Don Foreman wrote:
Horsepuckey! There is no such theoretical limit of efficiency. There
is no way the large turbine-driving generators enclosed in helium are
anywhere near as low as 85% efficient. They'd melt in minutes.
I think most are cooled by hydrogen, not helium. H is a better heat
conductor than He, although that is pretty good, too. The excitation in
a typical power house alternator is something like 1000 A at 100 V
across two strips of copper bar about 50 feet long, total. They are
usually something like 1/4" x 2" bar hammered into a pair of spiral
grooves cut into the solid steel rotor. So, the rotor has bars in it
that dissipate 100 KW anytime the alternator is excited.

Now, these numbers seem extreme util you compare them to the output of
the alternator, which can run to 1 GW, but something around 750 - 850 MW
is typical. Suddenly, that massive exciter dissipation is a tiny .01%
of the rated output!

I don't have a good figure handy for iron and copper losses in the
stator of these machines, but it is definitely no more than a couple %
of full output. Windage would be substantial if they weren't
hydrogen-cooled, as the air gap is an amazing ,002" or so, even though
the rotors are HUGE!

Jon