Looking through some references such as the Baldor Cowern papers, I find some info that helps but nothing really addresses this use of a generator. I'm keenly aware that the current spikes as soon as you do this, but that's why I want to know how to model the situation. This is clearly hard on switch contacts, and I don't want the current spike going higher than the locked-rotor amp rating (in motor terms) in the windings, either. I don't have equipment that can measure the spike, but I think I can do the math on it.
My understanding is that at all times, there is always an EMF (electromotive force) proportional to the speed of the generator, so for simplicity I'll say that it's (240V/1800RPM)= 0.13 V/rpm. If this is somehow oversimplified I hope you can point me to a better way.
Because the leads are shorted, there's no voltage at the leads. As a result, there is no EI power. However, a current is still driven by the EMF, through the winding resistance alone, demanding an I^2R power. I believe there will be no reactive power because E is zero, and for what it's worth, power factor is 1.
If the above is true, then the shaft power & torque of the generator can be determined by I^2R power alone, and this is rather simple because the current is only EMF/R, where R is the line resistance through the motor winding. Putting this into practice:
EMF = (240V/1800RPM)= 0.13 V/rpm
R = 1.0 Ohm
I = EMF / R = 0.13 A/rpm
For 100 RPM:
I = 0.133 A/rpm * 100 rpm = 13.3 A
P = I^2 * R = (13.3 A)^2 * (1.0 ohm) = 177 Watt
For 1000 RPM:
I = 0.133 A/rpm * 1000 rpm = 133 A
P = I^2 * R = (133 A)^2 * (1.0 ohm) = 17.7 kW
This looks reasonable and it also seems to reflect what I've been doing. Throwing the shorting switch when a wind turbine is turning fast is a recipe for disaster, and the numbers bear it out. On the other hand, when the wind is slow, shorting the leads is a convenient way to stop it. I like to have the dynamics and gyroscopic behaviour under control before lowering the tower, for example.
Putting this concept to use, if it's accurate, then to answer my concern about excessive current in the winding, I should consider the maximum rating and use the locked-rotor current ramp rate to find the maximum speed where this generator can be shorted.
I_max = 60 Amps
I_max / (0.133 A/rpm) = 450 rpm
Again, this seems to make sense. I'd still like a second opinion, if anyone has been able to follow this.
If this really is "so far so good" then I can go to the next step of specifying resistors to moderate the initial current spike, before switching to the dead-short that holds it. With 2 stages I believe I will be able to stop the turbine from any speed.