On Sun, 11 Nov 2007 20:36:36 -0600, SockPuppet
wrote:

I have to say I'm pretty surprised to hear that aircraft alternators
don't have permanent magnets. Wow. Of all places not to have permanent
magnets: an aircraft, where so much is usually done for redundancy. I
find it kind of amazing.

All the alternators I've ever seen had permanent magnets...

Huh? I can't think of an alternator that DOES have permanent magnets,
unless you count the "lighting coils" on 2-strokes.

I missed the start of this discussion, but the reason NOT to use
permanent magnets in an alternator is that the output voltage will
vary with the rpm. This means that the alternator has to have
permanent magnets strong enough to produce sufficient 12V current even
at idle, and regulator has to be able to regulate the full rpm voltage
down to 12V while handling the full load current... which, just after
the engine starts and the battery's charging, can be a lot. This
means a big heavy regulator that needs to dissapate a lot of heat.
The field current is a lot less than the alternator's output current,
OTOH, so a fairly small, light regulator can handle the job, and no
regulation is needed on the alternator's output.

And, of course, an aircraft engine DOES have redundancy: Even if the
alternator or regulator fails, the magnetos keep the fan turning.

-Dana
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