Question 1

Which of the following motor is preferred for operation in highly explosive atmosphere ?

Accepted Answer

A)  Series motor
B)  Shunt motor
C)  Air motor
D)  Battery operated motor C

Question 2

Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads?

Accepted Answer

A)  Series motor
B)  Shunt motor
C)  Cumulatively compounded motor
D)  Differentially compounded motor C

Question 3

For the same H.P. rating and full load speed, following motor has poor starting torque

Accepted Answer

A)  shunt
B)  series
C)  differentially compounded
D)  cumulativelyc'ompounded C

Question 4

In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be

Accepted Answer

A)  100%
B)  around 90%
C)  anywhere between 75% and 90%
D)  less than 50%

Question 5

The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is

Accepted Answer

A)  4.0
B)  2.0
C)  1.0
D)  0.5

Question 6

The power mentioned on the name plate of an electric motor indicates

Accepted Answer

A)  the power drawn in kW
B)  the power drawn in kVA
C)  the gross power
D)  the output power available at the shaft

Question 7

Which of the following load normally needs starting torque more than the rated torque?

Accepted Answer

A)  Blowers
B)  Conveyors
C)  Air compressors
D)  Centrifugal pumps

Question 8

In D.C. machines fractional pitch winding is used

Accepted Answer

A)  to improve cooling
B)  to reduce copper losses
C)  to increase the generated e.m.f.
D)  to reduce the sparking

Question 9

If the supply voltage for a D.C. motor is increased, which of the following will decrease?

Accepted Answer

A)  Starting torque
B)  Operating speed
C)  Full-load current
D)  All of the above

Question 10

A three point starter is considered suitable for

Accepted Answer

A)  shunt motors
B)  shunt as well as compound motors
C)  shunt, compound and series motors
D)  all D.C. motors

Question 11

For which of the following applications a D.C. motor is preferred over an A.C. motor ?

Accepted Answer

A)  Low speed operation
B)  High speed operation
C)  Variable speed operation
D)  Fixed speed operation

Question 12

Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?

Accepted Answer

A)  Lenz's law
B)  Faraday's law
C)  Coloumb's law
D)  Fleming's left-hand rule

Question 13

The speed of a D.C. series motor is

Accepted Answer

A)  proportional to the armature current
B)  proportional to the square of the armature current
C)  proportional to field current
D)  inversely proportional to the armature current

Question 14

Which one of the following is not the function of pole shoes in a D.C. machine ?

Accepted Answer

A)  To reduce eddy current loss
B)  To support the field coils
C)  To spread out flux for better unifor-mity
D)  To reduce the reluctance of the mag-netic path

Question 15

In case of conductively compensated D.C. series motors, the compensating winding is provided

Accepted Answer

A)  as separately wound unit
B)  in parallel with armature winding
C)  in series with armature winding
D)  in parallel with field winding

Question 16

The current drawn by the armature of D.C. motor is directly proportional to

Accepted Answer

A)  the torque required
B)  the speed of the motor
C)  the voltage across the terminals
D)  none of the above

Question 17

If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

Accepted Answer

A)  nothing will happen to th£ motor
B)  this will make armature to take heavy current, possibly burning it
C)  this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
D)  motor will run at very slow speed

Question 18

In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

Accepted Answer

A)  100% of the previous value
B)  50% of the previous value
C)  25% of the previous value
D)  10% of the previous value

Question 19

The ratio of starting torque to full-load torque is least in case of

Accepted Answer

A)  series motors
B)  shunt motors
C)  compound motors
D)  none of the above

Question 20

In D.C. motor which of the following can sustain the maximum temperature rise?

Accepted Answer

A)  Slip rings
B)  Commutator
C)  Field winding
D)  Armature winding

Exam 2: D.C. Motors