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Electrical Fundementals > AC Generators > Flashcards

AC Generators Flashcards

1
Q

In an alternating current Star wired system, the phase voltage is 115 volts. What is the line voltage approximately?

A

200 volts

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2
Q

An alternating current generator is operating at 50 kVA with a power factor of 0.8. What is the real power?

A

40kW

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3
Q

What two factors govern the output frequency of an AC generator?

A

the number of poles (in pairs) and the rotor RPM

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4
Q

In a delta-connected generator, how are line voltage and line current related to phase voltage and phase current?

A

line volts = phase volts, line current = root 3 times phase current

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5
Q

When measuring the phase and line voltages of a generator, if they are equal, what type of generator is it?

A

delta wound

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6
Q

What does the permanent magnet in an A.C generator induce?

A

A.C. in the exciter generator

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7
Q

If the phase voltage in a star connected generator is 200V, what will be the line voltage?

A

346V

Star connected generator - line voltage is 1.73 * phase voltage.

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8
Q

A generator is labeled as having 115V/200V, 20A and PF 0.8. What is the apparent power in each line?

A

4kVA

Apparent power is measured in VA. Line voltage is 200V (always the higher of the two voltages - and the presence of 2 voltages confirms it as star wound). So 200 * 20 = 4 KVA (PF not required).

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9
Q

AC generators are rated in what unit?

A

kVA

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10
Q

The frequency of the output of a 4 pole generator is 400 Hz. What is the generator turning at?

A

12000 RPM

Get the formula for frequency.

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11
Q

The voltage induced in the stator of an AC generator is produced by what?

A

a rotating magnetic field

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12
Q

A 30 KVA rated generator has a power factor of 0.8. What is its maximum consistent power?

A

24 KW

Power factor = true power / apparent power.

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13
Q

How are the three voltages of a three phase generator connected?

A

independently of each other

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14
Q

In a 2 phase generator, how far apart are the windings electrically?

A

90°

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15
Q

Where are the output windings on an AC generator?

A

Stator

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16
Q

The line voltage from a generator with a delta type connection will be?

A

the same as the phase voltage

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17
Q

What is an advantage of a star connected generator over a delta connected generator?

A

two potentials are available

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18
Q

What is the purpose of permanent magnets in an AC generator?

A

to provide initial excitation

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19
Q

The exciter winding in a brushless AC generator is supplied with electrical power from what?

A

a rotating generator integrally mounted on the same shaft as the AC windings

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20
Q

An AC generator’s frequency will be proportional to what?

A

the RPM at which it is driven

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21
Q

A frequency wild AC generator is used for what?

A

deicing loads

Frequency wild can only be used on resistive circuits (heating and lighting).

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22
Q

How many cycles of AC voltage are produced in a six-pole alternator with each revolution of the rotor?

A

Three

Each pair of poles produces 1 cycle.

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23
Q

The variable frequency output from an AC generator is used for what?

A

supplying non-inductive loads

Variable frequency (frequency wild) can only be used for resistive loads (i.e. non inductive and non capacitive).

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24
Q

In a single phase AC generator, frequency is determined by what?

A

controlling the current flow through the voltage field windings

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25
Q

The amount of electrical power for a given generator weight is greater for what type of generator?

A

AC generator

This is one of the advantages of AC power.

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26
Q

On a typical AC generator, where is the output taken from?

A

direct from the stator windings

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27
Q

If an AC generator is producing the required voltage but a higher frequency than required, what action must be taken?

A

Decrease the speed of the prime mover, and then increase the strength of the field

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28
Q

In a 3 phase generator with balanced load, what is the potential between the neutral point and earth?

A

zero volts

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29
Q

The power output of an AC parallel generator system is measured in what unit?

A

KVA

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30
Q

The shape of the output waveform of an AC generator is known as what?

A

sine wave

31
Q

The sum of the instantaneous E.M.F.’s in a three phase system is what?

A

zero

32
Q

One revolution of a three phase generator will produce how many cycles?

A

1 cycle

33
Q

The ‘A’ phase of a three phase power system is colour coded what?

A

red

34
Q

The tolerance of the AC generator frequency is what?

A

20 Hz

35
Q

The voltage of an AC generator rises to max in one direction, falls to zero and rises in the same direction. What is this describing?

A

the voltage behavior of an AC generator

36
Q

A generator supplies 25 A, 4 V and 50 W. What is the apparent power and the power factor?

A

100 VA and 0.5

PF = true power / apparent power. Apparent power = IV = 100 VA. PF = 50/100 = 0.5.

37
Q

A conventional aircraft generator will be what type?

A

star wound

38
Q

Power factor of a generator is what?

A

cos ø

39
Q

In a 3 phase generator, what angle are the phases to each other?

A

120°

40
Q

In a 2 phase generator, what angle are the phases to each other?

A

90°

41
Q

Delta wound line current is what?

A

root 3 * phase current

42
Q

Delta wound line current is.

A

root 3 * phase current

43
Q

An alternator delivers 500V RMS at 1 ampere. The power factor is 0.8. The true power is.

A

400W

Apparent Power = amps * volts = 1 * 500 = 500VA. Power Factor = TP/AP. 0.8 = TP/500. TP = 500 * 0.8 = 400W.

44
Q

On a brushless AC generator, the main rotating field is fed with.

A

DC via diodes on the rotor

45
Q

Single phase components in a three phase system may be connected between.

A

any phase and neutral OR between any two phases

46
Q

A generator rated at 30 KVA and power factor 0.8 has a maximum continuous power output of.

A

24 kW

True Power = PF * Apparent Power = 0.8 * 30 = 24kW.

47
Q

If there was a mainly inductive load on an AC generator, the voltage with respect to the current would.

A

lead

Remember CIVIL.

48
Q

On a brushless AC generator, the permanent magnet generator output is.

A

AC

The output is AC before it is rectified by the diodes inside the shaft.

49
Q

The output from a rotating armature AC generator would be taken from.

A

the rotor

View Comment/Reference (if available).

50
Q

A frequency wild AC generator system has.

A

variable frequency only

The voltage is held constant by a voltage regulator, which varies the resistance of the field current.

51
Q

An AC generator has four poles. To produce a frequency of 400Hz it must achieve a speed of.

A

12000 RPM

If it had the minimum of 2 poles (1 pair) it would require 400 revs/sec = 400 * 60 = 24,000 revs/min. With 4 poles (2 pairs), that speed can be halved.

52
Q

The stator of a permanent magnet rotor AC generator is usually wound in?

A

star

Star wound is more common due to the provision of two voltages.

53
Q

The power output of a similar sized 3 phase compared to a single phase generator is?

A

more

3 phase generators are lighter and more compact for their output power.

54
Q

In a voltage equalising circuit, the generators are regulated so that?

A

the lower is increased and the higher is decreased until they are equal.

55
Q

In a 3-phase delta wound circuit, line current is?

A

the vectorial sum of all 3 phase currents.

Line current is root 3 * phase current.

56
Q

In an aircraft brushless alternator, the armature is?

A

star wound.

The armature is normally Star Wound to give a choice of 2 voltages.

57
Q

If the phase voltage of a star wound generator is 115V, what would be the line to line voltage?

A

200V

Line voltage = 1.73 * phase voltage.

58
Q

What are the factors affecting the frequency of an AC generator?

A

Number of pole pairs and rpm.

59
Q

A generator with more than two phases is known as?

A

polyphase.

Aircraft Electricity and Electronics. Eismin 5th Edition Page 92.

60
Q

A generator supplies 150VAC phase and load is 10 Ohms per phase. What is the phase current?

A

15 A.

I = V/R = 150/10 = 15A.

61
Q

The phases on a three phase AC generator are spaced at?

A

120 degrees.

The phases are equally spaced.

62
Q

Increasing the load on a permanent magnet generator will?

A

reduce the terminal voltage by a small amount.

Aircraft Electricity and Electronics Eismin Pg 193 and Electrical Systems for A&Ps Pg 106.

63
Q

Two 3 phase generators operating in parallel with different phase and line voltages will be connected in?

A

star configuration.

64
Q

The Stability windings in a 3 phase AC generator are to sense?

A

overvolt.

Stability Winding CAIPs EEL/1-2 3.4.5 (d).

65
Q

Delta wound alternator currents are?

A

the phase current multiplied by the square root of 3.

Delta wound Line current = root 3 * phase current.

66
Q

In smaller single or twin engine aircraft, the primary DC power is supplied in the form of rectified output from?

A

frequency wild ac generator.

67
Q

Single phase components in a 3 phase system may be connected between?

A

any phase and earth or between any two phases.

Eismin - Aircraft Electricity & Electronics - page 220.

68
Q

Under frequency in an AC supply would cause?

A

overheating of inductive devices.

69
Q

The line voltage of a 3 phase star connected AC generator is?

A

greater than the phase voltage.

Eismin - Aircraft Electricity & Electronics - page 220.

70
Q

In an AC generator of the brushless type, the rotating field winding is fed with?

A

DC via diodes from the main exciter winding on the rotor.

Eismin - Aircraft Electricity & Electronics - page 220.

71
Q

An AC generator is rated at 30 kVA at 0.8 Power Factor: this means that the maximum continuous output should not exceed?

A

24 KW.

Eismin - Aircraft Electricity & Electronics - page 90.

72
Q

To adjust the voltage of an AC generator, whilst maintaining constant frequency, it is necessary to?

A

alter the field current.

Eismin - Aircraft Electricity & Electronics - page 199.

73
Q

In a Delta connected generator, the line current is equal to?

A

1.7 * phase current.

Electrical Fundementals (19 decks)

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