Induction Motor MCQs Part 13

Contents

- 1 The starting torque of a 3-phase induction motor is related to supply voltage as follows
- 2 The number of rotor slots in a squirrel cage motor are __________ rotor slots
- 3 An 8 pole alternator runs at 450 RPM and supplies electric power to an induction motor having 6 poles and has full load slip of 3%. The frequency of rotor EMF is
- 4 The input to a 3 phase induction motor is 40 kW. The stator losses amount to 900 W. If slip = 2%, the rotor Cu loss per phase is
- 5 If speed of 3-phase induction motor increases, its efficiency

The starting torque of a 3-phase induction motor is related to supply voltage as follows

- Starting torque is independent of supply voltage
- Startining torque is directly proportional supply voltage
- Starting torque is directly proportional to square of supply voltage
- Starting torque is inversely proportional to square of supply voltage

Correct answer: 3. Starting torque is directly proportional to square of supply voltage

## The number of rotor slots in a squirrel cage motor are __________ rotor slots

The number of rotor slots in a squirrel cage motor are __________ rotor slots

- More than
- Less than
- Either of above
- None of above

Correct answer: 3. Either of above

## An 8 pole alternator runs at 450 RPM and supplies electric power to an induction motor having 6 poles and has full load slip of 3%. The frequency of rotor EMF is

An 8 pole alternator runs at 450 RPM and supplies electric power to an induction motor having 6 poles and has full load slip of 3%. The frequency of rotor EMF is

- 0.5 Hz
- 0.9 Hz
- 1.3 Hz
- 1.5 Hz

Correct answer: 2. 0.9 Hz

Supply frequency = N*P/120 = (450*8)/120 = 30 Hz

Frequency of rotor EMF = f’ = sf = 0.03 * 30 = 0.9 Hz

## The input to a 3 phase induction motor is 40 kW. The stator losses amount to 900 W. If slip = 2%, the rotor Cu loss per phase is

The input to a 3 phase induction motor is 40 kW. The stator losses amount to 900 W. If slip = 2%, the rotor Cu loss per phase is:

- 182.54 W
- 260.67 W
- 292.53 W
- 305.63 W

Correct answer: 2. 260.67 W

Solution:

Stator output = Stator iput – Stator losses = 40 – 0.9 = 39.1 kW

———————

Rotor input = Stator input = 39.1 kW

Rotor Cu loss = s * Rotor input = 0.02 * 39.1 = 0.782 kW Rotor Cu loss per phase = 0.782 * 1000/3 = 260.67 W

## If speed of 3-phase induction motor increases, its efficiency

If speed of 3-phase induction motor increases, its efficiency:

- Increases
- Decreases
- Remains the same
- None of these

Correct answer: 1. Increases

Explanation: Efficient can be mathematically related to slip s as:

Efficiency = 1 – s

Is speed of induction motor increases, the slip (s0) decreases and hence the efficiency increases.