Good commutation means

Half the normal sparking at the brushes

Excessive sparking at the brushes

Half the normal sparking at the brushes

In case of dc motor, maximum mechanical power is developed when back emf equals:

One third of the applied voltage

In DC shunt motor, if the terminal voltage is reduced to half and torque remains the same then __________.

speed and armature current will remain the same

speed will be half and armature current also will be half

speed will be half, but armature current remains the same

speed will be decreased and armature current become double

speed and armature current will remain the same

Direction: The following item consists of two statements, one labelled as ‘Statement (I)’ and the other as ‘Statement (II)’. Examine these two statements carefully and select the answers to these items using the code given below:Statement (I): For constant applied voltage to its terminals, the effect of armature resistance in the operation of a dc shunt motor, is to reduce the operating speed, and cause a ‘drooping’ speed Vs. load characteristic.Statement (II): The effect or armature demagnetization with the decreasing load is to reduce the drop in operating speed, and can be designed to give a ‘rising’ speed Vs. load characteristic which may result in a possible ‘runaway’.

Statement I is false but Statement II is true

Both Statement I and Statement II are individually true and Statement II is the correct explanation of Statement I

Both Statement I and Statement II are individually true but Statement II is not the correct explanation of Statement I

Statement I is true but Statement II is false

Statement I is false but Statement II is true

By conducting Swinburne’s test on a DC machine, which of the following losses can be determined?

Copper losses in both armature and field

A DC shunt motor has rated rpm of 480. Certain industrial application requires this motor to run at 540 rpm for some time. Which speed control will be desirable?

It is not possible to run the motor at more than the rated rpm

Not suitable for high stating toque

Should always be started on load

Not suitable for high stating toque

May run away if the field becomes one

Directions: It consists of two statements one labeled as statement I and the other as statement II. Examine these two statements carefully and select the correct answer using the code given below.Statement I: Series DC motors are used for cranes and locomotives.Statement II: Series DC motors provide high starting torques, a requisite for such applications.

Both statement I and statement II are individually true but statement II is not the correct explanation of statement I

Both statements I and statement II are individually true and statement II is the correct explanation of statement I

Both statement I and statement II are individually true but statement II is not the correct explanation of statement I

Statements I is true but statement II is false

Statement I is false but statement II is true

_______ combined together are called stray losses.

Frictional loss and copper loss

Field copper loss and iron loss

A DC series motor should always be started with load because_________

At no load will not develop high starting torque

At no load, it will rotate with dangerously high speed

At no load will not develop high starting torque

It draws a small amount of current at no load

If supply voltage terminals for a DC shunt motor are reversed, then:

it will run in opposite direction

In the case of the DC series motor, it is possible to have finite no-load speed

if resistance is connected across its field and armature together

if resistance is connected across its filed terminals

if resistance is connected across its armature terminals

if resistance is connected across its field and armature together

Consider the following statements related to the speed control of a D.C. motor (a) Speed may be controlled by changing the pole flux.(b) Speed may be changed by changing voltage across the armature

Both (a) and (b) are not correct

A starter is necessary to start a DC motor because:

DC motors do not have starting torque

at starting, the value of back-EMF is zero

initially, high starting torque is required

it restricts initial high armature current that exists, as the value of starting back-EMF is zero

A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved then:

Both speed and armature current become 2 pu

The speed becomes 2 pu but armature current remains at 1 pu

The speed remains at 1 pu but armature current becomes 2 pu

Both speed and armature current become 2 pu

Both speed and armature current remain at 1 pu

132 + Mcqs in Dc Motor Starting in Electrical Machines Page 2 McqOptions

51.	In a DC motor drive, if the armature current is reversed by keeping field current positive producing a braking torque, then the drive is said to be operating in
A.	Motoring mode
B.	Regenerative braking mode
C.	Dynamic braking mode
D.	Plugging mode
Answer» E.

Discussion

52.	A separately excited dc generator rotating at 3000 rpm produces an emf of 157 V and delivers a current of 20 A. The braking torque exerted by the armature is
A.	17 N-m
B.	10 N-m
C.	12 N-m
D.	12.5 N-m
Answer» C. 12 N-m

Discussion

53.	Good commutation means
A.	Good sparking at the brushes
B.	Excessive sparking at the brushes
C.	No sparking at the brushes
D.	Half the normal sparking at the brushes
Answer» D. Half the normal sparking at the brushes

Discussion

54.	A cumulative compound d.c. motor runs at 1500 RPM on full load. if its series field is short circuited its speed:
A.	Became zero
B.	remain same
C.	increase
D.	Decrease
Answer» D. Decrease

Discussion

55.	In case of dc motor, maximum mechanical power is developed when back emf equals:
A.	The applied voltage
B.	Half the applied voltage
C.	One third of the applied voltage
D.	Double the applied voltage
Answer» C. One third of the applied voltage

Discussion

56.	A 250 V dc shunt motor has Rf = 125 Ω and Ra = 0.5 Ω. The motor operates on no load with full field flux at rated speed 1000 rpm with Ia = 5 A. Neglect saturation and armature reaction. The motor drives a load torque of 100 Nm. The armature current of the motor is
A.	80 A
B.	42.2 A
C.	67.8 A
D.	59.3 A
Answer» C. 67.8 A

Discussion

57.	At the time of starting the motor consumes current _______ than normal.
A.	Same
B.	High
C.	Low
D.	None of these
Answer» C. Low

Discussion

58.	In electric motor _________ is loss due to fans and the bearing friction.
A.	Iron loss
B.	Core loss
C.	Stray loss
D.	Friction and windage loss
Answer» E.

Discussion

59.	As the load is increases, the speed of a DC shunt motor will _______.
A.	remain constant
B.	Increase proportionately
C.	Increase slightly
D.	Reduce slightly
Answer» E.

Discussion

60.	Consider the following statements regarding a DC motor. When the speed of a DC motor increases:1. Back emf decreases2. Current drawn increases
A.	Only 2
B.	Only 1
C.	Both 1 and 2
D.	Neither 1 nor 2
Answer» E.

Discussion

61.	In DC shunt motor, if the terminal voltage is reduced to half and torque remains the same then __________.
A.	speed will be half and armature current also will be half
B.	speed will be half, but armature current remains the same
C.	speed will be decreased and armature current become double
D.	speed and armature current will remain the same
Answer» D. speed and armature current will remain the same

Discussion

62.	Direction: The following item consists of two statements, one labelled as ‘Statement (I)’ and the other as ‘Statement (II)’. Examine these two statements carefully and select the answers to these items using the code given below:Statement (I): For constant applied voltage to its terminals, the effect of armature resistance in the operation of a dc shunt motor, is to reduce the operating speed, and cause a ‘drooping’ speed Vs. load characteristic.Statement (II): The effect or armature demagnetization with the decreasing load is to reduce the drop in operating speed, and can be designed to give a ‘rising’ speed Vs. load characteristic which may result in a possible ‘runaway’.
A.	Both Statement I and Statement II are individually true and Statement II is the correct explanation of Statement I
B.	Both Statement I and Statement II are individually true but Statement II is not the correct explanation of Statement I
C.	Statement I is true but Statement II is false
D.	Statement I is false but Statement II is true
Answer» D. Statement I is false but Statement II is true

Discussion

63.	A DC shunt machine develops an AC EMF of 250 V at 1500 rpm. Find the torque developed for an armature current of 50 A.
A.	59.6 N-m
B.	79.6 N-m
C.	69.6 N-m
D.	49.6 N-m
Answer» C. 69.6 N-m

Discussion

64.	By conducting Swinburne’s test on a DC machine, which of the following losses can be determined?
A.	Friction and windage losses
B.	Constant losses
C.	Copper losses in both armature and field
D.	Armature copper losses
Answer» C. Copper losses in both armature and field

Discussion

65.	________ braking is generally used in breaking of battery-operated scooters.
A.	Plugging
B.	Regenerative
C.	Mechanical
D.	Rheostatic
Answer» D. Rheostatic

Discussion

66.	In a dc motor the windage loss is proportional to _____.
A.	supply voltage
B.	square of the supply voltage
C.	square of the flux density
D.	square of the armature speed
Answer» E.

Discussion

67.	A DC series motor develops a torque of 20 Nm at 3 A of load current. If the current is increased to 6 A, the torque developed will be
A.	10 Nm
B.	20 Nm
C.	80 Nm
D.	40 Nm
Answer» D. 40 Nm

Discussion

68.	A DC shunt motor has rated rpm of 480. Certain industrial application requires this motor to run at 540 rpm for some time. Which speed control will be desirable?
A.	Ward Leonard control
B.	Armature current control
C.	Field resistance control
D.	It is not possible to run the motor at more than the rated rpm
Answer» D. It is not possible to run the motor at more than the rated rpm

Discussion

69.	A dc series motor
A.	Always runs at constant speed
B.	Should always be started on load
C.	Not suitable for high stating toque
D.	May run away if the field becomes one
Answer» C. Not suitable for high stating toque

Discussion

70.	A 4-pole dc motor takes a 50 A armature current. The armature has lap connected 480 conductors. The flux per pole is 20 mWb. Calculate the gross torque developed by the armature of the motor.
A.	52.6 N-m
B.	39.5 N-m
C.	76.4 N-m
D.	15.2 N-m
Answer» D. 15.2 N-m

Discussion

71.	Directions: It consists of two statements one labeled as statement I and the other as statement II. Examine these two statements carefully and select the correct answer using the code given below.Statement I: Series DC motors are used for cranes and locomotives.Statement II: Series DC motors provide high starting torques, a requisite for such applications.
A.	Both statements I and statement II are individually true and statement II is the correct explanation of statement I
B.	Both statement I and statement II are individually true but statement II is not the correct explanation of statement I
C.	Statements I is true but statement II is false
D.	Statement I is false but statement II is true
Answer» B. Both statement I and statement II are individually true but statement II is not the correct explanation of statement I

Discussion

72.	_______ combined together are called stray losses.
A.	Frictional loss and copper loss
B.	Field copper loss and iron loss
C.	Iron and mechanical losses
D.	Copper and iron losses
Answer» D. Copper and iron losses

Discussion

73.	A 250 V dc shunt motor has an armature resistance of 0.2 Ω and a field resistance of 100 Ω. When the motor is operated on no-load at rated voltage, it draws an armature current of 5 A and runs at 1200 rpm. When a load is coupled to the motor, it draws total line current of 50 A at rated voltage, with a 5 % reduction in the air-gap flux due to armature reaction. Voltage drop across the brushes can be taken as 1 V per brush under all operating conditions. The speed of the motor, in rpm, under this loaded condition, is closest to:
A.	1200
B.	1000
C.	1220
D.	900
Answer» D. 900

Discussion

74.	A electric motor operating from 220 V supply takes a current of 8 A. The motor has an efficiency of 80%. Determine the output of the motor.
A.	1450 W
B.	1408 W
C.	1200 W
D.	1500 W
Answer» C. 1200 W

Discussion

75.	Find the current required by a 400 V, 10 H.P. DC motor at 73.55% efficiency.
A.	35 A
B.	20 A
C.	30 A
D.	25 A
Answer» E.

Discussion

76.	Enclosure of motor is made from
A.	Copper
B.	aluminum
C.	cast iron
D.	Laminated sheath
Answer» D. Laminated sheath

Discussion

77.	A DC series motor should always be started with load because_________
A.	At no load, it will rotate with dangerously high speed
B.	At no load will not develop high starting torque
C.	It cannot start without load
D.	It draws a small amount of current at no load
Answer» B. At no load will not develop high starting torque

Discussion

78.	If Ia is the armature current of a DC series motor, then the value of (torque developed by motor for small values of armature current)/(torque developed by motor for higher values of armature current) is:
A.	1/Ia
B.	1/I2a
C.	I2a
D.	Ia
Answer» E.

Discussion

79.	In a DC series motor, the ratio of back emf (Eb) to supply voltage (V) indicates:
A.	running torque
B.	efficiency
C.	speed regulation
D.	starting torque
Answer» C. speed regulation

Discussion

80.	In the DC motor, iron losses occur in
A.	Field
B.	Armature
C.	Brushes
D.	Commutator
Answer» C. Brushes

Discussion

81.	Energy-efficient motors generally have
A.	longer insulation
B.	less vibration
C.	lower heat output
D.	all option are correct
Answer» E.

Discussion

82.	If supply voltage terminals for a DC shunt motor are reversed, then:
A.	it will stop immediately
B.	it will run in same direction
C.	it will act as generator
D.	it will run in opposite direction
Answer» C. it will act as generator

Discussion

83.	A lap wound DC machine has 400 conductors and 8 poles. The voltage induced per conductor is 2 volts. The machine generates a voltage of
A.	100 V
B.	200 V
C.	400 V
D.	800 V
Answer» B. 200 V

Discussion

84.	A 250 V dc motor has an armature resistance of 0.5 Ω. It is drawing an armature current of 25 A, driving a certain load. Calculate the induced emf in the motor under this conditions.
A.	180.5 V
B.	237.5 V
C.	192.5 V
D.	625 V
Answer» C. 192.5 V

Discussion

85.	For constant load and supply voltage, two DC series motors will develop maximum torque when both are connected in
A.	series
B.	parallel
C.	series or parallel
D.	None of the above
Answer» B. parallel

Discussion

86.	A 200 V d.c. shunt motor with armature resistance of 0.2 Ω and carrying a current of 50 A is running at 960 r.p.m. If the flux is reduced by 10% at constant torque and with negligible iron and friction losses, the speed will be nearly.
A.	1280 r.p.m.
B.	1170 r.p.m.
C.	1100 r.p.m.
D.	1060 r.p.m.
Answer» E.

Discussion

87.	In the case of the DC series motor, it is possible to have finite no-load speed
A.	if resistance is connected across its filed terminals
B.	if resistance is connected across its armature terminals
C.	if resistance is connected across its field and armature together
D.	None of these
Answer» C. if resistance is connected across its field and armature together

Discussion

88.	Consider the following statements related to the speed control of a D.C. motor (a) Speed may be controlled by changing the pole flux.(b) Speed may be changed by changing voltage across the armature
A.	Only (a) is correct
B.	Only (b) is correct
C.	Both (a) and (b) are correct
D.	Both (a) and (b) are not correct
Answer» D. Both (a) and (b) are not correct

Discussion

89.	A 220 V, 10 kW, 900 rpm separately excited DC motor has an armature resistance Ra = 0.02 Ω. When the motor operates at rated speed and with rated terminal voltage, the electromagnetic torque developed by the motor is 70 Nm. Neglecting the rotational losses of the machine, the current drawn by the motor from the 220 V supply is
A.	34.2 A
B.	30 A
C.	22 A
D.	4.84 A
Answer» C. 22 A

Discussion

90.	A dc shunt motor operating in a linear region of magnetizing characteristics runs at 1500 rpm with a voltage rated over the entire portion of the armature in a linear region of magnetizing characteristics. If the applied voltage is reduced by 50%, what will be the speed of the motor?
A.	750 rpm
B.	2500 rpm
C.	3000 rpm
D.	1500 rpm
Answer» E.

Discussion

91.	A 200 V dc motor takes 50 A in series and is operating at a speed of 1000 rpm when full of electrical components (load). Its resistance is 0.2 Ω. What value of series resistance will offer a rated torque at 800 rpm?
A.	3.8 Ω
B.	0.76 Ω
C.	0.54 Ω
D.	2.6 Ω
Answer» C. 0.54 Ω

Discussion

92.	For DC motor magnets, _______ is preferred:
A.	Tungsten steel
B.	Silicon steel
C.	Carbon steel
D.	Iron cobalt alloy
Answer» B. Silicon steel

Discussion

93.	A ________ motor runs at the highest speed when the load is removed.
A.	cumulatively compound
B.	series
C.	shunt
D.	differentially compound
Answer» C. shunt

Discussion

94.	A starter is necessary to start a DC motor because:
A.	DC motors do not have starting torque
B.	at starting, the value of back-EMF is zero
C.	initially, high starting torque is required
D.	it restricts initial high armature current that exists, as the value of starting back-EMF is zero
Answer» E.

Discussion

95.	Consider the following statements regarding the starting of a dc shunt motor:a. External field resistance should be minimumb. External field resistance should be maximumc. Reduced voltage should be applied to the armatured. Increased voltage should be applied to the armatureWhich of the above statements are correct?
A.	a and c are correct
B.	a and d are correct
C.	b and c are correct
D.	b and d are correct
Answer» B. a and d are correct

Discussion

96.	A dc series motor with a resistance between terminals of 1 Ω, runs at 800 rpm from a 200 V supply taking 15 A. If the speed is to be reduced to 475 rpm for the same supply voltage and current the additional series resistance to be inserted would be approximately
A.	2.5 Ω
B.	3 Ω
C.	4.5 Ω
D.	5 Ω
Answer» E.

Discussion

97.	In electric machines, copper losses occur
A.	In rotor and stator windings
B.	In commutator
C.	Due to high speed
D.	Due to flux distortion
Answer» B. In commutator

Discussion

98.	A DC Motor develops a torque of 150 N-m. A 10 percent reduction in the field flux causes a 50 percent increase in armature current. The new value of torque is:
A.	102.5 N-m
B.	202.5 N-m
C.	172.5 N-m
D.	232.5 N-m
Answer» C. 172.5 N-m

Discussion

99.	Consider the following statements:In the above figure, the curves pertain to the DC motorA. Speed vs Armature current characteristic of a DC shunt motorB. Torque vs Armature current characteristic of a DC shunt motorC. Speed vs Armature current characteristic of a DC series motorD. Torque vs Armature current characteristic of a DC series motor
A.	A - 4, B - 1, C - 3, D - 2
B.	A - 2, B - 1, C - 3, D - 4
C.	A - 4, B - 3, C - 1, D - 2
D.	A - 2, B - 3, C - 1, D - 4
Answer» D. A - 2, B - 3, C - 1, D - 4

Discussion

100.	A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved then:
A.	The speed becomes 2 pu but armature current remains at 1 pu
B.	The speed remains at 1 pu but armature current becomes 2 pu
C.	Both speed and armature current become 2 pu
D.	Both speed and armature current remain at 1 pu
Answer» C. Both speed and armature current become 2 pu

Discussion

Explore topic-wise MCQs in Electrical Machines.

In a DC motor drive, if the armature current is reversed by keeping field current positive producing a braking torque, then the drive is said to be operating in

A separately excited dc generator rotating at 3000 rpm produces an emf of 157 V and delivers a current of 20 A. The braking torque exerted by the armature is

Good commutation means

A cumulative compound d.c. motor runs at 1500 RPM on full load. if its series field is short circuited its speed:

In case of dc motor, maximum mechanical power is developed when back emf equals:

A 250 V dc shunt motor has Rf = 125 Ω and Ra = 0.5 Ω. The motor operates on no load with full field flux at rated speed 1000 rpm with Ia = 5 A. Neglect saturation and armature reaction. The motor drives a load torque of 100 Nm. The armature current of the motor is

At the time of starting the motor consumes current _______ than normal.

In electric motor _________ is loss due to fans and the bearing friction.

As the load is increases, the speed of a DC shunt motor will _______.

Consider the following statements regarding a DC motor. When the speed of a DC motor increases:1. Back emf decreases2. Current drawn increases

In DC shunt motor, if the terminal voltage is reduced to half and torque remains the same then __________.

A DC shunt machine develops an AC EMF of 250 V at 1500 rpm. Find the torque developed for an armature current of 50 A.

By conducting Swinburne’s test on a DC machine, which of the following losses can be determined?

________ braking is generally used in breaking of battery-operated scooters.

In a dc motor the windage loss is proportional to _____.

A DC series motor develops a torque of 20 Nm at 3 A of load current. If the current is increased to 6 A, the torque developed will be

A DC shunt motor has rated rpm of 480. Certain industrial application requires this motor to run at 540 rpm for some time. Which speed control will be desirable?

A dc series motor

A 4-pole dc motor takes a 50 A armature current. The armature has lap connected 480 conductors. The flux per pole is 20 mWb. Calculate the gross torque developed by the armature of the motor.

_______ combined together are called stray losses.

A electric motor operating from 220 V supply takes a current of 8 A. The motor has an efficiency of 80%. Determine the output of the motor.

Find the current required by a 400 V, 10 H.P. DC motor at 73.55% efficiency.

Enclosure of motor is made from

A DC series motor should always be started with load because_________

If Ia is the armature current of a DC series motor, then the value of (torque developed by motor for small values of armature current)/(torque developed by motor for higher values of armature current) is:

In a DC series motor, the ratio of back emf (Eb) to supply voltage (V) indicates:

In the DC motor, iron losses occur in

Energy-efficient motors generally have

If supply voltage terminals for a DC shunt motor are reversed, then:

A lap wound DC machine has 400 conductors and 8 poles. The voltage induced per conductor is 2 volts. The machine generates a voltage of

A 250 V dc motor has an armature resistance of 0.5 Ω. It is drawing an armature current of 25 A, driving a certain load. Calculate the induced emf in the motor under this conditions.

For constant load and supply voltage, two DC series motors will develop maximum torque when both are connected in

A 200 V d.c. shunt motor with armature resistance of 0.2 Ω and carrying a current of 50 A is running at 960 r.p.m. If the flux is reduced by 10% at constant torque and with negligible iron and friction losses, the speed will be nearly.

In the case of the DC series motor, it is possible to have finite no-load speed

Consider the following statements related to the speed control of a D.C. motor (a) Speed may be controlled by changing the pole flux.(b) Speed may be changed by changing voltage across the armature

A dc shunt motor operating in a linear region of magnetizing characteristics runs at 1500 rpm with a voltage rated over the entire portion of the armature in a linear region of magnetizing characteristics. If the applied voltage is reduced by 50%, what will be the speed of the motor?

A 200 V dc motor takes 50 A in series and is operating at a speed of 1000 rpm when full of electrical components (load). Its resistance is 0.2 Ω. What value of series resistance will offer a rated torque at 800 rpm?

For DC motor magnets, _______ is preferred:

A ________ motor runs at the highest speed when the load is removed.

A starter is necessary to start a DC motor because:

A dc series motor with a resistance between terminals of 1 Ω, runs at 800 rpm from a 200 V supply taking 15 A. If the speed is to be reduced to 475 rpm for the same supply voltage and current the additional series resistance to be inserted would be approximately

In electric machines, copper losses occur

A DC Motor develops a torque of 150 N-m. A 10 percent reduction in the field flux causes a 50 percent increase in armature current. The new value of torque is:

A dc shunt motor is required to drive a constant power load at rated speed while drawing rated armature current. Neglecting saturation and all machine losses, if both the terminal voltage and the field current of the machine are halved then: