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

[Rhett George <rtg ee duke edu>]

> Date: Sun, 13 Mar 2011 17:27:54 -0400
> From: Chris Zach <cz alembic crystel com>
> To: elec-trak cosmos phy tufts edu
> Subject: (ET) Small E20 fire: What is it with MOVs?
> Message-ID: <4D7D36DA 8020909 alembic crystel com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 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
> 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 - Greetings to Chris and the others -

Gathering a few points from the first two paragraphs, the tractor with 
Chris
aboard was coasting downhill in L and going faster and faster.  The motor 
was engaged and "foom" was produced.  

It is a guess but since the transmission is not synchromeshed and there is
no clutch, the motor was engaged by stepping on the GO pedal.  If the 
tractor
was going twice as fast as it would with the cruise control ON, then the 
twice-as-fast spinning armature wouold be generating about 72 volts.  
Power 
dissipated (briefly) in the MOV which is not a resistor increases at an 
exponential rate greater than V squared.  The resulting heat escaped, 
taking
pieces of the MOV with it.  Like Humpty-Dumpty it cannot be put back 
together
again.  Sigh.

And cheers,

Rhett