Ed Huntress wrote:
"Jon Elson" wrote in message
...


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!


Efficiency of large power-plant generators runs around 98%, shaft input
power to electrical output.
That might not count the exciter, then. The exciter is
generally a HUGE transformer-rectifier set connected through
slip rings, although some systems use brushless excitation. Our
local utility uses all slip-ring coupled excitation for some
reason, maybe corporate inertia. But, I think all of them use
ono-rotary exciters, however they are coupled.

Jon