Top Dc Generators Objective Questions For Freshers
1.In D.C. generators the pole shoes are fastened to the pole core by
rivets
counter sunk screws
brazing
welding
ANSWER : : counter sunk screws
2.Iron losses in a D.C. machine are independent of variations in
speed
load
voltage
speed and voltage
ANSWER : : load
3.In lap winding, the number of brushes is always
double the number of poles
same as the number of poles
half the number of poles
two
ANSWER : : same as the number of poles
4.The armature of D.C. generator is laminated to
reduce the bulk
provide the bulk
insulate the core
reduce eddy current loss
ANSWER : : reduce eddy current loss
5.The resistance of armature winding depends on
length of conductor
cross-sectional area of the conductor
number of conductors
all of the above
ANSWER : : all of the above
6.Which of the following could be lamina-proximately the thickness of lamina-tions of a D.C. machine ?
0.005 mm
0.05 mm
0.5 m
5 m
ANSWER : : 0.5 m
7.The field coils of D.C. generator are usually made of
mica
copper
cast iron
carbon
ANSWER : : copper
8.In case of D.C. machines, mechanical losses are primary function of
current
voltage
speed
none of above
ANSWER : : speed
9.A separately excited generator as compared to a self-excited generator
is amenable to better voltage con-trol
is more stable
has exciting current independent of load current
has all above features
ANSWER : : has all above features
10.The commutator segments are connected to the armature conductors by means of
copper lugs
resistance wires
insulation pads
brazing
ANSWER : : copper lugs
11.Brushes of D.C. machines are made of
carbon
soft copper
hard copper
all of above
ANSWER : : carbon
12.The bearings used to support the rotor shafts are generally
ball bearings
bush bearings
magnetic bearmgs
needle bearings
ANSWER : : ball bearings
13.For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?
Lap winding
Wave winding
Either of (a) and (b) above
Depends on other features of design
ANSWER : : Wave winding
14.In D.C. generators, the cause of rapid brush wear may be
severe sparking
rough commutator surface
imperfect contact
any of the above
ANSWER : : any of the above
15.In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be
4
8
16
32
ANSWER : : 8
16.In D.C. generators, current to the external circuit from armature is given through
commutator
solid connection
slip rings
none of above
ANSWER : : commutator
17.In a commutator
copper is harder than mica
mica and copper are equally hard
mica is harder than copper
none of the above
ANSWER : : mica is harder than copper
18.If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by
Blv
Blv2
Bl2v
Bl2v2
ANSWER : : Blv
19.The material for commutator brushes is generally
mica
copper
cast iron
carbon
ANSWER : : carbon
20.Copper brushes in D.C. machine are used
where low voltage and high currents are involved
where high voltage and small cur-rents are involved
in both of the above cases
in none of the above cases
ANSWER : : where low voltage and high currents are involved
21.In a four-pole D.C. machine
all the four poles are north poles
alternate poles are north and south
all the four poles are south poles
two north poles follow two south poles
ANSWER : : alternate poles are north and south
22.According to Fleming’s right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of
motion of conductor
lines of force
either of the above
none of the above
ANSWER : : lines of force
23.While applying Fleming’s right-hand rule to And the direction of induced e.m.f., the thumb points towards
direction of induced e.m.f.
direction of flux
direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
direction of motion of conductor, if forefinger points along the lines of flux
ANSWER : : direction of motion of conductor, if forefinger points along the lines of flux
24.Fleming’s right-hand rule regarding direction of induced e.m.f., correlates
magnetic flux, direction of current flow and resultant force
magnetic flux, direction of motion and the direction of e.m.f. induced
magnetic field strength, induced voltage and current
magnetic flux, direction of force and direction of motion of conductor
ANSWER : : magnetic flux, direction of motion and the direction of e.m.f. induced
25.If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be
demagnetisation only
cross magnetisation as well as magnetisation
crossmagnetisation as well as demagnetising
cross magnetisation only
ANSWER : : crossmagnetisation as well as demagnetising
26.Armature reaction of an unsaturated D.C. machine is
crossmagnetising
demagnetising
magnetising
none of above
ANSWER : : crossmagnetising
27.In the case of lap winding resultant pitch is
multiplication of front and back pitches
division of front pitch by back pitch
sum of front and back pitches
difference of front and back pitches
ANSWER : : difference of front and back pitches
28.Welding generator will have
lap winding
wave winding
delta winding
duplex wave winding
ANSWER : : lap winding
29.The armature core of a D.C. generator is usually made of
silicon steel
copper
non-ferrous material
cast-iron
30.The voltage drop for which of the following types of brush can be expected to be least ?
Graphite brushes
Carbon brushes
Metal graphite brushes
None of the above
ANSWER : : Metal graphite brushes
31.For a D.C. machines laboratory following type of D.C. supply will be suitable
rotary converter
mercury are rectifier
induction motor D.C. generator set
synchronous motor D.C. generator set
ANSWER : : induction motor D.C. generator set
32.The number of brushes in a commutator depends on
speed of armature
type of winding
voltage
amount of current to be collected
ANSWER : : amount of current to be collected
33.Which of the following statement about D.C. generators is false ?
Compensating winding in a D.C. machine helps in commutation
In a D. C. generator interpoles winding is connected in series with the armature winding
Back pitch and front pitch are both odd and approximately equal to the pole pitch
Equilizing bus bars are used with parallel running of D.C. shunt generators
ANSWER : : Equilizing bus bars are used with parallel running of D.C. shunt generators
34.The demagnetising component of armature reaction in a D.C. generator
reduces generator e.m.f.
increases armature speed
reduces interpoles flux density
results in sparking trouble
ANSWER : : reduces generator e.m.f.
35.The e.m.f. generated by a shunt wound D.C. generator isE. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be
E/2
2E
slightly less than E
E
ANSWER : : 2E
36.In D.C. generators, lap winding is used for
high voltage, high current
low voltage, high current
high voltage, low current
low voltage, low current
ANSWER : : low voltage, high current
37.In D.C. generators, the brushes on commutator remain in contact with conductors which
lie under south pole
lie under north pole
lie under interpolar region
are farthest from the poles
ANSWER : : lie under interpolar region
38.The function of pole shoes in the case of D.C. machine is
to reduce the reluctance of the magnetic path
to spread out the flux to achieve uniform flux density
to support the field coil
to discharge all the above functions
ANSWER : : to discharge all the above functions
39.Equilizer rings are required in case armature is
wave wound
lap wound
delta wound
duplex wound
ANSWER : : lap wound
40.D.C. generators are connected to the busbars or disconnected from them only under the floating condition
to avoid sudden loading of the primemover
to avoid mechanicaljerk to the shaft
to avoid burning of switch contacts
all above
ANSWER : : all above
41.Magnetic field in a D.C. generator is produced by
electromagnets
permanent magnets
both (a) and (b)
none of the above
ANSWER : : electromagnets
42.In a D.C. generator the ripples in the direct e.m.f. generated are reduced by
using conductor of annealed copper
using commutator with large number of segments
using carbon brushes of superior quality
using equiliser rings
ANSWER : : using carbon brushes of superior quality
43.In case of D.C. machine winding, number of commutator segments is equal to
number of armature coils
number of armature coil sides
number of armature conductors
number of armature turns
ANSWER : : number of armature coils
44.A D.C. welding generator has
lap winding
wave moving
duplex winding
any of the above
ANSWER : : lap winding
45.Compensating windings are used in D.C. generators
mainly to reduce the eddy currents by providing local short-circuits
to provide path for the circulation of cooling air
to neutralise the cross-magnetising effect of the armature reaction
none of the above
ANSWER : : to neutralise the cross-magnetising effect of the armature reaction
46.Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator ?
Dummy coils
Commutator
Eye bolt
Equilizer rings
ANSWER : : Commutator
47.Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generator A and B will be
2 : 3
3 : 1
3 : 2
1 : 3
ANSWER : : 3 : 1
48.Eddy currents are induced in the pole shoes of a D.C. machine due to
oscillating magnetic field
pulsating magnetic flux
relative rotation between field and armature
all above
ANSWER : : relative rotation between field and armature
49.If a D.C. generator fails to build up the probable cause could not be
imperfect brush contact
field resistance less than the critical resistance
no residual magnetism in the generator
faulty shunt connections tending to reduce the residual magnetism
ANSWER : : field resistance less than the critical resistance
50.Permeance is the reciprocal of
flux density
reluctance
ampere-turns
resistance
ANSWER : : reluctance
51.In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when
r.p.m. is more than 300
r.p.m. is less than 300
number of poles is 4
number of poles is 2
ANSWER : : number of poles is 2
52.The purpose of providing dummy coils in a generator is
to reduce eddy current losses
to enhance flux density
to amplify voltage
to provide mechanical balance for the rotor
ANSWER : : to provide mechanical balance for the rotor
53.In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when
there is no load on|he generator
the generator runs on full load
the generator runs on overload
the generator runs on designed speed
ANSWER : : there is no load on|he generator
54.Interpole flux should be sufficient to
neutralise the commutating self induced e.m.f.
neutralise the armature reaction flux
neutralise both the armature reaction flux as well as commutating e.m.f. induced in the coil
perform none of the above functions
ANSWER : : neutralise both the armature reaction flux as well as commutating e.m.f. induced in the coil
55.In D.C. generators on no-load, the air gap flux distribution in space is
sinusoidal
triangular
pulsating
flat topped
ANSWER : : flat topped
56.The e.m.f. generated in a D.C. generator is directly proportional to
flux/pole
speed of armature
number of poles
all of the above
ANSWER : : speed of armature
57.Flashing the field of D.C. generator means
neutralising residual magnetism
creating residual magnetism by a D.C. source
making the magnetic losses of forces parallel
increasing flux density by adding extra turns of windings on poles
ANSWER : : creating residual magnetism by a D.C. source
58.Open circuited armature coil of a D.C. machine is
identified by the scarring of the commutator segment to which open circuited coil is connected
indicated by a spark completely around the commutator
both (a) and (b)
none of the above
ANSWER : : both (a) and (b)
59.For the parallel operation of two or more D.C. compound generators, we should ensure that
voltage of the incoming generator should be same as that of bus bar
polarity of incoming generator should be same as that of bus bar
all the series fields should be run in parallel by means of equilizer connection
series fields of all generators should be either on positive side or negative side of the armature
ANSWER : : series fields of all generators should be either on positive side or negative side of the armature
60.The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be
640 V
620 V
600 V
580 V
ANSWER : : 580 V
61.D.C. generator generally preferred for charging automobile batteries is
series generator
shunt generator
long shunt compound generator
any of’the above
ANSWER : : long shunt compound generator
62.D.C. series generator is used
to supply traction load
to supply industrial load at constant voltage
voltage at the toad end of the feeder
for none of the above purpose
ANSWER : : voltage at the toad end of the feeder
63.In a D.C. generator the critical resistance refers to the resistance of
brushes
field
armature
load
ANSWER : : field
64.In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except
interpoles
dummy coils
compensating winding
shifting of axis of brushes
ANSWER : : dummy coils
65.To avoid formation of grooves in the commutator of a D.C. machine
the brushes of opposite polarity should track each other
the brushes of same polarity should track each other
brush position has no effect on the commutator grooving
the brushes of opposite polarity should track each other
ANSWER : he following constitute short-circuit in the armature winding.
66.In D.C. generators the polarity of the interpoles
is the same as that of the main pole ahead
is the same as that of the immediately preceding pole
is opposite to that of the main pole ahead
is neutral as these poles do not play part in generating e.m.f.
ANSWER : : is the same as that of the main pole ahead
67.A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly
150 V
175 V
240 V
290 V
ANSWER : : 240 V
68.A cumulatively compounded long shunt generator when operating as a motor would be
cumulatively compounded long shunt
differentially compounded long shunt
cumulatively compounded short shunt
differentially compounded short shunt
ANSWER : : differentially compounded long shunt
69.To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them
just ahead of magnetic neutral axis
in magnetic neutral axis
just behind the magnetic neutral axis
just ahead of magnetic neutral axis
ANSWER : rmature coil is short circuited by brushes when it lies
70.Following D.C. generator will be in a position to build up without any residual magnetism in the poles
series generator
shunt generator
compound generator
self-excited generator
ANSWER : : self-excited generator
71.In a level compounded D.C. generator, full load terminal voltage is
negligibly low
equal to no-load terminal voltage
more than no-load terminal voltage
less than no-load terminal voltage
ANSWER : : equal to no-load terminal voltage
72.Which of the following generating machine will offer constant voltage on all loads ?
Self-excited generator
Separately excited generator
Level compounded generator .
All of the above
ANSWER : : Level compounded generator .
73.A D.C. generator can be considered as
rectifier
primemover
rotating amplifier
power pump
ANSWER : : rotating amplifier
74.A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be
zero
about 2 V
about 50 V
220 V
ANSWER : : about 2 V
75.Which of the following generator will have negligible terminal voltage while running on no-load ?
Series generator
Shunt generator
Compound generator
Separately excited generator
ANSWER : : Series generator
76.In case of a flat compounded generator
voltage generated is less than the rated voltage
generated voltage is proportional to the load on the generator
voltage remains constant irrespec�tive of the load
speed varies in proportion to the load on the generator
ANSWER : : voltage remains constant irrespec�tive of the load
77.In over compounded generator, full load terminal voltage is
almost zero
less than noload terminal voltage
more than noload terminal voltage
equal to no-load terminal voltage
ANSWER : : more than noload terminal voltage
78.A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armature voltage wave
will be zero
will be of 5 Hz
willbeof5xiVHz
will be of v Hz 5
ANSWER : : will be of 5 Hz
79.When two D.C. series generators are running in parallel, an equilizer bar is used
to increase the speed and hence generated e.m.f.
to increase the series flux
so that two similar machines will pass approximately equal currents to the load
to reduce the combined effect of ar-mature reaction of both machines
ANSWER : : so that two similar machines will pass approximately equal currents to the load
80.Wave winding is composed of
any even number of conductors
any odd number of conductors
that even number which is exact multiple of poles + 2
that even number which is exact multiple of poles
ANSWER : : that even number which is exact multiple of poles + 2
81.Following energized winding of a D.C. machine should not be opened as it would produce high inductive voltage which may be dangerous to personnel and may cause its own insulation failure.
Series field
Compensating field
Inter pole field
Shunt field
ANSWER : : Shunt field
82.Armature reaction in a generator results in
demagnetisation of leading pole tip and magnetisation of trailing pole tip
demagnetisation of trailing pole tip and magnetisation of leading pole tip
damagnetising the centre of all poles
magnetising the centre of all poles
ANSWER : : demagnetisation of leading pole tip and magnetisation of trailing pole tip
83.Which of the following D.C. generators will be in a position to build up without any residual magnetism in the poles ?
Series generator
Shunt generator
Compound generator
None of the above
ANSWER : : None of the above
84.Lap winding is composed of
any even number of conductors
any odd number of conductors
that even number which is exact multiple of poles + 2
that even number which is exact multiple of poles
ANSWER : : any even number of conductors
85.In a D.C. generator in case the resistance of the field winding is increased, then output voltage will
increase
decrease
remain unaffected
fluctuate heavily
ANSWER : : decrease
86.The terminal voltage of a D.C. shunt generator drops on load because of all of the following reasons except
armature reaction
armature resistance drop
field weakening due to armature reaction and armature
commutation
ANSWER : : commutation
87.An exciter for a turbo generator is a
separately excited generator
shunt generator
series generator
compound generator
ANSWER : : shunt generator
88.In a D.C. machine stray loss is the sum of
total copper loss and mechanical loss
armature copper loss and iron loss
shunt field copper loss and mechanical loss
iron loss and mechanical loss
ANSWER : : iron loss and mechanical loss
89.In a separately excited generator supplying rated load the armature reaction ,
is always present
is always absent
may be sometimes present
none of the above
ANSWER : : is always present
90.The critical resistance of the D.C. generator is the resistance of
field
brushes
armature
load
ANSWER : : field
91.In a D.C. generator
external resistance = internal char-acteristic – armature reaction
internal characteristic = magnetisation characteristic – ohmic drop
external characteristic = magnetisation characteristic – ohmic drop – armature reaction
magnetisation characteristic = external characteristic
ANSWER : : external characteristic = magnetisation characteristic – ohmic drop – armature reaction
92.Which of the following generators will be preferred if they are required to be run in parallel ?
Shunt generators
Series generators
Compound generators
None of the above
ANSWER : : Shunt generators
93.In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilised is called
rotor
stator
field
armature
ANSWER : : armature
94.If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be
zero
small
the same as rated voltage
high
ANSWER : : zero
95.If a self excited D.C. generator after being installed, fails to build up on its first trial run, the first thing to do is to
reverse the field connections
increase the field resistance
increase the speed of primemover
check armature insulation resis�tance
ANSWER : : reverse the field connections
96.Which generator would you prefer for feeding long D.C. transmission lines ?
Series generator
Shunt generator
Over compound generator
Flat compound generator
ANSWER : : Over compound generator
97.A D.C. generator works on the principle of
Lenz’s law
Ohm’s law
Faraday’s law of electromagnetic induction
none of the above
ANSWER : : Faraday’s law of electromagnetic induction
98.The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage
will be less than 250 V
will always be 250 V
may be greater or less than 250 V
none of the above
ANSWER : : may be greater or less than 250 V
99.Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be
270 V
267.5 V
265 V
257.4 V
ANSWER : : 267.5 V
100.When two D.C. generators are running in parallel an equilizer bar is used
to increase the series flux
to increase the generated e.m.f.
to reduce the combined effect of ar-mature reaction of both the machines
so that the two identical machines will pass approximately equal currents to the load
ANSWER : : so that the two identical machines will pass approximately equal currents to the load
101.The essential condition for parallel operation of two D.C. generators is that they have ‘
same kW rating
the same operation r.p.m.
the same drooping voltage charac-teristics
same percentage regulation
ANSWER : : the same drooping voltage charac-teristics
102.Shunt generators are most suited for stable parallel operation because of their
rising voltage characteristics
identical voltage characteristics
drooping voltage characteristics
linear voltage characteristics
ANSWER : : drooping voltage characteristics
103.A series generator can self-excite
only if the load current is zero
only if the load current is not zero
irrespective of the value of load current
none of the above
ANSWER : : only if the load current is not zero
104.In a D.C. generator the critical resistance can be increased by
increasing its field resistance
decreasing its field resistance
increasing its speed
decreasing its speed
ANSWER : : increasing its speed
105.The number of armature parallel paths in a two-pole D.C. generator having duplex lap winding is
2
4
6
8
ANSWER : : 4
106.The main factor which leads to unstable parallel operation of flat and over compounded generators is
their rising voltage characteristics
unequal number of turns in their series field windings
unequal speed regulation of their primemovers
unequal series field resistances
ANSWER : : their rising voltage characteristics
107.With a D.C. generator which of the following regulation is preferred ?
100% regulation
infinite regulation
50% regulation
1% regulation
ANSWER : : 1% regulation
108.For both lap and wave windings, there are as many commutator bars as the number of
slots
armature conductors
winding elements
poles
ANSWER : : winding elements
109.As a result of armature reaction, the reduction in the total mutual air gap flux in a D.C. generator is approximately
40 percent
25 percent
10 percent
5 percent
ANSWER : : 5 percent
110.The series field of a short-shunt D.C. generator is excited by
external current
armature current
shunt current
load current
ANSWER : : load current