This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Power Factor Control of Three Phase Induction Motors”.
1. The 3-phase induction motor operates at lagging power factor due to ________
I. Leading magnetizing current necessary to produce the working flux. II. Voltage drops in the leakage impedance. III. Lagging magnetizing current which lags by 90° to the voltage
a) II, III
b) I, II
d) I, II, III
Explanation: Magnetizing current is lagging in nature and also the leakage impedance increases the power factor angle.
2. Choose the most appropriate option regarding the power factor control of the induction motor.
a) A: with capacitor
b) B: with capacitor
c) A: with capacitor, B: with capacitor
d) A: with no capacitor, B: with capacitor
Explanation: Leading stator current shifts to the left.
3. Which tests are to be conducted to draw circle diagram for a given induction motor?
A. Running light test B. Blocked rotor test C. Voltmeter-Ammeter test
a) A, B, C
b) A, B
c) B, C
Explanation: All the three tests results are needed to plot the circle diagram.
4. Which tests are to be conducted to draw circle diagram for a given induction motor?
A. Running light test B. Slip test C. Voltmeter-Ammeter test
a) A, C
b) A, B, C
c) B, C
d) A, B
Explanation: Slip test is conducted on the synchronous machine.
5. If an unbalanced supply is fed to an induction motor, it will cause excessive heating of ____________
c) rotor shaft
d) all of the mentioned
Explanation: Unbalanced supply will cause most heating of rotor winding than any other part.
6. If one of the three phases of the induction motor fails due to fault. The motor will ___________
a) stop and do permanent damage if it was running overloaded
b) continue running but will draw more current if it was operating at rated load
c) will not be affect the operation if light loaded
d) any of the mentioned
Explanation: All the possibilities can happen if one of the phase is removed.
7. Which of the following will not happen if one of the phases to the induction motor is not available?
a) Motor will start but operate at lower speed
b) It will hum but not start
c) It will continue to operate below 57.7% of rated load
d) External means are needed to make it run at rated speed
Explanation: Motor will not start if one of the phases is missing.
8. Induction motor can be fed from ___________
a) either rotor or stator
d) neither stator nor rotor
Explanation: It can be fed from either side at one time.
9. In double cage induction motor, inner cage has ___________
a) small resistance and large reactance
b) high resistance and high reactance
c) small resistance and small reactance
d) high resistance and small reactance
Explanation: In the bottom cage, the air gap is less, so the leakage reluctance is less and more the leakage reactance.
10. Static capacitors are used to control power factor of slip ring induction motor only.
Explanation: Static capacitors are also used to improve power factor of the slip ring as well as wound rotor induction motors.
11. A 3-phase, 400 V, 50 Hz delta connected induction motor has power factor of 0.35 when delivering 30% of its load. If it is star connected, then its?
a) p.f. increased and the stator current decreases
b) p.f. worsened and the stator current decreases
c) p.f. increased,and the stator current increases
d) p.f. does not change,and the stator current increase
Explanation: In star connection, resistance decrease than delta connected.
12. A 3-phase induction machine is operating on 3-phase fixed frequency ac mains at a per unit slip of 1.5. Then which of the following statements follow the working condition?
I. It draws electrical power from the mains. II. It draws mechanical power through the shaft. III. It delivers electrical power to the mains. IV. It delivers mechanical power through the shaft.
a) I, IV
b) I, II
c) II, III
d) III, IV
Explanation: The slip is positive so it is operating as a motor and in the braking mode.
13. A 3-phase induction machine is operating on 3-phase fixed frequency ac mains at a per unit slip of 1.5. Then machine is operating in generating mode.
Explanation: It is braking mode.
14. As per the condition given, if a 3-phase squirrel-cage induction motor is operating at a slip of -0.05 then machine will?
a) draw mechanical strength and deliver electrical power to mains
b) draw electrical power from mains and deliver mechanical strength to shaft
c) draw electrical power from shaft and deliver mechanical strength through mains
d) any of the mentioned
Explanation: It is in generating mode as the slip is negative.
15. If the induction motor connected to a power grid draws real power P and reactive power Q. If it is made to run as a generator, then P and Q will be?
a) negative, positive
b) positive, positive
c) positive, negative
d) negative, negative
Explanation: Induction motor will always need reactive power as it is single excited machine.
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