Deck 3: D.C. Motors and Speed Control

Question 1

In D.C. machines the residual magnetism is of the order of

A)2 to 3 per cent
B)10 to 15 per cent
C)20 to 25 per cent
D)50 to 75 per cent

Answer 1

2 to 3 per cent

Question 2

Which D.C. motor is generally preferred for cranes and hoists ?

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

Answer 2

Series motor

Question 3

Three point starter can be used for

A)series motor only
B)shunt motor only
C)compound motor only
D)both shunt and compound motor

Answer 3

both shunt and compound motor

Question 4

Sparking, is discouraged in a D.C. motor because

A)it increases the input power con-sumption
B)commutator gets damaged
C)both (a) and (b)
D)none of the above

Question 5

Speed control by Ward Leonard method gives uniform speed variation

A)in one direction
B)in both directions
C)below normal speed only
D)above normal speed only.

Question 6

Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be

A)level compounded
B)under compounded
C)cumulatively compounded
D)differentially compounded

Question 7

Following motor is used where high starting torque and wide speed range control is required.

A)Single phase capacitor start
B)Induction motor
C)Synchronous motor
D)D.C. motor

Question 8

In a differentially compounded D.C. motor, if shunt field suddenly opens

A)the motor will first stop and then run in opposite direction as series motor
B)the motor will work as series motor and run at slow speed in the same direction
C)the motor will work as series motor and run at high speed in the same direction
D)the motor will not work and come to stop

Question 9

Which of the following motor has the poorest speed regulation ?

A)Shunt motor
B)Series motor
C)Differential compound motor
D)Cumulative compound motor

Question 10

Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of

A)D.C. series motor
B)D.C. shunt motor
C)induction motor
D)all of above motors

Question 11

As -the load is increased the speed of D.C. shunt motor will

A)reduce slightly
B)increase slightly
C)increase proportionately
D)remains unchanged

Question 12

The armature torque of the D.C. shunt motor is proportional to

A)field flux only
B)armature current only
C)both (a) and (b)
D)none of the above

Question 13

Which of the following method of speed control of D.C. machine will offer minimum efficiency ?

A)Voltage control method
B)Field control method
C)Armature control method
D)All above methods

Question 14

Usually wide and sensitive speed control is desired in case of

A)centrifugal pumps
B)elevators
C)steel rolling mills
D)colliery winders

Question 15

The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is

A)2.36%
B)4.76%
C)6.77%
D)8.84%

Question 16

The armature voltage control of D.C. motor provides

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

Question 17

As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

A)a resistance is connected in series with armature
B)a resistance is connected parallel to the armature
C)armature is temporarily open circuited
D)a high value resistor is connected across the field winding

Question 18

The speed of a D.C. shunt motor can be increased by

A)increasing the resistance in armature circuit
B)increasing the resistance in field circuit
C)reducing the resistance in the field circuit
D)reducing the resistance in the armature circuit

Question 19

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

Question 20

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

Question 21

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

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

Question 22

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)any of the above

Question 23

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 24

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 25

Which motor has the poorest speed control?

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