60/6 = 1200 RPM

————

Speed of rotating field due to 5th harmonic = Ns/5 = 1200/5 = 240 RPM

The breakdown torque of a 3-phase induction motor of negligible stator impedance

The breakdown torque of a 3-phase induction motor of negligible stator impedance:

(Rotor resistance (R2), Rotor reactance (X2))

  1. Is directly proportional to X2
  2. Is inversely proportional to X2
  3. Is directly proportional to R2
  4. Is inversely proportional to R2

Correct answer: 2. Is inversely proportional to X2

The induction motor has lagging power factor

The induction motor has lagging power factor:

  1. At starting only
  2. During operation
  3. Both of these
  4. None of these

Correct answer: 3. Both of these

Leave a Reply