Randall wrote:
>BTW, I don't know how they do it now, but early AC alternators (like those
>used to generate power for your home) also required DC excitation as it's
>called. They used to actually run a DC generator from the same power
>source, whose only purpose was to supply the rotor current to the
>alternator. The field current to the generator controlled the rotor
current
>to the alternator, which controlled the main output voltage.
<
This is called a rotary exciter. Most have been replaced with "static
exciters" which are usually three phase semi-converting bridges. (I
designed one of these once with a steady state output of 250Vdc @ 1,000
amps and a 1 minute forcing rating of 375 V @ 1,500 amps, but that's beside
the point).
Brushless generators (most common today) are actually two rotary machines
on one shaft. The voltage regulator controls one with a stationary field
whose output is rectified by diodes mounted on the shaft to power the main
field. Closed loop control of these is quite a bit more difficult due to
the two generators in series makes the plant model quite complex. (But
that is beside the point). The alternators in our cars does not have this
second "excitation" generator. The regulator feeds current to the field
(rotor) via slip rings.
If anyone is not overwhelmed by all this generator trivia, let me know.
There's more.
Dave
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