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Machine Design > MCQ in Alternators > Flashcards

MCQ in Alternators Flashcards

1
Q

A 50-Hz alternator will run at the greatest possible speed if it is wound for ______ poles.

A

2

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2
Q

The main disadvantage of using short-pitch in alternators is that it ___________.

A

reduces the total voltage around the armature coils

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3
Q

Three-phase alternators are invariably Y-connected because _________.

A

higher terminal voltage is obtained

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4
Q

The harmonic which would be totally eliminated from the alternator e.m.f. using a fractional pitch of 4/5 is _______.

A

5th

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5
Q

For eliminating 7th harmonic from the e.m.f. wave of an alternator, the fractional-pitch must be ______.

A

6/7

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6
Q

If, in an alternator, chording angle for fundamental flux wave is α, its value for 5th harmonic is ________.

A

5α

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7
Q

Regarding distribution factor of an armature winding of an alternator which is statement is false?

A. it decreases the distribution of coils (slot/pole) increases

B. higher its value, higher the induced e.m.f per phase

C. it is not affected by the type of winding either lap or wave

D. its not affected the number of turns per coil

A

B

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8
Q

When speed of alternator is changed from 3600 r.p.m. to 1800 r.p.m. the generated e.m.f./phase will become _______.

A

one-half

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9
Q

The magnitude of the three voltages drops in an alternator due to armature resistance, leakage reactance and armature reaction is solely determined by __________.

A

load current, Ia

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10
Q

Armature reaction in an alternator primarily affects __________.

A

generated voltage per phase

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11
Q

Under no-load condition, power drawn by the prime mover of an alternator goes to _________.

A

meet no load loss

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12
Q

As load p.f of an alternator becomes more leading, the value of generated voltage required to give rated terminal voltage _________.

A

decreases

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13
Q

With a load p.f. of unity, the effect of armature reaction on the main-field flux of an alternator is _________.

A

distortional

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14
Q

At lagging loads, armature reaction in an alternator is __________.

A

magnetizing

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15
Q

At leading p.f., the armature flux in an alternator ______ the rotor flux.

A

aids

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16
Q

The power factor of an alternator is determined by its _________.

A

load

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17
Q

For proper parallel operation, a.c. polyphase alternators must have the same _________.

A

voltage rating

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18
Q

Of the following conditions, the one which does NOT have to be met by the alternators working in parallel is __________.

A. terminal voltage of each machine must be the same

B. the machine must have the same phase rotation

C. the machine must operate at the same frequency

D. the machine must have equal rating

A

D

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19
Q

After wiring up two 3-Ф alternators, you checked their frequency and voltage and found them to be equal. Before connecting them in parallel, you would ________.

A

check phase rotation

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20
Q

Zero power factor method of an alternator is used to find its ________.

A

voltage regulation

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21
Q

Some engineers prefer ‘lamps bright’ synchronization to ‘lamps dark’ synchronization because ___________.

A

it gives sharper and more accurate synchronization

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22
Q

It is never advisable to connect a stationary alternator to live bus-bars because it _________.

A

will get short-circuited

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23
Q

Two identical alternators are running in parallel and carry equal loads. If excitation of one alternator is increased without changing its steam supply , then __________.

A

it will keep supplying almost the same load

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24
Q

Keeping its excitation constant, if steam supply of an alternator running in parallel with another identical alternator is increased, then __________.

A

it will supply greater portion of the load

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25
Q

The load sharing between two steam-driven alternators operating in parallel may be adjusted by varying the _________.

A

steam supply to their prime movers

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26
Q

Squirrel-cage bars placed in the rotor pole faces of an alternator help reduce hunting ___________.

A

above and below synchronous speed both

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27
Q

For a machine on infinite bus, active power can be varied by __________.

A

changing of prime mover speed

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28
Q

Majority of alternators in the use have _________.

A

revolving field type construction

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29
Q

The stator of an alternator is identical to that of a _________.

A

3-phase induction motor

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30
Q

The stator of an alternator rarely uses _____ slots.

A

closed type

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31
Q

The field winding of an alternator is ______ excited.

A

dc

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32
Q

The ac armature winding of an alternator operates at ______ the field winding.

A

much higher voltage than

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33
Q

The salient-pole construction for field structure of an alternator is generally used for ______ machine.

A

8-pole

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34
Q

An alternator is sometimes called ________ generator.

A

synchronous

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35
Q

A turbo-alternator uses __________.

A

non-salient-pole field structure

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36
Q

The non-salient pole field construction is used for ____ generator.

A

high-speed

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37
Q

The ac armature winding of an alternator is _________.

A

always star-connected

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38
Q

Low-speed alternators are driven by _________.

A

hydraulic turbines

39
Q

High-speed alternators are driven by _________.

A

steam turbines

40
Q

The air-gap in an alternator is ______ in an induction machine.

A

much longer than

41
Q

The stator of an alternator is wound for ______ on the rotor.

A

the same number of poles as

42
Q

Turbo-alternators have rotors of __________.

A

small diameter and long axial length

43
Q

The rotor of a turbo-alternator is made cylindrical in order to reduce ______.

A

windage losses

44
Q

For the same rating, the size of low-speed alternator is _______ that of high-speed alternator.

A

more than

45
Q

The synchronous reactance of an alternator is due to ________.

A

armature reaction

46
Q

The synchronous reactance of an alternator is generally _________ armature resistance.

A

10 to 100 times greater than

47
Q

The synchronous reactance of an alternator _________ as the iron is saturated.

A

decreases

48
Q

In an alternator, the effect of armature reaction is minimum at power factor of ______.

A

unity

49
Q

If the lagging load p.f. of an alternator is decreased, the demagnetizing effect of armature reaction _________.

A

is increased

50
Q

When load on an alternator is increased, the terminal voltage increases if the load pf is ________.

A

leading

51
Q

The efficiency of the turbo-alternator ________ with increase in speed.

A

increases

52
Q

The full-load efficiency of the alternator ________ with the size of the machine.

A

increases

53
Q

For the same power rating, an alternator is ________ that of a dc generator.

A

smaller in size than

54
Q

The disadvantage of a short-pitched coil is that _________.

A

voltage around the coil is reduced

55
Q

The voltage regulation of an alternator for a pf of 0.8 lagging is ______ at unity pf.

A

greater than

56
Q

The voltage regulation of an alternator is larger than that of a dc generator because of _________.

A

complex effects of armature reaction

57
Q

The under excited alternator supplies _______.

A

leading VAR

58
Q

If the excitation of an alternator operating in parallel with other alternators is decreased, its ___________.

A

pf becomes more leading

59
Q

The distribution of load between two alternators operating in parallel can be changed by changing _________.

A

driving torques of prime movers

60
Q

When a number of alternators are operating in parallel, the power factor at which each operates is determined by _________.

A

its field excitation

61
Q

If the excitation of an alternator operating in parallel with other alternators is increased above the normal value of excitation, its ___________.

A

pf becomes more lagging

62
Q

The voltage of the bus-bar to which several alternators are paralleled may be raised by simultaneously _______ of all alternators.

A

increasing field excitation

63
Q

The frequency of the system with which several alternators are paralleled can be increased by simultaneously ________ of all generators.

A

increasing the speed of prime movers

64
Q

The alternators driven by ________ do not have a tendency to hunt.

A

steam turbines

65
Q

Damper windings are used in alternators to _________.

A

prevent hunting

66
Q

The rating of an alternator is expressed in ________.

A

kVA

67
Q

The armature flux opposes the main field flux when the load power factor is ___________.

A

zero lagging

68
Q

The armature flux helps the main field flux when the load power factor is

A

zero leading

69
Q

A commercial alternator has ___________.

A

stationary armature and rotating field

70
Q

The stator core of a synchronous machine is made up of laminated sheets to __________.

A

minimize the eddy current loss

71
Q

In alternators damper windings are used to ____________.

A

prevent hunting

72
Q

The magnitude of the resultant magnetic field produced by a three-phase current flowing through a three-phase winding is equal to __________.

A

1.5 times the maximum value of flux due to any one phase

73
Q

Voltage regulation of an alternator may be negative when __________.

A

the load power factor is leading

74
Q

Armature reaction in an electrical machine is the effect of _________.

A

armature flux on the main flux

75
Q

An infinite bus-bar should maintain ____________.

A

constant frequency and constant voltage

76
Q

_________ refers to an AC generator.

A

Alternator

77
Q

In huge alternators, the moving part is ___________.

A

poles

78
Q

In case of alternator, the stator frame serves __________.

A

as a return path for the flux

79
Q

Alternators mostly wotk on rotating field type principle which is different from d.c. machines where the field is stationary. Therefore alternators _____________.

A

work on Faraday’s law

80
Q

High speed alternators have a rotor construction __________.

A

of non-salient type

81
Q

The voltage applied to the field of a rotating cylindrical rotor type alternators is __________.

A

low d.c. voltage

82
Q

Alternators are generally designed to generate ___________.

A

definite frequencies

83
Q

The standard practice nowadays in alternators is to have _____________.

A

rotating field

84
Q

In power generating stations (thermal stations) the usual number of poles employed in alternators are __________.

A. twenty four

B. forty eight

C. thirty two

D. none of these

A

D

85
Q

The diameter of high speed turbo alternators as compared to hydroelectric ones are ___________.

A

smaller because speed is high

86
Q

The slip rings employed in a three-phase alternators in hydrostation are insulated for ___________.

A

low voltage

87
Q

When the speed of alternator increases, the frequency ___________.

A

increases

88
Q

Steam turbo-alternators are much smaller in size than water-turbine alternators for a given output. This is so because ___________.

A

steam turbo alternators run at high speed

89
Q

Distributing the armature winding in alternators in more than one number of slots per pole per phase results in ____________.

A

reduction of irregularities produced in the waveform

90
Q

When the power factor of load is unity, the armature flux of an alternator will be __________.

A

cross-magnetizing

91
Q

An alternator is capable of delivering power at a particular frequency. The frequency can be increased by ____________.

A

increasing armature speed

92
Q

If the input of the prime mover of an alternator is kept constant but the excitation is increased then _________.

A

kVA will be lagging

93
Q

For two alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is __________.

A

to be kept constant but input torque should increase

94
Q

The best and the most accurate method for determining the voltage regulation is _________.

A

Pottier triangle method

Machine Design (3 decks)

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