# 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

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

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°

Explanation: The power factor angle would be atan(0.2/0.2) = 45°.

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

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°

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.

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

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

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.

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

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

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)

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

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