MCQ in DC Motors Flashcards
(139 cards)
1
Q
The DC motor armature whose speed is to be controlled by Ward-Leonard system is fed by ___________________.
A
DC generator
2
Q
What dc motor over-speeds at light loads?
A
Series Motor
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
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.
5
Q
The speed of a series motor at no load is ____________.
A
infinity
6
Q
The armature of a dc shunt motor is laminated to reduce the __________________.
A
eddy-current loss
7
Q
A dc series motor is started without load, the effect is a rapid increase in _____________.
A
its emf
8
Q
The rotation of a direct current motor may be reversed by _______________.
A
reversing either the armature or field
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
10
Q
___________ is called the electrical prime mover.
A
Motor
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
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
13
Q
The speed of a dc-shunt motor can be increased under which condition?
A
Increased armature voltage
14
Q
The speed of a dc shunt motor can be slowed down under which condition?
A
Increased field voltage
15
Q
The source of mechanical power to turn the rotors of alternators, large and small AC and DC.
A
Prime mover
16
Q
It refers to a form of DC motor generator with a common field coil for both motor and generator.
A
Dynamotor
17
Q
The power and the torque of an electric motor are linked by _________.
A
shaft
18
Q
In a d.c. motor, unidirectional torque is produced with the help of ______________.
A
brushes, commutator
19
Q
The counter e.m.f. of a d.c. motor _____________.
A
regulates its armature voltage
20
Q
The normal value of the armature resistance of a d.c. motor is _____________.
A
0.5 ohm
21
Q
The Eb/V ratio of a d.c. motor is an indication of its ___________.
A
efficiency
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.
23
Q
The induced e.m.f. in the armature conductors of a d.c. motor is ______________.
A
sinusoidal
24
Q
A d.c. motor can be looked upon as d.c. generator with the power flow _____________.
A
reversed
25
In a d.c. motor, the mechanical output power actually comes from ______________.
electrical input power
26
The maximum torque of d.c. motors is limited by ____________.
commutation
27
Which quantity maintains the same direction whether a d.c. motor runs as a generator or as a motor?
induced e.m.f.
28
Under constant load conditions, the speed of a d.c. motor is affected by ___________.
field flux
29
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.
armature current
30
The shaft torque of a d.c. motor is less than its armature torque because of ___________ losses.
rotational
31
If load on a d.c. shunt motor is increased, its speed is decreased due primarily to ___________.
decrease in back e.m.f.
32
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 _________.
increase by about 10%
33
If the pole flux of a d.c. motor approaches zero, its speed will _________.
approach infinity
34
If the field current of a loaded shunt motor is suddenly opened, _________.
circuit breaker or fuse will open the circuit before too much damage is done to the motor
35
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
C
36
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
D
37
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
38
As the load is increased, the speed of a d.c. shunt motor __________.
reduces slightly
39
Between no-load and full-load, ________ motor develops the least torque.
series
40
The Ta/Ia graph of a d.c. series motor is a __________.
parabola up to full-load and a straight line at overloads
41
As compared to shunt and compound motors, series motor has the highest torque because of its comparatively ________ at the start.
stronger series field
42
Unlike shunt motor, it is difficult for a series motor to stall under heavy loading because __________.
it develops high overload torque
43
When the load is removed, the ________ motor will run at the highest speed.
series
44
A series motor is best suited for driving _________.
cranes and hoists
45
The d.c. series motor should never be switched on at no load because __________.
the speed becomes dangerously high
46
A shunt d.c. motor works on a.c. mains __________.
unsatisfactory
47
The speed of a d.c. motor can be controlled by varying ________.
(i) its flux per pole
(ii) resistance of armature circuit
(iii) applied voltage
48
The most efficient method of increasing the speed of a 3.75 kW d.c. shunt motor would be the ____ method.
flux control
49
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
D
50
In the rheostatic method of speed control for a d.c. shunt motor, used of armature diverter makes the method _________.
suitable for rapidly changing loads
51
The chief advantage of Ward-Leonard system of d.c. motor speed is that it ___________.
can be used for small motors
52
The flux control method using paralleling of field coils when applied to a 4-pole series d.c. motor can give ____ speeds.
four
53
The series-parallel system of speed control of series motor widely used in traction work gives a speed range of about _______.
1:3
54
In practice, regenerative braking is used when _________.
load has overhauling characteristics
55
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
C
56
Ward-Leonard system of speed control is NOT recommended for ___________.
constant speed operation
57
Thyristor chopper circuits are employed for ____________.
lowering the level of a d.c. voltage
58
An inverter circuit is employed to convert _________.
d.c. voltage into a.c. voltage
59
The phase control rectifiers used for speed of d.c. motors to convert fixed a.c. supply of voltage into ____________.
variable d.c. supply voltage
60
If some of the switching devices in a converter are controlled devices and some are diodes, the converter is called ____________.
semi converter
61
A solid-state chopper converts a fixed-voltage d.c. supply into a __________.
variable-voltage d.c. supply
62
The d.c. motor terminal voltage supplied by a solid-state chopper for speed control purposes varies with the duty ratio of the chopper ___________.
indirectly
63
One of the main advantages of Swinburne’s test is that it _____________.
is very economical and convenient
64
The main disadvantage of Hopkinson’s test for finding efficiency of shunt d.c. motors is that it _________________.
requires two identical shunt machines
65
The most economical method of finding no-load losses of a large d.c. shunt motor is _____________.
Swinburne’s
66
Retardation test on a dc shunt motor is used for finding _______ losses.
stray
67
The main thing common between Hopkinson’s test and field’s test is that both ___________.
need two similar mechanically-coupled motors
68
The usual test for determining the efficiency of a traction number is the _________ test.
Field’s
69
A dc motor is used to _____________.
change electrical energy to mechanical energy
70
A dc motor is still used in industrial application because it ____________.
provides fine speed control
71
Carbon brushes are preferable to copper brushes because ___________.
they reduce sparking
72
The field poles and armature of a dc machine are laminated to ___________.
reduce eddy currents
73
The back emf in a dc motor __________.
opposes the applied voltage
74
The value of back emf (Eb) in a dc motor is maximum at ___________.
no load
75
The motor equation is given by __________.
V = Eb + IaRa
76
The quantity EbIa represents (for DC shunt motor) _____________.
total friction and windage and core losses
77
The mechanical power developed in a dc motor is maximum when back emf (Eb) is equal to ____ the applied voltage (V).
half
78
When the speed of a dc shunt motor increases, its armature current ___________.
decreases
79
The amount of back emf of a shunt motor will increase when ____________.
the field is strengthened
80
The speed of a dc motor is ___________.
inversely proportional to flux per pole
81
The torque developed by a dc motor is directly proportional to ___________.
flux per pole x armature current
82
The shaft torque (Tsh) in a dc motor is less than total armature torque (Ta) because of _________.
iron and friction losses
83
Armature reaction in a dc motor is increased _________.
when the armature current increases
84
With respect to the direction of rotation, interpoles on a dc motor must have the same polarity as the main poles __________.
behind them
85
In a dc motor, the brushes are shifted from the mechanical neutral plane in direction opposite to the rotation to ___________.
reduce sparking
86
In very large dc motors with severe heavy duty, armature reaction effects are corrected by __________.
using compensatory windings in addition to interpoles
87
The speed of a _______ motor is practically constant.
shunt
88
________ motor is a variable speed motor.
Series
89
The most commonly used method of speed control of a dc motor is by varying ________.
field strength
90
The running speed of a dc series motor is basically determined by ___________.
load
91
After a shunt motor is up to speed, the speed may be increased considerably by __________.
decreasing field circuit resistance
92
_______ motor has the best speed regulation.
Shunt
93
The deciding factor in the selection of a dc motor for a particular application is its _________ characteristics.
speed-torque
94
The demand for a large increase in torque of a dc shunt motor is met by a _________.
large increase in current
95
The demand for a large increase in torque of a dc series motor is met by a _________.
large decrease in speed
96
A series motor will over-speed when __________.
load is removed
97
As the load increases, a ______ motor will speed up.
differentially compounded
98
A cummulatively compounded motor does not run at dangerous speed at light loads because of the presence of _________.
shunt winding
99
DC shunt motors are used in applications where _______ is required.
practically constant speed
100
DC series motors are used in applications where ________ is required.
high starting torque
101
Cummulatively compounded motors are used where we require ___________.
sudden heavy loads for short duration
102
For the same rating, _______ motor has the highest starting torque.
series
103
________ motors never use belt-connected loads.
Series
104
For 20% increase in current, the motor that will give the greatest increase in torque is ________ motor.
series
105
The _______ motor is used to start heavy loads.
series
106
When load is removed, the motor that will run at the highest speed is __________.
series
107
For the same rating, _______ motor has the least starting torque.
shunt
108
A dc series motor is most suitable for _________.
cranes
109
________ is most suitable for punch presses.
Cummulatively compounded
110
In a vacuum cleaner, we generally used _______ motor.
series
111
The most suitable motor for elevators is the ________ motor.
cummulatively compounded
112
The friction and windage losses in a dc motor depends upon ___________.
speed
113
The iron losses in a dc motor depend upon _________.
both speed and flux
114
The greatest percentage of power loss in a dc motor is due to ____________.
copper loss
115
A motor takes a large current at starting because ___________.
back emf is low
116
Excessive sparking at the brushes may be cause due to __________.
dirt on the commutator
117
Excessive motor vibration is caused by __________.
worn bearings
118
Overheating of a dc motor is often due to _________.
overloads
119
Hot bearings of a dc motor may be caused by _________.
lack of dirty lubricant
120
Intermittent sparking at the brushes of a dc motor may be caused due to ___________.
an open armature coil
121
The function of a brush and commutator arrangement in a dc motor is ___________.
to produce unidirectional torque
122
The direction of rotation of a dc motor can be reversed ___________.
by reversing the connections of both armature or the filed winding connections with the supply
123
A dc series motor should always be started with load because ___________.
at no-load it will rotate at a dangerously high speed
124
Swinburne’s method cannot be used for determining the efficiency of a dc series motor because ____________.
it is not advisable to run a series motor on no-load
125
What is the effect produced by the electric motor?
magnetic as well as heating effect
126
The rotating part of a dc motor is known as _________.
armature
127
The function of a commutator in a dc machine is __________.
to change alternating voltage to direct voltage
128
Carbon brushes are used in electric motors to __________.
provide a path for the flow of current
129
Voltage equation of dc motor is __________.
V = Eb + IaRa
130
Condition for maximum power output for a dc motor is ___________.
Eb = V/2
131
The speed of a motor is __________.
directly proportional to back emf and inversely proportional to flux
132
The highest speed attained by a shunt motor is __________.
equal to no load speed at rated flux
133
In dc motor iron losses occur in ___________.
the yoke
134
The dummy coil in a dc machines is used to ___________.
bring about mechanical balance of armature
135
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 ___________.
to increase the speed of the motor
136
The speed of a dc shunt motor is required to be more than F.L. speed. This is possible by ________.
reducing the field current
137
If speed of a dc shunt motor increases, the back emf ___________.
increases
138
Interpole windings in direct current motors are used primarily __________.
to reduce the effect of armature reaction
139
In a series motor, what is the relation of the connection of motor field winding with respect to the motor armature?
series
