# Electric Drives Questions and Answers – Induction Motors – Regenerative Braking (3-phase IM)

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motors – Regenerative Braking (3-phase IM)”.

1. Power input to an induction motor is given by _________
a) 3VpIpcosΦ
b) 2VpIpcosΦ
c) VpIpcosΦ
d) 5VpIpcosΦ

Explanation: Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connection. The power is constant and has no vibrations.

2. For motoring mode, the phase angle between the stator phase voltage and stator phase current should be _______
a) < 90°
b) > 90°
c) < 40°
d) 180°

Explanation: For motoring mode, the phase angle between the stator phase voltage and stator phase current should be < 90°. The rotor magnetic field tries to catch up the stator magnetic field.

3. The relative speed between the stator magnetic field and rotor in the induction motor is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0

Explanation: In induction motor rotor tries to catch up the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. Stator magnetic field rotates at the speed of synchronous speed.

4. The relative speed between the stator magnetic field and rotor in the synchronous generator is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0

Explanation: In synchronous generator rotor and stator magnetic field rotates at speed of synchronous speed in order to produce the steady state torque.

5. The relative speed between the stator magnetic field and stator in the induction motor is ______
a) Ns
b) Ns-Nr
c) Ns+Nr
d) 0

Explanation: In Induction motor stator magnetic field rotates at the speed of synchronous speed. The stator is made up of Silicon steel and remains at rest. The relative speed is Ns-0 = Ns.

6. The relative speed between the rotor and stator in the induction motor is ______
a) Nr
b) Ns-Nr
c) Ns+Nr
d) 0

Explanation: In Induction motor rotor tries to catch up the stator magnetic field in order to satisfy Lenz law. Effect opposes the cause. Stator remains at rest. The relative speed is Nr-0=Nr.

7. What will happen if we increase the air gap in the induction motor?
a) Power factor will reduce
b) Power factor will increase
c) Reduction in harmonics
d) Speed will increase

Explanation: If we increase the air gap in the induction motor, the machine will draw more magnetizing current to maintain flux in the air gap. The power factor of the machine will reduce.

8. The ratio of the rotor copper loss and air gap power is _______
a) s
b) 1-s
c) 1+s
d) 0

Explanation: The ratio of the rotor copper loss and air gap power is s. The air gap power in the induction motor is Pg and the rotor copper loss is s×Pg. The ratio is s.

9. Magnetizing current in the induction motor is ______
a) more than the transformer
b) equal as a transformer
c) less than the transformer
d) 0

Explanation: The magnetizing current in the induction motor is (23-25)% of the full load current. The magnetizing current in the transformer is 3-5% of the full load.

10. Single phase induction motor is self-starting.
a) True
b) False

Explanation: The single-phase induction motor is not self-starting because of the equal and opposite forward & backward magnetic field. The net torque developed is zero.

11. What is the speed of the 6th order time harmonics in the induction motor?
a) Ns
b) Ns/6
c) 6×Ns
d) 0

Explanation: The time harmonics are present in the supply voltage. The speed of the 6th order time harmonics in the induction motor is Ns/6.

12. What is the speed of the 11th order harmonics in the induction motor?
a) Ns
b) Ns/11
c) 2×Ns
d) Nr

Explanation: The speed of the 6k±1 is Ns÷6k±1. The speed of the 11th order time harmonics in the induction motor is Ns/11.

13. A 3 – phase, 4 pole IM is supplied from a 40 Hz source. Calculate rotor frequency when the rotor runs 100 rpm.
a) 35 Hz
b) 36.6 Hz
c) 9 Hz
d) 8 Hz

Explanation: The slip of the induction motor can be calculated using the relation s = Ns-Nr÷ Ns. Ns=120F÷P=1200 rpm. The value of slip is .91. The rotor frequency is 36.6 Hz.

14. Calculate the three-phase power using the following data: Vp=23 V, Ip=23 A, ∅=60.
a) 793.5 W
b) 790 W
c) 792 W
d) 791 W

Explanation: Power input to a 3-Φ induction motor is 3VpIpcosΦ. The power remains the same for Delta and star connection. P=3×23×23×co60°=793.5 W.

Sanfoundry Global Education & Learning Series – Electric Drives.

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