For those turbines that can handle it, I would try a triac(s) across the windings, (before the rectifiers of course), and trigger the triac above a certain voltage, like you are doing in your circuit.  Using a triac will let it turn itself off every cycle when the current drops below the holding current of the triac and would be re-triggered next cyle.  This would just appear as a cliped DC voltage seen by at the battery side of the diodes if it was just connected to a big capacitor.

Using a relay when turning off the short leaves a big chance that it will turn off when the current is high and might damage the relay contacts from inductive flyback.

But, on the other hand, if you can sort of control the timing of the relay turning off, it would be a nice rugged way of shorting in emergencies.

  I may be a bit traumatized after watching my first 12' machine start up and accelerate to over 800 RPM (measured) with it completely shorted. True this was in 60 MPH winds and the furling was set too hot but keep in mind that your blade profile will have a lot to do with the effectiveness of shorting also.

  Higher torque lower TSR blades just might not care that you shorted the alternator and could start up and then fly being more than happy to burn up your stator and start the self destruct mode. I can hear it now from those who have manually tried to turn their alternator when shorted (no way you say) and good luck I say.

  Just another tip to those who frequently short out their alternators to actually slow the rotation : take a good hard look at your stator, stator mounting holes, spider mount, hardware and airgap etc. That huge amount of braking force is your magnets trying to move your coils (turn your stator) and can crack, bend, loosen or break the weakest link.

  This same force is acting on the stator when generating power but is not the same slam, bam all at once happening as when shorted.

In the old days with dc machines it was impossible to do this sort of thing. The machine wouldn't stand it even if you could maintain excitation ( which shunt machines wouldn't, they just shed load)

The same is true of industrial machines, nobody expected a dc machine to stand a sudden short. Alternators if built well enough could survive and were generally expected to be able to survive a few sudden shorts but in power generation it is a disaster situation and you would never expect it to happen.

For wind power the early alternators used feeble magnets and had to use slotted iron cores to get the output in a finite size machine. These had so much internal impedance that their current was levelling off at full load and a short may have very little effect on the maximum current and no way would it stop in a high wind, Some produced so little extra current that they wouldn't burn out at full speed when shorted.

Then came neo magnets and the prospects of higher field strengths and the chance of reducing the number of turns in the winding, this significantly reduces the reactance and with thicker wire the resistance is lower so the impedance drops. Many of these machines are below the impedance limiting point on full load and on short circuit they may produce perhaps 3 times full load current. Most of these will stop with a brake switch in most winds. In a gale you may have to dump resistance across it first  to develop enough power to stall the prop but then it will generally stop and will hold stopped in high wind.

The ironless axial machines have much less reactance and the characteristics are largely resistance dominated to the extent that in the working range the short circuit current rises linearly with speed. If these are built with high efficiency ( low internal resistance) they develop a braking torque that will stop them even without adding more resistance. Lightly built high resistance versions will fry the winding without producing a torque great enough to stall the prop and will not stop.

There is a limiting case where some will hold braked once stopped, less efficient ones will break loose at some wind speed and burn out.

With the conventional matching you have to have a fair bit of inefficiency some where and the cheapest place to put it is in the alternator where it reduces the cost and size very significantly. If you take this too far it is ok when running but you won't stop it in a gale with a brake switch without burning it out.

When working in the mode that you are thinking of you need the highest alternator efficiency you can get for a sensible cost and this trope of alternator will be capable of braking under virtually any condition.

Besides the ability to stop and not burn out there is also the mechanical consideration. This is a violent process and in any other application would be considered a disaster situation not an operating one. The shock mechanical load is very great and must severely stress everything. You have to be certain that this long term shock loading is not going to cause mechanical structure failure in the turbine, alternator and blades.

The iron cored machines limit to several times full load current and the severity of the shock is less. A very efficient ironless machine could momentarily exceed 10 times full load current and you may need limit resistors in the braking circuit to keep mechanical forces within limits. That can probably be determined on a commercial basis but for a one off machine you don't want to use it as a fatigue test bed for a year before you know that it will be ok.

As for burning the stator that largely depends on the available braking force, a reactance limited alternator with ceramic magnets won't stop and probably won't burn out. Make it more efficient and you may have enough current to burn it and still not have a reliable brake for all winds.

Ironless machines if lightly built may hold braked but it may not be possible to stop them in a high wind without burn out. If you can't stop it within 30 seconds it is safer to let it fly and let the furling take it.

Highly efficient ironless machines will stop so quickly that burn out will not occur but the mechanical shock may be more than you can tolerate and you may have to reduce speed in the first stage with load resistors before the final short.

Sorry this has gone on a bit but I hope it helps.

All of this goes out the window if you have a winding or cable fault and unless you are Bergey and can guarantee running in a storm on no load then I am with Dave B and like a back up, I prefer to turn it from the wind but a good reliable mechanical brake is ok in climates where it doesn't rust up or freeze up.

Anyway, if I recall, it was an 18" diameter stator, .5" thick. It was a 3 phase alternator using 12 poles (24 mags) and 9 coils. It had 45 turns per coil, and it was wound 'two in hand' with #14 wire. It was a 24v machine. It was used to charge batteries.

The feed line was three #10 wires coming down the tower (approx 95' run) AND three runs of #2 from the tower base to the shop (another 90' run). Rectifier located at the battery bank.

To answer your question regarding "backplate". Im assuming you are talking about the magnet rotor composition? If this is the case, the rotors were 1/2" steel, and they were 14" in diameter.

The prop on this turbine weighed quite a bit if I recall. I wish I had an exact figure for you but I recall it was aprox 40 - 45 lbs. Maybe a little more. It was made of doug fir. Anyway, ....alot of built up inertia like I was saying..

(Interestingly, variable load trying to hold an RPM is what a battery charging load approximates - and what it would achieve if the coils and drop wire were superconductors.  We already have trouble with burnout from that in the nontrivial-resistance non-ideal case.  If the non-ideal case puts burnout-level heating into the stator, adding a controller to INCREASE the current - and thus the heating - in the stator {while letting the mill spin fast enough that the blades aren't stalled} will make the problem worse, not better.)

You might want to consider, instead of trying to hold an RPM, trying to hold some maximum CURRENT and let the mill speed up - but only somewhat, since it's still under load.  This would mean an adjustable series dump load - that could take a lot of power and a lot of overvoltage.