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Machine Design > MCQ in DC Motors > Flashcards

MCQ in DC Motors Flashcards

1
Q

The DC motor armature whose speed is to be controlled by Ward-Leonard system is fed by ___________________.

A

DC generator

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2
Q

What dc motor over-speeds at light loads?

A

Series Motor

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3
Q

DC motor starters are used for the following functions EXCEPT one. Which one is this?

A. control the speed of the motor

B. start large DC motors

C. limit the starting current of the motor

D. prevent large voltage drop during starting

A

A

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4
Q

If you try to start a DC compound motor the shunt field of which is open, what would happen?

A

It will overspeed.

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5
Q

The speed of a series motor at no load is ____________.

A

infinity

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6
Q

The armature of a dc shunt motor is laminated to reduce the __________________.

A

eddy-current loss

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7
Q

A dc series motor is started without load, the effect is a rapid increase in _____________.

A

its emf

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8
Q

The rotation of a direct current motor may be reversed by _______________.

A

reversing either the armature or field

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9
Q

It is a motor generator unit that is built to convert low-voltage direct current to high-voltage current both uses the same field windings and share armature.

A

Dynamotor

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10
Q

___________ is called the electrical prime mover.

A

Motor

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11
Q

In D.C motors, a type of armature consisting of coils actually wound around the armature core so that all coil surfaces are exposed to the magnetic field.

A

Drum wound armature

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12
Q

Type of motor where the field windings are connected in series with armature coil and the field strength varies with changes in armature current.

A

Series motor

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13
Q

The speed of a dc-shunt motor can be increased under which condition?

A

Increased armature voltage

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14
Q

The speed of a dc shunt motor can be slowed down under which condition?

A

Increased field voltage

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15
Q

The source of mechanical power to turn the rotors of alternators, large and small AC and DC.

A

Prime mover

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16
Q

It refers to a form of DC motor generator with a common field coil for both motor and generator.

A

Dynamotor

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17
Q

The power and the torque of an electric motor are linked by _________.

A

shaft

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18
Q

In a d.c. motor, unidirectional torque is produced with the help of ______________.

A

brushes, commutator

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19
Q

The counter e.m.f. of a d.c. motor _____________.

A

regulates its armature voltage

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20
Q

The normal value of the armature resistance of a d.c. motor is _____________.

A

0.5 ohm

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21
Q

The Eb/V ratio of a d.c. motor is an indication of its ___________.

A

efficiency

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22
Q

The mechanical power developed by the armature of a d.c. motor is equal to _______________.

A

armature current multiplied by back e.m.f.

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23
Q

The induced e.m.f. in the armature conductors of a d.c. motor is ______________.

A

sinusoidal

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24
Q

A d.c. motor can be looked upon as d.c. generator with the power flow _____________.

A

reversed

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25
Q

In a d.c. motor, the mechanical output power actually comes from ______________.

A

electrical input power

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26
Q

The maximum torque of d.c. motors is limited by ____________.

A

commutation

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27
Q

Which quantity maintains the same direction whether a d.c. motor runs as a generator or as a motor?

A

induced e.m.f.

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28
Q

Under constant load conditions, the speed of a d.c. motor is affected by ___________.

A

field flux

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29
Q

It is possible to increase the field flux and, at the same time, increase the speed of a d.c. motor provided its ________________ is held constant.

A

armature current

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30
Q

The shaft torque of a d.c. motor is less than its armature torque because of ___________ losses.

A

rotational

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31
Q

If load on a d.c. shunt motor is increased, its speed is decreased due primarily to ___________.

A

decrease in back e.m.f.

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32
Q

If the load current and flux of a d.c. motor are held constant and voltage applied across its armature is increased by 10 %, its speed will _________.

A

increase by about 10%

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33
Q

If the pole flux of a d.c. motor approaches zero, its speed will _________.

A

approach infinity

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34
Q

If the field current of a loaded shunt motor is suddenly opened, _________.

A

circuit breaker or fuse will open the circuit before too much damage is done to the motor

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35
Q

Which of the following d.c. motor would be suitable for drives requiring high starting torque but only fairly constant speed such as crushers?

A. shunt

B. series

C. compound

D. permanent magnet

A

C

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36
Q

A d.c. shunt motor is found suitable to drive fans because they require

A. small torque at start up

B. large torque at high speeds

C. practically constant voltage

D. both A and B

A

D

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37
Q

Which of the following load would be best driven by a d.c. compound motor?

A. reciprocating pump

B. centrifugal pump

C. electric locomotive

D. fan

A

A

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38
Q

As the load is increased, the speed of a d.c. shunt motor __________.

A

reduces slightly

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39
Q

Between no-load and full-load, ________ motor develops the least torque.

A

series

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40
Q

The Ta/Ia graph of a d.c. series motor is a __________.

A

parabola up to full-load and a straight line at overloads

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41
Q

As compared to shunt and compound motors, series motor has the highest torque because of its comparatively ________ at the start.

A

stronger series field

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42
Q

Unlike shunt motor, it is difficult for a series motor to stall under heavy loading because __________.

A

it develops high overload torque

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43
Q

When the load is removed, the ________ motor will run at the highest speed.

A

series

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44
Q

A series motor is best suited for driving _________.

A

cranes and hoists

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45
Q

The d.c. series motor should never be switched on at no load because __________.

A

the speed becomes dangerously high

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46
Q

A shunt d.c. motor works on a.c. mains __________.

A

unsatisfactory

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47
Q

The speed of a d.c. motor can be controlled by varying ________.

A

(i) its flux per pole

(ii) resistance of armature circuit

(iii) applied voltage

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48
Q

The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be the ____ method.

A

flux control

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49
Q

Regarding Ward-Leonard system of speed control which statement is false?

A. it is usually used where wide and very sensitive speed control is required

B. it is used for motors having ratings from 750 kW to 4000 kW

C. capital outlay involved in the system is right since it uses two extra machine

D. it has low overall efficiency especially at light loads

A

D

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50
Q

In the rheostatic method of speed control for a d.c. shunt motor, used of armature diverter makes the method _________.

A

suitable for rapidly changing loads

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51
Q

The chief advantage of Ward-Leonard system of d.c. motor speed is that it ___________.

A

can be used for small motors

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52
Q

The flux control method using paralleling of field coils when applied to a 4-pole series d.c. motor can give ____ speeds.

A

four

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53
Q

The series-parallel system of speed control of series motor widely used in traction work gives a speed range of about _______.

A

1:3

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54
Q

In practice, regenerative braking is used when _________.

A

load has overhauling characteristics

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55
Q

Statement 1. A direct-on-line (DOL) starter is used to start a small d.c. motor because

Statement 2. it limits initial current drawn by the armature circuit.

A. both statements 1 and 2 are incorrect

B. both statements 1 and 2 are correct

C. statement 1 is correct but 2 is wrong

D. statement 2 is correct but 1 is wrong

A

C

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56
Q

Ward-Leonard system of speed control is NOT recommended for ___________.

A

constant speed operation

57
Q

Thyristor chopper circuits are employed for ____________.

A

lowering the level of a d.c. voltage

58
Q

An inverter circuit is employed to convert _________.

A

d.c. voltage into a.c. voltage

59
Q

The phase control rectifiers used for speed of d.c. motors to convert fixed a.c. supply of voltage into ____________.

A

variable d.c. supply voltage

60
Q

If some of the switching devices in a converter are controlled devices and some are diodes, the converter is called ____________.

A

semi converter

61
Q

A solid-state chopper converts a fixed-voltage d.c. supply into a __________.

A

variable-voltage d.c. supply

62
Q

The d.c. motor terminal voltage supplied by a solid-state chopper for speed control purposes varies with the duty ratio of the chopper ___________.

A

indirectly

63
Q

One of the main advantages of Swinburne’s test is that it _____________.

A

is very economical and convenient

64
Q

The main disadvantage of Hopkinson’s test for finding efficiency of shunt d.c. motors is that it _________________.

A

requires two identical shunt machines

65
Q

The most economical method of finding no-load losses of a large d.c. shunt motor is _____________.

A

Swinburne’s

66
Q

Retardation test on a dc shunt motor is used for finding _______ losses.

A

stray

67
Q

The main thing common between Hopkinson’s test and field’s test is that both ___________.

A

need two similar mechanically-coupled motors

68
Q

The usual test for determining the efficiency of a traction number is the _________ test.

A

Field’s

69
Q

A dc motor is used to _____________.

A

change electrical energy to mechanical energy

70
Q

A dc motor is still used in industrial application because it ____________.

A

provides fine speed control

71
Q

Carbon brushes are preferable to copper brushes because ___________.

A

they reduce sparking

72
Q

The field poles and armature of a dc machine are laminated to ___________.

A

reduce eddy currents

73
Q

The back emf in a dc motor __________.

A

opposes the applied voltage

74
Q

The value of back emf (Eb) in a dc motor is maximum at ___________.

A

no load

75
Q

The motor equation is given by __________.

A

V = Eb + IaRa

76
Q

The quantity EbIa represents (for DC shunt motor) _____________.

A

total friction and windage and core losses

77
Q

The mechanical power developed in a dc motor is maximum when back emf (Eb) is equal to ____ the applied voltage (V).

A

half

78
Q

When the speed of a dc shunt motor increases, its armature current ___________.

A

decreases

79
Q

The amount of back emf of a shunt motor will increase when ____________.

A

the field is strengthened

80
Q

The speed of a dc motor is ___________.

A

inversely proportional to flux per pole

81
Q

The torque developed by a dc motor is directly proportional to ___________.

A

flux per pole x armature current

82
Q

The shaft torque (Tsh) in a dc motor is less than total armature torque (Ta) because of _________.

A

iron and friction losses

83
Q

Armature reaction in a dc motor is increased _________.

A

when the armature current increases

84
Q

With respect to the direction of rotation, interpoles on a dc motor must have the same polarity as the main poles __________.

A

behind them

85
Q

In a dc motor, the brushes are shifted from the mechanical neutral plane in direction opposite to the rotation to ___________.

A

reduce sparking

86
Q

In very large dc motors with severe heavy duty, armature reaction effects are corrected by __________.

A

using compensatory windings in addition to interpoles

87
Q

The speed of a _______ motor is practically constant.

A

shunt

88
Q

________ motor is a variable speed motor.

A

Series

89
Q

The most commonly used method of speed control of a dc motor is by varying ________.

A

field strength

90
Q

The running speed of a dc series motor is basically determined by ___________.

A

load

91
Q

After a shunt motor is up to speed, the speed may be increased considerably by __________.

A

decreasing field circuit resistance

92
Q

_______ motor has the best speed regulation.

A

Shunt

93
Q

The deciding factor in the selection of a dc motor for a particular application is its _________ characteristics.

A

speed-torque

94
Q

The demand for a large increase in torque of a dc shunt motor is met by a _________.

A

large increase in current

95
Q

The demand for a large increase in torque of a dc series motor is met by a _________.

A

large decrease in speed

96
Q

A series motor will over-speed when __________.

A

load is removed

97
Q

As the load increases, a ______ motor will speed up.

A

differentially compounded

98
Q

A cummulatively compounded motor does not run at dangerous speed at light loads because of the presence of _________.

A

shunt winding

99
Q

DC shunt motors are used in applications where _______ is required.

A

practically constant speed

100
Q

DC series motors are used in applications where ________ is required.

A

high starting torque

101
Q

Cummulatively compounded motors are used where we require ___________.

A

sudden heavy loads for short duration

102
Q

For the same rating, _______ motor has the highest starting torque.

A

series

103
Q

________ motors never use belt-connected loads.

A

Series

104
Q

For 20% increase in current, the motor that will give the greatest increase in torque is ________ motor.

A

series

105
Q

The _______ motor is used to start heavy loads.

A

series

106
Q

When load is removed, the motor that will run at the highest speed is __________.

A

series

107
Q

For the same rating, _______ motor has the least starting torque.

A

shunt

108
Q

A dc series motor is most suitable for _________.

A

cranes

109
Q

________ is most suitable for punch presses.

A

Cummulatively compounded

110
Q

In a vacuum cleaner, we generally used _______ motor.

A

series

111
Q

The most suitable motor for elevators is the ________ motor.

A

cummulatively compounded

112
Q

The friction and windage losses in a dc motor depends upon ___________.

A

speed

113
Q

The iron losses in a dc motor depend upon _________.

A

both speed and flux

114
Q

The greatest percentage of power loss in a dc motor is due to ____________.

A

copper loss

115
Q

A motor takes a large current at starting because ___________.

A

back emf is low

116
Q

Excessive sparking at the brushes may be cause due to __________.

A

dirt on the commutator

117
Q

Excessive motor vibration is caused by __________.

A

worn bearings

118
Q

Overheating of a dc motor is often due to _________.

A

overloads

119
Q

Hot bearings of a dc motor may be caused by _________.

A

lack of dirty lubricant

120
Q

Intermittent sparking at the brushes of a dc motor may be caused due to ___________.

A

an open armature coil

121
Q

The function of a brush and commutator arrangement in a dc motor is ___________.

A

to produce unidirectional torque

122
Q

The direction of rotation of a dc motor can be reversed ___________.

A

by reversing the connections of both armature or the filed winding connections with the supply

123
Q

A dc series motor should always be started with load because ___________.

A

at no-load it will rotate at a dangerously high speed

124
Q

Swinburne’s method cannot be used for determining the efficiency of a dc series motor because ____________.

A

it is not advisable to run a series motor on no-load

125
Q

What is the effect produced by the electric motor?

A

magnetic as well as heating effect

126
Q

The rotating part of a dc motor is known as _________.

A

armature

127
Q

The function of a commutator in a dc machine is __________.

A

to change alternating voltage to direct voltage

128
Q

Carbon brushes are used in electric motors to __________.

A

provide a path for the flow of current

129
Q

Voltage equation of dc motor is __________.

A

V = Eb + IaRa

130
Q

Condition for maximum power output for a dc motor is ___________.

A

Eb = V/2

131
Q

The speed of a motor is __________.

A

directly proportional to back emf and inversely proportional to flux

132
Q

The highest speed attained by a shunt motor is __________.

A

equal to no load speed at rated flux

133
Q

In dc motor iron losses occur in ___________.

A

the yoke

134
Q

The dummy coil in a dc machines is used to ___________.

A

bring about mechanical balance of armature

135
Q

An external resistance is added in the series with the field of a dc shunt motor. When the motor runs, the effect of resistance is ___________.

A

to increase the speed of the motor

136
Q

The speed of a dc shunt motor is required to be more than F.L. speed. This is possible by ________.

A

reducing the field current

137
Q

If speed of a dc shunt motor increases, the back emf ___________.

A

increases

138
Q

Interpole windings in direct current motors are used primarily __________.

A

to reduce the effect of armature reaction

139
Q

In a series motor, what is the relation of the connection of motor field winding with respect to the motor armature?

A

series

Machine Design (3 decks)

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