Why 28V DC?

"Ben Jackson" wrote in message
news:veiNb.66937$xy6.125476@attbi_s02...
In article ,
S Narayan wrote:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.
But this was in the context of home/shop power where the single phase
in a standard home is inefficient with big motors.

I believe it's actually the converse on an aircraft. It's the generator
that's big and heavy and by generating 3 phase, you can get more power per
pound of generator.

To make things more interesting, the industry is now going to 120V, variable
frequency AC power. There are two variants, IIRC they are 320 to 480 Hz and
280 to 620 Hz (or thereabouts.)

Why? Because to get a fixed 400Hz, you have to have a constant speed drive
off the engine to drive the generator. Those drives are expensive, heavy,
and require lots of maintenance. Getting rid of them saves money, but now
frequency varies with engine speed.

So now we get to put variable frequency power supplies in all our new
avionics.

Gerry