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M3 > 3.17 AC Generators > Flashcards

3.17 AC Generators Flashcards

1
Q
A
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2
Q

What does the term ‘Alternating Current’ (AC) describe?

A

The flow of charge that changes direction periodically, causing the voltage level to reverse with the current.

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3
Q

What induces a current in a wire in an AC generator?

A

Spinning a loop of wire inside a magnetic field.

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4
Q

What can be used to rotate the wire in an AC generator?

A

A wind turbine, steam turbine, flowing water, or mechanical drive from a prime mover such as a gas turbine or reciprocating engine.

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5
Q

What happens when the loop of wire is at 12 and 6 o’clock positions?

A

It cuts no magnetic lines of force, resulting in no induced EMF.

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6
Q

At what angle does the loop of wire induce an EMF?

A

At 90° as it cuts the lines of flux.

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7
Q

How does the density of the magnetic field affect induced EMF?

A

The higher the density, the greater the value of induced EMF.

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8
Q

What is the nature of AC?

A

Bi-directional.

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9
Q

How is AC usually represented?

A

By a sinusoidal waveform.

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10
Q

What is the mathematical formula for a sine waveform in AC?

A

EMF = amplitude × sin(2πft)

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11
Q

What do AC waveforms vary according to?

A

The method of generation.

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12
Q

What is the most common type of AC waveform?

A

Sine waveform.

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13
Q

What do the axes represent in a sine waveform?

A

Amplitude on the y-axis and time on the x-axis.

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14
Q

How is amplitude sometimes referred to in the context of AC waveforms?

A

Voltage.

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15
Q

What is a key feature of the AC waveform regarding its cycles?

A

It has the same amplitude in the positive and negative half cycles.

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16
Q

What type of signals do AC waveforms represent?

A

Time-dependent signals or periodic signals.

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17
Q

Fill in the blank: The AC waveform is ______ dependent.

A

time

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18
Q

What are the two main parts of an AC generator?

A

Rotor and stator

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19
Q

What is the principle of operation of a permanent magnet generator?

A

The engine driven rotor is a permanent magnet that, when rotated, cuts the stationary output windings producing an alternating voltage and current output

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20
Q

What are the applications of permanent magnet generators?

A

Used as Tachogenerators and Ram Air Turbines (RAT) on aircraft

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21
Q

What type of generator is similar to a DC generator but uses slip rings instead of a commutator?

A

Rotating armature generator

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22
Q

What is a key disadvantage of rotating armature generators?

A

Relatively low limit output due to arc over and shorts from high voltages

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23
Q

What generates heat in rotating armature generators that limits output?

A

Combination of current load (demand) and friction on the brushes

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24
Q

In a rotating field generator, where is the field winding located?

A

On the rotor

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25
Q

In a rotating field generator, where is the armature located?

A

In the stator

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26
Q

What type of voltage is applied to the rotating field in a rotating field generator?

A

Low voltage DC

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27
Q

What is the function of the stator in a rotating field generator?

A

Holds the windings that are cut by the rotating magnetic field

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28
Q

What is generated in the armature of a rotating field generator as a result of the cutting action?

A

AC power

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29
Q

What is a benefit of using a rotating field generator compared to a rotating armature generator?

A

Requires smaller, less costly slip ring connections

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30
Q

How many contacts do DC rotors typically need?

A

Two contacts

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31
Q

What is the maximum voltage level that can be wound in the stationary armature of a rotating field generator?

A

Up to tens of thousands of volts

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32
Q

What is the main type of generator that uses a rotating armature?

A

DC generators

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33
Q

What is the advantage of rotating field generators for AC generation?

A

Especially advantageous at voltages of 11 kV or more

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34
Q

Fill in the blank: The rotating field generator switches the output and field around. Instead of _______ the stator, the rotor is magnetised.

A

magnetising

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35
Q

What does the term ‘single-phase’ refer to in generators?

A

The number of unique phases of the output windings of a generator

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36
Q

What is a single-phase generator?

A

A generator that produces a single, continuously alternating voltage

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37
Q

Where are the stator windings of a single-phase generator connected?

A

In series and inserted in slots cut in the inner surface of the laminated stator

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38
Q

How is the rotor of a single-phase generator driven?

A

By a prime mover carrying the exciter field windings fed from a DC source

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39
Q

In which applications are single-phase generators commonly used?

A

For relatively light loads, homes, portable tools, and small appliances

40
Q

True or False: All single-phase power comes from single-phase generators.

A

False

41
Q

What characterizes a two-phase generator?

A

Two or more single-phase windings spaced symmetrically around the stator

42
Q

What is the phase difference between the outputs of each winding in a two-phase generator?

A

90°

43
Q

What are the challenges associated with two-phase generators?

A

Control complexity, vibrational and uneven heating issues, and higher costs compared to three-phase systems

44
Q

What is the primary advantage of using three-phase generators?

A

More economical for higher ratings and better performance

45
Q

What is the phase difference between the voltages in a three-phase generator?

A

120°

46
Q

What is the configuration of the windings in a three-phase generator?

A

Three single-phase windings spaced so that their voltages run 120° out of phase

47
Q

How do the outputs of the three phases in a three-phase generator behave?

A

They are independent of each other

48
Q

Fill in the blank: A _______ generator produces multiple AC voltages.

A

multi-phase

49
Q

What is one main reason two-phase generators are not commonly used today?

A

Complexity and cost compared to three-phase generation

50
Q

What type of circuit is primarily used in aircraft generators?

A

Three-phase or polyphase circuit

51
Q

True or False: The windings in a two-phase generator are electrically connected.

A

False

52
Q

What are the two types of connections in three-phase circuits?

A

Star Connection and Delta Connection

53
Q

What is a Star Connection?

A

A four-wire system where one lead from each phase connects to a common junction

54
Q

What is a Delta Connection?

A

A three-wire system where phases are connected end-to-end

55
Q

What is the main advantage of using a three-phase power supply over a single-phase power supply?

A

It can transmit three times the power with just three conductors

56
Q

In a balanced Star Connection, how does the line voltage relate to the phase voltage?

A

Line voltage is 1.73 (√3) times the phase voltage

57
Q

What happens to line current and phase current in a Star Connection?

A

Line current is equal to phase current

58
Q

What are the typical AC voltages found in aircraft systems?

A

26 V AC, 115 V AC, 200 V AC

59
Q

What is a key characteristic of the Delta Connection?

A

Line voltages are equal to phase voltages

60
Q

In a balanced Delta Connection, how does the line current relate to the phase current?

A

Line current is 1.73 times the phase current

61
Q

What is a major advantage of a Star Connection?

A

Used for high voltage applications

62
Q

What is a major advantage of a Delta Connection?

A

More torque and efficient for heavy-duty applications

63
Q

Fill in the blank: In a three-phase Delta system, Phase Voltage = _______

A

Line Voltage

64
Q

Fill in the blank: In a three-phase Star system, √3 x Phase Voltage = _______

A

Line Voltage

65
Q

True or False: The Star Connection can distribute load evenly among phases.

A

True

66
Q

What is the effect of adding a single-phase device to a three-phase distribution system?

A

It creates an unbalanced condition in the generator

67
Q

What occurs if too many single-phase loads are added to one phase of a generator?

A

Overheating will occur

68
Q

What does the output of a generator depend on?

A

The velocity of the rotor

69
Q

How does a Star Connection affect insulation requirements?

A

Requires less insulation than Delta Connection

70
Q

What is the benefit of the ability to earth the neutral point in a Star Connection?

A

Improves safety and stability

71
Q

What is the implication of the Delta Connection allowing for one phase to be damaged?

A

Remaining two phases can still deliver power

72
Q

What factor contributes to the simplicity and low construction cost of Delta Connections?

A

Fewer conductors are needed

73
Q

What is the relationship between line and phase currents in a Star Connection?

A

Line current = Phase current

74
Q

What happens to the generator’s capacity if one phase becomes damaged in a Delta Connection?

A

Capacity is reduced to 57.7% of normal

75
Q

What is the significance of the common neutral point in a Star Connection?

A

Allows for unbalanced loading and dual voltage applications

76
Q

What is a Permanent Magnet Generator (PMG)?

A

A modern high-output brushless generator also known as a Brushless AC Generator

77
Q

What issues do Permanent Magnet Generators address?

A

Problems related to slip rings and brushes in carrying exciter current to the rotating field

78
Q

What are the advantages of a brushless generator?

A
  • Lower maintenance cost
  • High stability and consistency of output
  • Better performance at high altitudes
79
Q

How do brushless generators transfer current?

A

By using electromagnetic induction to transfer current from stationary components to rotating components

80
Q

What type of armature do brushless generators typically use?

A

A three-phase, star-connected armature

81
Q

What is the voltage across a single phase of a brushless generator?

A

115 V

82
Q

What is the voltage across two of the main output terminals?

A

200 V

83
Q

How is the circuit connected for aircraft circuits requiring 115 V single-phase power?

A

Between one main phase and ground

84
Q

What are the three separate generators within a PMG?

A
  • Permanent magnet generator
  • Exciter generator
  • Main generator
85
Q

What role does the permanent magnet play in a PMG?

A

It induces an alternating voltage into the stationary PMG three-phase armature winding

86
Q

What does the Generator Control Unit (GCU) do?

A

Rectifies AC armature current and regulates the exciter field current

87
Q

What frequency does the GCU typically work with?

A

1200 Hz

88
Q

What happens if more generator output is required?

A

The GCU increases the exciter field current

89
Q

What is the output voltage generated by the main generator armature?

A

115 V across a single phase and 200 V across two phases

90
Q

What are the typical labels for the output terminals of an aircraft AC generator?

A
  • A, B, C and N (Star Windings)
  • T1, T2, T3, T4, T5, T6 (Delta Windings)
91
Q

Fill in the blank: The brushless system induces voltage into the rotor using _______.

A

[magnetic flux lines]

92
Q

True or False: The main generator armature is connected to the output terminals of the generator.

A

True

93
Q

What does the exciter field induce?

A

An alternating voltage into the exciter armature

94
Q

True or False: The main field weakens the main armature’s output when more output is required.

A

False

95
Q

What is the function of the rotating rectifier in a PMG?

A

Changes AC to DC and sends a current to the main generator field

M3 (19 decks)

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