Electrical Basic Questions 7

Electrical Basic Questions 7

1.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

ANSWER : : inversely proportional to the armature current

2.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

ANSWER : : shunt motors

3.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

ANSWER : : Field winding

4.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

ANSWER : : Fleming’s left-hand rule

5.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

ANSWER : : 0.5

Electrical Basic Questions 7

6.The condition for maximum power in case of D.C. motor is

A. back e.m.f. = 2 x supply voltage
B. back e.m.f. = 1/2 x supply voltage
C. supply voltage = 1/2 x back e.m.f.
D. supply voltage = back e.m.f.

ANSWER : : back e.m.f. =1/2 x supply voltage

7.In D.C. m C. achines 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 : : to reduce the sparking

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

A. shunt
B. series
C. differentially compounded
D. cumulatively compounded

ANSWER : : differentially compounded

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

A. low
B. around 500 ohm
C. 1000 ohm
D. infinitely large

ANSWER : : low

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

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

ANSWER : : Cumulatively compounded motor

Electrical Basic Questions 7

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

ANSWER : : 25% of the previous value

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

A. nothing will happen to with 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

ANSWER : this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

13.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%

ANSWER : : less than 50%

14.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

ANSWER : : in series with armature winding

Electrical Basic Questions 7

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

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

ANSWER : : Conveyors

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

A. Series motor
B. Shunt motor
C. Cumulatively compounded ‘motor
D. Differentially compounded motor

ANSWER : : Differentially compounded motor

17.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 : : shunt as well as compound motors

18.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

ANSWER : : Full-load current

Electrical Basic Questions 7

19.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 uniformity
D. To reduce the reluctance of the mag-genetic path

ANSWER : : To reduce eddy current loss

20.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

ANSWER : : Air motor

21.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

ANSWER : : all of the above

Electrical Basic Questions 7