# Electrical Machines Expected MCQ PDF 1 For Coal India Limited (CIL) MT Exam 2017

1)   Iron losses are considered to be the constant losses. It depends on

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

2)   Copper losses in a rotating machine are

a. Variable losses
b. Constant losses
c. Both (a) or (b)
d. None of these

3)   Stray losses
are the losses which vary with the load but their relationship with load
current cannot be identified. Stay losses is maximum in

a. Synchronous machines
b. D.C. machines
c. Induction machines
d. Equal in all types of machines

4)   The brush contact losses in a d.c. Machine is

a. Inversely proportional to the square of current
b. Directly proportional to the square of current
c. Inversely proportional to the current
d. Directly proportional to the current

 ANSWER: Directly proportional to the current

5)   Electrical power output in a d.c. generator is equal to

a. Electrical power developed in armature – copper losses
b. Mechanical power input – iron and friction losses
c. Electrical power developed in armature – iron and copper losses
d. Mechanical power input – iron and friction losses – copper losses

 ANSWER: Mechanical power input – iron and friction losses – copper losses

6)   Mechanical power developed in armature of a d.c. motor is given by

a. Electrical power input to motor – copper losses
b. Motor output + iron and friction losses
c. Both (a) & (b)
d. None of these

7)   Out of electrical, mechanical and magnetic losses, the losses which is minimum is

a. All are equal
b. Electrical losses
c. Magnetic losses
d. Mechanical losses

8)   The hysteresis motor are mainly used in tape recorders because

a. Of its extremely steady torque
b. Constant speed
c. Reduced initial current
d. None of these

9)   If % ηm is the mechanical efficiency of a DC machine and % ηe is the electrical efficiency. Then the overall efficiency of DC machine is given by

a. % η m + % η e
b. % η m X % η e
c. (% η m + % η e) / % η m
d. (% η m + % η e) / % η e

 ANSWER: % η m X % η e

10)   The current flowing through the armature of a d.c. shunt machine at maximum efficiency is given by

a. √(Pi / Ra)
b. √(Ra / Pi)
c. √(Pi / R2a)
d. √(Ra / P2i)

11)   A 120 V
shunt generator running at 850 rpm has its armature and shunt field
resistance of 0.15 ohm and 50 ohm respectively. It supplies 200 lamps
each rated at 60 W, 100 V. The friction and windage and core loss of
the machine is 400 W. its armature copper loss on full load and shunt
field loss is

a. 2156.7 W, 200 W
b. 2232.6 W, 200 W
c. 2156.7 W, 240 W
d. 2232.6 W, 240 W

12)   A d.c. shunt
generator delivers 190 A at a terminal voltage of 220 V. The copper
losses and stray losses are 2000 W and 1000 W respectively. The
efficiency of the generator is

a. 91.35 %
b. 92.60 %
c. 93.30 %
d. 94.23 %

13)   A 4 pole
d.c. shunt generator having a wave winding supplies 45 lamps, each of 50
W at 100 V. The armature and field resistance are 0.15 ohm and 50 ohm
respectively. The current in armature conductor is

a. 11.25 A
b. 11.50 A
c. 12.25 A
d. 13.50 A

14)   A 120 V d.c.
shunt motor runs at speed of 1200 rpm. When the motor is operated
unloaded but an additional resistance of 4 ohm is connected in series
with the shunt field, then the speed increases and reaches to 1370 rpm
with the same terminal voltage. The value of series resistance is

a. 28.23 ohm
b. 32.48 ohm
c. 35.72 ohm
d. 36.82 ohm

15)   A 240 V, 17
kW d.c. shunt motor draws an armature current of 80 A at full load. The
armature and shunt field resistances are 0.2 ohm and 195 ohm
respectively. The rotational losses and efficiency of motor at full load
is

a. 87.20 %
b. 89.32 %
c. 89.67 %
d. 90.03 %

16)   Out of
different methods available for testing of d. c. motors, Swinburne’s
test and Hopkinson’s test are commonly used in practice on

a. Shunt generators
b. Series motors
c. Shunt motors
d. All of these

17)   Swinburne’s test and brake tests

a. Both are direct method of testing
b. Direct method of testing, indirect method of testing
c. Indirect method of testing, direct method of testing
d. Both are indirect method of testing

 ANSWER: Indirect method of testing, direct method of testing

18)   Swinburne’s test can be performed at

19)   While performing Swinburne’s test, the iron losses are assumed to be

a. Constant
b. Absent
c. Variable
d. None of these

20)   The speed in d.c. machine can be measured by using

a. Anemometer
b. Tachometer
c. Voltmeter
d. Ammeter

21)   While carrying out brake tests if the belt snaps, then the motor will

a. Rotate at reduced speed but in forward direction
b. Rotate at increased speed but in forward direction
c. Rotate at reduced speed but in backward direction
d. Rotate at increased speed but in backward direction

 ANSWER: Rotate at increased speed but in forward direction

22)   While performing retardation tests, the machine whose test is to be taken is run at a speed which is

a. Slightly less than its rated speed
b. Equal to its rated speed
c. Slightly greater than its rated speed
d. All of these

 ANSWER: Slightly greater than its rated speed

23)   The rotational or stray losses includes

a. Iron losses only
b. Iron losses, friction and windage losses
c. Iron losses, copper losses, friction and windage losses
d. None of these

 ANSWER: Iron losses, copper losses, friction and windage losses

24)   The rotational losses in d.c. machines is equal to the

a. Kinetic energy of armature
b. Half of the kinetic energy of armature
c. Square of the kinetic energy of armature
d. Rate of change of kinetic energy

 ANSWER: Rate of change of kinetic energy

25)   In regenerative test, the supply is given to the motor

a. To account for losses
b. To drive generator
c. Both (a) and (b)
d. None of these

26)   The back to back test is best suited for

a. Small machines
b. Medium size machines
c. Large machines
d. All of these

27)   For
performing Hopkinson’s test two identical machines are required which
are mechanically coupled. The iron losses in the two machines

a. Can be obtained separately
b. Cannot be separated
c. Both
d.None of these

28)   The test which can be performed on the d.c. series motor is

a. Brake test
b. Hokinson’s test
c. Swinburne’s test
d. Field test

29)   To perform field test on d.c. machines two motors are required. The motors used are

a. Series motors
b. One will be series motor and other will be shunt motors
c. Shunt motors
d. None of these

30)   The difference between field test and regenerative test is that

a. Field test requires two identical machines while regenerative test requires only one machine
b. In field test iron losses can be separated while in regenerative test it cannot
c. In field test generator
output is not fed back to the motor while in regenerative test generator
output is fed back to the motor
d. All of these

 ANSWER: In field test generator output is not fed back to the motor while in regenerative test generator output is fed back to the motor

31)   The most common method used to check for shorted windings is to perform

a. Field test
b. Drop test
c. Regenerative test
d. Brake test

32)   To have an
induced emf in the d.c. generator, there should be relative motion
between the conductor and flux. The plane of rotation and plane of flux

a. Should be parallel to each other
b. Should not be parallel to each other
c. Both (a) & (b)
d. None of these

 ANSWER: Should not be parallel to each other

33)   To have d.c.
voltage, a device is used in a d.c. generator to convert alternating
emf to unidirectional emf. This device is called

a. Armature
b. Commutator
c. Brushes
d. All of these

34)   Yoke in d.c. machine serves the outermost cover. For magnetic flux it provides

a. A low reluctance path
b. A high reluctance path
c. Both (a) & (b)
d. Does not provide path for magnetic flux

35)   Functions of commutator in d.c. machines are

a. To facilitate the collection of current from armature conductors
b. To convert internally developed induced emf to unidirectional emf
c. To produce unidirectional torque in case of motors
d. All of these
e. None of these

36)   Brushes in d.c. machines are made up of

a. Cast iron
b. Mild steel
c. Copper
d. Carbon

37)   If A is the
number of parallel paths and P is the number of poles, then the number
of parallel path in lap winding and in wave winding is

a. A = P, A = 2
b. A = 2P, A = P
c. A = 2, A = P
d. A = P, A = 2P

 ANSWER: A = P, A = 2

38)   A 4 pole,
d.c. generator has a wave wound armature with 812 conductors. The flux
per pole is 0.014 Wb. The speed at which it should be run to generate
240 V on no load is

a. 623.23 rpm
b. 633.35 rpm
c. 643.36 rpm
d. 645.53 rpm

39)   A 4 pole,
lap wound d.c. generators has 40 coils with 8 turns per coils. It is
driven at 1200 rpm. If the flux per pole is 0.022 Wb, then the generated
emf is

a. 265.8 V
b. 276.3 V
c. 281.6 V
d. 287.9 V

40)   In a d.c. machine, the current rating and voltage rating of wave winding is

a. Low, high
b. High, low
c. High, high
d. Low, low

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• Harikesh Yadav October 25, 2017