# Electrical Machines Expected MCQ PDF 3 For Coal India Limited (CIL) MT Exam 2017

1)   For the construction of the armature of a d.c. machine, the best suited material is

a. Cast iron
b. Silicon steel
c. Carbon
d. All of these

2)   In a d.c. machine, the form of armature reaction mmf is

a. Triangular
b. Sinusoidal
c. Saw tooth
d. Rectangular

3)   For low reluctance path for the flux in armature, the permeability of the material should be

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

4)   In a d.c. machine, the eddy current losses mainly occurs in

a. Armature
b. Windings
c. Yoke
d. At brush contact

5)   The conductors which will cause distortion in the main field flux of d.c. machine are

a. Demagnetizing armature conductors
b. Cross magnetizing armature conductors
c. Both (a) and (b)
d. None of these

6)   If the brush
shift angle is given in electrical degrees then for converting it into
mechanical degree by using the relation

a. θelectrical / P
b. 2θelectrical / P
c. θelectrical / 2P
d. 2P / θelectrical

7)   A 6 pole wave
wound d.c. generator has 460 armature conductors carrying armature
current of 180 A. The armature reaction demagnetizing and cross
magnetizing ampere turns per pole if brushes are shifted by 6 degree
electrical from G.N.A are

a. 3220, 230
b. 3220, 260
c. 260, 3220
d. 230, 3220

8)   For
neutralizing the demagnetizing effect of armature reaction, an extra
number of winding turns are provided on each pole. To calculate number
of extra turns per pole the equation used is

a. (ATd / I) * leakage coefficient
b. (ATd / I) / leakage coefficient
c. Leakage coefficient / (ATd / I)
d. (ATd / I)

 ANSWER: (ATd / I) * leakage coefficient

9)   Due to the effect of armature reaction in d.c. machine, the value of generated voltage and flux per pole

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

10)   The armature
reaction in d.c. machine causes distortion in the main field flux. This
effect of armature reaction can be reduced by

a. Increasing the length of air gap
b. Decreasing the length of air gap
c. Increasing the number of poles
d. Decreasing the number of poles

 ANSWER: Increasing the length of air gap

11)   Two 4 pole
d.c. machines of identical armature, one with wave winding and other
with lap winding are wound. Then

a. The lap wound machine will have more generated voltage
b. Wave wound machine will have more generated voltage
c. Both will have same generated voltage
d. None of these

 ANSWER: Wave wound machine will have more generated voltage

12)   The armature in d.c. machines are always placed on rotor

a. Otherwise commutation will not be possible
b. Otherwise there will not be any induced emf
c. Otherwise current will not flow
d. All of these

 ANSWER: Otherwise commutation will not be possible

13)   In a d.c. machine, the commutator provides

a. Half wave rectification
b. Full wave rectification
c. Semi controlled rectification
d. Uncontrolled rectification

14)   To have
perfect neutralization of armature mmf under the pole shoe, the ampere
conductors of compensating winding must be

a. Not equal to the total armature ampere conductors under the pole shoe
b. Equal to the total armature ampere conductors under the pole shoe
c. Half of the total armature ampere conductors under the pole shoe
d. Twice of the total armature ampere conductors under the pole shoe

 ANSWER: Equal to the total armature ampere conductors under the pole shoe

15)   Ampere turns per pole for compensating winding in d.c. machines is equal to

a. (pole arc / pole pitch) * armature ampere turns per pole
b. (pole pitch / pole arc) * armature ampere turns per pole
c. (pole arc / pole pitch) * total ampere conductors per pole
d. None of these

 ANSWER: (pole arc / pole pitch) * armature ampere turns per pole

16)   The
compensating windings are used to neutralize the armature reaction in
d.c. machines. These windings are connected

a. Across armature
b. In series with armature
c. Both (a) and (b)
d. None of these

17)   The armature reaction effect in d.c. machines can be completely neutralized by using

a. Only compensating winding
b. Only interpoles
c. Both compensating winding and interpoles
d. None of these

 ANSWER: Both compensating winding and interpoles

18)   Practically
in d.c. machines, it may happen that the emf induced in different
parallel paths may not be same. This will cause inequality in brush arm
currents and will give rise to copper losses. These effects can be
avoided by using

a. Compensating windings
b. Interpoles
c. Equalizer rings
d. All of these

19)   In d.c. machines, equalizer connections are provided in

a. Only wave winding
b. Only lap winding
c. Both wave winding and lap winding
d. None of these

20)   If in the
d.c machine, the reversal of current in the coil is faster than ideal or
linear commutation then the commutation is said to be

a. Retarded commutation
b. Under commutation
c. Accelerated commutation
d. Ideal commutation

21)   In d.c. machines, the interpoles have tapering shape which results in

a. Simpler design
b. Reduction in the weight
c. Increase in acceleration of commutation
d. All of these

 ANSWER: Increase in acceleration of commutation

22)   The generating action and motoring action in d.c. motor is determined by

a. Fleming’s left hand rule, Fleming’s right hand rule
b. Both by Fleming’s left hand rule
c. Both by Fleming’s right hand rule
d. Fleming’s right hand rule, Fleming’s left hand rule

 ANSWER: Fleming’s right hand rule, Fleming’s left hand rule

23)   In a practical motor, to reverse the direction of rotation

a. Reverse the direction of main field produced by the field winding
b. Reverse the direction of current passing through the armature is reversed
c. Either (a) or (b)
d. None of these

24)   A 220 V,
d.c. motor draws an armature current of 20 A. Its armature resistance is
0.6 ohm. Then the induced emf in the motor will be

a. 195 V
b. 202 V
c. 208 V
d. 215 V

25)   If the flux
is increased by 50% and speed is reduced by 50% of a d.c. motor (keeping
the other parameters constant), then its back emf will become

a. 50 % of the original back emf
b. 0.75 % of the original back emf
c. 100 % of the original back emf
d. 150 % of the original back emf

 ANSWER: 0.75 % of the original back emf

26)   If Eb is the back emf of d.c. motor and V is the terminal voltage, then the condition for maximum power is

a. Eb = V
b. Eb = 2V
c. Eb = ( V / 2 )
d. Eb = V ∧ 2

 ANSWER: Eb = ( V / 2 )

27)   A 4 pole,
lap wound d.c. motor drawing an armature current of 20 A has 360
conductors. If the flux per pole is 0.015 Wb then the gross torque
developed by the armature of motor is

a. 10.23 N-m
b. 15.56 N-m
c. 17.17 N-m
d. 19.08 N-m

28)   If the no
load speed of d.c. motor is 1350 rpm and full load speed is 1150 rpm,
then its voltage regulation is

a. 11.56 %
b. 15.36 %
c. 17.39 %
d. 19.39 %

29)   As the load on d.c. motor increases, the current drawn by motor

a. Increases
b. Decreases
c. Remains same
d. None of these

30)   If the back
emf of a d.c. motor is doubled and flux is halved (keeping other
parameters constant) then its speed will become

a. Double of the original speed
b. Square of the original speed
c. Four times of the original speed
d. Half of the original speed

 ANSWER: Four times of the original speed

31)   Keeping the
field excitation constant, the constant of proportionality between back
emf and armature speed of d.c. motor is

a. Twice of the proportionality constant between developed torque and armature current
b. Half of the proportionality constant between developed torque and armature current
c. Same as that of proportionality constant between developed torque and armature current
d. None of these

 ANSWER: Same as that of proportionality constant between developed torque and armature current

32)   D.C. shunt motor is also called as

a. Constant flux motor
b. Constant voltage motor
c. Variable voltage motor
d. Constant current motor

33)   The torque developed in d.c. shunt motor is

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

 ANSWER: Directly proportional to the armature current

34)   For a
constant torque load, if the armature resistance of shunt motor is
doubled (keeping the shunt field constant) then the armature current
will

a. Double
b. Get halved
c. Remain same
d. None of these

35)   A 4 pole,
220 V, 10 H.P, d.c. shunt motor is rotating at a speed of 320 rpm. Then
the developed useful torque is

a. 210.647 N-m
b. 215.547 N-m
c. 219.484 N-m
d. None of these

36)   If a shunt motor is started with its field winding open then

a. It will rotate at the same speed as that with its field winding closed
b. It will rotate at less speed as that with its field winding closed
c. It will rotate at dangerously high speed
d. None of these

 ANSWER: It will rotate at dangerously high speed

37)   D.C. shunt motors are commonly used in

a. Cranes
b. Electric traction
c. Elevators
d. Lathe machines

38)   For d.c. series motor, if Ia is the armature current then the torque developed is

a. α Ia
b. α Ia2
c. α (1 / Ia2)
d. α (1 / Ia)

39)   Neglecting
saturation, if the current drawn from by a d.c. series motor is
increased from 15 A to 18 A, then the increase in the torque expressed
as percentage of initial torque will be

a. 22 %
b. 33%
c. 44%
d. 56.78%

40)   A 4 pole 240
V, d.c. series motor has a wave wound armature with 180 conductors. The
armature resistance and the field winding resistance are 0.10 ohm and
0.2 ohm respectively. If the motor is drawing 40 A current at 0.015 Wb
flux per pole, then the speed under this condition is

a. 2533.3 rpm
b. 2644.4 rpm
c. 2756.9 rpm
d. 2987.6 rpm

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