Electrical Machines Questions and Answers – Induction Motor as a Transformer

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motor as a Transformer”.

1. An induction motor can be said analogous to _________
a) transformer
b) synchronous motor
c) universal motor
d) stepper motor

Explanation: An induction motor has similar operation as a transformer.

2. A 3-phase induction motor with its rotor blocked behaves similar to a _________
a) transformer under short circuit of secondary terminals
b) transformer under open circuit of secondary
c) synchronous motor under slip test
d) synchronous motor under open circuit

Explanation: It is analogous to transformer under the shorted terminals of the secondary.

3. No load current in induction motor is 10-20% of full load current and the no load current of transformer is 2-6%.
a) True
b) False

Explanation: The air gap in induction motor is more prominent than transformer and so the flux requirement will also be more.
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4. The rated current in induction motor for a three phase system is 100A. What can be the no load estimated current for the machine?
a) 12 A
b) 20 A
c) 30 A
d) 5 A

Explanation: No load current in induction motor is 10-20% of full load current.

5. The no load current of the transformer is very less due to _________
a) mutual flux having low reluctance iron core
b) mutual flux having high reluctance iron core
c) leakage flux having low reluctance iron core
d) leakage flux having high reluctance iron core

Explanation: It is due to the low reluctance path, the flux requirement is low in the transformer.
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6. The no load current of the induction motor is high due to _________
a) long and high reluctance path between stator and rotor
b) mutual flux having moderate reluctance path between stator and rotor
c) leakage flux having low reluctance iron core
d) leakage flux having high reluctance iron core

Explanation: It is due to the higher reluctance path, the flux requirement is low in the induction motor.

7. At no load induction motor has possible power factor as _________
a) 0.2
b) 0.5
c) 0.65
d) 0

Explanation: At no load the lower power factor is low and lagging in nature.

8. Mechanically air gaps in induction motor are kept very low to avoid _________
a) lower power factor
b) lagging nature
c) magnetizing current
d) all of the mentioned

Explanation: Air gap is kept lower to avoid the low value power factor, lagging behaviour and to reduce the magnetizing current.

9. The low no load power factor _________
a) reduces full load operating pf
b) increases full load operating pf
c) reduces full load excitation voltage
d) increases full load excitation voltage

Explanation: The low no-load power factor reduces full load operating pf.

10. An induction motor when started on load, it does not accelerate up to full speed but runs at 1/7th of the rated speed. The motor is said to be _________
a) Locking
b) Plumming
c) Crawling
d) Cogging

Explanation: Running stably at low speed around 1/7th of normal motor speed is known as crawling.

11. The great advantage of the double squirrel-cage induction motor over single cage rotor is that its _________
a) efficiency is higher
b) power factor is higher
c) slip is larger
d) starting current is lower

Explanation: The outer cage has higher resistance and low reactance while vice versa for the inner cage. So at starting the current is confined to outer cage with the reduction in starting current improving the torque.

12. The rotor of 3-phase slip ring induction motor is fed from a 3-phase supply with its stator winding short circuited having rotor rotating clockwise at a speed of Nr, then the _________
a) speed of air gap field w.r.t. stator is Ns-Nr anticlockwise
b) speed of air gap field w.r.t. stator is Ns-Nr clockwise
c) speed of airgap field w.r.t rotor is Ns clockwise
d) speed of airgap field w.r.t. stator is Ns-Nr clockwise

Explanation: The air gap field of stator = -(Ns-Nr).

13. (I) Even at no load a large 3-phase squirrel- cage induction motor is started at reduced voltage than rated.
(II) If a large 3–phase squirrel-cage induction motor with no load is started at full voltage, it will be damaged.
a) I is true, II is false
b) I is true and II is also true
c) I is false, II is true
d) I and II are false

Explanation: Large current of short duration are not harmful to induction motor but it may cause voltage drop in the power supply.

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