Electrical Machines Multiple Choice Questions 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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%
Answer: A
Clarification: 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
Answer: A
Clarification: s = 0.04, rotor frequency = s*f = 0.04*50 = 2 Hz.
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
Answer: A
Clarification: 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
Answer: C
Clarification: 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?
A. 157 rad/s
B. 150 rad/s
C. 6.28 rad/s
D. 145 rad/s
Answer: C
Clarification: 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
Answer: A
Clarification: 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
Answer: B
Clarification: 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