First up, we need to understand wind power. If we double the wind speed, we don't double the power we can harness from it ( or push through it ). The power is actually cubed. To put it simply...

Power = Windspeed³

So if a hypothetical windmill can get 10 watts from a 10kmh wind, then it could get 80 watts from a 20kmh wind, or 270 watts from a 30kmh wind. Please note, all figures used on this page are simplified and don't represent real world data.

Our typical windmill alternator, on the other hand, has a linear output. If the windmill RPM doubles, so does the output power. The wind power curve and alternator power curve mismatch, so the alternator is built to run in a "band" of wind speed where it can make the most power. There is more power in high winds, but these happen less often than low winds. If we built a windmill to only use high winds, we would miss out on the more common low wind power. If we build a windmill to use low winds, we miss out on the big power in high winds.

So how do we take advantage of high and low winds? One way is by using different alternators, each designed to run in a different band of wind speed.

This is a standard F&P alternator, a 1X14C ( 1 times 14 coils ). It has many coils in series, so has a low cut in RPM and will start making current in very low winds. Due to its high coil resistance, its current/power ability is limited, so it cant make high power in high wind.

If we electrically split the alternator in half, and connect the two halves in parallel ( a 2X7C ), we will double its current capacity, but also double its cut in speed, meaning we are missing out on any power we could be making in low winds.

So what if we use two circuits of different coil counts? Each circuit will have a different cut in speed. In this example, in light winds the 9 coil side will start making power, without loading down the windmill too much. As the wind increases, the 5 coil side will start making power and this will add to the current already produced by the 9 coil alternator. Our alternator power curve is starting to follow the winds power curve. This alternator is a 1X9C 1X5C. Note we need to use separate bridge rectifiers for each circuit, as they run at different voltages.

We can try different combinations to get a power curve that works best with our wind conditions and turbine performance. Here we have one 6 coil circuit and 2 4 coil circuits in parallel.

Its a 1X6C 2X4C.