Re: Alternator.

PC Paul wrote:

Aha, a bit more research, and I see now. The reason
alternators work evenly well over a huge rev range compared to
dynamos is that they use coils everywhere instead of having
permanent magnets. This means that by controlling the (low)
current flow in the exciter coils the overall output is
regulated - so the regulator isn't dumping excess power, it's
reducing the exciter coil field strength so less power is
generated in the first place. Clever.

In this respect, alternators are no different from the
generators they replaced. Both use regulated field coils
instead of permanent magnets as a means of controlling output.

In an alternator the field coils spin, and the AC generated in
the stator is rectified by diodes. In the earlier generator
the field coils are fixed and the armature rotates inside, with
rectification to DC achieved _mechanically_ by use of a
commumator and brushes.

It's the use of an armature and communator which limits both
rotational speed and maximum current handling (it needs to pass
full output current through the brushes). Such a generator
uses a regulator which is both a voltage and current limiting
device, whereas an alternator uses just a voltage regulator.

An unregulated alternator will build up to 120Vish at high
RPM. But when it's being used for 14.4V car battery charging
it doesn't get anywhere near that high internally, it
regulates it down first, making the alternator easier to turn
and hence making the overall system more efficient than with a
dynamo.

If the generator used permanent magnets, this might be the case.

John
.

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