Electrical Power

AC/DC generation (IDG/CSD, TRUs, batteries, RAT), distribution, load management and protection.

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Main ideas

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.

Solved examples

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.

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