Electrical Machines Questions and Answers – Rotor Frequency

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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
View Answer

Answer: a
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.
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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
View Answer

Answer: a
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
View Answer

Answer: a
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.
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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%
View Answer

Answer: a
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
View Answer

Answer: a
Explanation: s = 0.04, rotor frequency = s*f = 0.04*50 = 2 Hz.
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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?
a) 157 rad/s
b) 150 rad/s
c) 6.28 rad/s
d) 145 rad/s
View Answer

Answer: a
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?
a) 157 rad/s
b) 150 rad/s
c) 6.28 rad/s
d) 145 rad/s
View Answer

Answer: c
Explanation: Speed of the stator field revolving rotor structure = 1550-1440 = 60 rpm = 6.28 rad/s.
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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?
a) 157 rad/s
b) 150 rad/s
c) 6.28 rad/s
d) 145 rad/s
View Answer

Answer: c
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
b) 150 rad/s
c) 6.28 rad/s
d) 145 rad/s
View Answer

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

Answer: b
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: a
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.

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