So what are they using than inside an airplane or aircraft? a AC generator or an AC alternator? a car uses a 12VDC battery with an alternator , do planes have a battery connected to the alternator?
The electrical network of a typical single-isle or wide-body jet is such:
Each engine will typically have two generators.
Pre-2000 these generators were coupled to the engine via a constant-speed gearbox giving an almost constant rotor speed == supply freq
Post-2000 these EXTREMELY heavy gearbox was removed for a simpler, lighter gearbox and thus the supply frequency varies from about 360Hz to 800Hz across a flight as the rev's of the jet changes.
All these generators go via distribution panels so one set of equipment will be fed from one and others from another...
They can be switched in and out at will depending on loading and engine performance.
There is also a 28Vdc bus fed from a battery. This powers the Avionics and will also fire the APU in the event of a significant powerloss.
This battery is continuously tricklecharged from the main generators.
That battery, along with the RAT is actually capable of landing an aircraft will full generator failure - assuming immediate divergance to the closest airport. This is why atlantic flights, esp dual engine, fly nearer to greenland
And why not 400Hz and not say 1kHz or 1MHz since increase freq results in smaller magnetics?
Engineering tradeoff. Increase freq increases the impedance of a cable so lower voltage seen at the loads and reduced ability to regulate the PCC
Decrease frequency means larger magnetics but tighter PCC regulation