Induction Motor MCQs Part 19

Contents

- 1 If air gap in induction motor is increased, its
- 2 A 3-phase induction motor is connected to a normal supply voltage
- 3 The stator of 6 pole, 3-phase induction motor is fed from 3-phase, 60 Hz supply which contains a pronounced fifth harmonic
- 4 The breakdown torque of a 3-phase induction motor of negligible stator impedance
- 5 The induction motor has lagging power factor

## If air gap in induction motor is increased, its

If air gap in induction motor is increased, its:

- Efficiency will improve
- Speed will reduce
- Power factor will decrease
- 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.

- 0.1 lag
- 0.2 lag
- 0.3 lag
- 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:

- 100 RPM
- 200 RPM
- 240 RPM
- 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))

- Is directly proportional to X2
- Is inversely proportional to X2
- Is directly proportional to R2
- 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:

- At starting only
- During operation
- Both of these
- None of these

Correct answer: 3. Both of these