Electrical Systems, Generation and Distribution
Generation
Aircraft use 28 V DC (small aircraft) or 115 V AC 400 Hz (transports), with 235 V AC on some large types. AC is generated by integrated drive generators (IDG) or variable frequency generators; the IDG holds constant output frequency despite engine speed variation. DC comes from transformer-rectifier units (TRU) fed from AC, or from battery and starter-generators on light aircraft. The battery is mainly for starting, emergency power and transient loads; it is not a primary source in flight on a transport.
Distribution and protection
Busbars distribute power: main buses, essential/emergency buses, and battery bus. Circuit breakers and fuses protect wiring; a bus tie allows one generator to feed both sides. Parallel operation of AC generators requires matching voltage, frequency and phase; the generator control unit (GCU) handles this and trips a faulty generator off the bus. Reverse current protection prevents a battery or bus feeding back into a failed generator.
Traps
A frequent trap is the difference between a generator failure and a bus failure: loss of one generator may leave all buses powered via the tie, whereas a bus fault removes the consumers on that bus. Another is that AC generators cannot be paralleled if frequency differs; DC generators can share load more simply. Also, external power is normally connected only on the ground and must be monitored for correct polarity and voltage.
Worked example
A 28 V system with a 100 A alternator gives 2.8 kW maximum. If a 500 W load is added, current rises by about 18 A; check the remaining capacity against the bus limit, not just the generator rating.
Emergencies
On total generation loss, the battery and emergency bus feed essential instruments and radios; the RAT may drive an emergency generator. Know which items are on the emergency bus.
Rule of thumb
Voltage × current = power; the battery ampere-hour rating tells endurance only at a stated discharge rate.