Deck 5: D.C. Generator and Motor Efficiency, Testing and Braking Techniques

Question 1

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

Answer 1

variable losses = constant losses

Question 2

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

Answer 2

full-load

Question 3

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

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

Answer 3

armature rotor

Question 4

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

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

Question 5

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 6

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

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

Question 7

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

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

Question 8

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)none

Question 9

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 10

Which method of braking is generally used in elevators ?

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

Question 11

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 12

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 13

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 14

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

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

Question 15

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

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

Question 16

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)any of the above

Question 17

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

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

Question 18

In the D.C. motor the iron losses occur in

A)the field
B)the armature
C)the brushes
D)the commutator

Question 19

The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by

A)reducing the field current
B)decreasing the armature current
C)increasing the armature current
D)increasing the excitation current

Question 20

One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven

A)runs as a generator
B)does not run as a generator
C)also runs as a motor comes to stop after sometime
D)none