# Electrical Machines Expected MCQ PDF 4 For Coal India Limited (CIL) MT Exam 2017

1)   In squirrel cage rotor, bars are usually brazed to the end rings. The end rings are used to

a. Prevent rotor leakage current
b. Give reduced starting torque
c. Provide good mechanical strength
d. All of these

2)   Slots of squirrel cage rotor are generally skewed to

a. Reduce magnetic hum
b. Reduce magnetic locking
c. Make motor operation smooth
d. All of these

3)   The wound rotor induction motor gives

a. high stating torque
b. moderate starting torque
c. reduced starting torque
d. all of these

4)   The slip ring or wound rotor induction motor find its applications in

a. lifts, cranes
b. lathes
c. drilling machines
d. printing machines

5)   For the speed control of squirrel cage induction motor, the most suitable method is

a. Rotor resistance control
b. Rotor reactance control
c. Stator resistance control
d. Stator reactance control

6)   “The direction of induced current in the rotor is so as to oppose the cause producing it”. This is given by

b. Lenz’s law
c. Electromagnetic law
d. Ampere’s law

7)   If the rotor of the machine catches the speed of the rotating magnetic field, then the motor will

a. Rotate at the speed of rotating magnetic field
b. Rotate at double the speed of the rotating magnetic field
c. Eventually stop
d. None of these

8)   Slip speed of
the motor decides the magnitude of the induced emf and the rotor
current, which in turn decides the torque produced. If Ns is the synchronous speed and N is the motor speed in rpm, then the slip speed is given by

a. Ns
b. Ns – N
c. Ns + N
d. N – Ns

9)   At start, the slip of the induction motor is

a. 1
b. 0
c. 0.5
d. None of these

10)   Slip of a induction motor will be zero,

a. At start
b. When machine is rotating at its maximum speed
c. When it is rotating at half of the maximum speed
d. Cannot be zero

11)   A 4 pole, 3
phase, 50 Hz star connected induction motor has a full load slip of 5%.
Then full load speed of the motor is

a. 1500
b. 1400
c. 1405
d. 1425

12)   A 50 Hz, 3
phase slip ring induction motor, has 6 poles on stator and 4 poles on
rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1400 rpm
d. Machine will not run at all

 ANSWER: Machine will not run at all

13)   A 3 phase,
50 Hz squirrel cage induction motor, has 6 poles on stator and 4 poles
on rotor. Then the machine will run at

a. 1000 rpm
b. 1500 rpm
c. 1750 rpm
d. Machine will not run

14)   The rotor frequency of a three phase induction motor is

a. Slip times the supply frequency
b. 1 / slip times the supply frequency
c. Equal to the supply frequency
d. None of these

 ANSWER: Slip times the supply frequency

15)   The rotor
resistance per phase on standstill and rotor reactance per phase on
standstill of induction motor is

a. Slip times the rotor resistance at standstill and slip times rotor reactance at standstill
b. Same as rotor resistance at standstill and slip times rotor reactance at standstill
c. Same as rotor resistance at standstill and same as rotor reactance at standstill
d. Slip times the rotor resistance at standstill and same as rotor reactance at standstill

 ANSWER: Same as rotor resistance at standstill and slip times rotor reactance at standstill

16)   Nature of the rotor power factor in running condition is always

b. Lagging
c. Both (a) and (b)
d. None of these

17)   In terms of slip, the actual speed (N) of the induction motor can be expressed as

a. Ns (1-s)
b. Ns (s-1)
c. Ns / (1-s)
d. Ns / (s-1)

18)   If a
voltmeter when connected to the rotor of an induction motor gives 150
oscillations per minute and stator frequency is 50 Hz. Then the slip of
induction motor will be

a. 3%
b. 4.5%
c. 5%
d. 5.5%

19)   For a three phase induction motor, the two rotor frequencies are given by

a. sf, (1 – s) f
b. sf, (2 – s) f
c. (1 – s) f, (2 – s) f
d. Sf , sf

 ANSWER: sf, (2 – s) f

20)   The torque produced in the induction motor depends on the following factors
1. The part of rotating magnetic field which reacts with rotor and is responsible for producing emf.
2. The magnitude of rotor current in running condition.
3. The power factor of the rotor circuit in running condition.
Which of the above statement is true?

a. Only 1
b. 1 & 2
c. 2 & 3
d. 1, 2 and 3

21)   The torque produced in a three phase induction motor is

a. Directly proportional to the square of the induced emf in rotor
b. Directly proportional to the induced emf in rotor
c. Inversely proportional to the square of the induced emf in rotor
d. Inversely proportional to the induced emf in rotor

 ANSWER: Directly proportional to the square of the induced emf in rotor

22)   For maximum torque, the only parameter which controls the torque is slip. The maximum torque is

a. directly proportional to the rotor resistance
b. inversely proportional to the rotor resistance
c. directly proportional to the slip times the rotor resistance
d. independent of the rotor resistance

 ANSWER: independent of the rotor resistance

23)   When the torque produced by the induction motor is at its maximum, the slip is given by

a. X2 / R2
b. (R2 + X2) / X2
c. R2 / X2
d. R2 / (R2 + X2)

24)   A 400v, 4
pole, 3 phase, 50 Hz star connected induction motor has a rotor
resistance and reactance per phase equal to 0.02 ohm and 0.2 ohm
respectively. Then the slip at which maximum torque occurs and the
corresponding maximum torque is (assuming stator to rotor turns ratio as
6)

a. 10%, 159.08 N – m
b. 20%, 318.16 N – m
c. 11%, 159.08 N – m
d. 10%, 318.16 N – m

 ANSWER: 10%, 159.08 N – m

25)   When an induction motor is loaded from no load to full load, its speed and slip

a. Increases, decreases
b. Decreases, increases
c. Both increases
d. Both decreases

26)   An induction
= 1 to s = 0. Then the torques in the low slip region and in high slip
region is

a. Both are directly proportional to the slip
b. Both are inversely proportional to the slip
c. Directly proportional to the slip, inversely proportional to the slip
d. Independent of slip, proportional to the slip

 ANSWER: Directly proportional to the slip, inversely proportional to the slip

27)   The curve
obtained by plotting torque against slip from s = 1 to s = 0 is called
torque slip characteristics. The nature of the graph in the low slip
region and in the high slip region is

a. Rising exponential, decaying exponential
b. Both will be straight line
c. Straight line, rectangular parabola
d. Straight line, decaying exponential

28)   If an induction motor is operating at a point in the high slip region, then the motor will be

a. Stable
b. Unstable
c. Either stable or unstable depending on the torque
d. None of these

29)   The value of slip for motoring, generating and braking region respectively are

a. S > 1, 0 = s = 1, s < 0
b. S < 0, 0 = s = 1, s > 1
c. 0 = s = 1, s > 1, s < 1
d. 0 = s = 1, s < 0, s > 1

 ANSWER: 0 = s = 1, s < 0, s > 1

30)   When the slip of an induction machine is negative, it is running as a

a. Motor
b. Generator
c. Synchronous machine
d. None of these

31)   If sf is the full load slip and sm is the slip for maximum torque, then the ratio of full load torque to the maximum torque is given by

a. 2sfsm / ( sm2 + sf2 )
b. 2sf / ( sm2 + sf2 )
c. 2sm / ( sm2 + sf2 )
d. sfsm / ( sm2 + sf2 )

 ANSWER: 2sfsm / ( sm2 + sf2 )

32)   If sst is the starting slip and sm is the slip for maximum torque, then the ratio of starting torque to the maximum torque is

a. sm / ( sm2 + 1 )
b. 2sm / ( sm2 + 2 )
c. 2sm / ( sm2 + 1 )
d. 1 / ( 1 + sm2 )

 ANSWER: 2sm / ( sm2 + 1 )

33)   A 16 pole,
50 Hz, star connected three phase induction motor has rotor resistance
of 0.012 ohm per phase and rotor reactance of 0.220 ohm per phase at
standstill. Its full load torque is at speed of 220 rpm. Ratio of its
starting torque to maximum torque is

a. 0.1208
b. 0.1087
c. 0.1102
d. 0.1012

34)   If the rotor
resistance of an induction motor is doubled, keeping the other
parameters constant, then the maximum torque of the induction motor will
become

a. Double
b. Halved
c. One fourth
d. Remains same

external resistance to rotor till it becomes equal to rotor reactance,
the maximum torque can be achieved at start”. This statement holds for

a. Slip ring induction motor
b. Squirrel cage induction motor
c. Both a and b
d. Neither a nor b

36)   A 4 pole, 50
Hz, wound rotor inductor is supplied at rated voltage and frequency
with slip ring open circuited. If the rotor is driven by an external
means at 1500 rpm opposite to the direction of stator field, the
frequency of voltage across slip rings will be

a. 50 Hz
b. 80 Hz
c. 100 Hz
d. 0 Hz

37)   The
approximate value of torque angle at which maximum pull out torque will
occur under practical circumstances is

a. 90 degree
b. 75 degree
c. 60 degree
d. 30 degree

38)   If P2 is the rotor input power, Pm is the gross mechanical power developed and s is the slip of induction motor, then the gross mechanical power developed is

a. S P2
b. (1 – s) P2
c. P2 / s
d. P2 / (1 – s)

39)   For slip s of an induction motor, the ratio of rotor copper loss to the gross mechanical output is

a. 1 / ( 1 – s)
b. 1 / s
c. ( 1 – s ) / s
d. s / ( 1 – s )

 ANSWER: s / ( 1 – s )

40)   The torque equation of an induction motor is given as
ksE22R2 / R22 + (sX2)2
The value of k in the above equation is,

a. 3 / (2 Π ns)
b. 2 / (Π ns)
c. 2 / (3 Π ns)
d. 3 / (Π ns)

 ANSWER: 3 / (2 Π ns)

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