Electrical Machines Questions and Answers – Flux and MMF Phasors and Waves in Induction Motors

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This set of Electrical Machines Questions and Answers for Campus interviews focuses on “Flux and MMF Phasors and Waves in Induction Motors”.

1. A three phase induction motor is not self starting.
a) True
b) False
View Answer

Answer: b
Explanation: A three phase induction motor is self starting. Because air gap flux and rotor mmf produce electromagnetic torque in the direction of rotating magnetic field.
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2. Mark the correct waveform for the emf produced in an induction motor and the resultant flux density wave for a resistive load.
a) Waveform for resultant flux density wave for a resistive load - option a
b) Waveform for resultant flux density wave for a resistive load - option b
c) Waveform for resultant flux density wave for a resistive load - option c
d) Waveform for resultant flux density wave for a resistive load - option d
View Answer

Answer: a
Explanation: The flux will lead the induced current and emf by 90°.

3. Mark the most appropriate phasor for the induction machine operation as motor.
a) Phasor for the induction machine operation as motor - option a
b) Phasor for the induction machine operation as motor - option b
c) Phasor for the induction machine operation as motor - option c
d) none of the mentioned
View Answer

Answer: a
Explanation: The flux will lead the induced current and emf by 90°.
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4. In a three phase induction motor, rotor current in each phase lags behind the generated emf in that phase to rotor by a power factor angle of _________
a) atan(r2/x2)
b) atan(x2/r2)
c) atan(x1/r1)
d) atan(r1/x1)
View Answer

Answer: b
Explanation: The power factor angle is atan(x2/r2).

5. In a three phase induction motor of rotor impedance as (0.2+j0.2), rotor current in each phase lags behind the generated emf in that phase to rotor by a power factor angle of _________
a) 45°
b) 135°
c) 90°
d) 0°
View Answer

Answer: a
Explanation: The power factor angle would be atan(0.2/0.2) = 45°.
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6. For the three phase induction motor having leakage impedance, the maximum generated emf would be attained at an angle of θ having a purely resistive load is?
a) 90 + θ
b) θ
c) 90 – θ
d) -θ
View Answer

Answer: a
Explanation: Due to leakage reactance there will be a lagging of angle θ in the excitation voltage with the main flux. So the maximum is attained at 90°+θ.

7. For a slip ring induction motor having leakage impedance of (1+j), the maximum generated emf would be attained at _________
a) 135°
b) 45°
c) -45°
d) 0°
View Answer

Answer: a
Explanation: Due to leakage reactance there will be a lagging of angle θ in the excitation voltage with the main flux. So the maximum is attained at 90°+θ.
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8. In an induction motor, the stator is also known as _________ and the rotor as __________
a) field winding, armature winding
b) armature winding, field winding
c) armature winding, compensating winding
d) armature winding, interpole winding
View Answer

Answer: a
Explanation: In an induction motor, the stator is also known as field winding, and the rotor as armature winding.

9. For a 6-pole three phase squirrel cage induction motor has flux density wave of four poless, then the rotor induced poles will be?
a) 4
b) 6
c) 2
d) 10
View Answer

Answer: a
Explanation: For a squirrel-cage induction motor the rotor will have the induced poles will be same as flux density poles which are induced inside the machine while working.
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10. The external resistance can be inserted in rotor circuit of _________
a) wound rotor induction motor
b) slip ring induction motor
c) wound rotor as slip ring induction motor
d) neither of motors
View Answer

Answer: a
Explanation: External resistance can be inserted only in the wound rotor as does not get shorted as like squirrel cage motor.

11. Starting torque induced in a three phase induction motor in a direction of rotating magnetic field depends on _________
a) number of poles squared
b) inverse of number of poles
c) number of poles
d) number of number poles cubic
View Answer

Answer: a
Explanation: T = (pi/8).(P2) φ (F2)cos(θ).

12. Starting torque induced in a three phase induction motor in a direction of rotating magnetic field depends on _________

(i) rotor mmf per pole
(ii) rotor power factor angle
(iii)type of rotor winding

a) (i), (ii)
b) (ii), (iii)
c) (i), (iii)
d) (i), (ii), (iii)
View Answer

Answer: a
Explanation: Starting torque induced in a three phase induction motor depends on rotor mmf per pole and rotor power factor angle.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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

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