# Electrical Machines Questions and Answers – Rotor Frequency

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Rotor Frequency”.

1. Slip is defined as _________ (Ns as the synchronous speed and Nr is the rotor speed)
a) Ns-Nr/Ns
b) Ns-Nr/Nr
c) Nr-Ns/Ns
d) Ns-Nr

Explanation: Slip is the ratio of the relative speed of between the stator mmf and the rotor speed to the synchronous speed of the machine.

2. For a 4 pole three phase induction motor having synchronous speed of 1500 rpm is operating at 1450rpm. The frequency of the induced emf in rotor is?
a) 100 Hz
b) 50 Hz
c) 150 Hz
d) 0 Hz

Explanation: The induced frequency will be f = P(Ns-Nr)/2.
f = 4(1500-1450)/2 = 100 Hz.

3. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. The number of poles of this motor is?
a) 4
b) 6
c) 5
d) 2

Explanation: P = 120f/N = 120*50/1440 = 4(1/6). The number of the poles should be whole number and must be even. So number of poles must be 4.

4. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. The full load slip will be?
a) 4%
b) 5%
c) 2.4%
d) 3%

Explanation: s = 1500-1440/1500 = 4%.

5. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Rotor frequency is?
a) 2 Hz
b) 50 Hz
c) 52 Hz
d) 58 Hz

Explanation: s = 0.04, rotor frequency = s*f = 0.04*50 = 2 Hz.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!

6. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the stator filed with respect to stator structure is?

Explanation: Speed of the stator filed with respect to stator structure = Ns = 1500 rpm = 157 rad/s.

7. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the stator field revolving rotor structure is?

Explanation: Speed of the stator field revolving rotor structure = 1550-1440 = 60 rpm = 6.28 rad/s.

8. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the rotor field w.r.t. rotor structure is?

Explanation: Speed of the rotor field w.r.t. rotor structure = 120*rotor frequency/Poles = 120*2/4 = 60 rpm = 6.28 rad/s.

9. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the rotor field w.r.t. stator field is?
a) zero

Explanation: As both the rotor field and stator file rotate at synchronous speed, the relative speed between them is zero.

10. A properly shunted center zero galvanometer is connected in the rotor circuit of a 6-pole, 50 hz wound rotor induction motor. If the makes 90 complete oscillations in one minute, the rotor speed is?
a) 1000 rpm
b) 970 rpm
c) 950 rpm
d) 930 rpm

Explanation: Rotor frequency = s*f1 = 90/60 = 1.5 Hz.
s = 1.5/50 = 0.03
rotor speed = 1000(1-0.03) = 970 rpm.

11. A 4-pole, 3-phase slip ring induction motor is used as a frequency changer. Its stator is excited from 3-phase, 50 Hz supply. A load requiring 3-phase, 20 Hz supply is connected to the star-connected rotor through three slip-rings of SRIM. At what two speeds the prime mover should drive the rotor of this SRIM?
a) 900 rpm
b) 600 rpm
c) 1500 rpm
d) 0

Explanation: Ns = 1500 rpm. Speed of the rotor field with respect to rotor structure = 120 * f2/P = 120*20/4 = 600 rpm
Nr = 1500-600
= 900 rpm.

12. For a 4-pole, 3-phase induction motor the rotor speed found is 2100 rpm at 50 Hz. Then the phase sequence of the emf generated will be?
a) acb
b) abc
c) no emf will be induced
d) cba

Explanation: Stator field will be running at the speed of 600 rpm anticlockwise wrt to rotor. So the phase sequence generated at the slip rings is reversed.

13. Stator flux induces emf in the rotor conductors _________
a) magnitude depending on the load
b) rotating at synchronous speed around stator
c) constant in magnitude
d) none of the mentioned

Explanation: The emf will be induced based on the slip which depends on the load connected to the rotor.

14. The 3-phase induction motor with rotor circuit open will _________
a) not run
b) run normally
c) get over heated
d) make noise

Explanation: It will not run because there will not be any closed path for the flux in rotor circuit of the machine to generate emf.

15. The rotor of th 3-phase induction motor rotates in the same direction as that of stator field. This can be explained by _________
a) Newton’s laws of motion
b) Farady’s laws of electromagnetic induction
c) Lenz’s law
d) Fleming’s right hand rule

Explanation: Newtons’s law explains the over all behaviour of the inertia involved in the machine and a unidirectional torque generated.

16. In a three phase slip ring induction motor, brushes are connected to _________
a) external star connected resistors
b) dc supply
c) 3-phase ac supply
d) any of the mentioned

Explanation: The rotor external resistances must e connected in star and at the external end of the rotor.

17. The rotor winding for 3-phase slip ring induction motor having delta connected stator must be connected in _________
a) star
b) delta
c) tertiary
d) any of the mentioned

Explanation: We should use star connected resistors as the initial requirement is low resistance for high starting torque.

18. The squirrel cage induction motor of 6-pole can be used as ________ induction motor.
a) any number of poles
b) 6 pole
c) 6 or 12 pole
d) integral number of 4 poles

Explanation: The poles in a squirrel cage induction motor can be adjusted.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]