wire size and current

I got an email question today and it seems important enough to answer with a new post.  Here is the question “How many amp of 14,15,17,18 and 20 awg wire for wind turbine. Can you give me any chart? Please!!!!! ”

If you are winding a stator according to my style – about 13mm thick coils with some glass cloth on the faces and plenty of wind blowing past it – then my rule of thumb is about 6A RMS current per sqmm of wire cross section. If the stator is star/wye connected, with a single wire in hand and all coils in series then the DC output will be about 22% higher than the RMS AC current.

The relationship between AC current in each phase and the DC output of the rectifier depends on the waveform. But the simplest way to think of it is to say that each wire carries the full DC current for 2/3 of the time, and is resting for 1/3 of the time. The result is that the rms AC current in each wire is root(2/3) = 0.82 times the DC current. In reality the wires share current some of the time, so the % is very slightly lower in relation to the DC.  But this 0.82 is the worst case, and it is very close to the reality usually.
So maximum DC current should be about 1.22 x maximum allowed AC current as in the table.

So here is my answer:

It’s OK if the current surges way above this value in a gust, but this is the maximum sustained current I would design for, as a safe target. Your furling tail should prevent the machine from kicking out more than this on a continuous basis. If output is higher then make the tail lighter to reduce amps.

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