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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
The amount of electrical power for a given generator weight is greater for what type of generator?
AC generator ## Footnote This is one of the advantages of AC power.
26
On a typical AC generator, where is the output taken from?
direct from the stator windings
27
If an AC generator is producing the required voltage but a higher frequency than required, what action must be taken?
Decrease the speed of the prime mover, and then increase the strength of the field
28
In a 3 phase generator with balanced load, what is the potential between the neutral point and earth?
zero volts
29
The power output of an AC parallel generator system is measured in what unit?
KVA
30
The shape of the output waveform of an AC generator is known as what?
sine wave
31
The sum of the instantaneous E.M.F.'s in a three phase system is what?
zero
32
One revolution of a three phase generator will produce how many cycles?
1 cycle
33
The 'A' phase of a three phase power system is colour coded what?
red
34
The tolerance of the AC generator frequency is what?
20 Hz
35
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?
the voltage behavior of an AC generator
36
A generator supplies 25 A, 4 V and 50 W. What is the apparent power and the power factor?
100 VA and 0.5 ## Footnote PF = true power / apparent power. Apparent power = IV = 100 VA. PF = 50/100 = 0.5.
37
A conventional aircraft generator will be what type?
star wound
38
Power factor of a generator is what?
cos ø
39
In a 3 phase generator, what angle are the phases to each other?
120°
40
In a 2 phase generator, what angle are the phases to each other?
90°
41
Delta wound line current is what?
root 3 * phase current
42
Delta wound line current is.
root 3 * phase current
43
An alternator delivers 500V RMS at 1 ampere. The power factor is 0.8. The true power is.
400W ## Footnote Apparent Power = amps * volts = 1 * 500 = 500VA. Power Factor = TP/AP. 0.8 = TP/500. TP = 500 * 0.8 = 400W.
44
On a brushless AC generator, the main rotating field is fed with.
DC via diodes on the rotor
45
Single phase components in a three phase system may be connected between.
any phase and neutral OR between any two phases
46
A generator rated at 30 KVA and power factor 0.8 has a maximum continuous power output of.
24 kW ## Footnote True Power = PF * Apparent Power = 0.8 * 30 = 24kW.
47
If there was a mainly inductive load on an AC generator, the voltage with respect to the current would.
lead ## Footnote Remember CIVIL.
48
On a brushless AC generator, the permanent magnet generator output is.
AC ## Footnote The output is AC before it is rectified by the diodes inside the shaft.
49
The output from a rotating armature AC generator would be taken from.
the rotor ## Footnote View Comment/Reference (if available).
50
A frequency wild AC generator system has.
variable frequency only ## Footnote The voltage is held constant by a voltage regulator, which varies the resistance of the field current.
51
An AC generator has four poles. To produce a frequency of 400Hz it must achieve a speed of.
12000 RPM ## Footnote 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
The stator of a permanent magnet rotor AC generator is usually wound in?
star ## Footnote Star wound is more common due to the provision of two voltages.
53
The power output of a similar sized 3 phase compared to a single phase generator is?
more ## Footnote 3 phase generators are lighter and more compact for their output power.
54
In a voltage equalising circuit, the generators are regulated so that?
the lower is increased and the higher is decreased until they are equal.
55
In a 3-phase delta wound circuit, line current is?
the vectorial sum of all 3 phase currents. ## Footnote Line current is root 3 * phase current.
56
In an aircraft brushless alternator, the armature is?
star wound. ## Footnote The armature is normally Star Wound to give a choice of 2 voltages.
57
If the phase voltage of a star wound generator is 115V, what would be the line to line voltage?
200V ## Footnote Line voltage = 1.73 * phase voltage.
58
What are the factors affecting the frequency of an AC generator?
Number of pole pairs and rpm.
59
A generator with more than two phases is known as?
polyphase. ## Footnote Aircraft Electricity and Electronics. Eismin 5th Edition Page 92.
60
A generator supplies 150VAC phase and load is 10 Ohms per phase. What is the phase current?
15 A. ## Footnote I = V/R = 150/10 = 15A.
61
The phases on a three phase AC generator are spaced at?
120 degrees. ## Footnote The phases are equally spaced.
62
Increasing the load on a permanent magnet generator will?
reduce the terminal voltage by a small amount. ## Footnote Aircraft Electricity and Electronics Eismin Pg 193 and Electrical Systems for A&Ps Pg 106.
63
Two 3 phase generators operating in parallel with different phase and line voltages will be connected in?
star configuration.
64
The Stability windings in a 3 phase AC generator are to sense?
overvolt. ## Footnote Stability Winding CAIPs EEL/1-2 3.4.5 (d).
65
Delta wound alternator currents are?
the phase current multiplied by the square root of 3. ## Footnote Delta wound Line current = root 3 * phase current.
66
In smaller single or twin engine aircraft, the primary DC power is supplied in the form of rectified output from?
frequency wild ac generator.
67
Single phase components in a 3 phase system may be connected between?
any phase and earth or between any two phases. ## Footnote Eismin - Aircraft Electricity & Electronics - page 220.
68
Under frequency in an AC supply would cause?
overheating of inductive devices.
69
The line voltage of a 3 phase star connected AC generator is?
greater than the phase voltage. ## Footnote Eismin - Aircraft Electricity & Electronics - page 220.
70
In an AC generator of the brushless type, the rotating field winding is fed with?
DC via diodes from the main exciter winding on the rotor. ## Footnote Eismin - Aircraft Electricity & Electronics - page 220.
71
An AC generator is rated at 30 kVA at 0.8 Power Factor: this means that the maximum continuous output should not exceed?
24 KW. ## Footnote Eismin - Aircraft Electricity & Electronics - page 90.
72
To adjust the voltage of an AC generator, whilst maintaining constant frequency, it is necessary to?
alter the field current. ## Footnote Eismin - Aircraft Electricity & Electronics - page 199.
73
In a Delta connected generator, the line current is equal to?
1.7 * phase current.

Electrical Fundementals (19 decks)

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