Aircraft Electrical Systems

Aircraft Electrical Systems
The function of the aircraft electrical system is to generate, regulate and distribute electrical power throughout the aircraft
New-generation aircraft rely heavily on electrical power because of the wide use of electronic flight instrument systems

Electrical Power Uses
Aircraft electrical power is used to operate:
Aircraft Flight Instruments
Essential Systems
Passenger Services

Electrical Power Uses (cont.)
Essential power is power that the aircraft needs to be able to continue safe operation
Passenger services power is the power that used for:
Cabin lighting
Operation of entertainment systems
Preparation of food

Power Used
Aircraft electrical components operate on many different voltages both AC and DC
However, most of the systems use:
115 VAC @ 400 Hz
28 VDC
26 VAC is also used in some aircraft for lighting

Power Sources
There are sever different power sources on large aircraft to be able to handle excessive loads, for redundancy, and for emergency situations.
These power sources include:
Engine driven AC generators
Auxiliary Power Units
External power
Ram Air Turbines

Engine Driven AC Generators
Each of the engines on an aircraft drives an AC generator
The power produced by these generators is used in normal flight to supply the entire aircraft with power

APU Power
Most often the APUs power is used while the aircraft is on the ground during maintenance or for engine starting
However, most aircraft can use the APU while in flight as a backup power source
One exception to this is the B272, which only allows APU operation in the ground

External Power
External power may only be used with the aircraft on the ground
This system utilizes a Ground Power Unit (GPU) to provide AC power through an external plug on the nose of the aircraft
GPUs may be either portable or stationary units

Ram Air Turbine
Some aircraft are equipped with Ram Air Turbines, or “RATs”
These may be used, in the case of a generator or APU failure, as an emergency power source
When necessary, the RAT may be deployed to be used as an AC power source

Aircraft Batteries
The aircraft’s nickel cadmium battery is final source of backup power
The battery provides 28 VDC
It is also possible to change the 28 VDC into 115 VAC 400Hz with the use of a static inverter
When using the battery, power usage is limited by the short life of the battery

Electrical Power System Components
AC Generator
Constant Speed Drive
Integrated Drive Generator
Transformer Rectifier Unit
Generator Control Unit

Constant Speed Drive
The purpose of the Constant Speed Drive (CSD) is to take rotational power from the engine 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 a differential assembly and hydraulic pumps to turn the generator

Integrated Drive Generator
Another method of regulating the generator speed is with the use of an Integrated Drive Generator (IDG)
An IDG is simply a CSD and generator combined into one unit
There are two ways to mount the IDG:
Co-axially
Side-by-side

Transformer Rectifier Unit
Transformer Rectifier Units (TRUs) are utilized 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, a rectifier is used
Each aircraft AC bus feeds a TRU which feeds a DC bus

Other Generator Controls and Monitoring Devices
A Generator Control Unit (GCU), or voltage regulator, is used to control generator output
Generator circuit protection monitors electrical system parameters
Voltage
Frequency
Overcurrent
Undercurrent
Differential Fault

Other Generator Controls and Monitoring Devices
Load controls sense real system load to provide a signal to the CSD for frequency control
Current transformers are used for current load sensing and differential fault protection
The electrical system control panel may be found either on the pilot’s overhead panel or on the flight engineer’s panel

Function of System Components
The basic functions of the electrical system’s components are to:
Generate Power
Control Electrical Power
Protect the Electrical System
Distribute Electrical Power Throughout the Aircraft