Chu Chu:
None of this has anything to do with aircraft. But I have read that aircraft AC systems are 400 HZ rather than 50 or 60 because the higher frequency makes transformers more efficient. More efficient transformers can be built smaller and lighter without causing excessive losses.
This is true of all 'magnetic' devices such as motors. Also in AC to DC conversion. Ripple filtering capacitors can be much smaller at higher frequencies.
Well, sort of true. The part about lighter weight anyway. Voltage regulation is worse at higher frequencies. So are magnetic core losses. But the weight factor wins out on aircraft. Voltage regulation is a factor for longer circuit lengths. So you will see its use on long distance transmission lines, where the cost of regulation and losses offsets the higher cost of AC/DC and DC/AC conversion. These conversion costs are coming down rapidly with the advent of higher power, more reliable and cheaper solid state conversion*. You will begin to see more utility DC transmission and distribution as transformers are replaced by solid state conversion.
*Switching power supplies are an example. Old style transformer wall warts have been replaced my much lighter, smaller and more efficient switching power supplies.