Aircraft Electrical Systems-OV

Aircraft Electrical Aircraft Electrical SystemsSystems

General ViewGeneral View

Aircraft Electrical SystemsAircraft Electrical Systems

The function of the aircraft electrical The function of the aircraft electrical system is to generate, regulate and system is to generate, regulate and distribute electrical power distribute electrical power throughout the aircraftthroughout the aircraft

New-generation aircraft rely heavily New-generation aircraft rely heavily on electrical power because of the on electrical power because of the wide use of electronic flight wide use of electronic flight instrument systemsinstrument systems

Electrical Power UsesElectrical Power Uses

Aircraft electrical power is used to Aircraft electrical power is used to operate:operate:– Aircraft Flight InstrumentsAircraft Flight Instruments– Essential SystemsEssential Systems– Passenger ServicesPassenger Services

Electrical Power Uses (cont.)Electrical Power Uses (cont.)

Essential power is power that the Essential power is power that the aircraft needs to be able to continue aircraft needs to be able to continue safe operationsafe operation

Passenger services power is the Passenger services power is the power that used for:power that used for:– Cabin lightingCabin lighting– Operation of entertainment systemsOperation of entertainment systems– Preparation of foodPreparation of food

Power UsedPower Used

Aircraft electrical components Aircraft electrical components operate on many different voltages operate on many different voltages both AC and DCboth AC and DC

However, most of the systems use:However, most of the systems use:– 115 VAC @ 400 Hz115 VAC @ 400 Hz– 28 VDC28 VDC

26 VAC is also used in some aircraft 26 VAC is also used in some aircraft for lightingfor lighting

Vaughn CollegeVaughn College

WHY 400 Hz ?WHY 400 Hz ?

Higher than usual frequencies, Higher than usual frequencies, such as 400 Hz, offer several such as 400 Hz, offer several advantages over 60 Hz – notably advantages over 60 Hz – notably in allowing smaller, lighter power in allowing smaller, lighter power supplies to be used for military supplies to be used for military hardware, commercial aircraft hardware, commercial aircraft operations and computer operations and computer applicationsapplications. .



As aircraft space is at a premium and As aircraft space is at a premium and weight is critical to aircraft engine weight is critical to aircraft engine thrust and fuel burn (and thus the thrust and fuel burn (and thus the aircraft range and engine aircraft range and engine horsepower per pound), 115 volts at horsepower per pound), 115 volts at 400 Hz offers a distinct advantage 400 Hz offers a distinct advantage and is much better than the usual 60 and is much better than the usual 60 Hz used in utility power generation.Hz used in utility power generation.



However, higher frequencies are also more However, higher frequencies are also more sensitive to voltage drop problems.sensitive to voltage drop problems.

Reactive drops, on the other hand, are Reactive drops, on the other hand, are caused by the inductive properties of the caused by the inductive properties of the conductor. Reactive drops are a function conductor. Reactive drops are a function of both cable length and the AC frequency of both cable length and the AC frequency flowing through the conductor. With high flowing through the conductor. With high frequencies such as 400 Hz, reactive drops frequencies such as 400 Hz, reactive drops are up to seven times greater are up to seven times greater

at 60 Hz.at 60 Hz.

Power SourcesPower Sources

There are sever different power sources on There are sever different power sources on large aircraft to be able to handle large aircraft to be able to handle excessive loads, for redundancy, and for excessive loads, for redundancy, and for emergency situations.emergency situations.

These power sources include:These power sources include:– Engine driven AC generatorsEngine driven AC generators– Auxiliary Power UnitsAuxiliary Power Units– External powerExternal power– Ram Air TurbinesRam Air Turbines

Typical Aircraft Electrical SystemTypical Aircraft Electrical System

Primary (main) power sourcePrimary (main) power source Emergency power sourceEmergency power source Secondary power conversion Secondary power conversion

equipmentequipment System control and protection devicesSystem control and protection devices Interconnection networkInterconnection network Power distribution system Power distribution system

Engine Driven AC GeneratorsEngine Driven AC Generators

Each of the engines on an aircraft Each of the engines on an aircraft drives an AC generatordrives an AC generator

The power produced by these The power produced by these generators is used in normal flight to generators is used in normal flight to supply the entire aircraft with powersupply the entire aircraft with power

APU PowerAPU Power

Most often the APUs power is used Most often the APUs power is used while the aircraft is on the ground while the aircraft is on the ground during maintenance or for engine during maintenance or for engine startingstarting

However, most aircraft can use the However, most aircraft can use the APU while in flight as a backup power APU while in flight as a backup power sourcesource– One exception to this is the B-727, which One exception to this is the B-727, which

only allows APU operation in the groundonly allows APU operation in the ground


APU on a standAPU on a stand


Functions of APUFunctions of APU

The primary purpose of an aircraft The primary purpose of an aircraft APU is to provide power to start the APU is to provide power to start the main engines. Turbine engines have main engines. Turbine engines have large, heavy rotors that must be large, heavy rotors that must be accelerated to a high rotational accelerated to a high rotational speed in order to provide sufficient speed in order to provide sufficient air compression for self-sustaining air compression for self-sustaining operation. This process takes operation. This process takes significantly longer and requires significantly longer and requires much more energy than starting a much more energy than starting a reciprocating enginereciprocating engine. .



Smaller turbine engines are usually Smaller turbine engines are usually started by an electric motor, while started by an electric motor, while larger turbine engines are usually larger turbine engines are usually started by an air turbine motor. started by an air turbine motor. Whether the starter is electrically or Whether the starter is electrically or pneumatically powered, however, pneumatically powered, however, the amount of energy required is far the amount of energy required is far greater than what could be provided greater than what could be provided by a storage device (battery or air by a storage device (battery or air tank) of reasonable size and weighttank) of reasonable size and weight



An APU solves this problem by powering An APU solves this problem by powering up the aircraft in two stages. First, the APU up the aircraft in two stages. First, the APU is started by an electric motor, with power is started by an electric motor, with power supplied by a battery or external power supplied by a battery or external power source (ground power unit). After the APU source (ground power unit). After the APU accelerates to full speed, it can provide a accelerates to full speed, it can provide a much larger amount of power to start the much larger amount of power to start the aircraft's main engines, either by turning aircraft's main engines, either by turning an electrical generator or by providing an electrical generator or by providing compressed air to the air turbine of the compressed air to the air turbine of the starter motor.starter motor.


Functions of APUFunctions of APUAPUs also have several auxiliary APUs also have several auxiliary

functions. Electrical and functions. Electrical and pneumatic power are used to run pneumatic power are used to run the heating, cooling, and the heating, cooling, and ventilation systems prior to ventilation systems prior to starting the main engines. This starting the main engines. This allows the cabin to be allows the cabin to be comfortable while the comfortable while the passengers are boarding without passengers are boarding without the expense, noise, and danger the expense, noise, and danger of running one of the aircraft's of running one of the aircraft's main enginesmain engines. .


Functions of APUFunctions of APU Electrical power is also used to power Electrical power is also used to power

up systems for preflight checks. up systems for preflight checks. Some APUs are also connected to a Some APUs are also connected to a hydraulic pump, allowing hydraulic pump, allowing maintenance and flight crews to maintenance and flight crews to operate the flight controls and power operate the flight controls and power equipment without running the main equipment without running the main engines. This same function is also engines. This same function is also used as a backup in flight in case of used as a backup in flight in case of an engine failure or hydraulic pump an engine failure or hydraulic pump failure.failure.

External PowerExternal Power

External power may only be used External power may only be used with the aircraft on the groundwith the aircraft on the ground

This system utilizes a Ground Power This system utilizes a Ground Power Unit (GPU) to provide AC power Unit (GPU) to provide AC power through an external plug on the nose through an external plug on the nose of the aircraftof the aircraft

GPUs may be either portable or GPUs may be either portable or stationary unitsstationary units

Ground powerGround power

Emergency powerEmergency power

In the event of a primary power In the event of a primary power source failure, source failure, emergency poweremergency power is is usually provided from independent usually provided from independent auxiliary power unit (APU)-driven auxiliary power unit (APU)-driven generator(s), ram air or generator(s), ram air or hydraulically-driven generator(s), or hydraulically-driven generator(s), or batteries. batteries.

Ram Air TurbineRam Air Turbine

Some aircraft are equipped with Ram Some aircraft are equipped with Ram Air Turbines, or “RATs”Air Turbines, or “RATs”

These may be used, in the case of a These may be used, in the case of a generator or APU failure, as an generator or APU failure, as an emergency power sourceemergency power source

When necessary, the RAT may be When necessary, the RAT may be deployed to be used as an AC power deployed to be used as an AC power sourcesource

Emergency Power SourceEmergency Power Source

Aircraft BatteriesAircraft Batteries

The aircraft’s nickel cadmium battery The aircraft’s nickel cadmium battery is final source of backup poweris final source of backup power

The battery provides 28 VDCThe battery provides 28 VDC It is also possible to change the 28 It is also possible to change the 28

VDC into 115 VAC 400Hz with the VDC into 115 VAC 400Hz with the use of a static inverteruse of a static inverter

When using the battery, power usage When using the battery, power usage is limited by the short life of the is limited by the short life of the battery battery

Electrical Power System Electrical Power System ComponentsComponents

AC GeneratorAC Generator Constant Speed DriveConstant Speed Drive Integrated Drive GeneratorIntegrated Drive Generator Transformer Rectifier UnitTransformer Rectifier Unit Generator Control UnitGenerator Control Unit

Constant Speed DriveConstant Speed Drive

The purpose of the Constant Speed Drive The purpose of the Constant Speed Drive (CSD) is to take rotational power from the (CSD) is to take rotational power from the engine and, no matter the engine speed, engine and, no matter the engine speed, turn the generator at a constant speedturn the generator at a constant speed

This is necessary because the generator This is necessary because the generator output must be 400Hzoutput must be 400Hz

CSD OperationCSD Operation– The engine turns the CSD which uses a The engine turns the CSD which uses a

differential assembly and hydraulic pumps to differential assembly and hydraulic pumps to turn the generatorturn the generator

Integrated Drive GeneratorIntegrated Drive Generator

Another method of regulating the Another method of regulating the generator speed is with the use of an generator speed is with the use of an Integrated Drive Generator (IDG)Integrated Drive Generator (IDG)

An IDG is simply a CSD and An IDG is simply a CSD and generator combined into one unitgenerator combined into one unit

There are two ways to mount the There are two ways to mount the IDG:IDG:– Co-axiallyCo-axially– Side-by-sideSide-by-side

Transformer Rectifier UnitTransformer Rectifier Unit

Transformer Rectifier Units (TRUs) Transformer Rectifier Units (TRUs) are utilized to 115 VAC, 400Hz into are utilized to 115 VAC, 400Hz into 28 VDC.28 VDC.

A transformer is used to reduce the A transformer is used to reduce the voltage from 115 volts to 28 volts.voltage from 115 volts to 28 volts.

At this point the 28 volts is still AC At this point the 28 volts is still AC current.current.

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

Each aircraft AC bus feeds a TRU Each aircraft AC bus feeds a TRU which feeds a DC bus.which feeds a DC bus.

Other Generator Controls and Other Generator Controls and Monitoring DevicesMonitoring Devices

A Generator Control Unit (GCU), or voltage A Generator Control Unit (GCU), or voltage regulator, is used to control generator regulator, is used to control generator outputoutput

Generator circuit protection monitors Generator circuit protection monitors electrical system parameterselectrical system parameters– VoltageVoltage– FrequencyFrequency– OvercurrentOvercurrent– UndercurrentUndercurrent– Differential FaultDifferential Fault

Other Generator Controls and Other Generator Controls and Monitoring DevicesMonitoring Devices

Load controls sense real system load Load controls sense real system load to provide a signal to the CSD for to provide a signal to the CSD for frequency controlfrequency control

Current transformers are used for Current transformers are used for current load sensing and differential current load sensing and differential fault protectionfault protection

The electrical system control panel The electrical system control panel may be found either on the pilot’s may be found either on the pilot’s overhead panel or on the flight overhead panel or on the flight engineer’s panelengineer’s panel

Function of System Function of System ComponentsComponents

The basic functions of the electrical The basic functions of the electrical system’s components are to:system’s components are to:– Generate Power.Generate Power.– Control Electrical Power.Control Electrical Power.– Protect the Electrical System.Protect the Electrical System.– Distribute Electrical Power Distribute Electrical Power

Throughout the Aircraft.Throughout the Aircraft.

As a engineer, we should know the As a engineer, we should know the following informationfollowing information

Electrical system operation, Electrical system operation, which describes primary and which describes primary and secondary power sources, bus secondary power sources, bus configuration with circuit configuration with circuit breakers and connected. loads breakers and connected. loads for each bus. A copy of the bus for each bus. A copy of the bus wiring diagram or electrical wiring diagram or electrical schematic should also be schematic should also be considered for inclusion in the considered for inclusion in the reportreport..

As a engineer, we should know the As a engineer, we should know the following informationfollowing information

Alternators and other power Alternators and other power source description and source description and related data (including such related data (including such items as battery discharge items as battery discharge curves, Transformer Rectifier curves, Transformer Rectifier Unit (TRU), Inverter, APU, Unit (TRU), Inverter, APU, RAT, etc.)RAT, etc.)

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