Induction Motor MCQs Part 8

Induction Motor MCQs Part 8. Multiple choice questions on Induction motor with brief explanatory answers and solutions.

If one of the lines of a 3 phase induction motor is interchanged, next to that

If one of the lines of a 3 phase induction motor is interchanged, next to that:

1. Flux speed increases
2. Flux speed decreases
3. Flux speed remains keeping same direction
4. Flux speed remains same but direction is reversed

Correct answer: 4. Flux speed remains same but direction is reversed

The magnetizing current drawn by 3-phase induction motor is about what percentage of full-load stator current

The magnetizing current drawn by 3-phase induction motor is about what percentage of full-load stator current:

1. 5%
2. 10 to15%
3. 15 to 30%
4. 30 to 50%

Correct answer: 4. 30 to 50%

The conditions of an induction motor at no-load closely resemble to those of a transformer whose secondary is

Correct answer: 2. Open-circuited

Under running conditions, the maximum torque of 3-phase induction motor will happen at that value of slip at which rotor resistance/phase is X times the rotor reactance/phase. Here X implies

Under running conditions, the maximum torque of 3-phase induction motor will happen at that value of slip at which rotor resistance/phase is X times the rotor reactance/phase. Here X implies:

1. 1 time (equal)
2. 5 times
3. 10 times
4. 15 times

Correct answer: 1. Time (equal)

What is value of supply frequency of a 4-pole induction motor that has a synchronous speed of 1500 RPM

What is value of supply frequency of a 4-pole induction motor that has a synchronous speed of 1500 RPM:

1. 0 Hz
2. 50 Hz
3. 60 Hz
4. 100 Hz

Correct answer: 2. 50 Hz

From formula: Speed = 120*f/P; f = Speed * P/120 = 1500*4/120 = 50 Hz

If N and Ns are speeds of rotor and rotating fields of a 3-phase induction motor respectively. Then ratio rotor input/rotor output is expressed in the form

If N and Ns are speeds of rotor and rotating fields of a 3-phase induction motor respectively. Then ratio rotor input/rotor output is expressed in the form:

1. Ns/N
2. N/Ns
3. N-Ns
4. Ns-N

Correct answer: 1. Ns/N

Speed of a squirrel cage induction motor can be changed only by

Speed of a squirrel cage induction motor can be changed only by:

1. Rheostatic control
2. Cascade control
3. Changing poles
4. None of the above

Correct answer: 3. Changing poles

A 3-phase induction motor having synchronous speed of 1000 RPM develops 10 mechanical HP at an RPM of 950. If stator loss is 500 W then, What is stator input

A 3-phase induction motor having synchronous speed of 1000 RPM develops 10 mechanical HP at an RPM of 950. If stator loss is 500 W then, What is stator input:

1. 4.5 kW
2. 8.35 kW
3. 12.75 kW
4. 15.25 kW

Correct answer: 2. 8.35 kW

Solution:

Stator loss in kW = 500 W = 0.5 kW

Slip (s) = (Ns – N)/Ns = (1000 – 950)/1000 = 0.05

Rotor input = (Mechanical HP * 0.746)/(1-s) = 7.85 kW

Stator input = Rotor input + Stator loss = 7.85 kW + 0.5 kW = 8.35 kW

Rotor copper losses of a 3-phase induction motor running at some decimal slip s are mathematically expressed in the form

Rotor copper losses of a 3-phase induction motor running at some decimal slip s are mathematically expressed in the form:

1. s* rotor input
2. 1-s * rotor input
3. 1+s * rotor input
4. None of these

Correct answer: 1. s * rotor input

What is frequency of induced EMF in a 4-pole, 60 Hz induction motor operates at 5% slip

What is frequency of induced EMF in a 4-pole, 60 Hz induction motor operates at 5% slip:

1. 30Hz
2. 60 Hz
3. 3 Hz
4. None of these

Correct answer: 3. 3 Hz

Solution: Induced rotor frequency (f’) = s* supply frequency (f)

f’ = 0.05 * 60 = 3 Hz