Deck 10: D.C Motors and Generators

Question 1

Which of the following loss in a D.C. generator varies significantly with the load current ?

A)Field copper loss
B)Windage loss
C)Armature copper loss
D)None of the above

Answer 1

Armature copper loss

Question 2

Torque developed by a D.C. motor depends upon

A)magnetic field
B)active length of the conductor
C)current flow through the conductors
D)all above factors

Answer 2

all above factors

Question 3

D.C. shunt motors are used for driving

A)trains
B)cranes
C)hoists
D)machine tools

Answer 3

machine tools

Question 4

In a manual shunt motor starter

A)over load relay is connected in series and no volt relay in parallel with the load
B)over load relay is connected in paral¬lel and no volt relay in series with the load
C)over load relay and no volt relay are both connected in series with the load
D)over load relay and no volt relay are both connected in parallel with the load

Question 5

Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?

A)Providing laminations in armature core
B)Providing laminations in stator
C)Using non-magnetic material for frame
D)Using material of low hysteresis co-efficient for armature corematerial

Question 6

Which of the following loss in a D.C. generator is dissipated in the form of heat?

A)Mechanical loss
B)Core loss
C)Copper loss
D)All of the above

Question 7

Which of the following losses are significantly reduced by laminating the core of a D.C. generator ?

A)Hysteresis losses
B)Eddy current losses
C)Copper losses
D)Windage losses

Question 8

The total losses in a well designed D.C. generator of 10 kW will be nearly

A)100 W
B)500 W
C)1000 W
D)1500 W

Question 9

The condition for maximum efficiency for a D.C. generator is

A)eddy current losses = stray losses
B)hysteresis losses = eddy current losses
C)copper losses = 0
D)variable losses = constant losses

Question 10

D.C. generators are normally designed for maximum efficiency around

A)full-load
B)rated r.p.m.
C)rated voltage
D)all of the above

Question 11

In a D.C. generator, the iron losses mainly take place in

A)yoke
B)commutator
C)armature conductors
D)armature rotor

Question 12

D.C. generators are installed near the load centres to reduce

A)iron losses
B)line losses
C)sparking
D)corona losses

Question 13

The purpose of retardation test on D.C. shunt machines is to find out

A)stray losses
B)eddy current losses
C)field copper losses
D)windage losses

Question 14

Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?

A)Swinburne'stest
B)Hopkinson'stest
C)Field test
D)Brake test

Question 15

Hopkinson's test on D.C. machines is conducted at

A)no-load
B)part load
C)full-load
D)overload

Question 16

During rheostat braking of D.C. series motors

A)motor is run as a generator
B)motor is reversed in direction
C)motor is run at reduced speed
D)all

Question 17

For which types of D.C. motor, dynamic braking is generally used ?

A)Shunt motors
B)Series motors
C)Compound motors
D)All of the above

Question 18

Which method of braking is generally used in elevators ?

A)Plugging
B)Regenerative braking
C)Rheostatic braking
D)None of the above

Question 19

In variable speed motor

A)a stronger commutating field is needed at low speed than at high speed
B)a weaker commutating field is needed at low speed than at high speed
C)same commutating field is needed at low speed than at high speed
D)none of the above is correct

Question 20

When the armature of a D.C. motor rotates, e.m.f. induced is

A)self-induced e.m.f.
B)mutually induced e.m.f.
C)back e.m.f.
D)none of the above

Question 21

Where D.C. motor of H.P. 12 or more requires frequent starting, stopping,reversing and speed control

A)drum type controller is used
B)three point starter is used
C)four point starter is used
D)all above can be used

Question 22

If a D.C. shunt motor is working at full load and if shunt field circuit suddenlyopens

A)this will make armature to take heavy current, possibly burning it
B)this will result in excessive speed, possibly destroying armature due toexcessive centrifugal stresses
C)nothing will happen to motor
D)motor will come to stop

Question 23

D.C. motor is to drive a load which has certain minimum value for most of thetime and some peak value for short duration. We will select the

A)series motor
B)shunt motor
C)compound motor
D)All of the above

Question 24

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this

A)series motor
B)shunt motor
C)compound motor
D)All of the above

Question 25

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

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