Did some calculations. But I did it a little different, so I'd like to share it with you guys to see if my calculations are right before I do anything.
I read that to get started, you should first make the turbine but without the stator. Then make a test coil and place it in between the rotors and spin it at your desired cut in speed and measure voltage.
Well, here's what I did different. My cut in speed will be 250RPMs, but I have no way to spin it at that speed. The only known speed I have is the drill's maximum speed which is 800RPMs. And at that speed I get 3.6V.
Now if the voltage is 3.6 at 800 RPMs, then will it be 1.8V (half) at 400 RPMs? If so, then the following table should be correct..
RPMs Voltage
-----------------
800 3.6
400 1.8
200 .9
100 .45
50 .225
If the above is correct, then at cut in speed of 250RPMs it should produce 1.125V (.9V + .225V). Which isn't enough for my application.
Chris Olson made an example in a thread in the FAQ section. According to his example, then for a 12 pole 9 coil 12 Volt PMA, I need:
14 / 1.4 = 10 AC volts at cut in.
10 / 1.732 = 5.77 volts per phase, in star.
5.77 / 3 = 1.92 volts per coil.
So I need 1.92V per coil. But the test coil only produces 1.125V, which means the coil needs more turns than the 200 it has now. Below is how I calculated how many more turns it needs.
1.125V devided by 200 turns = .005625 volt per turn.
1.92 - 1.125 = .795V
.795 devided by .005625 = 141 more turns.
So the coils need 200 + 141 = 341 turns. Wow.. that's alot of turns.
Note that the 1.92V per coil is when winding for full charging voltage(14V). If I used 12V in the calculations, I'd come out to 293 turns. And 318 turns for 13V.
Chris Olson mentioned the following:
"It's important to note that full charging voltage on a 12 volt is 14 volts, 24 is 28 volts, and 48 is 56 volts. If you wind for full charging voltage and you hammer the batteries down to 12, 24 or 48 respectively, the turbine will run in stall at cut-in trying to bring the batteries back up. It's not as big of a deal with 12 volt as it is for 48. So if you're building a 48 you might want to use the low end (48-50 volts) as the cut-in DC voltage so you don't end up with turbines like I got that are overwound and stall badly unless the batteries are fully charged."I'm not sure what he means by "hammer the batteries down to 12", but I don't want the turbine stalling at cut in, so perhaps I should wind for 13V and use 318 turns.
Well, that's about it. I hope I got it right because it took me long enough to figure it out. I don't know anything about this electrical stuff
Oh, I took some more pics.