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Re: (ET) Small E20 fire: What is it with MOVs?
On 3/15/2011 2:22 PM, David Roden (Akron OH USA) wrote:
I haven't looked at an E20 diagram yet, but as a first guess, I'm with
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).
Maybe. Let's think about this.
I was rolling down the hill with the throttle up, tractor in L. This is
enough to allow the motor to pick up some speed without allowing it to
run away. Once it was at about 1.5 times full speed I hit the brakes.
Tractor stopped, but slowly. I need to re-adjust the brakes.
I then took off the brake, went to speed 5, then took foot off the pedal
and let it build up to about 1.5 speed. I then put the pedal down to
speed 4, at which point the MOV blew.
It's possible that what happened is that the voltage at the motor went
up higher than the voltage at the meter or the batteries since there is
wire between them and it does have resistance. Also my E20 is configured
with the front power wires directly connected since the batteries are
all in the rear. That might have raised the voltage at the MOV to a
point where it started to short. Once it did, it was gone.
Maybe a bigger MOV.
Hitting the pedal while rolling backward, even slowly, has released the
smoke from controller freewheel diodes in road EVs with series motors.
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
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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