Gijs,
I had assumed that you have a gearbox, and that it is the large blue component on the left of your nacelle photograph. Then I assumed that the generator is out of the field of view, but you did mention an asynchronous generator (to my vocabulary an induction generator).
If I am wrong, please correct me.
The speed ratios that you describe, when you say that blades cut-in at 170 RPM and a cut-in speed of 1500 RPM for the generator, implies that you use a 8:1 gearbox. Would it be simpler to find a gearbox with a 10:1 or 12:1 ratio?
Another question: If you are over-driving an induction generator to produce power, then presumably you need 5% to 10% above the synchronous speed to do so. Then why isn't your generator's cut-in speed 1600-1650 RPM? That makes a difference in selecting the right gearbox ratio, too.
This isn't to discourage you from building an axial-flux alternator. I only want to understand the problem thoroughly, before discussing the solution.
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Yes you are right, it is a induction generator.
I have do a lot of test with different gearboxes but the problems stays, a other gearbox wil not give a real better result, for a induction generator the wind must be enough to let the generator go at least 1600rpm(without connected to the grid), because wenn you connected to the grid the generator goes back in rpm the first vew seconds. But wenn the windpower is enough he generated up to 3,5kw.
Wenn the generator make 1500rpm you get 50hz and it is synchronyse with the grid, wenn you connected at 1600rpm, your generator rpm should always go down, the wind will let the generator turn faster and make more hz and the generated power goes up.
So the grid will try to turn the rpm to 50hz, and the wind will try to make more rpm and more hz, and the result is power.
Is a little difficult to explain because my english isn`t so good