Then try it (same number of turns, with the same stator thickness, a gauge or 2 larger wire), with the 13mm opened up to say 25mm. If I understand WilliB's work, pretty sure that would be a decent improvement.
Nope, it's not. This is the second one of these 16 pole units that I've built. The other one had wider spacing between the mags with 375 mm rotors. The pin spacing at the bottom was 22 mm on that one. The internal resistance came out to .61 ohm and 8.45 rpm/volt. I got 118.3 open volts @ 1,000 rpm from it.
On this one, which is an improvement over the first experiment, I cut the rotors down to 355 mm and scrunched the mags in closer. I got 8.58 rpm/volt from this one with the 13 mm spacing on the bottom pins @ .55 ohm. I get 116.5 open volts from this one @ 1,000 rpm.
If you do the calculations, the first one yielded:
118.3 - 100 / .61 = 30.0 amps and the power dissipated in the stator was 549 watts, or 84.5% efficiency @ 3 kW output.
This one yields:
116.5 - 100 / .55 = 30.0 amps and the power dissipated in the stator is 495 watts, or 85.8% efficiency @ 3.0 kW output. Plus it has a lower polar moment of inertia and a lighter rotating assembly.
I think WilliB also would say the magnets are too close together, which could make my line of thinking way off base... I don't know.
Yeah, I know. That's what the "conventional wisdom" says. That's not what I've found with my experimentation however. Sure, by using the "conventional wisdom" you get more voltage from the setup, per turn. But you have to figure the rest to see what the actual efficiency is to determine if that extra voltage actually does anything.
In either setup, I got room for bigger wire. But I don't need bigger wire in this one with MPPT. What I need is a low polar moment of inertia, less weight in the rotating assembly, and about 85% efficiency at full power. It took a lot of experimenting to arrive at a combination that gives me the specs I needed for the 3.2 meter rotor.
So what I'm say is, you have to look at the whole picture. Concentrating on, and getting hung up on, a couple aspects like mag spacing and coil shape - trying to use "conventional wisdom" to design the thing - may not work out in the end. I was looking at things like how much power does it take from the rotor to spin this thing up to 1,000 rpm. And how much power does the rotating assembly store in its mass when it's spun up to operating speed. How fast can it accelerate, and how much torque does it take to accelerate it, to take advantage of increased power available in gusts, and hence total kWh production at the end of the day.
If you space the mags wider, sure you'll get better voltage performance from the mag surface area. But will it actually produce more power at the end of the day when you take into consideration the higher polar moment of inertia in the rotating assembly? Under ideal conditions with a constant wind speed and perfectly clean air thru the rotor it might. But in the real world on a real tower in real wind, it won't because acceleration time and how much power is "wasted" to get the rotating assembly up to speed, is important to total power production. And that's where experience doing this sort of thing comes in. You can slap a geared drive on anything and make it go around. Tweaking it for peak performance is another matter.
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Chris
This is my coil winder cheek piece. It keeps getting more holes in it all the time. LOL!
The holes on the outside of where the bottom roll pins are now are the ones I used for the first 16 pole ferrite gen I built.
