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

["David Roden (Akron OH USA)" <etpost drmm net>]

I haven't looked at an E20 diagram yet, but as a first guess, I'm with 
Rhett 
on this one.  Even with nothing more than residual field magnetism, 
voltage 
can rise to truly astonishing levels in a spinning motor, and if I 
understand aright, here the field was powered (even if weakened).  

Hitting the pedal while rolling backward, even slowly, has released the 
smoke from controller freewheel diodes in road EVs with series motors.  
With 
a sepex motor, where the armature current isn't reversed through the 
field, 
I can see how rolling forward downhill at high speed could do something 
similar. 

Also remember that MOVs lose their ability to absorb surges as they age.  
They're rather like sponges; eventually they sort of become saturated.

Disclaimer : I am not an engineer.