Induction Motor MCQs

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Induction motor is one of the most popular motors in the electrical industry. Due to its rugged and reliable operations, it is widely employed for various processes and industries. Unlike the synchronous motor, it doesn’t need a separate dc field for excitation purposes. Given below are the top 10 Induction Motor MCQs with answers and solved solutions:

1. The synchronous speed of 60 Hz, three phase four pole motor is:

  1. 60 RPM
  2. 80 RPM
  3. 1800 RPM
  4. 2880 RPM

Correct answer: 3

Solution: Using formula: Speed = (120 * f)/P = 120 * 60 / 4 = 1800 RPM

2. In an induction motor the speed of rotor is always:

  1. Equal to synchronous speed
  2. Smaller than synchronous speed
  3. Greater than synchronous speed
  4. Very large than synchronous speed

Correct answer: 2

3. Correct statement about wound rotor motor:

  1. It is cheaper than squirrel cage motor
  2. External resistance can be inserted in its rotor circuit
  3. It has low starting torque
  4. It requires less maintenance than squirrel cage motor

Correct answer: 2

4. The power factor of squirrel cage motor under light load conditions is:

  1. Poor
  2. Average
  3. Good
  4. Excellent

Correct answer: 1

5. Intermediate duty on nameplate of induction motor means:

  1. It can run at full load throughout the day
  2. It will run at full load for some specified time
  3. It can run at full speed
  4. It can run at some specific speed only

Correct answer: 2

6. The induction motor which contains slip ring and brushes:

  1. Squirrel cage induction motor
  2. Wound rotor induction motor
  3. Both of these
  4. None of these

Correct answer: 2

7. An input power of 10 kW is provided to a motor having stator copper losses of 700 watts. The air gap power is:

  1. 10.7 kW
  2. 10 kW
  3. 9.3 kW
  4. 0.7 kW

Correct answer: 3

Explanation: P (AG) = P (in) – P (SCL) = 10 kW – 0.7 kW

= 0.3 kW

8. The induction motors rotor with low resistance has:

  1. Low starting torque and low starting current
  2. Low starting torque and high starting current
  3. High starting torque and low starting current
  4. High starting torque and high starting current

Correct answer: 2

9. Induction motor is sometimes also referred as:

  1. Singly excited machine
  2. Doubly excited machine
  3. Triply excited machine
  4. Quad excited machine

Correct answer: 1

In induction motor power is only supplied to stator circuit and no external dc source is required for excitation of the rotor, therefore the term singly excited machine is also used to represent induction motor

10. The rotor of a 4 pole, 50 Hz induction motor having rotor slip of 3 percent has mechanical shaft speed of:

  1. 45 r/min
  2. 1455 r/min
  3. 1500 r/min
  4. None of above

Correct answer: 2

Rotor shaft speed (ηm) = (1 – s).ηsync

ηsync = 120 * f / P = 120 * 50 /4 = 1500 r/min

ηm = (1- 0.03) * 1500 r/min

ηm = 1455 r/min

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