Homebrewed Electricity > Wind
New build- 20 foot diameter variable pitch windmill
Adriaan Kragten:
I have checked your calculation of the unloaded AC voltage in between two phases and this is correct if the winding is connected in star and if the rotational speed is 200 rpm. If you rectify the voltage with a 3-phase rectifier, you will get an open DC voltage which is a factor 1.35 higher (if the voltage drop over the rectifier diodes is neglected). The open voltage increases linear with the rotational speed. The loaded voltage is much lower than the open voltage and how much lower depends on the resistance of the load. The lower the resistance, the larger the current and the lower the loaded voltage.
The rotational speed is given by formula 4.8 of report KD 35 if the rotor is perpendicular to the wind and by formula 7.1 of KD 35 if the rotor is turned out of the wind by a yaw angle delta as the result of a certain safety system. So the maximum rotational speed for a certain rotor at high wind speeds depends on the delta-V curve of the safety system but also on the generator load. Don't think that you can limit the maximum rotational speed without a proper safety system because there is always a wind speed for which the rotor torque is higher than the peak torque of the generator.
bigrockcandymountain:
In my opinion, 200 rpm would be a good maximum for a 20' diameter.
It is approximately tsr7 at a wind speed of 20 mph. After that, the safety system should start to limit rpm.
If the hub and blades are really well made, you might go a bit higher.
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