I've been researching grid tie induction generation for windmills. This is how most of the commercial systems do it. From what can be seen on the web, I think the breezy looks pretty good. Their power numbers are impressive. Wish they had one in service longer than 1 1/2 yrs.
The controller they speak of only connects the motor/generator to the grid when it reaches synchronous speed and sets the brake for over speed conditions. Don't confuse that with sychronizing with the grid. The generator does that all by itself.
The generator operates on the principle of slip. If the poles rotate at the same rate as the frequency, no power is used or generated. As the poles rotate faster than the line frequency, power is generated. If it rotates slower, then its acts as a motor. This is why you need their controller. (if you didn't mind standing by your turbine all the time, you could watch the wind speed on an anemometer and throw a switch to generate power and do almost as well as their controller)
Since the generator operates with less than 5% slip, it stays at pretty much the same speed regardless of power, so what controls the power output is the characteristics of the blades. For the PM alternator guys, I think output and rotor speed and wind speed are pretty much proportional. That's not the case with the induction setup. The speed is prett close to constant assuming you have enough wind to freewheel the turbine up to synchronous speed. (There may be more slip since the third leg is connected via capacitors) The power produced is proportional to the wind speed. As the mill freewheels up to synch, the wind is not putting much power into the blades. The angle of attack for the blades relative to the wind is small. When synch is reached, the generator engages. As the wind speed further increases, the apparent angle of attack increases though the blade speed is almost the same. The power also increases as the angle of attack increases. This angle of attack increases until the blade can't produce any more lift. this is called stall. This is a valid, tested turbine control technique called stall regulation.
All this means that your generator must be geared such that cut in speed is at the bottom of useful power production relative to wind speed. At that point you are limited essentially by the blades ability to accept high angles of attack as far as pulling all the power possible from higher wind speeds.
Now, if the slip was higher or able to be regulated, faster blade speeds would be possible which would allow more power to be captured before the stall angle was reached for the blades. I've been looking at designing my own direct drive induction wind turbine with a wound rotor to allow for variable slip. Not a lot of info this type of coil design out there though.
Howie
