(ET) Small E20 fire: What is it with MOVs?

[Chris Zach <cz alembic crystel com>]

So I was using the E20 pretty heavily today in the yard, mostly to tow 
the trailer full of dirt. Once done I drove it up to the top of the 
driveway to get the paper, then let it coast in L down to check the brakes.

Brakes stink like normal but can still stop the tractor (new brake kit 
going on after spring work). Fired it up again, let it build up speed, 
then engaged the motor to slow it down.

*Foom* Big cloud of white smoke from under hood. That's always bad. 
Motor still had power, but I stopped the tractor and hit the main 
disconnect. Smoke continued to pour out for a moment, then it lightened up.

Hm. Opened the main panel, and looked at card 4. Everything was intact 
(I have bigger diodes on that) however on the resistor card there was a 
smoking disc where the MOV once was. Looks like it blew up.

So what is it with E20's and MOVs? I know they're there to catch the 
back emf from the motor or the spikes or whatever but they just seem to 
explode every year. Can one get bigger/higher capacity MOVs? Can one 
parallel them? Do something else?

Chris