First of all forget working with AC, as with wind turbines we often reffer to it as "wild AC" because it has no fixed frequency and the best way is to rectifie it to a stable DC.
Now the voltage will fluctuate with RPM so you need some way of controlling this, and the best way is to charge a battery as it will keep load on the mill and give a stable voltage to work with.
Then you need some way of controlling the charge to the battery, so not to over charge the battery, this can be done with a simple opamp circuit.
Then once the battery is charged you need to divert the power to be able to control the mill, as you CAN NOT just disconnect the mill and let it spin free, it will self distruct.( known as a run away mill)
The best way is to divert the power to a DUMP load, and this is normally some form of heavy resistor capable of handling the power produced under all wind conditions.
Another way is to simply short out ALL the AC leads and hit the brakes so to speak, this can have problems with iron core machines (as a stepper is) as the iron core can saturate and the mill over speed past the point of control.
The first rule of building a wind turbine should be looking at how you will control it in all wind conditions, as this might largely effect how you go about construction in the first place.
It has been noted and tested on many forums that wild ac can have larger losses over wire distances than DC.
Personally i dont like VAWT's (vertical axis wind turbines) as they are a ineffecent design and are around 10 time less efficent with extracting energy form the wind compared to a HAWT.
You have a lot to learn about wind turbines and their use, but we all start somewhere and learn as we go, but remember short cuts will result in large failures with wind energy.
Many of us have played with wind energy for a long time and often have learn the lessons the hard way, and its a sad feeling seeing your mill scatted all over the yard and will scare the **** out of you when you see a runaway mill in good winds.