120+ DC Motor Solved MCQs

1.

B. DC MOTOR 1. No-load speed of which of the following motor will be highest ?

A. Shunt motor
B. Series motor
C. Cumulative compound motor
D. Differentiate compound motor
Answer» B. Series motor
2.

The direction of rotation of a D.C. series motor can be changed by

A. interchanging supply terminals
B. interchanging field terminals
C. either of (a) and (b) above
D. None of the above
Answer» B. interchanging field terminals
3.

Which of the following application requires high starting torque ?

A. Lathe machine
B. Centrifugal pump
C. Locomotive
D. Air blower
Answer» C. Locomotive
4.

If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?

A. Series motor
B. Shunt motor
C. Differentially compound motor
D. Cumulative compound motor
Answer» A. Series motor
5.

Which D.C. motor will be preferred for machine tools ?

A. Series motor
B. Shunt motor
C. Cumulative compound motor
D. Differential compound motor
Answer» B. Shunt motor
6.

Differentially compound D.C. motors can find applications requiring

A. high starting torque
B. low starting torque
C. variable speed
D. frequent on-off cycles
Answer» B. low starting torque
7.

Which D.C. motor is preferred for elevators ?

A. Shunt motor
B. Series motor
C. Differential compound motor
D. Cumulative compound motor
Answer» D. Cumulative compound motor
8.

According to Fleming's left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of

A. current in the conductor aovtaat of conductor
C. resultant force on conductor
D. none of the above
Answer» A. current in the conductor aovtaat of conductor
9.

If the field of a D.C. shunt motor gets opened while motor is running

A. the speed of motor will be reduced %
B. the armature current will reduce
C. the motor will attain dangerously high speed 1
D. the motor will continue to nuvat constant speed
Answer» C. the motor will attain dangerously high speed 1
10.

Starters are used with D.C. motors because

A. these motors have high starting torque
B. these motors are not self-starting
C. back e.m.f. of these motors is zero initially
D. to restrict armature current as there is no back e.m.f. while starting
Answer» D. to restrict armature current as there is no back e.m.f. while starting
11.

In D.C. shunt motors as load is reduced

A. the speed will increase abruptly
B. the speed will increase in proportion to reduction in load
C. the speed will remain almost/constant
D. the speed will reduce
Answer» C. the speed will remain almost/constant
12.

A D.C. series motor is that which

A. has its field winding consisting of thick wire and less turns
B. has a poor torque
C. can be started easily without load
D. has almost constant speed
Answer» A. has its field winding consisting of thick wire and less turns
13.

For starting a D.C. motor a starter is required because

A. it limits the speed of the motor
B. it limits the starting current to a safe value
C. it starts the motor
D. none of the above
Answer» B. it limits the starting current to a safe value
14.

The type of D.C. motor used for shears and punches is

A. shunt motor
B. series motor
C. differential compoutid D.C. motor
D. cumulative compound D.C. motor
Answer» D. cumulative compound D.C. motor
15.

If a D.C. motor is connected across the A.C. supply it will

A. run at normal speed
B. not run
C. run at lower speed
D. burn due to heat produced in the field winding by .eddy currents
Answer» D. burn due to heat produced in the field winding by .eddy currents
16.

To get the speed of D.C, motor below the normal without wastage of electrical energy is used.

A. Ward Leonard control
B. rheostatic control
C. any of the above method
D. none of the above method
Answer» A. Ward Leonard control
17.

When two D.C. series motors are connected in parallel, the resultant speed is

A. more than the normal speed
B. loss than the normal speed
C. normal speed
D. zero
Answer» C. normal speed
18.

The speed of a D.C. shunt motor more than its full-load speed can be obtained by

A. decreasing the field current
B. increasing the field current
C. decreasing the armature current
D. increasing the armature current
Answer» A. decreasing the field current
19.

In a D.C. shunt motor, speed is

A. independent of armature current
B. directly proportional to the armature current
C. proportional to the square of the current
D. inversely proportional to the armature current
Answer» A. independent of armature current
20.

A direct on line starter is used: for starting motors

A. iip to 5 H.P.
B. up to 10 H.P.
C. up to 15 H.P.
D. up to 20 H.P.
Answer» A. iip to 5 H.P.
21.

What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

A. The motor will stop
B. The motor will continue to run
C. The armature may burn
D. The motor will run noisy
Answer» C. The armature may burn
22.

In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

A. back e.m.f. is equal to armature drop
B. armature drop is negligible
C. flux is proportional to armature current
D. flux is practically constant in D:C. shunt motors
Answer» D. flux is practically constant in D:C. shunt motors
23.

In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be

A. almost negligible
B. rated full-load current
C. less than full-load current
D. more than full-load current
Answer» D. more than full-load current
24.

These days D.C. motors are widely used in

A. pumping sets
B. air compressors
C. electric traction
D. machine shops
Answer» C. electric traction
25.

By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?

A. Frame
B. Shaft
C. Commutator
D. Stator
Answer» C. Commutator
26.

In which of the following applications D.C. series motor is invariably tried?

A. Starter for a car
B. Drive for a water pump
C. Fan motor
D. Motor operation in A.C. or D.C.
Answer» A. Starter for a car
27.

In D.C. machines fractional pitch winding is used

A. to improve cooling
B. to reduce copper losses
C. to increase the generated e.m.f.
D. to reduce the sparking
Answer» D. to reduce the sparking
28.

A three point starter is considered suitable for

A. shunt motors
B. shunt as well as compound motors
C. shunt, compound and series motors
D. all D.C. motors
Answer» B. shunt as well as compound motors
29.

In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be

A. 100%
B. around 90%
C. anywhere between 75% and 90%
D. less than 50%
Answer» D. less than 50%
30.

The ratio of starting torque to full-load torque is least in case of

A. series motors
B. shunt motors
C. compound motors
D. none of the above
Answer» B. shunt motors
31.

In D.C. motor which of the following can sustain the maximum temperature rise?

A. Slip rings
B. Commutator
C. Field winding
D. Armature winding
Answer» C. Field winding
32.

Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?

A. Lenz's law
B. Faraday's law
C. Coloumb's law
D. Fleming's left-hand rule
Answer» D. Fleming's left-hand rule
33.

Which of the following load normally needs starting torque more than the rated torque?

A. Blowers
B. Conveyors
C. Air compressors
D. Centrifugal pumps
Answer» B. Conveyors
34.

The starting resistance of a D.C. motor is generally

A. low
B. around 500 Q
C. 1000 Q
D. infinitely large
Answer» A. low
35.

The speed of a D.C. series motor is

A. proportional to the armature current
B. proportional to the square of the armature current
C. proportional to field current
D. inversely proportional to the armature current
Answer» D. inversely proportional to the armature current
36.

In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

A. 100% of the previous value
B. 50% of the previous value
C. 25% of the previous value
D. 10% of the previous value
Answer» C. 25% of the previous value
37.

The current drawn by the armature of D.C. motor is directly proportional to

A. the torque required
B. the speed of the motor
C. the voltage across the terminals
D. none of the above
Answer» A. the torque required
38.

The power mentioned on the name plate of an electric motor indicates

A. the power drawn in kW
B. the power drawn in kVA
C. the gross power
D. the output power available at the shaft
Answer» D. the output power available at the shaft
39.

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

A. Series motor
B. Shunt motor
C. Cumulatively compounded 'motor
D. Differentially compounded motor
Answer» D. Differentially compounded motor
40.

Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads?

A. Series motor
B. Shunt motor
C. Cumulatively compounded motor
D. Differentially compounded motor
Answer» C. Cumulatively compounded motor
41.

If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

A. nothing will happen to th£ motor
B. this will make armature to take heavy current, possibly burning it
C. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
D. motor will run at very slow speed
Answer» C. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
42.

D.C. series motors are used

A. where load is constant
B. where load changes frequently
C. where constant operating speed is needed
D. in none of the above situations.
Answer» D. in none of the above situations.
43.

For the same H.P. rating and full load speed, following motor has poor starting torque

A. shunt
B. series
C. differentially compounded
D. cumulativelyc'ompounded
Answer» C. differentially compounded
44.

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» C. in series with armature winding
45.

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» D. all of the above
46.

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» C. Air motor
47.

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

A. Starting torque
B. Operating speed
C. Full-load current
D. All of the above
Answer» C. Full-load current
48.

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
Answer» A. To reduce eddy current loss
49.

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

A. 4.0
B. 2.0
C. 1.0
D. 0.5
Answer» D. 0.5
50.

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
Answer» C. Variable speed operation
51.

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» A. 2 to 3 per cent
52.

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» A. Series motor
53.

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» D. both shunt and compound motor
54.

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
Answer» B. commutator gets damaged
55.

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.
Answer» B. in both directions
56.

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
Answer» C. cumulatively compounded
57.

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
Answer» D. D.C. motor
58.

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
Answer» A. the motor will first stop and then run in opposite direction as series motor
59.

Which of the following motor has the poorest speed regulation ?

A. Shunt motor
B. Series motor
C. Differential compound motor
D. Cumulative compound motor
Answer» B. Series motor
60.

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
Answer» A. D.C. series motor
61.

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
Answer» A. reduce slightly
62.

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
Answer» B. armature current only
63.

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
Answer» C. Armature control method
64.

Usually wide and sensitive speed control is desired in case of

A. centrifugal pumps
B. elevators
C. steel rolling mills
D. colliery winders
Answer» D. colliery winders
65.

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%
Answer» B. 4.76%
66.

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
Answer» A. constant torque drive
67.

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
Answer» A. a resistance is connected in series with armature
68.

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
Answer» B. increasing the resistance in field circuit
69.

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» A. independent of Ia
70.

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» A. remain unchanged
71.

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
Answer» B. a low resistance circuit
72.

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
Answer» D. any of the above
73.

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.
Answer» A. Low cost
74.

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
Answer» B. torque will change but power will remain constant
75.

Which motor has the poorest speed control?

A. Differentially compounded motor
B. Cumulatively compounded motor
C. Shunt motor
D. Series motor
Answer» D. Series motor
76.

The plugging gives the

A. zero torque braking
B. smallest torque braking
C. highest torque braking
D. none of the above
Answer» C. highest torque braking
77.

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» C. constant torque drive
78.

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» B. plugging
79.

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
Answer» D. Synchronous motor
80.

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
Answer» A. increase
81.

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
Answer» D. The speed is inversely proportional to the torque and the torque is proportional to square of armature current
82.

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
Answer» C. Single phase induction motor (split phase start or induction run motor)
83.

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
Answer» D. Reluctance motor
84.

Which motor should not be started on no-load ?

A. Series motor
B. Shunt motor
C. Cumulatively compounded motor
D. Differentially compounded motor.
Answer» A. Series motor
85.

Ward-Leonard control is basically a

A. voltage control method
B. field divertor method
C. field control method
D. armature resistance control method
Answer» A. voltage control method
86.

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
Answer» B. Armature voltage control
87.

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
Answer» A. residual magnetism of the generator
88.

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
Answer» D. all of the above
89.

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
Answer» B. back e.m.f. is equal to the applied voltage
90.

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
Answer» D. Rate of flow of ventilating air
91.

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
Answer» C. Increase in terminal voltage
92.

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
Answer» C. armature copper loss
93.

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
Answer» C. Armature copper loss
94.

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
Answer» D. all above factors
95.

D.C. shunt motors are used for driving

A. trains
B. cranes
C. hoists
D. machine tools
Answer» D. machine tools
96.

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
Answer» A. over load relay is connected in series and no volt relay in parallel with the load
97.

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
Answer» D. Using material of low hysteresis co-efficient for armature core material
98.

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
Answer» D. All of the above
99.

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
Answer» B. Eddy current losses
100.

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
Answer» B. 500 W
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