On Thu, 28 Dec 2006 06:53:08 GMT, James Sweet
wrote:

I'm working on a generator for someone that recently just quit producing
electricity. On the label it touted the fact that it's a brushless
alternator; this is actually the first time I'd worked on one but I was
expecting much more inside it, certainly some sort of regulator module
but in fact it seems the only parts are a stator very much like that of
a large induction motor, a simple 2 pole armature with a diode mounted
to a heatsink, and a capacitor connected to two of the leads from the
stator. The capacitor is open circuit so that's an obvious problem,
diode checks out fine as do the windings so I'm assuming replacing the
cap will get it going.

What I'm curious though is how exactly does this thing work? The
armature has no connection at all to anything. I imagine it must receive
power through induction but how is the output regulated? Is there a
trick to manufacturing these? Given there's no brushes or slip rings I'd
have thought all alternators would be made this way unless there was a
disadvantage.

Search for "induction generators"

There's a common trick of using an ordinary induction motor as an AC
sine wave generator. It requires one cap across the winding and that
is usually a non-polar type with a low dissipation factor (oil filled
are ideal)

They won't start under load - have to be started then the load
applied. They balk at running other induction motors unless the motor
is ~1/6 the size of the generator.

No rectifier necessary. (but that may be part of some self-excitation
scheme, to insure the iron keeps some residual magnetism)

They can lose excitation and stop producing even without any defective
parts. They usually depend on some residual magnetism to kick things
off - and that may not be there - in which case you connect a DC
source to the generator when it is stopped to magnetize the iron.

http://www.linux-host.org/energy/einductge.html gives some info on
them.


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