DC MACHINES MCQS
1) The developed electromagnetic
force and/or torqe in electromechanical energy conversion system act in a
direction that tends
a) to
increase the stored energy at constant flux.
b) to
decrease the stored energy at constant flux.
c) to
decrease the coenergy at constant mmf.
d) to
decrease the stored energy at constant mmf.
ANS: B
2) For a linear electromagnetic
circuit,the following statement is true
a) field
energy is equal to the coenergy.
b) field
energy is greater than the coenergy.
c) field
energy is lesser than the coenergy.
d)
coenergy is zero.
ANS: A
3) In an electromechanical energy
conversion device, the developed torque depends upon
a) stator
field strength and torque angle.
b) stator
field and rotor field strength.
c) stator
field and rotor field strengths and the torque angle.
d) stator
field strength only.
4) In electromechanical energy
conversion devices(e.g. generators and motors),a small air gap is left between
the stator and the rotor ignored to
a) reduce
the reluctance of the magnetic path.
b)
increase fiux density in the air gap.
c) permit
mechanical clearance.
d) avoid
saturation of field.
ANS: C
5) Reluctance torque in rotating
machines is present,when
a) air gap
is not uniform.
b)
reluctance seen by stator mmf varies.
c) reluctance
seen by rotor mmf varies.
d)
reluctance seen by the working mmf varies.
ANS:D
6) A rotating electrical machine
having its self inductance of both the stator and the rotor winding,
independent of rotor position will definitely not developed
a) starting
torque.
b)
synchronizing torque.
c)
hysteresis torque.
d)
reluctance torque.
ANS:D
7) A circular metallic disc is
placed in a vertical magnetic field of constant induction in
the downward direction.If the disc is rotated in a
horizontal plane,the emf induced will be
a) zero.
b) constant independent of disc
size.
c) increasing radially in the
outward direction.
d) decreasing radially in the
outward direction.
ANS:A
8) The emf induced in a conductor
rotating in a bipolar field is
a)
dc.
b) ac.
c) dc and ac
both. d) none of these.
ANS:B
9) 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.
ANS:D
10) Which of the following are the
variable losses in a rotating machine?
a) core
loss and mechanical loss.
b) core
loss and stray load loss.
c) copper
and mechanical losses.
d) copper
loss and stray load loss.
ANS:D
11) stray losses are sum of:
a) iron
and mechanical losses.
b) copper
and iron losses.
c) copper
and mechanical losses.
d) none of
these.
ANS:A
12) The principle of dynamically
induced emf is utilized in
a)
transformer.
b) choke.
c)
generator.
d) thermocouple.
ANS:C
13) Which of the following is not a
part of dc machine?
a)
armature.
b) commutator.
c) field
winding. d) damping winding.
ANS:D
14) The field coils of a dc
generator are usually made of
a)
mica.
b) copper.
c) cast
iron.
d) carbon.
ANS:B
15) Laminated yoke in dc motor can
reduce
a) speed
regulation.
b) iron loss.
c) temperature
rice.
d) sparking on load.
ANS:B
16) pole shoe of a dc machine is
laminated for the purpose of
a) decreasing
hysteresis loss.
b)
decreasing eddy current loss.
c)
decreasing both hysteresis and eddy current loss.
d)
manufacturing ease.
ANS:B
17) Poles of dc machine are often
laminated to
a) reduce
pulsation.
b) reduce
armature reaction.
c) reduce
iron weight.
d)
dissipate more heat.
18) The ventilating ducts in most of
the dc machines are
a)
radial.
b) longitudinal.
c) radial as well as
longitudinal. d) none of
these.
ANS:C
19) Armature of a dc machine is made
of
a)
conducting material.
b)
insulating material.
c)
non-ferrous material.
d) silicon
steel.
ANS:D
20) Consider the following parts of
a dc machine:
1.
yoke 2.
armature core.
3.
brushes. 4. pole core.
Which of following subject to iron
loss?
a) 1 and
2
only
b) 2 only
b) 1
only
d)1,2,3 and 4
ANS:B
21) The armature of a dc machine is
laminated
a) to
reduce the hysteresis loss.
b) to
reduce eddy current loss.
c) to
reduce the mass.
d) to
reduce the inductance.
ANS:B
22) What are slot wedges in a dc
machine made of?
a) mild
steel.
b) silicon steel.
c)
fibre.
d) cast iron.
ANS:C
23) The teeth in the armature of a
dc machine are something skewed.This is done to reduce
a) hysteresis loss.
b) copper loss.
c) eddy current loss.
d) vibrations.
ANS:D
24) The commutator of a dc motor
serves the purpose of
a) changing ac to dc.
b) converting dc into ac.
c) reducing friction.
d) avoiding arc at the brushes.
ANS:B
25) Wave winding is employed in a dc
machine of
a) high current & low voltage
rating.
b) low current & high voltage
rating.
c) high current & high voltage
rating.
d) low current & low voltage
reting.
ANS:C
26) In lap winding,the number of
brushes is always------the number of poles.
a) double
b) same as
c) half
d) none of these
ANS:B
27) A 4 pole generator with 16 coils
has a two layer lap winding.The pole pitch is
a) 32
b)
16
c) 8
d) 4
ANS:C
28) Welding generator will have
a) lap
winding.
b) wave winding.
c) either lap or wave
winding.
d) none of these.
ANS:A
29) The number of parallel paths for
a 4-pole duplex lap winding will be
a)
2
b)
4
c) 6
d) 8
ANS:D
30) Minimum number of coils for
400v,6-poles lap connected dc machine for a maximum voltage of 15v between
adjacent commutator segments should be
a)
80
b)
160
c) 200
d) 400
ANS:B
31) Width of a carbon brush should
be equal to
a) less than the width of one
commutator segment
b) the width of 1to 2 commutator
segment
c) the width of 2 to 3 commutator
segment
d) the width of more than 3
commutator segments
ANS:C
32) Wave winding is composed of
a) any odd number of conductors
b) any even number of conductors
c) that even number which is exact
multiple of poles +or- 2
d) that even number which is exactly
multiple of poles
ANS:C
33) For a 4-pole machine wave
winding is impossible with --- armature conductors
a)
30
b) 32
c)
34
d) 38
ANS:B
34) A homopolar generator usually
has
a) high voltage and high
current
b) low voltage and high current
c) low voltage and low
current
d) high voltage and low current
ANS:D
35) In a dc generator,the ripples in
the direct emf generated are reduced by employing
a) cpmmutator with large number of
segments
b) equalizer
rings c) carbon
brushes d) graphite brushes
ANS:A
36) The coil span
a) must be exactly equal to pole
pitch
b) can never be equal to pole pitch
c) may or may not be exactly equal
to pole pitch
d) none of the above
ANS:C
37) The commutator pitch for simplex
wave winding is equal to
a) +1 or -1
b) twice average
pitch
c) average pitch d) half
average pitch
ANS:C
38) The commutator pitch for simplex
lap winding is
a) +1 b)
-1 c) +1 or -1 d)
average pitch
ANS:C
39) In a dc machine armature
windind,the number of commutator segments is equal to the number of armature
a)
coils
b) coils sides c) conductors d) turns
ANS:A
40) How many segments will be there
for the commutator of a 6-pole dc machine having a simple wave-wound armature
with 72 slots?
a) 73
b)72 c) 71 d) 70
ANS:B
41) For both lap and wave windings,
there are as many commutator bar as the number of
a) armature
conductors b) winding
elements
c)
poles
d) slots
ANS:B
42) In a drum type dc armature
winding the back pitch and front pitch in terms of the coil-sides must be
a) even and odd respectively
b) odd and even respectively
c) both
even
d) both odd
ANS:D
43) In a dc machine, fractional
pitch winding is employed to
a) increase generated voltage
b) reduce sparking
c) save
copper
d) both b and c
ANS:D
44) Aluminum is not used as winding
wire in dc machine armature as it
a) is
costlier
b) is of low resistivity
c) is of lower thermal
conductivity d) requires large winding space
ANS:D
45) In a lap wound dc generator
having P poles and Z conductors the maximum number of equalizes rings will be
a)
Z/P
b) 2Z/P c) Z/2P
d) Z/4P
ANS:B
46) Number of tapping’s to each
equalizer ring is equal to number of
a)
poles b) pole
pairs c) parallel
paths d) commutator
segments
ANS:B
47) A 250 V dc generator is run at
rated speed with no excitation. The open-ckt voltage will be
a)
zero
b) very small,say about 2 or 3 V
c) about
100V d)
250 V
ANS:B
48) The field coils of a dynamo have
an inductance of 8H and have a R of 60 ohms and are connected to120 V dc
source.The steady state current in the field coils is
a) 15
A
b) 2
A
c) 0 A
d) none of the above
ANS:B
49) In a dc machine,the air gap flux
distribution in space at no load is
a) sinusoidal b)
triangular c) flat topped d) pulsating
ANS:C
50) The armature MMF waveform of a
dc machine is
a)
pulsating
b)
rectangular
c) triangular
d) sinusoidal
ANS:C
51) The armature current in a
dc shunt generator is gives as
a) EG+V/RA
b) EG/RA
c) d) IL+ISH
ANS:C
52) When the shunt field of a dc
compound generator is connected across both the series field and armature. Such
a connection is known as
a) short
shunt
b) long shunt
c) cumulative
compounding
d) differential compounding
ANS:B
53) The series field of a short
shunt dc generator is excited by ---------current
a)
armature
b) shunt
field
c) load
d) Il+Ish
ANS:C
54) With the increase in field
excitation of dc generator,is generated emf
a)
increases
b)
decreases
c) remains constant
d) increases up to a limit and the remain almost constant
ANS:D
55) The slight curvature at the
lower end of the OCC of a self excited dc generator is due to
a) magnetic
inertia
b) residual flux
c) high
speed
d) high field resistance
ANS:B
56) The fall in speed of a dc
generator due to increase in load can be connected by
a) cooling the
armature
b) increasing the excitation
c) reducing the load
voltage
d) increasing the input to the prime mover
ANS:D
57) The magnetic field which
initially emf in the armature in a self excited dc generator is because of
a) permanent
magnets
b) armature rotation
c) residual
magnetism
d) none of the above
ANS:C
58) A generator may loose residual
magnetism is of the oder of
a)
heating
b)
vibrations
c) over excitation
d) any of these
ANS:A
59) In dc generators the residual
magnetism is of the order of
a)
2.5%
b)
10%
c) 15%
d) 25%
ANS:A
60) The voltage at the terminals of
a dc series generator running at rated rpm and no load will be
a) more then the rated
voltage
b) full rated voltage
c) a very small voltage
d) zero
ANS:C
61) A dc series generator is
provided with divertor and is delivering its rated current.If the divertor
switch is opened, the terminal voltage will
a) remain
constant
b)
decrease
c) increase
d) none of these
ANS:C
62) A 4 pole, 25 kw, 200 v wave
wound dc shunt generator the current in each parallel path will be
a) 62.5
A
b) 125
A
c) 31.25
A
d) 250 A
ANS:A
63) A dc shunt generator is
supplying a load of 1.8 kw at 200 V. Its armature and field resistances are 0.4
ohms and 200 ohms respectively. What is the generated emf>
a) 190
V
b) 196
V
c) 204
V
d) 210 V
ANS:C
64) If the field circuit resistance
of a dc shunt generator exceeds its critical value, the generator
a) fails to build
up
b) builds up a very high voltage
c) exceeds its current
capacity d)
produces power beyoud its rating
ANS:A
65) The factor that is not
responsible for drop in terminal voltage of a dc shunt generator on load is
a)
commutation
b) armature reaction
c) armature resistance
drop d) field weakening due to (b) and (c)
ANS:A
66) In a dc shunt generator, the
voltage build up is generally restricted due to
a) armature
heating
b) insulation restrictions
c) saturation of
iron
d) speed limitation
ANS:C
67) An ideal dc generator has a
regulation of ------ per cent
a)
zero b)
10
c) 25
d) 100
ANS:A
68) The terminal voltage of an over
compounded dc generator
a) decreases with the increase in
load current
b) increases with the increase in
load current
c) remains constant at all load
current
d) none of the above
ANS:B
69) In a dc compound generator, flat
compound characteristic, required for certain applications,may be obtained by
connecting a variable resistance
a) across the series
field
b) in series with the series field
c) in oarallel with the shunt
field d) in series with the shunt
field
ANS:A
70) A cumulatively compounded dc
generator is supplying 20 A at 200 V. Now if the series field winding is short
circuited, the terminal voltage
a) will remain unaltered at 200 V
b) will rise to 220 V
c) will shoot upto a very high value
d) will become less than 200 V
ANS:A
71) Over or under compounding of a
dc generator is achieved by
a) shunting more or less current
from the shunt field
b) shunting more or less current
from the series field
c) connecting its as long shunt
d) connecting its as short-shunt
ANS:B
72) An over-compounded dc generator
is supplying 800 A at full load on 500 V. The resistance of each conductor is
0.02 ohm. The percentage of compounding is
a) 1.6
%
b)
3.2%
c)
6.4%
d) 12.8%
ANS:C
73) Which of the following dc
generators will be in a position to build up without any residual magnetism in
the field?
a)
series
b)
shunt
c) separately
excited
d) compound
ANS:C
74) Which of the following dc
generators has rising v-I characteristics?
a)
series
b)
shunt
c)
compound
d) none
ANS:A
75) Which of the following dc
generators will have negligible terminal voltage on no load?
a)
shunt
b)
series
c)
compound
d) none
ANS:B
76) An exciter for turbo-generator
is a dc ---------
a)
shunt
b)
series
c)
compound
d) separately excited
ANS:A
77) For a specific open-ckt voltage
of a dc generator, the short-ckt current will be maximum when it is
a) separately
excited
b) shunt connected
c) cumulatively
compounded
d)differentially compounded
ANS:C
78) The voltage regulation of a dc
generator at full load being zero implies that the generator is
a)
shunt-connected
b) differentially compounded
c) cumulatively
compounded
d) series-connected
ANS:C
79) The ------ generator has the
poorest voltage regulation
a)
shunt
b) over compounded
c) under
compounded
d) differential compounded
ANS:D
80) The voltmeter connected across a
generator reads voltage same at no load and at full load. The generator is of
the type
a) shunt
generator
b) series generator
c) level
compound
d) short-shunt compound
ANS:C
81) Flashing of field of dc
generator means
a) neutralization of residual
magnetism
b) creation of residual magnetism by
a dc source
c) increasing flux density by
providing extra ampere-turns in field
d) none of the above
ANS:B
82) a dc series generator is
employed
a) as a booster to maintain constant
voltage at the load end of the feeder
b) for supplying traction load
c) for supplying industrial load a
constant voltage
d) for battery charging
ANS: A
83) The type of dc generator used
for arc welding purposes is a
a) series
generator
b) shunt generator
c) differentially compounded generator
d) cumulatively compounded generator
ANS: D
84) The essential condition for
satisfactory parallel operation of two dc generators is that they should have
same
a) kw output rating
b) drooping voltage characteristics
c) percentage regulation
d) speed of operation
ANS: B
85) When two dc series generators
are operating in parallel, an equalizer bar is used
a) to reduce armature
reaction
b) to increase emf
c) to increase the
speed
d) so that the two similar machines take approximately equal load current
ANS: D
86) The simplest way of shifting
load from one shunt generator to the other operating in parallel is by
a) Adjustment of
speeds
b) adjustment of armature resistances
c) Adjustment of field
rheostats d) using equalizer
connections
ANS: C
87) If field of one of two
generators operating in parallel is made very weak, then it wil
a) not take any
load
b) take major share of load
c) operate as a motor and run in the
same direction d) operate as a motor and run in the opposite
direction
ANS: C
88) DC generators are connected to
or disconnected from the busbar only under the floating condition. This is done
to avoid
a) burning of switch
constants
b) sudden loading of the prime mover
c) mechanical jerk to the
shaft
d) all of the above
ANS: D
89) The main factor that leads to
unstable parallel operation of flat or over compounded dc generators is
a) their rising voltage
characteristics
b) unequal series field resistance
c) unequal speed regulation of their
prime movers d) their drooping voltage characteristics
ANS: A
90) In the case of parallel
operation of dc compound wound generators, for proper division of load from no
load to fill load it is essential that they should have the same
a) KW output
rating
b) percentage regulation
c) series field
resistance d) speed of
operation
ANS: B
91) In parallel operation two dc
compound generator provided with interpoles, the equalizing bar must be
connected to
a) inter pole field
only
b) series field only
c) junction of series and interpole
fields d) none of the above
ANS: C
92) For parallel operation, the dc
generator normally preferred are
a)
shunt
b)series c)
over-compound d) under compound
e) both shunt and under compound
ANS: E
93) Equalizer bar is necessary for
parallal operation of dc ----------- generators
a)
series b)
shunt c)
over-compound d) under
compound
e) series and over-compound
ANS: E
94) In the block diagram of a
separately excited dc motor, how does the armature induced emf appear as?
a) positive
feedback
b) negative feedback
c) disturbance
input
d) output
ANS: B
95) A 220 V dc machine has an
armature resistance of 1ohm if the full load current is 20 A, the difference of
induced voltage, when the machine is running as a motor and generator, is
a) 20
V
b) 0
V
c) 40
V
d) 60 V
ANS: C
96) The current drawn by a 220 V dc
motor of armature resistance 0.5 ohms and back emf 200 V is
a) 40
A
b) 44 A
c) 400
A
d) 440 A
ANS: A
97) The current drawn by a 120 V dc
motor with back emf of 110 V and armature resistance of 0.4 ohms is
a) 4
A
b) 25
A
c) 275
A
d) 300 A
ANS: B
98) In a dc motor, energy conversion
would not have been possible but for
a) input energy from the
supply
b) production of back emf in the armature
c) use of
commutator
d) rotation of the machine
ANS: B
99) With the increase in speed of a
dc motor
a) both back emf as well as line
current increase
b) both back emf as well as line
current fall
c) back emf increases but line
current falls
d) back emf falls and line current
increases
ANS: C
100) If the back emf in a dc motor
vanishes suddenly, the motor will
a) start
hunting
b) burn
c) run at very high
speed d) run at very slow speed
ANS: B
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