DMRC JE/SSC JE/CIL MT ELECTRICAL ENGINEERING IMPORTANT MCQ PART 6(DC MOTORS)

1.The current drawn by the armature of D.C. motor is directly proportional to
the torque required
the speed of the motor
the voltage across the terminals
none of the above
ANSWER : : the torque required
2.The power mentioned on the name plate of an electric motor indicates
the power drawn in kW
the power drawn in kVA
the gross power
the output power available at the shaft
ANSWER : : the output power available at the shaft
3.The speed of a D.C. series motor is
proportional to the armature current
proportional to the square of the armature current
proportional to field current
inversely proportional to the armature current
ANSWER : : inversely proportional to the armature current
4.The ratio of starting torque to full-load torque is least in case of
series motors
shunt motors
compound motors
none of the above
ANSWER : : shunt motors
5.In D.C. motor which of the following can sustain the maximum temperature rise?
Slip rings
Commutator
Field winding
Armature winding
ANSWER : : Field winding 

6.Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
Lenz’s law
Faraday’s law
Coloumb’s law
Fleming’s left-hand rule
ANSWER : : Fleming’s left-hand rule
7.The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
4.0
2.0
1.0
0.5
ANSWER : : 0.5
8.The condition for maximum power in case of D.C. motor is
back e.m.f. = 2 x supply voltage
back e.m.f. = 1/2 x supply voltage
supply voltage = 1/2 x back e.m.f.
supply voltage = back e.m.f.
ANSWER : : back e.m.f. =1/2 x supply voltage
9.In D.C. machines fractional pitch winding is used
to improve cooling
to reduce copper losses
to increase the generated e.m.f.
to reduce the sparking
ANSWER : : to reduce the sparking
10.For the same H.P. rating and full load speed, following motor has poor starting torque
shunt
series
differentially compounded
cumulatively compounded
ANSWER : : differentially compounded
11.The starting resistance of a D.C. motor is generally
low
around 500 ohm
1000 ohm
infinitely large
ANSWER : : low
12.Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
ANSWER : : Cumulatively compounded motor
13.In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to
100% of the previous value
50% of the previous value
25% of the previous value
10% of the previous value
ANSWER : : 25% of the previous value
14.If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
nothing will happen to with motor
this will make armature to take heavy current, possibly burning it
this
will result in excessive speed, possibly destroying armature due to
excessive centrifugal stresses
motor will run at very slow speed
ANSWER : this
will result in excessive speed, possibly destroying armature due to
excessive centrifugal stresses
15.In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
100%
around 90%
anywhere between 75% and 90%
less than 50%
ANSWER : : less than 50% 

16.In case of conductively compensated D.C. series motors, the compensating winding is provided
as separately wound unit
in parallel with armature winding
in series with armature winding
in parallel with field winding
ANSWER : : in series with armature winding
17.Which of the following load normally needs starting torque more than the rated torque?
Blowers
Conveyors
Air compressors
Centrifugal pumps
ANSWER : : Conveyors
18.Which D.C. motor has got maximum self loading property?
Series motor
Shunt motor
Cumulatively compounded ‘motor
Differentially compounded motor
ANSWER : : Differentially compounded motor
19.A three point starter is considered suitable for
shunt motors
shunt as well as compound motors
shunt, compound and series motors
all D.C. motors
ANSWER : : shunt as well as compound motors 

20.If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
Starting torque
Operating speed
Full-load current
All of the above
ANSWER : : Full-load current 

21.Which one of the following is not the function of pole shoes in a D.C. machine ?
To reduce eddy current loss
To support the field coils
To spread out flux for better uniformity
To reduce the reluctance of the mag-genetic path
ANSWER : : To reduce eddy current loss
22.Which of the following motor is preferred for operation in highly explosive atmosphere ?
Series motor
Shunt motor
Air motor
Battery operated motor
ANSWER : : Air motor
23.Sparking at the commutator of a D.C. motor may result in
damage to commutator segments
damage to commutator insulation
increased power consumption
all of the above
ANSWER : : all of the above

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