induction-motor-mcq

Induction Motor MCQ Part 8

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Electrical Engineering MCQ shares multiple choice questions on Induction Motor MCQ Part 8.

Frequency of Induced EMF in Rotor of Induction Motor

A 4 pole, 50 Hz 3-phase induction motor has slip value of 4%. The frequency of induced emf in rotor is:

  1. 1 Hz
  2. 2 Hz
  3. 12.5 Hz
  4. 100 Hz

Correct answer: 2 Hz

Solution: f’ = s*f

where f’ = rotor frequency

s = decimal slip

f = supply frequency

f’ = 0.04 * 50 = 2 Hz

A four speed squirrel cage motor uses how many stator windings:

A four speed squirrel cage motor uses how many stator windings:

  1. One
  2. Two
  3. Three
  4. Four

Correct answer: 2. Two

Explanation: A four speed squirrel cage motor uses two stator windings. Each of these two provides two speeds.

Induction Motor Speed Calculation Numerical

The full load slip of a 60 Hz, 3 phase, 6 pole induction motor is 4%. If rotor resistance is 0.02 per phase and standstill reactance is 0.2 per phase. What is the value of speed for which maximum torque will occur:

  1. 400 RPM
  2. 982 RPM
  3. 1080 RPM
  4. 1200 RPM

Correct answer: 3. 1080 RPM

Solution:

Step 1: Synchronous Speed = 120*f/P = 120 * 60/6 = 1200 RPM

Step 2: Slip at which maximum torque occurs = s = R2/X2 = 0.02/0.2 = 0.1

Step 3: Rotor speed at max torque = N = Ns * (1 – s) = 1200 * (1 – 0.1) = 1080 RPM

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