The rotors have a diameter of 4" and are 1/8" thick.
For the number of turns for the coils its going to depend on what you want for output, your wind conditions, and your blades. It becomes quite the balancing act trying to match the alternator output with the blade output.
I made my blades before building the alternator and had a rough idea what the blades could output in terms of power vs wind speed vs blade rpm. The blades I made have a calculated Tip Speed Ratio (TSR) of 4.5 at which maximum power extraction occurs. Knowing the TSR at which max power extraction occurs for each wind speed allows you to determine the cutin speed for the alternator.
I wanted a very low cut in of about 4.5 mph but also wanted high output in high winds. This goal can not be attained using just coils and rectifiers hooked up to a battery. With just coils and rectifiers you have to decide if you want low, middle, or high wind response. If you use some form of load matching mechanism then you can have it all, sort of.
I used a boost converter for the low end of the power curve to match the load better so that the turbine can extract power from low and high winds without burning out. Each coil has 80 turns of 22awg wire which gives a wind speed cut in (using main rectifiers) of about 13 mph which is about 990 RPM. I could have used 20awg wire with 75 turns per coil since there was more room available in the stator but I had lots of 22 awg wire on hand. 20awg would make a slightly more efficient alternator but at a higher cost which I wasn't willing to do at the time. I made use of what I had on hand.
The boost converter allows the turbine to start generating power at about 400 rpm (4.5 mph) and can go up to 1650 rpm (30 mph) before furling starts. At 30 mph the alt is putting 46 watts into the battery. At about 33 mph (when it starts to furl) the alt is putting about 55 watts into the battery. The data logger has recorded 3 second peaks of 63 watts (4.7A @ 13.5 volts) into the battery when high wind gusts occur and the turbine is in the processes of furling.
With out the boost converter, I tried a number of different coils to see what kind of output I could get.
Using coils with 135 turns 22awg gave a cut in of 600 rpm ( 7mph). Unfortunately the blades start to stall around 12 mph and by 30 mph are severely stalled (TSR down to 1.9) and only putting 27 watts into the battery.
Coils with a turn count of 110 gave a good middle ground response. Cut in was 750 rpm (9mph). At 30 mph the rpm was 1220 and the alt was putting 35 watts into the battery which is about 11 watts less than the boost converter configuration.