Deck 9: D.C Motors and Controls

Question 1

If I2 be the armature current, then speed of a D.C. shunt motor is

A)independent of Ia
B)proportional to la
C)varies as (Ia)
D)varies as la

Answer 1

independent of Ia

Question 2

In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will

A)remain unchanged
B)reduce to one-fourth value
C)increase four folds
D)be doubled

Answer 2

remain unchanged

Question 3

At the instant of starting when a D.C. motor is put on supply, it behaveslike

A)a highly resistive circuit
B)a low resistance circuit
C)a capacitive circuit
D)none of the above

Answer 3

a low resistance circuit

Question 4

The speed of a D.C. motor can be varied by varying

A)field current
B)applied voltage
C)resistance in series with armature
D)All of the above

Question 5

Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?

A)Low cost
B)Wide speed range
C)Stability
D)High starting torque.

Question 6

For a D.C. shunt motor if the excitation is changed

A)torque will remain constant
B)torque will change but power will remain constant
C)torque and power both will change
D)torque, power and speed, all will change

Question 7

Which motor has the poorest speed control?

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

Question 8

The plugging gives the

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

Question 9

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

Question 10

If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor

A)of lower H.P. should be selected
B)of higher H.P. should be selected
C)can be used for 50°C ambient temperature also
D)is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor

Question 11

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

Question 12

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 13

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 14

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 15

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 16

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 17

Which motor should not be started on no-load ?

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

Question 18

Ward-Leonard control is basically a

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

Question 19

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 20

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 21

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 22

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 23

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 24

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 25

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