Field excitation comes from a small dc generator coupled to the same shaft
as the alternator (usually 220 V-1000 A dc for a 300 MW altenator,21kV,
10kA).Usually it's shunt excitation.THAT'S a case where a generator
generates without external excitation;there is some remaining magnetism in
the stator of the dc generator, so when it rotates the voltage
increases..and increases.

--
Tzortzakakis Dimitriïs
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
Ï "Joel Kolstad" Ýãñáøå óôï ìÞíõìá
...
"Dimitrios Tzortzakakis" wrote in
message ...
If one generator needs applying power to it to generate
electricity, it loses the purpose of being a generator.

Hardly; you can make a pretty good case that the most efficient generators
often DO require some external power for the sake of field excitation,
monitoring/control equipment, etc. This initially comes from a bank of
batteries, a smaller 'starter' generator where you don't care about
efficiency since it'll only be used briefly, etc.

I was talking to a guy here in Oregon whose job is to maintain and monitor
a
small (a half dozen or so MW, I believe) hydroelectric power plant; one of
the points he took pride in was that at _his_ plant he still had enough
equipment around that he could get the plant going without any external
power whatsoever. He said that many plants have been 'modernized' such
that
almost all of the control is computerized these days, but the downside is
that they require external grid power to get the main generator going at
all. I can see that being a defensible engineering choice, although if I
had the guy's job I'd also be a little more comfortable knowing I could
re-start from an off-grid condition.

---Joel Kolstad