# Electrical Machines Questions and Answers – Rotor EMF, Current and Power

This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Rotor EMF, Current and Power”.

1. Rotor leakage reactance for a three phase slip ring induction motor with slip ‘s’ is?
a) s(x2)
b) (x2)/s
c) x2
d) (x2)(s2)

Explanation: It is s*x2.

2. In an induction motor, when the number of stator slots is equal to an integral multiple of rotor slots?
a) machine will fail to start
b) machine will get heated
c) high starting torque will be achieved
d) discontinuity in torque slip characteristic

Explanation: Machine fails to start as the stator slots and rotor slots become equal.

3. Two of the supply terminals to a three phase induction motor gets interchanges while regular scheduling work. When the machine is switched on, then it will _________
a) rotate in opposite direction
b) rotate in same direction
c) not start
d) get heated and winding will burn

Explanation: As the supply phase sequence is reversed, the machine’s developed emf will also be reversed.

4. The rotor of the induction motor must never run at synchronous speed because then the relative speed between the rotating flux and rotor will be _________
a) zero and hence torque will be zero
b) zero, and torque will be maximum
c) maximum and hence, torque will be zero
d) maximum, and torque will also be zero

Explanation: At synchronous speed the slip will be zero and so the torque.

5. The voltage actually used to set up the working flux in the three phase induction motor is _________
a) equal to applied voltage
b) less than applied voltage
c) more than applied voltage
d) equal to rotor induced emf.

Explanation: The actual working flux requirement is met by the applied voltage and it is same as the applied voltage to it.

6. If the three phase supply is fed to the stator and it is running at its normal operating conditions. Then the synchronous speed can be defined as speed at which?
a) stator magnetic field rotates
b) rotor magnetic field rotates
c) rotor rotates
d) flux is set up in the rotor

Explanation: If it is fed from stator then the synchronous speed is also w.r.t. stator.

7.If the three phase supply is fed to the rotor and it is running at its normal operating conditions. Then the synchronous speed can be defined as speed at which?
a) stator magnetic field rotates
b) rotor magnetic field rotates
c) rotor rotates
d) flux is set up in the rotor

Explanation: If it is fed from rotor then the synchronous speed is also w.r.t. rotor.

8. The mmf produced by the current of the three phase induction motor _________
a) is standstill with respect to stator mmf
b) rotates at the speed of rotor in air gap
c) rotates at slip speed with respect to stator mmf
d) rotates at synchronous speed with respect to rotor

Explanation: The mmf produced will also be at synchronous speed and it will be stationary with respect to stator mmf which itself rotates at synchronous speed.

9. In an induction motor slip will be negative when the _________
a) rotor rotates at speed more than synchronous speed and in the direction of the rotation of stator field
b) stator magnetic field and rotor rotate in opposite direction
c) rotor rotates at speed less than synchronous speed and in the opposite direction of the rotation of stator field
d) rotor rotates at speed more than synchronous speed and in the opposite direction of the rotation of stator field

Explanation: s = (Ns-Nr)/Ns; Slip is negative when rotor rotates at speed more than synchronous speed and in the direction of the rotation of stator field.

10. A 3-phase induction motor is operating at slip ‘s’. If the slip is reversed, its slip at the instant will be _________
a) 2-s
b) zero
c) 2+s
d) 1-s

Explanation: For the reversed phase sequence the slip becomes 2-s.

11. In a three phase induction motor, voltage between the slip rings at standstill is 50 V. At full load the slip is 0.04. The voltage between slip rings at full load is?
a) 2V
b) 50V
c) 20V
d) 5V

Explanation: E2 = s*E = 0.04*50 = 2V.

12. A three phase induction motor is connected to 400V, 50 hz supply. If the stator to rotor turn ratio is 2, the standstill rotor induced voltage per phase is?
a) 115.5 V
b) 231 V
c) 346 V
d) 200 V

Explanation: Emf at secondary per phase = (400/1.73)/2 = 115.47 V.

13. The torque developed in the three phase induction motor depends on _________

```(i) standstill rotor phase emf
(ii) rotor power factor
```

a) (i) only
b) (ii) only
c) both (i) and (ii)
d) none of the mentioned

Explanation: The electromagnetic torque developed in the ac machine depends on the rotor emf and the power factor.

14. If a 400 V, 50 hz star connected, 3-phase squirrel cage induction motor is operated from 400V and 75 Hz supply. The torque that the motor can now provide while drawing rated current from the supply will _________
a) reduce
b) increase
c) remains same
d) increases or reduces based on the rotor resistance.

Explanation: The reactance will increase and so the torque will decrease.

15. For a constant load torque, the supply voltage of a squirrel cage induction motor is reduced by a factor of 0.5, its rotor current is modified by a factor is?
a) 2
b) 0.5
c) 4
d) 0.25

Explanation: For constant torque the product of emf and current should be constant.

Sanfoundry Global Education & Learning Series – Electrical Machines.

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