# Mechatronics Questions and Answers – Electrical Actuating Systems – Induction Motor Speed Control

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This set of Mechatronics Multiple Choice Questions & Answers (MCQs) focuses on “Electrical Actuating Systems – Induction Motor Speed Control”.

1. What is the synchronous speed of a 3-phase induction motor if has 12 poles and its frequency is 60 Hz?
a) 700 rpm
b) 600 rpm
c) 650 rpm
d) 500 rpm

Explanation: Given:
Number of poles of motor(P)=12
Frequency(f)=60 Hz
Synchronous speed(Ns) = 120f/P
Synchronous speed(Ns) = 600 rpm.

2. What is the speed of a 3-phase induction motor if has a synchronous speed of 500 rpm and has percentage slip of 4% ?
a) 700 rpm
b) 600 rpm
c) 480 rpm
d) 500 rpm

Explanation: Given:
Synchronous speed(Ns) = 500 rpm
percentage slip=4% =0.04
Speed(N) = Ns*(1-s)
Speed(N) = 480 rpm.

3. What is the frequency of a 3-phase induction motor if has 10 poles and its synchronous speed is 720 rpm?
a) 60 Hz
b) 50 Hz
c) 80 Hz
d) 40 Hz

Explanation: Given:
Number of poles of motor(P)=10
Synchronous speed(Ns) = 720 rpm
Synchronous speed(Ns) = 120f/P
Frequency (f)=(Ns*P)/120
Frequency(f)=60 Hz.

4. Speed of an induction motor is dependent on the frequency of the input supply.
a) True
b) False

Explanation: Speed of an induction motor is dependent on the frequency of the input supply. Since the formula for synchronous speed is given by Ns = 120*f/P, where Ns represents the synchronous speed, f represents the frequency and P is the number of poles. And the Speed of induction motor is given by N = Ns*(1-s) , where N represents the speed of induction motor, Ns represents the synchronous speed and s is the slip. Therefore it can be clearly seen that it is dependent on frequency of input supply.

5. Speed of an induction motor is independent of the number of stator poles.
a) True
b) False

Explanation: Speed of an induction motor is dependent of the number of stator poles. Since the formula for synchronous speed is given by Ns = 120*f/P, where Ns represents the synchronous speed, f represents the frequency and P is the number of stator poles. And the Speed of induction motor is given by N = Ns*(1-s) , where N represents the speed of induction motor, Ns represents the synchronous speed and s is the slip. Therefore it can be clearly seen that it is dependent on the number of stator poles.

6. Rotor rheostat speed control method can be used for controlling the speed of squirrel cage motors.
a) True
b) False

Explanation: Rotor rheostat speed control method cannot be used for controlling the speed of squirrel cage motors. It can only be used with the slip ring motors as an external resistance cannot be added to the squirrel cage motors.

7. What happens to the synchronous speed of the induction motor if the number of stator poles increases?
a) It remains same
b) It increases
c) It decreases
d) It does not work

Explanation: The synchronous speed of the induction motor decreases if the number of stator poles increases. Since the formula for synchronous speed is given by Ns = 120f/P, where Ns represents the synchronous speed, f represents the frequency and P is the number of poles. Therefore it can be clearly seen that, synchronous speed inversely proportional to the number of stator poles.

8. What happens to the synchronous speed of the induction motor if the frequency of the supply increases?
a) It remains same
b) It increases
c) It decreases
d) It does not work

Explanation: The synchronous speed of the induction motor increases if the frequency of the supply increases. The synchronous speed of the induction motor is directly proportional to the input frequency. Since the formula for synchronous speed is given by Ns = 120f/P, where Ns represents the synchronous speed, f represents the frequency and P is the number of poles. So if the frequency of the supply increases, then the synchronous speed also increases.

9. Which speed control method can only be applied to squirrel cage type induction motors?
a) Multiple Stator winding method
b) Rotor rheostat speed control method
c) Pole amplitude modulation method
d) Frequency control method

Explanation: Multiple stator winding method can only be applied to squirrel cage type induction motors. Other all speed control methods can also be applied in slip ring type induction motors. Rotor rheostat speed control method can only be used for controlling the speed of slip ring type induction motors.

10. What does PAM stands for, in context to speed control method of induction motor?
a) Pole amplitude maintenance
b) Passive amplitude modulation
c) Pole amplification modulation
d) Pole amplitude modulation

Explanation: PAM stands for Pole amplitude modulation, in context to speed control method of induction motor. It is a speed control method of 3 phase induction motor in which the original sinusoidal wave is modulated by another sinusoidal wave having the different number of poles.

11. What is the synchronous speed of a 3-phase induction motor if has 8 poles and its frequency is 40 Hz?
a) 700 rpm
b) 600 rpm
c) 650 rpm
d) 500 rpm

Explanation: Given:
Number of poles of motor(P)=8
Frequency(f)=40 Hz
Synchronous speed(Ns) = 120f/P
Synchronous speed(Ns) = 600 rpm.

12. What is the synchronous speed of a 3-phase induction motor if has 9 poles and its frequency is 50 Hz?
a) 700 rpm
b) 600 rpm
c) 650 rpm
d) 666.667 rpm

Explanation: Given:
Number of poles of motor(P)=9
Frequency(f)=50 Hz
Synchronous speed(Ns) = 120f/P
Synchronous speed(Ns) = 666.667 rpm.

13. What is the speed of a 3-phase induction motor if has a synchronous speed of 400 rpm and has percentage slip of 5% ?
a) 700 rpm
b) 380 rpm
c) 480 rpm
d) 500 rpm

Explanation: Given:
Synchronous speed(Ns) = 400 rpm
percentage slip=5%=0.05
Speed(N) = Ns*(1-s)
Speed(N) = 380 rpm.

14. What is the frequency of a 3-phase induction motor if has 12 poles and its synchronous speed is 620 rpm?
a) 60 Hz
b) 62 Hz
c) 80 Hz
d) 58 Hz

Explanation: Given:
Number of poles of motor(P)=12
Synchronous speed(Ns) = 620 rpm
Synchronous speed(Ns) = 120f/P
Frequency (f)=(Ns*P)/120
Frequency(f)=62 Hz.

15. What is the speed of a 3-phase induction motor if has a synchronous speed of 300 rpm and has percentage slip of 0.2% ?
a) 200 rpm
b) 380 rpm
c) 280 rpm
d) 294 rpm

Explanation: Given:
Synchronous speed(Ns) = 300 rpm
percentage slip=5%=0.02
Speed(N) = Ns*(1-s)
Speed(N) = 294 rpm.

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