I must assume that his equation makes assumptions, which may or may not be applicable to our situation.
I think it does too, and I agree with what you say. I assume that calculation is slanted towards windings in conventional air-over cooled electric motors and generators, and may not apply 100% to the axial style generators we (mostly) all build. And it may not apply to some stators where they are very thick, like you say. I have always tried to make the stator as thin as possible and these high speed gear driven generators I have used for quite awhile have less cooling problems than direct drives do. The rotors on these things move a lot of air because they spin so fast, and I think that helps some. And I usually include holes in the rotors so the magnets, which act as centrifugal fan blades, can draw more air in from the center to blow it out.
I have run various versions of these at very high outputs on my test stand without overheat problems so I am pretty confident in them installed on the turbine where more air is flowing over them than on a test stand.
One of the bigger problems for my climate is how to keep snow out of it. If the turbine is running in a snow storm with the generator warm, and snow blows in it it melts and freezes on cold metal things like magnets, builds up in there, and locks it up when the ice gets too thick between the mags and the stator. I have to figure something out for this new machine because it's got big holes in the rotors where snow will blow in and not very much space around the magnets to let it back out. I can see where it could build up ice on the inside of the magnets, then a piece of ice come loose and lock it up. Or if it stops when the stator is warm, it will melt the ice that accumulates on the inside of the magnets, the water will run down between the mags and stator, freeze, and then it won't turn. I might just duct tape the front holes shut for winter time operation.
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Chris