Induction Motor MCQs Part 17

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Induction Motor MCQs Part 17

An induction motor is favored over a DC motor since it

An induction motor is favored over a DC motor since it:

  1. Provides fine speed control
  2. Provides high starting torque
  3. Is simple and rugged in construction
  4. None of these

Correct answer: 3. Is simple and rugged in construction

The slip of three phase induction motor at no-load is

The slip of three phase induction motor at no-load is:

  1. Practically zero
  2. 0.4
  3. 0.525
  4. 1

Correct answer: 1. Practically zero

A 3-phase slip-ring induction motor develops starting torque at slip of 0.04 with slip-rings short-circuited

A 3-phase slip-ring induction motor develops starting torque at slip of 0.04 with slip-rings short-circuited. If rotor resistance is increased to 6 times by inserting external resistance in its rotor circuit, then at rated at rated torque, the slip would be:

  1. 0.12
  2. 0.24
  3. 0.52
  4. 0.68

Correct answer: 2. 0.24

Solution:

The rated torque of 3-phase induction motor

T = KsR2/(R2² + s² X2²)

——————-

At normal speed s is so small that sX2 is neglible as compared to R2.

Therefore,

T∝s/R2

T/T’ = (0.04/R2) * (6R2/ s)

1 = 0.24/s or s = 0.24

When rotor of three phase induction motor is blocked, the value of slip is

When rotor of three phase induction motor is blocked, the value of slip is:

  1. 0.1
  2. 0.5
  3. 1
  4. Zero

Correct answer: 3. 1

Explanation: When rotor is blocked N = 0 and s = Ns/Ns = 1

A 3-phase has 4 pole star-connected stator winding and runs on 230 V, 60 Hz supply

A 3-phase has 4 pole star-connected stator winding and runs on 230 V, 60 Hz supply. The rotor resistance is 0.2 Ω/phase. The standstill reactance is 0.8 Ω/phase. The full load slip and ratio of stator to rotor turns are 4% and 2 respectively. The speed at maximum torque is:

  1. 1220 RPM
  2. 1280 RPM
  3. 1310 RPM
  4. 1350 RPM

Correct answer: 4. 1350 RPM

Solution:

Slip at maximum torque (s) = R2/X2 = 0.2/0.8 = 0.25

Speed at maximum torque (N) = (1-s) * Ns

————————–

Ns = 120f/P = 120 * 60 / 4 = 1800

————————-

N = (1 – s) * Ns = ( 1 – 0.25) * 1800 = 1350 RPM

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