How is the design of a Low RPM alternator different from a high RPM?

[adrian]

Assuming we're talking about a 3 phase alternator with stator coil disk and 2 rotor magnet disks.
So the first rule is that it should have more coils, correct? let's say 27 instead of 9
But this doesn't mean that it will not perform even better at high speeds.
How to make peak alternator efficiency to be at low RPM?
Should the air gap between magnet rotor disks be smaller?

[Mary B]

3:4 ratio magnets to coils is typical.

[joestue]

The only difference is at low speeds and low frequencies, iron loss is lower, so you can run the flux density higher. A motor or generator's torque density follows flux squared, so every bit helps.

lower speed means less centrifugal acceleration so it is possible to epoxy magnets to a rotor without any mechanical backup.

[adrian]

The only difference is at low speeds and low frequencies, iron loss is lower, so you can run the flux density higher. A motor or generator's torque density follows flux squared, so every bit helps.

Could you please provide a few more details and examples on how to run the flux density higher in my alternator? i'm afraid i didn't fully understand how to implement it in design.  Thank you in advance!

[SparWeb]

Hi Adrian,

This should help:

http://www.sparweb.ca/Forum/AXIAL_FLUX_HowItWorks.pdf

One thing that determines the speed of the alternator is the voltage it is intended to
work at.  If you have an axial flux alternator designed to charge a 48V system, and
sized to handle a 10-foot diameter rotor, then it is probably designed to reach peak
power at 600 RPM or so.

Take that same turbine, but hook it up to a 12-volt battery system, and suddenly it
will try to operate in a speed range 1/4 as fast.  Once you get familiar with wind
turbines and the range of speeds they need to run at - especially to have an efficient
tip-speed ratio - then you will see how this turbine will fail.  It will perpetually be in a
"stalled" condition because as soon as the blades start to turn, the alternator will
cut-in immediately but there won't be enough wind speed to sustain the power that
the alt needs to keep turning.  It will just creep along, never getting up to a reasonable
running speed.

This turbine would need a different alternator - one with coils wound with much
heavier wire.  There would be fewer turns of wire, but much less resistance, too. 
It would allow it to operate at 600 RPM again, but this time clamped down to 12
Volts, not 48 like the first example I started with. 

I chose this example, rather than an exact answer to your question, because this
is much easier to explain, and yet has the same concepts as you need for your own
question.  The cause-and-effect just works in the opposite way.

If you take both of these alternators off of their respective turbines now, you will
have nearly-identical alt's.  One is wound with fine wire, the other has thick wire.
If you hooked them both up to charge the same battery, one would require 4x the
speed of the other to do the same charging power, but the power would be the
almost same.

[kitestrings]

So the first rule is that it should have more coils, correct?

No.  More often it is the other way around.  Mary explained that 3:4 ratio is common, but that is coil to magnet ratio, so a 12-pole alternator would have 12 pairs of magnets and 9 coils.

Hugh Piggot has some really good books describing other combinations.  The Dan's also have lots to contribute with there's (lower right side):

http://www.otherpower.com/

~ks

[Mary B]

Oops! bit dyslexic on that!

[adrian]

Hi Adrian,

This should help:

http://www.sparweb.ca/Forum/AXIAL_FLUX_HowItWorks.pdf

One thing that determines the speed of the alternator is the voltage it is intended to
work at.  If you have an axial flux alternator designed to charge a 48V system, and
sized to handle a 10-foot diameter rotor, then it is probably designed to reach peak
power at 600 RPM or so.

Take that same turbine, but hook it up to a 12-volt battery system, and suddenly it
will try to operate in a speed range 1/4 as fast.  Once you get familiar with wind
turbines and the range of speeds they need to run at - especially to have an efficient
tip-speed ratio - then you will see how this turbine will fail.  It will perpetually be in a
"stalled" condition because as soon as the blades start to turn, the alternator will
cut-in immediately but there won't be enough wind speed to sustain the power that
the alt needs to keep turning.  It will just creep along, never getting up to a reasonable
running speed.

This turbine would need a different alternator - one with coils wound with much
heavier wire.  There would be fewer turns of wire, but much less resistance, too. 
It would allow it to operate at 600 RPM again, but this time clamped down to 12
Volts, not 48 like the first example I started with. 

I chose this example, rather than an exact answer to your question, because this
is much easier to explain, and yet has the same concepts as you need for your own
question.  The cause-and-effect just works in the opposite way.

If you take both of these alternators off of their respective turbines now, you will
have nearly-identical alt's.  One is wound with fine wire, the other has thick wire.
If you hooked them both up to charge the same battery, one would require 4x the
speed of the other to do the same charging power, but the power would be the
almost same.

Thank you very much for details!
I also found a lot of useful information in attached file