As i see it one would have to guess at a number of windings far from required and then keep reducing eg make coil 50 turns, place in air gap spin, measure,eh eh too many , reduce to 45 and repeat , ending up with a length of wire 5 wraps long. That's how i'm thinking . Please put me right
You do the test coil to determine the proper number of turns for cut-in. You have to have the mag rotors assembled with the correct air gap, stick the coil in there and spin it at the rpm that the rotor is supposed to cut in at. You can guess on the number of turns and go from there. The voltage you get from the test coil is proportional to the number of turns in it - for example, if you wind 50 turns and get 5 volts, if you reduce it to 30 turns you'll get 3 volts, or if you increase it to 60 turns you'll get 6 volts. So you only need to do one test.
Once you find out what the test coil makes you multiply that voltage times the number of coils in one phase, then if it's star configuration you multiply that by 1.732. This will be the open AC voltage of the generator in star. Then you multiply that by 1.4 to get the DC voltage that you'll get out of the rectifiers.
So here's an example with a 12 pole 9 coil 24 volt:
To get full charging system voltage you'll need 20 AC volts at cut-in = 28 / 1.4
The voltage per phase, in star, would be 11.55 = 20 / 1.732
The voltage per coil would be 3.85 = 11.55 / 3
Then, whatever number of turns you come up with, I usually throw in one more turn to compensate for voltage drop across the rectifier. That should get you right in the ballpark.
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.
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Chris