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Aircraft Electrical Systems

The function of the aircraft electricalsystem is to generate, regulate anddistribute electrical power throughout

the aircraft

New-generation aircraft rely heavilyon electrical power because of the

wide use of electronic flightinstrument systems

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Electrical Power Uses

Aircraft electrical power is used tooperate:

Aircraft Flight Instruments

Essential Systems

Passenger Services

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Electrical Power Uses (cont.)

Essential power is power that theaircraft needs to be able to continuesafe operation

Passenger services power is thepower that used for:

Cabin lighting

Operation of entertainment systems

Preparation of food

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Power Used

Aircraft electrical componentsoperate on many different voltagesboth AC and DC

However, most of the systems use:

115 VAC @ 400 Hz

28 VDC

26 VAC is also used in some aircraftfor lighting

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Power Sources

There are sever different power sourceson large aircraft to be able to handleexcessive loads, for redundancy, and for

emergency situations. These power sources include:

Engine driven AC generators

Auxiliary Power Units

External power

Ram Air Turbines

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Engine Driven AC Generators

Each of the engines on an aircraftdrives an AC generator

The power produced by thesegenerators is used in normal flight tosupply the entire aircraft with power

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APU Power

Most often the APUs power is usedwhile the aircraft is on the groundduring maintenance or for engine

starting

However, most aircraft can use theAPU while in flight as a backup power

sourceOne exception to this is the B272, which

only allows APU operation in the ground

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External Power

External power may only be usedwith the aircraft on the ground

This system utilizes a Ground PowerUnit (GPU) to provide AC powerthrough an external plug on the noseof the aircraft

GPUs may be either portable orstationary units

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Ram Air Turbine

Some aircraft are equipped with RamAir Turbines, or RATs

These may be used, in the case of agenerator or APU failure, as anemergency power source

When necessary, the RAT may bedeployed to be used as an AC powersource

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Aircraft Batteries

The aircrafts nickel cadmium batteryis final source of backup power

The battery provides 28 VDC

It is also possible to change the 28VDC into 115 VAC 400Hz with theuse of a static inverter

When using the battery, powerusage is limited by the short life ofthe battery

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Electrical Power System

ComponentsAC Generator

Constant Speed Drive

Integrated Drive GeneratorTransformer Rectifier Unit

Generator Control Unit

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Constant Speed Drive

The purpose of the Constant Speed Drive(CSD) is to take rotational power from theengine and, no matter the engine speed,

turn the generator at a constant speed This is necessary because the generator

output must be 400Hz

CSD Operation

The engine turns the CSD which uses adifferential assembly and hydraulic pumps toturn the generator

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Integrated Drive Generator

Another method of regulating thegenerator speed is with the use of anIntegrated Drive Generator (IDG)

An IDG is simply a CSD andgenerator combined into one unit

There are two ways to mount the

IDG:Co-axiallySide-by-side

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Transformer Rectifier Unit

Transformer Rectifier Units (TRUs) areutilized to 115 VAC, 400Hz into 28 VDC

A transformer is used to reduce the

voltage from 115 volts to 28 volts At this point the 28 volts is still AC current

To change the current from AC to DC, arectifier is used

Each aircraft AC bus feeds a TRU whichfeeds a DC bus

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Other Generator Controls and

Monitoring Devices A Generator Control Unit (GCU), or

voltage regulator, is used to controlgenerator output

Generator circuit protection monitorselectrical system parameters

Voltage

Frequency

Overcurrent

Undercurrent

Differential Fault

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Other Generator Controls and

Monitoring DevicesLoad controls sense real system load

to provide a signal to the CSD forfrequency control

Current transformers are used forcurrent load sensing and differentialfault protection

The electrical system control panelmay be found either on the pilotsoverhead panel or on the flightengineers panel

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Function of System

ComponentsThe basic functions of the electrical

systems components are to:

Generate Power

Control Electrical Power

Protect the Electrical System

Distribute Electrical Power Throughout

the Aircraft

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Aircraft Electrical Systems

The function of the aircraft electricalsystem is to generate, regulate anddistribute electrical power throughout

the aircraft

New-generation aircraft rely heavilyon electrical power because of the

wide use of electronic flightinstrument systems

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Electrical Power Uses

Aircraft electrical power is used tooperate:

Aircraft Flight Instruments

Essential Systems

Passenger Services

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Electrical Power Uses (cont.)

Essential power is power that theaircraft needs to be able to continuesafe operation

Passenger services power is thepower that used for:

Cabin lighting

Operation of entertainment systems

Preparation of food

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Power Used

Aircraft electrical componentsoperate on many different voltagesboth AC and DC

However, most of the systems use:

115 VAC @ 400 Hz

28 VDC

26 VAC is also used in some aircraftfor lighting

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WHY 400 Hz ?

Higher than usual frequencies,such as 400 Hz, offer severaladvantages over 60 Hz notably

in allowing smaller, lighter powersupplies to be used for militaryhardware, commercial aircraftoperations and computer

applications.

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WHY 400 Hz ?

As aircraft space is at a premium andweight is critical to aircraft enginethrust and fuel burn (and thus the

aircraft range and enginehorsepower per pound), 115 volts at400 Hz offers a distinct advantage

and is much better than the usual 60Hz used in utility power generation.

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WHY 400 Hz ?

However, higher frequencies are alsomore sensitive to voltage drop problems.

Reactive drops, on the other hand, are

caused by the inductive properties of theconductor. Reactive drops are a functionof both cable length and the AC frequencyflowing through the conductor. With highfrequencies such as 400 Hz, reactivedrops are up to seven times greater

at 60 Hz.

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Power Sources

There are different power sources on largeaircraft to be able to handle excessiveloads, for redundancy, and for emergency

situations. These power sources include:

Engine driven AC generators

Auxiliary Power Units

External power

Ram Air Turbines

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Typical Aircraft Electrical System

Primary (main) power source

Emergency power source Secondary power conversion

equipment

System control and protectiondevices

Interconnection network

Power distribution system

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Engine Driven AC Generators

Each of the engines on an aircraftdrives an AC generator

The power produced by thesegenerators is used in normal flight tosupply the entire aircraft with power

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APU Power

Most often the APUs power is usedwhile the aircraft is on the groundduring maintenance or for engine

starting

However, most aircraft can use theAPU while in flight as a backup power

sourceOne exception to this is the B-727,

which only allows APU operation in the

ground

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Vaughn College

APU on a stand

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Functions of APU

The primary purpose of an aircraftAPU is to provide power to start themain engines. Turbine engines havelarge, heavy rotors that must beaccelerated to a high rotationalspeed in order to provide sufficientair compression for self-sustainingoperation. This process takes

significantly longer and requiresmuch more energy than starting areciprocating engine.

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Functions of APU

Smaller turbine engines are usuallystarted by an electric motor, whilelarger turbine engines are usuallystarted by an air turbine motor.Whether the starter is electrically orpneumatically powered, however,

the amount of energy required is fargreater than what could be providedby a storage device (battery or air

tank) of reasonable size and weight

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Functions of APU

An APU solves this problem by poweringup the aircraft in two stages. First, theAPU is started by an electric motor, withpower supplied by a battery or external

power source (ground power unit). Afterthe APU accelerates to full speed, it canprovide a much larger amount of power tostart the aircraft's main engines, either by

turning an electrical generator or byproviding compressed air to the air turbineof the starter motor.

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Functions of APU

APUs also have several auxiliaryfunctions. Electrical andpneumatic power are used to runthe heating, cooling, and

ventilation systems prior tostarting the main engines. Thisallows the cabin to becomfortable while thepassengers are boarding withoutthe expense, noise, and dangerof running one of the aircraft's

main engines

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Functions of APU

Electrical power is also used to powerup systems for preflight checks.Some APUs are also connected to a

hydraulic pump, allowingmaintenance and flight crews tooperate the flight controls and powerequipment without running the main

engines. This same function is alsoused as a backup in flight in case ofan engine failure or hydraulic pump

failure.

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External Power

External power may only be usedwith the aircraft on the ground

This system utilizes a Ground PowerUnit (GPU) to provide AC powerthrough an external plug on the noseof the aircraft

GPUs may be either portable orstationary units

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Ground power

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Emergency power

In the event of a primary powersource failure, emergency poweris usually provided from

independent auxiliary power unit(APU)-driven generator(s), ramair or hydraulically-driven

generator(s), or batteries.

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Ram Air Turbine

Some aircraft are equipped with RamAir Turbines, or RATs

These may be used, in the case of agenerator or APU failure, as anemergency power source

When necessary, the RAT may be

deployed to be used as an AC powersource

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Emergency Power Source

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Aircraft Batteries

The aircrafts nickel cadmium batteryis final source of backup power

The battery provides 28 VDC

It is also possible to change the 28VDC into 115 VAC 400Hz with theuse of a static inverter

When using the battery, powerusage is limited by the short life ofthe battery

El t i l P S t

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Electrical Power System

ComponentsAC Generator

Constant Speed Drive

Integrated Drive GeneratorTransformer Rectifier Unit

Generator Control Unit

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Constant Speed Drive

The purpose of the Constant Speed Drive(CSD) is to take rotational power from theengine and, no matter the engine speed,

turn the generator at a constant speed This is necessary because the generator

output must be 400Hz

CSD Operation

The engine turns the CSD which uses adifferential assembly and hydraulic pumps toturn the generator

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Integrated Drive Generator

Another method of regulating thegenerator speed is with the use of anIntegrated Drive Generator (IDG)

An IDG is simply a CSD andgenerator combined into one unit

There are two ways to mount the

IDG:Co-axiallySide-by-side

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Transformer Rectifier Unit

Transformer Rectifier Units (TRUs)are utilized to 115 VAC, 400Hz into28 VDC.

A transformer is used to reduce thevoltage from 115 volts to 28 volts.

At this point the 28 volts is still ACcurrent.

To change the current from AC toDC, a rectifier is used.

Each aircraft AC bus feeds a TRU

which feeds a DC bus.

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Oth G t C t l d

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Other Generator Controls and

Monitoring Devices A Generator Control Unit (GCU), or

voltage regulator, is used to controlgenerator output

Generator circuit protection monitorselectrical system parameters

Voltage

Frequency

Overcurrent

Undercurrent

Differential Fault

Oth G t C t l d

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Other Generator Controls and

Monitoring DevicesLoad controls sense real system load

to provide a signal to the CSD forfrequency control

Current transformers are used forcurrent load sensing and differentialfault protection

The electrical system control panelmay be found either on the pilotsoverhead panel or on the flightengineers panel

F ti f S t

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Function of System

ComponentsThe basic functions of the electrical

systems components are to:

Generate Power.

Control Electrical Power.

Protect the Electrical System.

Distribute Electrical PowerThroughout the Aircraft.

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As a engineer, we should know the

following information

Electrical system operation,which describes primary andsecondary power sources, bus

configuration with circuitbreakers and connected. loadsfor each bus. A copy of the bus

wiring diagram or electricalschematic should also beconsidered for inclusion in the

report.

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As a engineer, we should know the

following information

Alternators and other powersource description and

related data (including suchitems as battery dischargecurves, Transformer Rectifier

Unit (TRU), Inverter, APU,RAT, etc.)

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