Deck 7: D.C Motors and Starters

Question 1

In a D.C. shunt motor, speed is

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

Answer 1

independent of armature current

Question 2

A direct on line starter is used: for starting motors

A)iip to 5 H.P.
B)up to 10 H.P.
C)up to 15 H.P.
D)up to 20 H.P.

Answer 2

iip to 5 H.P.

Question 3

What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

A)The motor will stop
B)The motor will continue to run
C)The armature may burn
D)The motor will run noisy

Answer 3

The armature may burn

Question 4

In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

A)back e.m.f. is equal to armature drop
B)armature drop is negligible
C)flux is proportional to armature current
D)flux is practically constant in D:C. shuntmotors

Question 5

In a D.C. shunt motor, under the conditions of maximum power, the current inthe armature will be

A)almost negligible
B)rated full-load current
C)less than full-load current
D)more than full-load current

Question 6

These days D.C. motors are widely used in

A)pumping sets
B)air compressors
C)electric traction
D)machine shops

Question 7

By looking at which part of the motor, it can be easily confirmedthat a particular motor is D.C. motor?

A)Frame
B)Shaft
C)Commutator
D)Stator

Question 8

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.

Question 9

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

Question 10

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

Question 11

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 12

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 13

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 14

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

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

Question 15

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

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

Question 16

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

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

Question 17

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 18

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 19

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 20

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 21

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 22

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

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

Question 23

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 24

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 25

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