So you aren't wrong, both of you: tests to measure torque is not ABSOLUTELY necessary. But when you have 99% of the set-up needed to get the mechanical power measured, and you don't bother, then IMO you have thrown away the opportunity to spend 30 minutes to get the blade match right, and decided to spend hours and hours guessing instead.
Not all of the mechanical power gets converted into electricity. A lot is lost in heating the wire inside the motor. It's pretty easy to miss the fact that generators and motors can be anywhere from 90% to 30% efficient, and that this varies depending on the load on the generator. I really don't know if this is one or the other, so guessing about the rotor blade size based on the electrical output means we could be way off.
![](https://www.fieldlines.com/proxy.php?request=http%3A%2F%2Fwww.sparweb.ca%2F3_Gen_MoCo%2FBaldor%2FTests%2FBaldor_Efficiency.gif&hash=b2c12a5423e6e814128cbf49304e3134a357f382)
More details of what I did:
http://www.sparweb.ca/3_Gen_MoCo/Baldy.htmlScroll to the bottom to find the test on the lathe.
I hope you can make out that the 2x4 is clamped to the motor's mounting feet, that the free end of the 2x4 has another c-clamp on it, and the handle of that c-clamp pushes down on a scale. The length from the motor shaft to the clamp at the scale is measured exactly - that's the arm. The scale measures the force to restrain the motor.
Force X arm = torque
Torque X RPM = Power (if you use US units you have to fuss with "550" or "5252" or some other unit that depends on units of measure)
I'm sure it goes without saying.... be very very careful not to turn the lathe backwards!
I suggest you set (as close as you can) about 4 speed increments to make a good clear curve. Say, 200, 400, 600, 800 or thereabouts on the speed selections your lathe has. 800 RPM will really max out the generator, so don't make anything too hot. If that's what you see on the lathe test, then this should tell you something about max speed for making the WT tail furl in strong winds.
Once we understand the servo-motor's efficiency when driven as a generator, then we can be confident with the power you want the rotor blades to give it. With the information so far, I can't narrow it down closer than 10 to 8 feet diameter, but that's a 40% difference in swept area.
Battery-charging with wind is a bit more forgiving than resistance heating. If the rotor is too small, the battery cut-in unloads everything in light wind so the system can still start. With resistance heating, you have to rely on a disconnect system to get it right, and a small rotor just makes the fine-tuning harder. OTOH, having a rotor too big lets it spin up fast in strong wind. You'll need a furling tail or some sort of shut-down system anyway, no matter what, but again, battery charge is a bit more forgiving than heating.