UNIT 3

UNIT 3BEE SET-1PART A1. Define an ideal transformer draw and explain the no-load phasor diagram of an ideal single phase transformer2. Develop the equivalent circuit of a single phase transformer. What is the use of equivalent circuit?3 Derive the induced e.m.f equation of a transformer4. Draw the equivalent circuit of a single phase transformer and explain.5. Develop the phasor diagram of a single phase transformer under lagging power factor loadConditions6. Derive the induced EMF equation of a single phase transformer.PART B

[1] which of the following motor has the constant speed?A. Series motorB. Shunt motorC. Cumulatively compound motorD. All of the aboveE. None of the aboveAns:B

[2] The usual test to find the efficiency of the traction motor isA. Field's testB. Retardation testC. Hopkinson's testD. Swinburn's testE. None of the aboveAns: A

[3] A DC series motor is best for drivingA. LathesB. Cranes and hoistsC. Shears and punchesD. Machine toolsE. None of the aboveAns: B

[4] Retardation test on a dc shunt motor is used for findingA. Stray lossB. Copper lossC. Friction lossD. Iron lossE. None of the aboveAns: A

[7] In a DC series motor increasing the load current willA. Decrease the speedB. Increase the speedC. Better commutationD. Increase the back emfE. None of the aboveAns: A

[8] One of the main advantage of the swinburn's test isA. It is applicable both shunt and compound motorsB. It needs one running testC. It is very economical and convenientD. It ignores any charge in iron lossE. None of the aboveAns:C

[9] the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that itA. Requires full load powerB. Ignores any change in iron lossC. Needs one motor and one generatorD. Requires two identical shunt machinesE. None of the aboveAns: D

[10] The most economical method of finding no losses of a large dc shunt motor isA. Hopkinson's testB. Swinburn's testC. Retardation testD. Field's testE. None of the aboveAns: B 11.In a D.C. shunt motor, speed is

independent of armature current

directly proportional to the armature current

proportional to the square of the current

inversely proportional to the armature current

ANSWER : : independent of armature current Explanation :

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

shunt motor

series motor

differential compoutid D.C. motor

cumulative compound D.C. motor

ANSWER : : cumulative compound D.C. motor Explanation :

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

run at normal speed

not run

run at lower speed

burn due to heat produced in the field winding by .eddy currents

ANSWER : : burn due to heat produced in the field winding by .eddy currents Explanation :

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

almost negligible

rated full-load current

less than full-load current

more than full-load current

ANSWER : : more than full-load current Explanation :

15.Differentially compound D.C. motors can find applications requiring

high starting torque

low starting torque

variable speed

frequent on-off cycles

ANSWER : : low starting torqu

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

decreasing the field current

increasing the field current

decreasing the armature current

increasing the armature current

ANSWER : : decreasing the field current Explanation :

17.A direct on line starter is used: for starting motors

iip to 5 H.P.

up to 10 H.P.

up to 15 H.P.

up to 20 H.P.

ANSWER : : iip to 5 H.P. Explanation :

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

Frame

Shaft

Commutator

Stator

ANSWER : : Commutator Explanation :

19.These days D.C. motors are widely used in

pumping sets

air compressors

electric traction

machine shops

ANSWER : : electric traction Explanation :

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

Ward Leonard control

rheostatic control

any of the above method

none of the above method

ANSWER : : Ward Leonard control

21.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

current in the conductor aovtaat of conductor

resultant force on conductor

none of the above

current in the conductor aovtaat of conductor

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

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

back e.m.f. is equal to armature drop

armature drop is negligible

flux is proportional to armature current

flux is practically constant in D:C. shunt motors

ANSWER : : flux is practically constant in D:C. shunt motors Explanation :

23.A D.C. series motor is that which

has its field winding consisting of thick wire and less turns

has a poor torque

can be started easily without load

has almost constant speed

ANSWER : : has its field winding consisting of thick wire and less turns Explanation :

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

more than the normal speed

loss than the normal speed

normal speed

zero

ANSWER : : normal speed Explanation :

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

Series motor

Shunt motor

Differentially compound motor

Cumulative compound motor

ANSWER : : Series motor

26.Starters are used with D.C. motors because

these motors have high starting torque

these motors are not self-starting

back e.m.f. of these motors is zero initially

to restrict armature current as there is no back e.m.f. while starting

ANSWER : : to restrict armature current as there is no back e.m.f. while starting Explanation :

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

Series motor

Shunt motor

Cumulative compound motor

Differential compound motor

ANSWER : : Shunt motor Explanation :

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

interchanging supply terminals

interchanging field terminals

either of (a) and (b) above

None of the above

ANSWER : : interchanging field terminals Explanation :

29.Which of the following application requires high starting torque ?

Lathe machine

Centrifugal pump

Locomotive

Air blower

ANSWER : : Locomotive Explanation :

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

it limits the speed of the motor

it limits the starting current to a safe value

it starts the motor

none of the above

ANSWER : : it limits the starting current to a safe value

31.Which D.C. motor is preferred for elevators ?

Shunt motor

Series motor

Differential compound motor

Cumulative compound motor

ANSWER : : Cumulative compound motor Explanation :

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

The motor will stop

The motor will continue to run

The armature may burn

The motor will run noisy

ANSWER : : The armature may burn Explanation :

33.In D.C. shunt motors as load is reduced

the speed will increase abruptly

the speed will increase in proportion to reduction in load

the speed will remain almost/constant

the speed will reduce

ANSWER : : the speed will remain almost/constant

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

the torque required

the speed of the motor

the voltage across the terminals

none of the above

ANSWER : : the torque required Explanation :

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

the power drawn in kW

the power drawn in kVA

the gross power

the output power available at the shaft

ANSWER : : the output power available at the shaft Explanation :

36.The speed of a D.C. series motor is

proportional to the armature current

proportional to the square of the armature current

proportional to field current

inversely proportional to the armature current

ANSWER : : inversely proportional to the armature current Explanation :

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

series motors

shunt motors

compound motors

none of the above

ANSWER : : shunt motors Explanation :

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

Slip rings

Commutator

Field winding

Armature winding

ANSWER : : Field winding

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

Lenz's law

Faraday's law

Coloumb's law

Fleming's left-hand rule

ANSWER : : Fleming's left-hand rule Explanation :

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

4.0

2.0

1.0

0.5

ANSWER : : 0.5 Explanation :

BEE SET-2PART A1. Draw the approximate equivalent circuit of a transformer referred to the primary side and indicates how it differs from the exact equivalent circuit.2. What is a transformer? Differentiate between step-up and step-down transformers.3. Explain the construction and principle of operation of a 1-phase transformer.4. Explain the phasor diagram for 1-phase transformer under loaded and unloaded conditions.5. Derive the induced EMF equation of a single phase transformer.6. Describe the operation of a single-phase transformer, explaining clearly the functions of the different parts. Why are the cores laminated?PART B

[1] which of the following motor has the constant speed?A. Series motorB. Shunt motorC. Cumulatively compound motorD. All of the aboveE. None of the aboveAns:B

[2] The usual test to find the efficiency of the traction motor isA. Field's testB. Retardation testC. Hopkinson's testD. Swinburn's testE. None of the aboveAns: A

[3] A DC series motor is best for drivingA. LathesB. Cranes and hoistsC. Shears and punchesD. Machine toolsE. None of the aboveAns: B

[4] Retardation test on a dc shunt motor is used for findingA. Stray lossB. Copper lossC. Friction lossD. Iron lossE. None of the aboveAns: A

[7] In a DC series motor increasing the load current willA. Decrease the speedB. Increase the speedC. Better commutationD. Increase the back emfE. None of the aboveAns: A

[8] One of the main advantage of the swinburn's test isA. It is applicable both shunt and compound motorsB. It needs one running testC. It is very economical and convenientD. It ignores any charge in iron lossE. None of the aboveAns:C

[9] the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that itA. Requires full load powerB. Ignores any change in iron lossC. Needs one motor and one generatorD. Requires two identical shunt machinesE. None of the aboveAns: D

[10] The most economical method of finding no losses of a large dc shunt motor isA. Hopkinson's testB. Swinburn's testC. Retardation testD. Field's testE. None of the aboveAns: B 11.In a D.C. shunt motor, speed is

independent of armature current

directly proportional to the armature current

proportional to the square of the current

inversely proportional to the armature current

ANSWER : : independent of armature current Explanation :

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

shunt motor

series motor

differential compoutid D.C. motor

cumulative compound D.C. motor

ANSWER : : cumulative compound D.C. motor Explanation :

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

run at normal speed

not run

run at lower speed

burn due to heat produced in the field winding by .eddy currents

ANSWER : : burn due to heat produced in the field winding by .eddy currents Explanation :

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

almost negligible

rated full-load current

less than full-load current

more than full-load current

ANSWER : : more than full-load current Explanation :

15.Differentially compound D.C. motors can find applications requiring

high starting torque

low starting torque

variable speed

frequent on-off cycles

ANSWER : : low starting torqu

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

decreasing the field current

increasing the field current

decreasing the armature current

increasing the armature current

ANSWER : : decreasing the field current Explanation :

17.A direct on line starter is used: for starting motors

iip to 5 H.P.

up to 10 H.P.

up to 15 H.P.

up to 20 H.P.

ANSWER : : iip to 5 H.P. Explanation :

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

Frame

Shaft

Commutator

Stator

ANSWER : : Commutator Explanation :

19.These days D.C. motors are widely used in

pumping sets

air compressors

electric traction

machine shops

ANSWER : : electric traction Explanation :

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

Ward Leonard control

rheostatic control

any of the above method

none of the above method

ANSWER : : Ward Leonard control

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

back e.m.f. = 2 x supply voltage

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

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

supply voltage = back e.m.f.

ANSWER : : back e.m.f. = | x supply voltage Explanation :

42.In D.C. machines fractional pitch winding is used

to improve cooling

to reduce copper losses

to increase the generated e.m.f.

to reduce the sparking

ANSWER : : to reduce the sparking Explanation :

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

shunt

series

differentially compounded

cumulativelyc'ompounded

ANSWER : : differentially compounded

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

low

around 500 Q

1000 Q

infinitely large

ANSWER : : low Explanation :

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

Series motor

Shunt motor

Cumulatively compounded motor

Differentially compounded motor

ANSWER : : Cumulatively compounded motor Explanation :

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

100% of the previous value

50% of the previous value

25% of the previous value

10% of the previous value

ANSWER : : 25% of the previous value Explanation :

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

nothing will happen to th motor

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

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

ANSWER : D.C. series motors are used Explanation :

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

100%

around 90%

anywhere between 75% and 90%

less than 50%

ANSWER : : less than 50%

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

as separately wound unit

in parallel with armature winding

in series with armature winding

in parallel with field winding

ANSWER : : in series with armature winding Explanation :

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

Blowers

Conveyors

Air compressors

Centrifugal pumps

ANSWER : : Conveyors Explanation :

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

Series motor

Shunt motor

Cumulatively compounded 'motor

Differentially compounded motor

ANSWER : : Differentially compounded motor Explanation :

52.A three point starter is considered suitable for

shunt motors

shunt as well as compound motors

shunt, compound and series motors

all D.C. motors

ANSWER : : shunt as well as compound motors Explanation :

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

Starting torque

Operating speed

Full-load current

All of the above

ANSWER : : Full-load current

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

To reduce eddy current loss

To support the field coils

To spread out flux for better unifor-mity

To reduce the reluctance of the mag-netic path

ANSWER : : To reduce eddy current loss Explanation :

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

Series motor

Shunt motor

Air motor

Battery operated motor

ANSWER : : Air motor Explanation :

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

damage to commutator segments

damage to commutator insulation

increased power consumption

all of the above

ANSWER : : all of the above

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

Voltage control method

Field control method

Armature control method

All above methods

ANSWER : : Armature control method Explanation :

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

it increases the input power con-sumption

commutator gets damaged

both (a) and (b)

none of the above

ANSWER : : commutator gets damaged Explanation :

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

field current

applied voltage

resistance in series with armature

any of the above

ANSWER : : any of the above Explanation :

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

independent of Ia

proportional to la

varies as (Ia)

varies as la

ANSWER : : independent of Ia

BEE SET-3PART A1. Define an ideal transformer draw and explain the no-load phasor diagram of an ideal single phase transformer2. Develop the equivalent circuit of a single phase transformer. What is the use of equivalent circuit?3 Derive the induced e.m.f equation of a transformer4. Explain the phasor diagram for 1-phase transformer under loaded and unloaded conditions.5. Derive the induced EMF equation of a single phase transformer.6. Describe the operation of a single-phase transformer, explaining clearly the functions of the different parts. Why are the cores laminated?

PART B

31.Which D.C. motor is preferred for elevators ?

Shunt motor

Series motor

Differential compound motor

Cumulative compound motor

ANSWER : : Cumulative compound motor Explanation :

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

The motor will stop

The motor will continue to run

The armature may burn

The motor will run noisy

ANSWER : : The armature may burn Explanation :

33.In D.C. shunt motors as load is reduced

the speed will increase abruptly

the speed will increase in proportion to reduction in load

the speed will remain almost/constant

the speed will reduce

ANSWER : : the speed will remain almost/constant

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

the torque required

the speed of the motor

the voltage across the terminals

none of the above

ANSWER : : the torque required Explanation :

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

the power drawn in kW

the power drawn in kVA

the gross power

the output power available at the shaft

ANSWER : : the output power available at the shaft Explanation :

36.The speed of a D.C. series motor is

proportional to the armature current

proportional to the square of the armature current

proportional to field current

inversely proportional to the armature current

ANSWER : : inversely proportional to the armature current Explanation :

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

series motors

shunt motors

compound motors

none of the above

ANSWER : : shunt motors Explanation :

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

Slip rings

Commutator

Field winding

Armature winding

ANSWER : : Field winding

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

Lenz's law

Faraday's law

Coloumb's law

Fleming's left-hand rule

ANSWER : : Fleming's left-hand rule Explanation :

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

4.0

2.0

1.0

0.5

ANSWER : : 0.5 Explanation :

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

back e.m.f. = 2 x supply voltage

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

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

supply voltage = back e.m.f.

ANSWER : : back e.m.f. = | x supply voltage Explanation :

42.In D.C. machines fractional pitch winding is used

to improve cooling

to reduce copper losses

to increase the generated e.m.f.

to reduce the sparking

ANSWER : : to reduce the sparking Explanation :

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

shunt

series

differentially compounded

cumulativelyc'ompounded

ANSWER : : differentially compounded

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

low

around 500 Q

1000 Q

infinitely large

ANSWER : : low Explanation :

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

Series motor

Shunt motor

Cumulatively compounded motor

Differentially compounded motor

ANSWER : : Cumulatively compounded motor Explanation :

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

100% of the previous value

50% of the previous value

25% of the previous value

10% of the previous value

ANSWER : : 25% of the previous value Explanation :

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

nothing will happen to th motor

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

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

ANSWER : D.C. series motors are used Explanation :

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

100%

around 90%

anywhere between 75% and 90%

less than 50%

ANSWER : : less than 50%

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

as separately wound unit

in parallel with armature winding

in series with armature winding

in parallel with field winding

ANSWER : : in series with armature winding Explanation :

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

Blowers

Conveyors

Air compressors

Centrifugal pumps

ANSWER : : Conveyors Explanation : 21.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

current in the conductor aovtaat of conductor

resultant force on conductor

none of the above

current in the conductor aovtaat of conductor

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

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

back e.m.f. is equal to armature drop

armature drop is negligible

flux is proportional to armature current

flux is practically constant in D:C. shunt motors

ANSWER : : flux is practically constant in D:C. shunt motors Explanation :

23.A D.C. series motor is that which

has its field winding consisting of thick wire and less turns

has a poor torque

can be started easily without load

has almost constant speed

ANSWER : : has its field winding consisting of thick wire and less turns Explanation :

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

more than the normal speed

loss than the normal speed

normal speed

zero

ANSWER : : normal speed Explanation :

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

Series motor

Shunt motor

Differentially compound motor

Cumulative compound motor

ANSWER : : Series motor

26.Starters are used with D.C. motors because

these motors have high starting torque

these motors are not self-starting

back e.m.f. of these motors is zero initially

to restrict armature current as there is no back e.m.f. while starting

ANSWER : : to restrict armature current as there is no back e.m.f. while starting Explanation :

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

Series motor

Shunt motor

Cumulative compound motor

Differential compound motor

ANSWER : : Shunt motor Explanation :

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

interchanging supply terminals

interchanging field terminals

either of (a) and (b) above

None of the above

ANSWER : : interchanging field terminals Explanation :

29.Which of the following application requires high starting torque ?

Lathe machine

Centrifugal pump

Locomotive

Air blower

ANSWER : : Locomotive Explanation :

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

it limits the speed of the motor

it limits the starting current to a safe value

it starts the motor

none of the above

ANSWER : : it limits the starting current to a safe value

31.Which D.C. motor is preferred for elevators ?

Shunt motor

Series motor

Differential compound motor

Cumulative compound motor

ANSWER : : Cumulative compound motor Explanation :

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

The motor will stop

The motor will continue to run

The armature may burn

The motor will run noisy

ANSWER : : The armature may burn Explanation :

33.In D.C. shunt motors as load is reduced

the speed will increase abruptly

the speed will increase in proportion to reduction in load

the speed will remain almost/constant

the speed will reduce

ANSWER : : the speed will remain almost/constant

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

the torque required

the speed of the motor

the voltage across the terminals

none of the above

ANSWER : : the torque required Explanation :

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

the power drawn in kW

the power drawn in kVA

the gross power

the output power available at the shaft

ANSWER : : the output power available at the shaft Explanation :

36.The speed of a D.C. series motor is

proportional to the armature current

proportional to the square of the armature current

proportional to field current

inversely proportional to the armature current

ANSWER : : inversely proportional to the armature current Explanation :

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

series motors

shunt motors

compound motors

none of the above

ANSWER : : shunt motors Explanation :

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

Slip rings

Commutator

Field winding

Armature winding

ANSWER : : Field winding

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

Lenz's law

Faraday's law

Coloumb's law

Fleming's left-hand rule

ANSWER : : Fleming's left-hand rule Explanation :

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

4.0

2.0

1.0

0.5

ANSWER : : 0.5 Explanation :

BEE SET-4PART A1. Draw the approximate equivalent circuit of a transformer referred to the primary side and indicates how it differs from the exact equivalent circuit.2. What is a transformer? Differentiate between step-up and step-down transformers.3. Explain the construction and principle of operation of a 1-phase transformer.4. Draw the equivalent circuit of a single phase transformer and explain.5. Develop the phasor diagram of a single phase transformer under lagging power factor loadConditions6. Derive the induced EMF equation of a single phase transformer.

PART B

21.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

current in the conductor aovtaat of conductor

resultant force on conductor

none of the above

current in the conductor aovtaat of conductor

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

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

back e.m.f. is equal to armature drop

armature drop is negligible

flux is proportional to armature current

flux is practically constant in D:C. shunt motors

ANSWER : : flux is practically constant in D:C. shunt motors Explanation :

23.A D.C. series motor is that which

has its field winding consisting of thick wire and less turns

has a poor torque

can be started easily without load

has almost constant speed

ANSWER : : has its field winding consisting of thick wire and less turns Explanation :

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

more than the normal speed

loss than the normal speed

normal speed

zero

ANSWER : : normal speed Explanation :

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

Series motor

Shunt motor

Differentially compound motor

Cumulative compound motor

ANSWER : : Series motor

26.Starters are used with D.C. motors because

these motors have high starting torque

these motors are not self-starting

back e.m.f. of these motors is zero initially

to restrict armature current as there is no back e.m.f. while starting

ANSWER : : to restrict armature current as there is no back e.m.f. while starting Explanation :

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

Series motor

Shunt motor

Cumulative compound motor

Differential compound motor

ANSWER : : Shunt motor Explanation :

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

interchanging supply terminals

interchanging field terminals

either of (a) and (b) above

None of the above

ANSWER : : interchanging field terminals Explanation :

29.Which of the following application requires high starting torque ?

Lathe machine

Centrifugal pump

Locomotive

Air blower

ANSWER : : Locomotive Explanation :

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

it limits the speed of the motor

it limits the starting current to a safe value

it starts the motor

none of the above

ANSWER : : it limits the starting current to a safe value

31.Which D.C. motor is preferred for elevators ?

Shunt motor

Series motor

Differential compound motor

Cumulative compound motor

ANSWER : : Cumulative compound motor Explanation :

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

The motor will stop

The motor will continue to run

The armature may burn

The motor will run noisy

ANSWER : : The armature may burn Explanation :

33.In D.C. shunt motors as load is reduced

the speed will increase abruptly

the speed will increase in proportion to reduction in load

the speed will remain almost/constant

the speed will reduce

ANSWER : : the speed will remain almost/constant

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

the torque required

the speed of the motor

the voltage across the terminals

none of the above

ANSWER : : the torque required Explanation :

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

the power drawn in kW

the power drawn in kVA

the gross power

the output power available at the shaft

ANSWER : : the output power available at the shaft Explanation :

36.The speed of a D.C. series motor is

proportional to the armature current

proportional to the square of the armature current

proportional to field current

inversely proportional to the armature current

ANSWER : : inversely proportional to the armature current Explanation :

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

series motors

shunt motors

compound motors

none of the above

ANSWER : : shunt motors Explanation :

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

Slip rings

Commutator

Field winding

Armature winding

ANSWER : : Field winding

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

Lenz's law

Faraday's law

Coloumb's law

Fleming's left-hand rule

ANSWER : : Fleming's left-hand rule Explanation :

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

4.0

2.0

1.0

0.5

ANSWER : : 0.5 Explanation :

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

back e.m.f. = 2 x supply voltage

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

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

supply voltage = back e.m.f.

ANSWER : : back e.m.f. = | x supply voltage Explanation :

42.In D.C. machines fractional pitch winding is used

to improve cooling

to reduce copper losses

to increase the generated e.m.f.

to reduce the sparking

ANSWER : : to reduce the sparking Explanation :

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

shunt

series

differentially compounded

cumulativelyc'ompounded

ANSWER : : differentially compounded

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

low

around 500 Q

1000 Q

infinitely large

ANSWER : : low Explanation :

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

Series motor

Shunt motor

Cumulatively compounded motor

Differentially compounded motor

ANSWER : : Cumulatively compounded motor Explanation :

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

100% of the previous value

50% of the previous value

25% of the previous value

10% of the previous value

ANSWER : : 25% of the previous value Explanation :

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

nothing will happen to th motor

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

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed

ANSWER : D.C. series motors are used Explanation :

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

100%

around 90%

anywhere between 75% and 90%

less than 50%

ANSWER : : less than 50%

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

as separately wound unit

in parallel with armature winding

in series with armature winding

in parallel with field winding

ANSWER : : in series with armature winding Explanation :

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

Blowers

Conveyors

Air compressors

Centrifugal pumps

ANSWER : : Conveyors Explanation :

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

Series motor

Shunt motor

Cumulatively compounded 'motor

Differentially compounded motor

ANSWER : : Differentially compounded motor Explanation :

52.A three point starter is considered suitable for

shunt motors

shunt as well as compound motors

shunt, compound and series motors

all D.C. motors

ANSWER : : shunt as well as compound motors Explanation :

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

Starting torque

Operating speed

Full-load current

All of the above

ANSWER : : Full-load current

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

To reduce eddy current loss

To support the field coils

To spread out flux for better unifor-mity

To reduce the reluctance of the mag-netic path

ANSWER : : To reduce eddy current loss Explanation :

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

Series motor

Shunt motor

Air motor

Battery operated motor

ANSWER : : Air motor Explanation :

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

damage to commutator segments

damage to commutator insulation

increased power consumption

all of the above

ANSWER : : all of the above

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

Voltage control method

Field control method

Armature control method

All above methods

ANSWER : : Armature control method Explanation :

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

it increases the input power con-sumption

commutator gets damaged

both (a) and (b)

none of the above

ANSWER : : commutator gets damaged Explanation :

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

field current

applied voltage

resistance in series with armature

any of the above

ANSWER : : any of the above Explanation :

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

independent of Ia

proportional to la

varies as (Ia)

varies as la

ANSWER : : independent of Ia