Yes it is a black art and until the coming of the modern computer programmes to simulate things it was very much something that you learnt the hard way and each design was based on the back of the previous.
Someone capable of designing excellent turbo alternators for a power station would probably make a complete mess of a small wind turbine alternator.
There are many books crammed full of maths and other things that basically help very little except to keep Phd students active.
In the old days no machine designer working for a major manufacturer was going to give the competition any worthwhile advice.
If you can drive the modern computer simulation programmes they are probably excellent but if you feed them crap information they will give you crap answers. As always the trick is to spot crap answers before wasting time building them.
Probably at the low power end most designs will work reasonably well if you use a bit of common sense. If you get 70% instead of 80% it's not the end of the world for a home project but on a production line with thousands of machines it could be a disaster.
I can't think of a definitive book that I could recommend, even if there was one it would probably be out of print or not available where you are. I have some by Philip Kempe that have some reasonably understandable information but they were in the 1940's and long out of print.
Most of the axial air gap stuff used here is way off the normal designs of industry and I notice commercial people borrowing the work done by Hugh and Dan and a few others for small wind machines.
Flux
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