When the aero engine is not working (such as ground test), the main power supply does not work, and it is powered by the auxiliary power supply. Aircraft batteries or auxiliary power units (a power unit consisting of a small airborne engine, generator and hydraulic pump) are commonly used auxiliary power sources. When the main power supply fails in flight, the battery or emergency generator becomes the emergency power source.
Airborne electrical equipment requires high power quality, voltage adjustment accuracy, frequency adjustment accuracy, AC voltage waveform sinusoid, voltage surge and spikes have certain technical standards.
Usually there are 1 or 2 generators on one engine, so many engines are equipped with many generators. The generators in the DC power system are all operating in parallel. Some alternators work in parallel (such as the four generators of the Boeing 707 aircraft), and some do not work in parallel (such as the three generators of the "Trident" aircraft). The AC power system that does not work in parallel is relatively simple; the parallel system is more complicated, but the power supply capacity is large, and the fluctuation of the load has less influence on the power supply voltage and frequency, so the power quality is high and the power supply is not easily interrupted.