DC Motor Questions: Multiple Choice Questions and Answers

DC Motor Questions. DC Motors Questions and Answers in Electrical Engineering.

 

DC Motor Questions

 

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

View Answer

D. cumulative compound D.C. motor

 

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

View Answer

B. low starting torque

 

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
B. resultant force on conductor
C. Both
D. None of the above

View Answer

A. current in the conductor

 

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%

View Answer

D. less than 50%

 

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

View Answer

B. shunt motors

 

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

View Answer

D. Fleming’s left-hand rule

 

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 DC shunt motors

View Answer

D. flux is practically constant in DC shunt motors

 

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

A. Blowers
B. Conveyors
C. Air compressors
D. Centrifugal pumps

View Answer

B. Conveyors

 

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

View Answer

D. more than full-load current

 

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

View Answer

C. Commutator

 

DC Motor Questions

 

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

View Answer

C. 25% of the previous value

 

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

View Answer

D. to reduce the sparking

 

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

View Answer

D. 0.5

 

The condition for maximum power in case of D.C. motor is ?

A. back e.m.f. = 2 x supply voltage
B. back e.m.f. = | x supply voltage
C. supply voltage = | x back e.m.f
D. supply voltage = back e.m.f

View Answer

B. back e.m.f. = | x supply voltage

 

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

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

View Answer

C. Cumulatively compounded motor

 

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

A. nothing will happen to the 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. None of the above

View Answer

C. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

 

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

A. shunt
B. series
C. differentially compounded
D. cumulatively compounded

View Answer

C. differentially compounded

 

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

View Answer

C. in series with armature winding

 

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

View Answer

B. commutator gets damaged

 

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

View Answer

C. Air motor

 

DC Motor Questions

 

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

View Answer

C. cumulatively compounded

 

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 uniformity
D. To reduce the reluctance of the magnetic path

View Answer

A. To reduce eddy current loss

 

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

View Answer

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

 

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

View Answer

A. independent of Ia

 

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

View Answer

A. D.C. series motor

 

Which of the following motor has the poorest speed regulation ?

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

View Answer

B. Series motor

 

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

View Answer

B. torque will change but power will remain constant

 

The plugging gives the ?

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

View Answer

C. highest torque braking

 

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

View Answer

C. Armature control method

 

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 recommended by manufacturer and select the next higher H.P. motor

View Answer

D. is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor

 

DC Motor Questions

 

Usually wide and sensitive speed control is desired in case of ?

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

View Answer

D. colliery winders

 

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

View Answer

B. plugging

 

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%

View Answer

B. 4.76%

 

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

View Answer

A. increase

 

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

View Answer

A. a resistance is connected in series with armature

 

DC Motor Questions

 

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

View Answer

C. Increase in terminal voltage

 

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

View Answer

C. armature copper loss

 

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

View Answer

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

 

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

View Answer

C. Armature copper loss

 

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

View Answer

D. Reluctance motor

 

DC Motor Questions

 

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

View Answer

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

 

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 generator together
D. all of the above

View Answer

A. residual magnetism of the generator

 

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

View Answer

D. All of the above

 

The main disadvantage of the Ward-Leonard control method is ?

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

View Answer

D. all of the above

 

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

View Answer

B. Eddy current losses

 

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

View Answer

B. 500 W

 

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

View Answer

D. Rate of flow of ventilating air

 

D.C. generators are normally designed for maximum efficiency around ?

A. full-load
B. rated r.p.m
C. rated voltage
D. all of the above

View Answer

A. full-load

 

Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control ?

A. drum type controller is used
B. three point starter is used
C. four point starter is used
D. all above can be used

View Answer

A. drum type controller is used

 

In a D.C. generator, the iron losses mainly take place in ?

A. yoke
B. commutator
C. armature conductors
D. armature rotor

View Answer

D. armature rotor

 

DC Motor Questions

 

If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens ?

A. this will make armature to take heavy current, possibly burning it
B. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
C. nothing will happen to motor
D. motor will come to stop

View Answer

A. this will make armature to take heavy current, possibly burning it

 

D.C. generators are installed near the load centres to reduce ?

A. iron losses
B. line losses
C. sparking
D. corona losses

View Answer

B. line losses

 

D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the ?

A. series motor
B. shunt motor
C. compound motor
D. any of the above

View Answer

A. series motor

 

The purpose of retardation test on D.C. shunt machines is to find out ?

A. stray losses
B. eddy current losses
C. field copper losses
D. windage losses

View Answer

A. stray losses

 

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this ?

A. series motor
B. shunt motor
C. compound motor
D. any of the above

View Answer

C. compound motor

 

Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ?

A. Swinburne’s test
B. Hopkinson’s test
C. Field test
D. Brake test

View Answer

C. Field test

 

During rheostat braking of D.C. series motors ?

A. motor is run as a generator
B. motor is reversed in direction
C. motor is run at reduced speed
D. None of these

View Answer

A. motor is run as a generator

 

The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by ?

A. reducing the field current
B. decreasing the armature current
C. increasing the armature current
D. increasing the excitation current

View Answer

A. reducing the field current

 

For which types of D.C. motor, dynamic braking is generally used ?

A. Shunt motors
B. Series motors
C. Compound motors
D. All of the above

View Answer

D. All of the above

 

One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven ?

A. runs as a generator
B. does not run as a generator
C. also runs as a motor comes to stop after sometime
D. None of these

View Answer

A. runs as a generator

 

DC Motor Questions

 

Ward Leonard method of speed control is basically a ?

A. field control method
B. armature controlled method
C. field diverter
D. frequency control method

View Answer

B. armature controlled method

 

If the load on a DC shunt motor is increases, its speed is slightly reduced due to ?

A. increase in armature current
B. increase in flux
C. decrease in reactance
D. decrease in back emf

View Answer

D. decrease in back emf

 

In which of the following device the generated emf and armature current are in the opposite directions ?

A. dc generator
B. dc motor
C. transformer
D. all of the above

View Answer

B. dc motor

 

With the increase in temperature, the speed of DC shunt motor ?

A. decreases
B. increases
C. remain same
D. none of the above

View Answer

B. increases

 

DC Motor Questions

 

With the increase in temperature, the speed of DC series motor ?

A. decreases
B. increases
C. remain same
D. none of the above

View Answer

A. decreases

 

If the field winding resistance of a DC shunt motor is increases, its speed will ?

A. decreases
B. increases
C. remains same
D. none of the above

View Answer

B. increases

 

Which of the following method is used for the speed control of DC shunt motor ?

A. voltage control method
B. flux control method
C. rheostat control method
D. all of the above

View Answer

D. all of the above

 

A dc series motor should not be started on no load because it would ?

A. not develop starting torque
B. fail to start without load
C. run at dangerously high speed
D. draw more current without producing sufficient torque

View Answer

C. run at dangerously high speed

 

Which of the following test can be performed on DC series motor ?

A. Swinburne test
B. Hopkinson test
C. Brake test
D. Field test

View Answer

D. Field test

 

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