Deck 4: D.C. Motors and Generators

Question 1

The plugging gives the

A)zero torque braking
B)smallest torque braking
C)highest torque braking
D)none of the above

Answer 1

highest torque braking

Question 2

The armature voltage control of D.C. motor provides

A)constant voltage drive
B)constant current drive
C)constant torque drive
D)none of the above

Answer 2

constant torque drive

Question 3

If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking

A)regenerative
B)plugging
C)dynamic braking
D)none of the above

Answer 3

plugging

Question 4

Which of the following motors one will choose to drive the rotary compressor ?

A)D.C. shunt motor
B)D.C. series motor
C)Universal motor
D)Synchronous motor

Question 5

If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will

A)increase
B)decrease
C)remain same
D)become zero

Question 6

Why are the D.C. motors preferred for traction applications ?

A)Torque and speed are inversely proportional to armature current
B)Torque is proportional to armature current
C)Torque is proportional to square root of armature current
D)The speed is inversely proportional to the torque and the torque is proportional to square of armature current

Question 7

Which of the following motors is usually used in house-hold refrigerators ?

A)D.C. shunt motor
B)D.C. series motor
C)Single phase induction motor (split phase start or induction run motor)
D)Reluctance motor

Question 8

Which of the following motors is most suitable for signalling devices and many kinds of timers ?

A)D.C. shunt motor
B)D.C. series motor
C)Induction motor
D)Reluctance motor

Question 9

Which motor should not be started on no-load ?

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

Question 10

Ward-Leonard control is basically a

A)voltage control method
B)field divertor method
C)field control method
D)armature resistance control method

Question 11

For constant torque drive which speed control method is preferred ?

A)Field control
B)Armature voltage control
C)Shunt armature control
D)Mechanical loading system

Question 12

In Ward-Leonard control the lower limit of speed is imposed by

A)residual magnetism of the generator
B)core losses of motor
C)mechanical losses of motor and gen¬erator together
D)all of the above

Question 13

The main disadvantage of the Ward-Leonard control method is

A)high initial cost
B)high maintenance cost
C)low efficiency at Hght loads
D)all of the above

Question 14

Regenerative method of braking is based on that

A)back e.m.f. is less than the applied voltage
B)back e.m.f. is equal to the applied voltage
C)back e.m.f. of rotor is more than the applied voltage
D)none of the above

Question 15

The hysteresis loss in a D.C. machine least depends on

A)Frequency of magnetic reversals
B)Maximum value of flux density
C)Volume and grade of iron
D)Rate of flow of ventilating air

Question 16

In a D.C. generator all of the following could be the effects of iron losses except

A)Loss of efficiency
B)Excessive heating of core
C)Increase in terminal voltage
D)Rise in temperature of ventilating air

Question 17

The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?

A)hysteresis loss
B)field copper loss
C)armature copper loss
D)eddy current loss

Question 18

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

Question 19

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

Question 20

D.C. shunt motors are used for driving

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

Question 21

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 22

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 core material

Question 23

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 24

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 25

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