Working Principle & Slip of Three Phase Induction Motor - MCQs with Answers

Working Principle & Slip of Three Phase Induction Motor - MCQs with Answers

Q1. “The direction of induced current in the rotor is so as to oppose the cause producing it”. This is given by

a. Faraday’s law
b. Lenz’s law
c. Electromagnetic law
d. Ampere’s law

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Q2. If the rotor of the machine catches the speed of the rotating magnetic field, then the motor will

a. Rotate at the speed of rotating magnetic field
b. Rotate at double the speed of the rotating magnetic field
c. Eventually stop
d. None of these

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Q3. Slip speed of the motor decides the magnitude of the induced emf and the rotor current, which in turn decides the torque produced. If N_s is the synchronous speed and N is the motor speed in rpm, then the sip speed is given by

a. N_s
b. N_s – N
c. N_s + N
d. N-N_s

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Q4. At star, the slip of the induction motor is

a. 1
b. 0
c. 0.5
d. None of these

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Q5. Slip of a induction motor will be zero,

a. At start
b. When machine is rotating at its maximum speed
c. When it is rotating at half of the maximum speed
d. Cannot be zero

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Q6. A 4 pole, 3 phase, 50 Hz star connected induction motor has a full load slip of 5%. Then full load speed of the motor is

a. 1500
b. 1400
c. 1405
d. 1425

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Q7. A 50 Hz, 3 phase slip ring induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1400 rpm
d. Machine will not run at all

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Q8. A 3 phase, 50 Hz squirrel cage induction motor, has 6 poles on stator and 4 poles on rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1750 rpm
d. Machine will not run

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Q9. The rotor frequency of a three phase induction motor is

a. Slip times the supply frequency
b. 1 / slip times the supply frequency
c. Equal to the supply frequency
d. None of these

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Q10. The rotor resistance per phase on standstill and rotor reactance per phase on standstill of induction motor is

a. Slip times the rotor resistance at standstill and slip times rotor reactance at standstill
b. Same as rotor resistance at standstill and slip times rotor reactance at standstill
c. Same as rotor resistance at standstill and same as rotor reactance at standstill
d. Slip times the rotor resistance at standstill and same as rotor reactance at standstill

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Q11. Nature of the rotor power factor in running condition is always

a. Leading
b. Lagging
c. Either leading or lagging
d. Neither leading nor lagging

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Q12. In terms of slip, the actual speed (N) of the induction motor can be expressed as

a. N_s (1-s)
b. N_s (s-1)
c. N_s / (1-s)
d. N_s / (s-1)

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Q13. If a voltmeter when connected to the rotor of an induction motor gives 150 oscillations per minute and stator frequency is 50 Hz. Then the slip of induction motor will be

a. 3%
b. 4.5%
c. 5%
d. 5.5%

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Q14. For a three phase induction motor, the two rotor frequencies is given by

a. sf , (1-s) f
b. sf , (2-s) f
c. (1-s) f , (2-s) f
d. Sf , sf

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