Induction machine_Objectives

1.If two induction motors A and B are identical except that the air-gap of motor ‘A’ is 50% greater than that of motor ‘B’ then

a. The no-load power factor of A will be better than that of B
b. The no-load power factor of A will be poorer than that of B
c. The core losses of A will be more than those of B
d. The operating flux of A will be smaller than that of B

2.Which of the following statements regarding skewing of motor bars in a squirrel-cage induction motor are correct?
1. It prevents cogging.
2. It produces more uniform torque.
3. It increases starting torque.
4. It reduces motor ‘hum’ during its
operations
Select the correct answer using the codes given below:
a. 2,3 and 4
b. 1,2, and 3
c. 1, 2 and 4
d. 1, 3 and 4


3.The rotor power output of a 3-phase induction motor is 15kW and the corresponding slip is 4%. The rotor copper loss will be

a. 600 W
b. 625 W
c. 650W
d. 700 W

4.A 3-phase wound rotor induction motor, when stated with load connected to its shaft, was found to start but settee down at about half synchronous speed. If the rotor winding as well as the stator winding were star connected, then the cause of the malfunctioning could be attributed to

a. One of the stator phase windings being short-circuited
b. One of the supply fuses being blown
c. One of the rotor phases being open-circuited
d. Two of the rotor phases being open-circuited


5.In the case of a converter- inverter speed control arrangement for an induction motor operating with v/f constant and with negligible stator impedance

a. The maximum torque is independent of frequency
b. The maximum torque is proportional to frequency
c. The slip at maximum torque is proportional to frequency
d. The starting torque is proportional to frequency

6.A 3-phase induction machine draws active power ‘P’ and reactive power ‘Q’ from the grid. If it is operated as a generator, P’ and Q will respectively be
a. positive and negative
b. negative and negative
c. positive and positive
d. negative and positive

7.The deep bar rotor of an induction motor is considered equivalent to a double cage rotor for the purposes of starting and performance. At the time of starting, the current distribution in deep bar rotor slot will be as shown in



8.A 3-phase induction motor fed f a 3-phase voltage controller is suitable for driving loads whose torque is

a. constant irrespective of speed
b. inversely proportional to the square of speed
c. directly proportional to the square of speed
d. inversely proportional to speed



9. A 3-phase induction motor is used as an adjustable speed drive from zero to 2.0 per unit speed using a variable frequency inverter. In the speed range of 1.0 to. 2.0
per unit, in order to ensure satisfactory operation. Which one of the following sets of quantities is to be maintained approximately constant?

a. Voltage and power
b. Flux and torque
c. Voltage and torque
d. Flux and power


10. A 3-phase slip-ring induction motor has chopper controlled resistance in the rotor circuit. Its power factor and efficiency can be characterized respectively as

a. good and poor
b. poor and good
c. good and good
d. poor and poor


11.A 3-phase slip-ring induction motor when started picks up speed but runs stably at about half the normal speed. This is because of

a. unbalance in the supply voltages
b. non-sinusoidal nature of the supply voltage
c. stator circuit asymmetry
d. rotor circuit asymmetry


12. The phenomenon of crawling in a 3-phase cage induction motor may be due to

a. unbalance supply voltages
b. 7th space harmonic of air-gap field
c. 7th time harmonic of voltage wave
d. 5th space harmonic


13. Zero sequence impedance of a 3-phase cage induction motor is due to

a. stator circuit 3rd time harmonic
b. stator circuit 3rd space harmonic
c. rotor circuit 3rd time harmonic
d. rotor circuit 3rd space harmonic


14.Consider the following statements:
Star-delta starter is used in 3-phase induction motor because it
1. prevents heating of the motor windings.
2. ensures permissible minimum starting current.
3. is regulated by electricity authorities.
4. ensures smooth run-up to full speed.

Of these statements :
a. 1, 2 and 3 are correct
b. 2, 3 and 4 are correct
c. 1, 3 and 4 are correct
d. 1 and 2 are correct

15.Match List I with List II and select the correct answer:
List I
A. Single-phase inductor motor
B. 3-phase slip ring induction motor
C. 3-phase squirrel cage induction motor

List II
1. Rotor resistance starting
2. Not self-starting
3. Auto-transformer starting

Codes;
---A B C
a. 2 1 3
b. 1 2 3
c. 2 3 1
d. 3 1 2

16.A 3-phase induction motor is driving a content torque load of 1 Pu at 5% slip. It has a maximum torque of 2 Pu at 10% slip. Torque-speed variation in stable zone is assumed to be linear. For the stable operation of the motor, the minimum pu supply voltage is approximately

a. 0.25
b. 0.50
c. 0.707
d. 0.80

17.100. As compared to DOL starting, a cage induction motor with star-delta starting shall have

a. more starting torque
b. more starting time
c. reduced starting current
d. smoother acceleration


18. The speed of a 3-phase induction motor is controlled from 1 pu to 2 pu using a variable frequency inverter. The equivalent circuit parameter most likely to vary is

a. stator leakage inductance
b. rotor leakage inductance
c. magnetizing inductance
d. core loss resistance

19.‘Crawling’ is an induction motor is due to

a. time harmonics in supply
b. slip ring rotor
c. space harmonics produced y winding currents
d. insufficient starting torque

20.The mmf produced by the current of a 3-phase induction motor

a. rotates at the speed of rotor in the air gap
b. is standstill with respect to stator mmf
c. rotates at slip speed with respect to stator mmf
d. rotates at synchronous speed with respect to rotor


21. A 3-phase slip-ring induction motor is wound for 4 poles on stator and 6 poles on
rotor. When 3-phase balanced voltage source at 50 Hz is applied to the motor, it will run at :

a. 1500rpm
b. 1000rpm
c. 750 rpm
d. zero speed


22. If the rotor power factor of a 3-phase induction motor is 0.866, the spatial displacement between the stator magnetic field and the rotor magnetic field will be

a. 30°
b. 90°
c. 120°
d. 150°


23. The stator of a 6-pole, 3-phase induction motor is fed from a 3phase, 50 Hz supply which contains a pronounced fifth time harmonic. The speed of the fifth space harmonic field produced by the fifth time harmonic in the stator supply will be

a. 200 rpm
b. 1500 rpm
c. 1000 rpm
d. 5000 rpm


24. A 6-pole, 50 Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-ring opencircuited, developed a voltage of 100 V between any wo rings. If the rotor is driven by an external means at 1000 rpm opposite to the direction of stator field, the frequency of voltage across slip-rings will be

a. zero
b. 50 Hz
c. 100 Hz
d. 200 Hz


25. In a 3-phase squirrel cage induction motor, skewing of rotor slots reduces

a. parasitic torque and noise but increases pullout torque
b. parasitic torque and noise but increases starting torque
c. noise but increases pullout torque and parasitic torque
d. parasitic torque, noise, pullout torque and starting torque


26.The 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


27. An induction motor when started on load 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. Pluming
c. Crawling
d. Cogging


28.The rotor power output of 3-phase induction motor is 15 kW The rotor copper losses at a s1p of 4% will be

a. 600W
b. 625W
c. 650W
d. 700W


29. Skewing of the rotor in a three-phase squirrel-cage induction motor reduces

a. Noise, parasitic torque, starting torque and pullout torque
b. Noise and parasitic torque, but increases starting torque and pullout torque.
c. Noise and pullout torque, but increases parasitic torque and starting torque
d. Noise, parasitic torque and starting torque, but increases pullout torque

30.The starting current of a 3 phase induction motor is 5 times the rated current, while the rated slip is 4%. The ratio of starting torque to full-load torque is

a. 0.6
b. 0.8
c. 1.0
d. 1.2