Re: Why Size of transformer/motor reduces as frequency of operation increases

Excellent question, and one that I used to wonder about a lot.

The reason is that most of the loads in an aircraft (transformers, motors) rely on *inductance* as one of their primary methods to limit current. Now, the 'resistance' (it's really called reactance) presented to AC current by an inductor goes UP linearly as the frequency increases; the formula is "R" = 2 * pi * L * f. So we can see that the reactance due to a given inductor goes up by a factor of 8 as the frequency changes from 50 Hz to 400 Hz. Or another way to look at it is that for a given reactance, you need 1/8 the amount of inductance at 400Hz as opposed to 50Hz. This in turn means that the inductive components can be ~1/8 the volume at 400 Hz and, more importantly for aircraft, can be ~1/8 of the mass (weight), increasing the available payload. The same technique is often used on ships or submarines to reduce the size of the components necessary. If you extend the frequency up to a few MHz, as is done in switching power supplies (used in your computer or cellphone) to convert AC to DC, the same rule applies and the inductors can be tiny. Hope this helps!