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Module 13 EASA Part 1 > 13.5 Electrical Power > Flashcards

13.5 Electrical Power Flashcards

1
Q

What are the main sources of DC and AC power on an aircraft?

A
  • DC alternator
  • DC generator
  • AC alternator
  • Battery
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2
Q

When is an aircraft’s battery typically used?

A
  • No other sources of power are available
  • For emergency operations
  • Intermittent system overloads
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3
Q

What is a busbar?

A

A copper bar, located in distribution boxes, designed to carry the entire electrical load and distribute that load to the individual power users

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4
Q

What is the busbar connected to on almost all aircraft?

A

Positive output terminal of the generator and/or battery

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5
Q

What is the earth return?

A

The path of the negative voltage, distributed through the metal structure of the aircraft

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6
Q

What are the nominal voltages/frequencies of aircraft power?

A
  • 28v DC
  • 115v AC 400 Hz single phase
  • 200v AC 400 Hz three phase
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7
Q

What do we do for the earth on a composite airframe?

A
  • Two wires, one positive and one negative
  • In other cases, a ground plane is added to the structure of the aircraft
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8
Q

How are busbars categorised, and further subdivided?

A
  • AC and DC
  • Subdivided to essential and non-essential
  • Further, there are other buses which may be connected directly or indirectly to the battery, known as ‘hot bus’, ‘vital bus’ and ‘emergency bus’
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9
Q

What is a hot busbar?

A

A busbar that is permanently connected to the battery

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10
Q

What protective devices are used on an aircraft and what are we protecting against?

A
  • Fuses
  • Circuit breakers
  • Cut-out relays

Protecting against short circuits, hazardous overvoltage and other malfunctions

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11
Q

How many extra fuses should be carried?

A

10% of the number of each rating (if one fuse is used on the aircraft of a particular rating, three spares should be provided)
or
three of each rating

whichever is greater

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12
Q

What is meant by the term trip-free circuit breaker?

A

A circuit breaker, which when an overload or circuit fault exists, they will open the circuit irrespective of the position of the operating control

Must not be of the type that can be overridden manually

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13
Q

What are essential and non-essential services?

A

Essential services are those that are required to ensure safe flight and landing in cases of emergency, such as; essential lighting, flight control systems, and communication and navigation radios

Non-essential services are all of those wouldn’t be included in the class above

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14
Q

What is the circuit protection requirement of essential loads?

A

Individual circuit protecting

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15
Q

What is the role of a static inverter?

A

Converts DC, supplied by the battery, into AC power

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16
Q

What is the role of a transformer rectifier unit?

A

AC power produced by the generator is converted to direct current

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17
Q

What is a split-bus system?

A

Gearbox driven generators can never be connected to the same distribution bus at the same time

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18
Q

What is a parallel bus system?

A

Entire electrical load is equally shared by all the working generators

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19
Q

Fuses/circuit breakers for essential circuits must be accessible to whom?

A

To the pilot or some other member of the crew

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20
Q

What is the physical relationship between batteries and busbars?

A

Located as near as possible to the main and battery busbars as physically possible

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21
Q

What type of batteries are usually used with metal trays without surface protection (a coating of a PVC)?

A

Alkaline batteries

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22
Q

If the flight crew are complaining of an acrid smell in the cockpit, what could be wrong as it pertains to the battery system?

A

Non-return valve is US, allowing corrosive fumes from the battery charging and discharging cycle to escape into the aircraft

Would be detected when using cabin pressurisation as a means to vent battery, and when negative pressure exists

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23
Q

What problems do we monitor batteries for?

A
  • Temperature
  • Current output
  • Charging rate (linked with temperature monitoring)
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24
Q

What do the regulations state about the capacity of aircraft batteries?

A

Must provide 30 minutes of power to essential services

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25
Q

Describe the standby battery system?

A
  • Independent of the DC power system
  • Provides 12v power to the DC power system protection circuitry
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26
Q

Which circuits, normally supplied by the aircraft’s DC power system, are switched to the standby battery in the event of a DC power failure?

A
  • Emergency exits
  • Cabin notices
  • Cabin lighting
  • Standby artificial horizon
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27
Q

What two methods are used to vent the gasses from a battery?

A

Venturi action or cabin pressurisation systems

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28
Q

Why should the test function of standby batteries not be repeated at intervals of less than 3 minutes?

A

May cause the battery temperature to rise and there is the possibility of causing thermal runaway

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29
Q

Why is a ‘cooling-off period’, of approximately 10 to 25 seconds, provided for an on-board battery charging unit, where an overvoltage exists, while no such period is provided for undervoltage conditions?

A

Overvoltage conditions tend to cause an increase in current, which generally results in an increase in temperature

Undervoltage conditions don’t usually cause excessive current, so no overtemperature conditions exist

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30
Q

In a battery protection circuit, what is the ideal method by which to isolate the battery?

A

Relays on both terminals of the battery

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31
Q

Why does a battery charger not start unless the voltage sensed at the output terminals of the battery is above approximately 4v DC?

A

A battery at such a low charge will almost certainly have resulted in damage to the battery

If an attempt is made to charge it, a large current will flow into the battery and there may be a dangerous build-up of temperature or gases

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32
Q

Why is there a difference between a cut-off and cut-on temperature of the battery at which the battery charger will switch on and switch off?

A

If they were the same temperature, the battery charger will cycle on and off at a fast rate

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33
Q

What is the test provision sequence for a standby battery?

A

Pressing the charge test pushbuttons causes:

0.5 seconds controlled discharge through the current flow circuit. After this discharge, a measurement circuit checks that the battery voltage remains above 70% of the rated value and test OK light illuminates if successful

Simultaneously, overheat condition is simulated, lighting the fail lights

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34
Q

What is the power supply to the battery charging circuit?

A

115/200 Volts AC 3 phase

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35
Q

What is the charging current of a battery charging circuit limited to?

A

Approximately 65A

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36
Q

What two precautions should be taken with regards to aircraft battery switch positions?

A
  • Set the BATT/GND PWR switch to ON unless the aircraft battery or a ground power unit is connected
  • Set the NORM/EMERG switch to EMERG unless the aircraft power system is energised, if not, the standby battery will be discharged
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37
Q

Why are alkaline batteries more susceptible to thermal runaway?

A

Can handle high charge and discharge currents

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38
Q

What is thermal runaway?

A

At higher than normal temperatures, the heat loss of the battery through radiation and conduction is lower than the heat generated

As a result, internal resistance is lowered which increases current, this increases temperature further and the cycle continues to destruction

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39
Q

What factors can cause overheating in a battery?

A
  • Incorrect voltage regulation
  • High load current (multiple engine starts)
  • Loose links between cells
  • Low electrolyte
  • Leakage currents between cells and container
  • Cell unbalance
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40
Q

What are the advantages of alkaline over lead acid batteries?

A
  • Can handle higher charge and discharge currents
  • Can be left in any state of charge with no detrimental effects
  • Normally more robust
  • Terminal voltage between charged and discharged changes very little
  • Alkaline cell is lighter than lead acid cell (however 24v lead acid is actually lighter than 24v alkaline)
  • Normally have a longer life
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41
Q

How are batteries charged in a servicing bay, and how are they charged on an aircraft?

A

Servicing bay - Constant current method

Aircraft - lead acid charged by constant voltage method, alkaline charged by high initial current then pulse charged to allow for temperature stabilisation before switching to constant voltage

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42
Q

What is the purpose of the suppressor in a general DC generator?

A

Reduces radio interference, which may be caused by sparking between the brushes and commutator

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43
Q

What is the purpose of the trimmer resistor in a carbon pile regulator?

A

Adjusted by the engineer during a ground run, to ensure that, at the correct generator output voltage, the current in the voltage coil will produce the correct amount of magnetic force to balance the leaf spring

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44
Q

What is the principle of operation of the carbon pile voltage regulator?

A

The resistance of a carbon pile is balanced by a voltage coil acting against a leaf spring, which adjusts the excitation voltage in a coil to the generator

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45
Q

In what ways are solid state or transistorised voltage regulator superior to older types of regulators?

A
  • Cost
  • Reliability
  • Weight
  • Less radio interference
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46
Q

If the voltage coil in a carbon pile voltage regulator became open circuit, what would happen to the generator’s output voltage?

A

The leaf spring would have nothing acting against it, so resistance in the carbon pile decreases, allowing increased current in the excitation field and increased voltage

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47
Q

In an aircraft with fourteen 5A, seven 15A and two 30A fuses, how many fuses should the crew carry as spares?

A

9

as the rule states ‘10% or 3, whichever is greater’

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48
Q

Resistive loads can be supplied by which what types of AC generating systems?

A

Frequency wild

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49
Q

What conditions are necessary in the run-up of a generator?

A
  • Allowed to run-up in an off-load situation
  • At a specified voltage output, usually slightly higher than the bus bar, before being brought online
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50
Q

In a direct switching (differential cut-out) generator cut-out arrangement, what initially causes the differential cut-out contacts to close?

A

Thinner ‘differential’ coil

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51
Q

In a direct switching (differential cut-out) generator cut-out arrangement, what is the purpose of the heavy-duty current series coil?

A

When generator is ‘on-line’ and producing enough current for forward flow, magnetic effects of the coil assist the differential coil

When generator is ‘on-line’ but not producing enough so as to resist reverse flow, reverse current flow through coil creates magnetic field which opposes that of the shunt coil, and so forces the cut-out contacts open, disconnecting the generator from the busbar

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52
Q

What are the components of a basic single engined generator system?

A
  • Generator
  • Voltage regulator
  • Differential cut-out
  • Battery
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53
Q

What is the distinction between a differential relay and an RCCB?

A

Differential relay will operate on reverse currents of 20-30 amps

RCCBs will operate on reverse currents of approximately 500 amps and at very high speed

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54
Q

What might be the cause of a DC generator having a defect of sparking at all brushes?

A
  • Dirty commutator
  • Excessive load
  • Incorrect brush position
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55
Q

What is the basic function of hold-off systems?

A
  • Hold off or prevent the generator paralleling with external power
  • Prevent aircraft batteries paralleling with external power
56
Q

In general terms, what controls the hold-off circuits?

A

Small pin on the NATO 3 pin DC ground supply socket

57
Q

What are the advantages of paralleling a DC generator system?

A
  • No break in supplies if a generator fails
  • System can handle the switching of high transient loads
58
Q

What are the disadvantages of paralleling a DC generator system?

A
  • Necessitates additional circuitry to ensure that both machines share the loads equally
  • A generator side failing may prove difficult to diagnose as its effects can be seen in the other generators
59
Q

What is the purpose of an ammeter connected to each generator in a parallel DC generator system?

A

Allows the pilot to regularly check that load sharing is correct

60
Q

In a parallel DC generator system, what is the purpose of the voltage boost resistor and where is it in the circuit?

A

Artificially raises the output of generator No 2 above generator No 1, after No 1 has been brought on-line and prior to No 2 being brought likewise

Located in series with the voltage coil of No 2 regulator

61
Q

Why is it important to ensure that all connections in a parallel DC generator equalising line between generators are secure and have a minimum contact resistance?

A

Equalising potential differences are small, so small changes in circuit resistance will have drastic effects on load sharing between generators

62
Q

What is the general acceptable paralleled load difference between generators?

A

No more out of balance than a 60/40 split

63
Q

How does a voltage pick-up relay ensure the correct polarity?

A

A diode connected in series with the voltage pick-up relay

64
Q

In AC power generation, what are the two main categories on aircraft?

A
  • Frequency wild
  • Constant frequency
65
Q

What can frequency wild AC generation systems be used for?

A
  • To supply resistive AC loads, for example anti-icing/de-icing systems
  • Supply DC via a TRU
66
Q

In order to parallel two AC generators, what properties must be the same?

A
  • Frequency
  • Voltage
  • Phase rotation
67
Q

What are the two main types of constant frequency power generation systems?

A
  • Split busbar
  • Parallel busbar
68
Q

What are the disadvantages of constant frequency as compared to frequency wild?

A
  • More expensive
  • More complex
  • Requires maintenance due to additional components required
69
Q

What are the advantages of frequency wild AC systems as compared to DC systems?

A
  • Better power to weight ratio
  • Much less affected by poor brush performance at high altitude, especially if a brushless generator is used
70
Q

What is the frequency swing of a frequency wild AC system, connected directly to the engine gearbox?

A

Approximately 200 Hz either side of 400 Hz

71
Q

What are the main components of a frequency wild AC generator system?

A
  • AC generator
  • Generator control unit (GCU)
  • Compounding transformer
  • Voltage regulator
72
Q

Why are compounding transformers and rectifiers incorporated in a frequency wild AC generator system?

A

Increase the range of regulation

73
Q

What is the effect of the weight on wheels switch on a frequency wild AC generator system?

A

Alternator is held to a low level phase voltage of approximately 75v AC

74
Q

What is the purpose of the interpole windings in a parallel DC generator system?

A

Aid or oppose the voltage coil, thus increasing or decreasing the generator output voltage

75
Q

What are the two methods of constant frequency which supply large quantities of power for aircraft?

A
  • Integrated drive generator
  • Variable speed constant frequency system
76
Q

If the springs pushing a carbon pile voltage regulator snap, what would happen to the output voltage?

A

The only force acting on the carbon pile would be the voltage coil, which forces the pile apart and increases the effective resistance

This reduces the excitation voltage across the field coil, thus reducing the output voltage

77
Q

On what type of aircraft are frequency wild systems most employed?

A

Turbo prop and small jet aircraft

78
Q

What is an integrated drive generator (IDG)?

A

Constant speed drive unit and brushless generator

79
Q

What is a variable speed constant frequency (VSCF) system?

A

A variable speed/frequency generator which feeds an electronic network to produce a constant frequency supply

80
Q

What is the purpose of the CSDU and how is this achieved?

A

To drive the brushless generator at a constant RPM for varying engine speeds

Hydro-mechanical unit fitted between the engine gearbox and the brushless generator

81
Q

What is the principle of operation of a CSDU?

A

Engine driven hydraulic pump will drive a hydraulic motor at a desired speed, and therefore frequency

Increase of engine speed will cause increase in pump output, while a mechanical governor will sense the increase and port oil to the servo piston which will move a swash plate on the pump to underdrive

Reducing the pump action and therefore the motor drops back to the desired frequency

82
Q

How is the fine trim of the CSDU set?

A

Electromagnetic coil receives signals from the electrical system’s frequency control, which modifies the position of the flyweights in the governor

83
Q

Name the two types of automatic disconnect in a CSDU?

A

Thermal - internal oil temperature above a threshold is detected which causes a eutectic soldered ring to melt, actuating the disconnect mechanism

Shear - shear disconnect results when a detected torque of greater than a pre-determined inch pounds is present

84
Q

What parameters can the pilot monitor in order to make a decision to actuate a disconnect of the CSDU from the engine?

A

Oil pressure and temperature

85
Q

In the brushless generator of the IDG, what powers the GCU?

A

The permanent magnet generator (PMG)

86
Q

What is included in a VSCF generator which provides protection to the accessory gearbox in the event of VSCF generator mechanical failure?

A

Spindle between the generator input spline and the speed increaser gearbox has a shear section

87
Q

Premature bearing wear may be caused by an engine due to what type of fault in the electrical distribution system and why?

A

Earth leakage, leading to current flowing from a phase to the neutral line, resulting in unequal loads being presented between the 3 phases

Unbalanced loads would cause vibration similar to an unbalanced wheel on a motor vehicle passing vibrations on

88
Q

To discriminate between earth leakage faults and undervoltage conditions, what would the flight crew make reference to?

A

Voltmeter

An undervoltage will be a slight drop in voltage

An earth leakage will be a very large drop

89
Q

When can a CSDU be reset?

A

When the aircraft is on the ground with the engine stopped

90
Q

Where does a Bus Power Control Unit (BPCU) receive information from?

A
  • The GCUs about any generator malfunction
  • Load controllers
  • Current transformers
91
Q

In a three-phase AC generation system, what is the purpose of the capacitor and resistor in the overvoltage protection circuit?

A

Provide a time delay so that a transient voltage spike will not trip the generator off-line, only sustained overvoltages

92
Q

In a three-phase AC generation system, how does an over-voltage sensing circuit deal with an overvoltage?

A

Triggers an SCR which de-excites the generator and removes it from the busbar

93
Q

In a three-phase AC generation system, what component will sense over-load to protect the system?

A

Current transformers wrapped around the conductors, which sense when too much current is flowing, and de-excite the generator and remove it from the busbar

94
Q

In a three-phase AC generation system, how does differential protection work?

A

Current transformers sense current flow at the generator and at the circuit breaker immediately before the load

Differences in current flow suggest earth or line fault between the current transformers

95
Q

In a three-phase AC generation system, what lines are monitored?

A
  • Line to line faults
  • Line to earth faults
96
Q

What power supply will a GCU receive, if there is PMG failure of a brushless generator?

A

Battery backup power supply from an aircraft battery

97
Q

What tolerance does the GCU in a three-phase AC generation system maintain the generator output to?

A

115V ± 5 volts

98
Q

How does a GCU operate in a three-phase AC generation system?

A
  • Samples 3 phase AC output
  • Changing to a DC level
  • Compares to a reference value
  • Adjusts excitation to the field of the generator via the generator field relay (GFR)
99
Q

What steps are followed when a generator fails, to remove it from the busbar in a three-phase AC generation system?

A
  • GCU senses partial loss of power
  • Automatically signals the bus power control unit (BPCU)
  • BPCU removes the generator from the busbars
100
Q

What is the principle of a TRU used on aircraft?

A

AC voltage input is stepped down by three-phase transformer, rectified by the six rectifier bridge assembly, providing full wave rectified, relatively smooth DC

101
Q

In an aircraft TRU, what components ensure the smoothest rectified output possible?

A

2 rectifiers, one delta, the other star

102
Q

A TRU has what basic protection?

A
  • Overheat
  • Voltage
  • Reverse current
103
Q

At what upper and lower voltages might a TRU be automatically removed from the busbar?

A

33v and 22v

104
Q

When will the reverse/low current protection disconnect the TRU from the DC busbar automatically?

A

When fault current reaches approximately 1 amp

105
Q

In a transistorised AC voltage regulator, how is the strength of the exciter controlled?

A

Pulse width modulated signal which is a constant amplitude

At high generator voltage, pulse width is small

106
Q

What is the point of regulation on larger type aircraft?

A

The point at which the regulator derives its reference is as close to the busbars as possible, but upstream of the generator circuit breaker (GCB)

107
Q

What are the advantages of a relay over direct switching?

A
  • Remote switching
  • Relay can switch many contacts at once
  • Low current can be used to switch a large current
108
Q

What are the advantages of the main power breaker or latching relay?

A
  • Only needs power when changing state
  • Not affected by ‘G’ forces
109
Q

Why is a spike suppression fitted to a power contactor?

A

Discharge energy in the coil and stop hold-in when power is removed

110
Q

What are the advantages of the parallel generator system over the split bus system?

A
  • Better able to handle shock loads
  • Continuity of electrical supplies; without paralleling, there may be a break in services when switching to a correctly functioning generator
  • Generator life is prolonged, since each generator is normally run on part load
111
Q

What are the disadvantages of the parallel generator system over the split bus system?

A
  • Fault propagation; avoided by adequate circuit protection units
  • Parallel operation does not, without modification, meet requirements for totally independent supplies
112
Q

Why is it usual to incorporate an interlock circuit in a parallel generator system which becomes more prevalent on the ground?

A

Paralleling of the APU, external supply with each other or the aircaft generators is to be prevented as these sources are not considered compatible

113
Q

What are the four stages of a VSCF?

A
  • Generator
  • Rectifier
  • Inverter
  • AC filter
114
Q

What stages does a GCU modify in the VSCF cycle?

A

Generator - field excitation

Inverter - frequency control

115
Q

What are the conditions required for synchronising parallel generators?

A
  • Voltages within tolerance
  • Frequency within tolerance
  • Phase displacement within tolerance
  • Phase rotation correct
116
Q

If generators in a parallel system are not synchronised, what would be the result?

A

Serious damage to the generators could occur due to high circulating currents

117
Q

When is the ground handling bus powered and when is it used?

A

Automatically whenever external power or APU power is available, and mainly to power lights and the refuelling system

118
Q

What is the typical charging current of a typical battery charger?

A

45-50 amps

119
Q

What does protection circuitry in the battery charging system provide protection against?

A
  • Overvoltage
  • Overheating
  • Battery disconnect
120
Q

What is active load also known as?

A

Real load

121
Q

How is active load sharing achieved by active load controllers?

A

One active load controller for each IDG, varies the current to the magnetic trim coil of the associated speed governor’s trim head of the CSDU

  • Increase the torque of the light active load generator
  • Decrease the torque of the highly active load generator
122
Q

How is reactive load sharing achieved by reactive load controllers?

A

Output of the reactive load controllers circuit is applied to the voltage regulator GCU as a bias to:

  • Increase the excitation of generators carrying too little reactive load
  • Decrease the excitation of generators carrying too much reactive load
123
Q

What is the sensing element in an active load sharing loop?

A

Current transformers fitted to monitor the current in ‘B’ phase of each of the three generator outputs

124
Q

How is sensing achieved in a reactive load sharing loop?

A

Mutual reactor whose secondary winding is fitted in series with one of the phases feeding to the voltage reference detector in the voltage regulator of the GCU

125
Q

In a parallel AC generator system, with an over excitation (parallel fault) condition, what is the likely fault?

A

Fault voltage regulator or reactive load sharing circuit

If tripping the BTB cleared the problem, the fault was most likely due to the reactive load sharing circuit

126
Q

In a parallel AC generator system, what function does the overvoltage protection system operate on?

A

Inverse time function

Magnitude of voltage determines the time in which the offending generator will be de-energised

127
Q

What is the configuration of the auxiliary contacts and main contacts in a main power or latched breaker?

A

When the main contacts are open, the auxiliary contacts are closed

128
Q

What is the typical twin generator system for a helicopter?

A
  • Number 1 generator supplies the essential AC loads
  • Number 2 generator supplies the non-essential AC loads
129
Q

What is the purpose of the transformers on a helicopter?

A

Provide different values of AC voltage, typically 26v, for such things as lights and synchros

130
Q

At what height will a BTB not operate, even if a generator is lost?

A

Below 200 ft radio altitude

131
Q

What is the purpose of the auxiliary contacts in an active load sharing loop?

A

Shorts out the generator, thus ensuring the remaining generators take the active power

132
Q

At what frequency will instability protection in a parallel generation system delay trip all the BTB?

A

Approximately 8 Hz

133
Q

What usually causes frequency instability in a parallel generation system?

A

Low oil level in the CSD

134
Q

What part of the reactive load will be damaged with an over frequency in a parallel generation system?

A

Capacitive

135
Q

What part of the reactive load will be damaged with an under frequency in a parallel generation system?

A

Inductive

Module 13 EASA Part 1 (20 decks)

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