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

["Pieter Litchfield" <pieter_litch yahoo com>]

This exactly describes my ET behavior.  I have a very heavy E15 (with 
bucket
loader), and if I move from reverse to forward while the tractor has ANY
backward "freewheeling" (non powered movement) going on, one of the relays
will weld itself immediately, requiring replacement.  I suspect the MOV is
long gone.  The workaround for me is to apply the foot brake and wait for a
complete stop when reversing.  But it can easily generate enough current to
weld the relays.

-----Original Message-----
From: David Roden (Akron OH USA) [mailto:etpost drmm net] 
Sent: Tuesday, March 15, 2011 2:22 PM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Small E20 fire: What is it with MOVs?

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.



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