Electrical Power
AC/DC generation (IDG/CSD, TRUs, batteries, RAT), distribution, load management and protection.
Electrical power, summary notes
- Large aircraft use AC as the primary supply, generated by engine-driven generators. To keep a CONSTANT FREQUENCY (400 Hz) despite varying engine speed, the generator is driven through a constant-speed drive, the CSD/IDG (integrated drive generator).
- DC is produced from AC by TRANSFORMER-RECTIFIER UNITS (TRUs); batteries provide start power and a last-resort emergency supply; other sources are the APU generator, external/ground power and the RAM AIR TURBINE (RAT) for emergencies.
- DISTRIBUTION is via busbars with bus-tie breakers; loads are prioritised so that if generation is lost, non-essential loads are shed and the ESSENTIAL/BATTERY bus keeps the critical systems (and standby instruments) alive.
- PROTECTION: generator control units (GCUs) regulate voltage and provide differential/overvoltage/underfrequency protection; circuit breakers protect the wiring.
- Frequency-wild (variable-frequency) systems are used on some modern aircraft, with electronics tolerating the varying frequency.
- ⚠ Exam trap: the CSD/IDG keeps the AC generator output at a CONSTANT FREQUENCY (nominally 400 Hz) regardless of engine RPM; TRUs convert AC to DC; the battery/essential bus feeds the critical loads when generation fails.
Why is an engine-driven AC generator connected through a constant-speed drive rather than straight to the engine?
AC electrical equipment (and paralleling of generators) needs a stable 400 Hz frequency, but the engine speed varies with thrust. The constant-speed drive (in an IDG, integrated with the generator) uses a hydromechanical unit to hold the generator's rotational speed constant despite the changing engine speed, so the output frequency stays at 400 Hz.
Electrical power concept map
Electrical Power
Electrical power quiz
Electrical Power, quiz
1. In the distribution diagram shown, the component marked T lies between the two busbars. It is the:
Bus tie breakerGenerator control breakerBattery isolation relay2. An IDG/CSD keeps the AC generator output at:
A constant frequency (400 Hz)A constant currentA variable frequency3. A TRU converts:
AC to DCDC to ACHydraulic to electric4. The nominal AC frequency on large aircraft is:
400 Hz50 Hz60 Hz5. If all generators fail, critical loads are kept alive by the:
Battery/essential busGalley busLanding-light bus6. A RAT (ram air turbine) provides:
Emergency electrical/hydraulic power from the airflowExtra thrust, by recovering energy from the airflow and returning it through the accessory driveCabin air, drawing in ram air and delivering it to the packs when bleed supply is lost7. An inverter converts:
DC to ACAC to DCDC to hydraulic8. Bus-tie breakers are used to:
Connect/isolate busbars for load sharing/fault isolationCool the generators, by switching cooling airflow to whichever machine is most heavily loadedMeter fuel to the engines in proportion to the electrical load being drawn from each generator