I think the most simple way, without some type of dynamometer, is to calculate usable torque. Use the formula:
hp = torque x rpm / 5252
and therefore:
torque (in lb-ft) = hp * 5252 / rpm
The electric horsepower possible for a given rotor size can be estimated by using:
watts / ( .00508 x windspeed^3 x turbine efficiency ) = (rotor swept area)
1 hp = 745.7 watts
The windspeed is in mph and the typical well-built turbine will be around 30% efficient (.3 in the formula). An excellent performing wind turbine will maybe approach 32% efficient.
Note that this will only give you an approximation of usable torque, not actual shaft torque at the turbine mainshaft.
In a generator, you don't actually spin the stator - it's stationary and the rotor that carries the magnetic field is what requires the torque to spin it. If, for some odd reason, you'd need to know the torque required to spin the generator at a given speed, that would be more complicated. One way I can think of is to run an engine, that's properly matched to the maximum generator output, on a dyno and map its hp and torque curves. Then run the generator with the same engine, and using the dyno data from that engine, calculate the actual shaft torque required to run the generator.
But again, the torque required to turn the generator can be calculated fairly closely by using its electrical output in watts, then adding in estimated losses in heat, which can vary by generator design. Using the calculated method will give you a fairly close approximation of the actual shaft torque required to run it, and therefore engine or turbine shaft horsepower (which is always more than the electric hp generated) required to the spin the generator.
The only way I can think of to measure the torque exactly and accurately, is with a dynamometer. I don't know of anybody who makes one for a wind turbine - at least I've never seen one.
--
Chris
