Deck 8: D.C Motors and Speed Control

Question 1

In case of conductively compensated D.C. series motors, the compensating winding is provided

A)as separately wound unit
B)in parallel with armature winding
C)in series with armature winding
D)in parallel with field winding

Answer 1

in series with armature winding

Question 2

Sparking at the commutator of a D.C. motor may result in

A)damage to commutator segments
B)damage to commutator insulation
C)increased power consumption
D)all of the above

Answer 2

all of the above

Question 3

Which of the following motor is preferred for operation in highly explosive atmosphere ?

A)Series motor
B)Shunt motor
C)Air motor
D)Battery operated motor

Answer 3

Air motor

Question 4

If the supply voltage for a D.C. motor is increased, which of the following willdecrease ?

A)Starting torque
B)Operating speed
C)Full-load current
D)All of the above

Question 5

Which one of the following is not the function of pole shoes in a D.C. machine ?

A)To reduce eddy current loss
B)To support the field coils
C)To spread out flux for better unifor-mity
D)To reduce the reluctance of the mag-netic path

Question 6

The mechanical power developed by a shunt motor will be maximum when theratio of back e.m.f. to applied voltage is

A)4.0
B)2.0
C)1.0
D)0.5

Question 7

For which of the following applications a D.C. motor is preferred over an A.C. motor?

A)Low speed operation
B)High speed operation
C)Variable speed operation
D)Fixed speed operation

Question 8

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

Question 9

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

Question 10

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

Question 11

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 12

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 13

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 14

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 15

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 samedirection
D)the motor will not work and come to stop

Question 16

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 17

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 18

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 19

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 20

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 21

Usually wide and sensitive speed control is desired in case of

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

Question 22

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 23

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 24

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 fieldwinding

Question 25

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