Re: alternator design information needed

On 26 Jan 2006 17:18:23 -0800, hrvspooner@xxxxxxx wrote:

>Hi all,
>I'm looking into a small scale hydro project just to get my feet wet
>with renewable energy. Does anyone have any ideas to share on how to
>design an efficient alternator in the 100 to 250 watt range? I know I
>can use motors etc, but I'm interested in knowing how to get the
>maximum performance from the little bit of water energy I've got
>available. Especially, since I have other plans for what to do with
>low power alternators :)
>
>Any sources of high efficiency alternators I can simply buy? If
>nothing else, suggested reading material?
>
>Regards, Hrv

Motor and alternator design are a compromise of size, weight, cost and
efficiency. One designed to operate efficiently at relatively high
speed may not give you good system efficiency because of losses in the
mechanisms (belts, gears etc) necessary to drive it at speed from wind
or hydro. Efficiency is a matter of how much iron, how much copper
and how powerful the permanent magnets. Large copper windings have
low IR drop and loss. Lots of iron and strong magnets reduce the
number of turns per winding to produce given back EMF at given speed.

Automotive alternators use an excited field, which is a built in
inefficiency. That loss doesn't matter much in an automobile.

I would explore electric trolling motors. They are all at least 250
watts. Larger ones, as 700 watts or more, are typically 24-volt
motors. They use very strong magnets and multipole designs because
they are designed to run fairly slowly (direct drive to a propellor),
and efficiency is valued above cost in this application. You can
probably scrounge them for little or nothing if you live near a repair
shop. Many motors brought in for repair have either bent support
shafts or burnt speed control elex, both of which are uneconomical to
repair and neither of which is necessary for your application. The
best time to look for them is midsummer to late summer, because they
often clean up shop toward the end of the season.

Most are brush DC motors, but a few may be brushless now. Brushless
would be better because they are intrinsically polyphase motors with
electronic commutation so would work well as alternators just by
removing the commutation elex. But even one with brushes and a
commutator might serve very well in your application. Electronic
conversion of DC to AC at that power level (if even necessary) can
be done very efficiently.

.

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