Deck 2: D.C. Motors

Question 1

In which of the following applications D.C. series motor is invariably tried?

A)Starter for a car
B)Drive for a water pump
C)Fan motor
D)Motor operation in A.C. or D.C.

Answer 1

Starter for a car

Question 2

In D.C. machines fractional pitch winding is used

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

Answer 2

to reduce the sparking

Question 3

A three point starter is considered suitable for

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

Answer 3

shunt as well as compound motors

Question 4

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

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

Question 5

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

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

Question 6

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

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

Question 7

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

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

Question 8

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

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

Question 9

The starting resistance of a D.C. motor is generally

A)low
B)around 500 Q
C)1000 Q
D)infinitely large

Question 10

The speed of a D.C. series motor is

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 11

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

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 12

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

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

Question 13

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

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 14

Which D.C. motor has got maximum self loading property?

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

Question 15

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

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

Question 16

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

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 17

D.C. series motors are used

A)where load is constant
B)where load changes frequently
C)where constant operating speed is needed
D)in none of the above situations.

Question 18

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

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

Question 19

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

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

Question 20

Sparking at the commutator of a D.C. motor may result in

A)damage to commutator segments
B)damage to commutator insulation
C)increased power consumption
D)all of the above

Question 21

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

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

Question 22

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

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

Question 23

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

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 24

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

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

Question 25

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

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