Induction Motor MCQs Part 13

Induction Motor MCQs Part 13

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

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

1. Starting torque is independent of supply voltage
2. Startining torque is directly proportional supply voltage
3. Starting torque is directly proportional to square of supply voltage
4. 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

1. More than
2. Less than
3. Either of above
4. 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

1. 0.5 Hz
2. 0.9 Hz
3. 1.3 Hz
4. 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:

1. 182.54 W
2. 260.67 W
3. 292.53 W
4. 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:

1. Increases
2. Decreases
3. Remains the same
4. 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.