Re: (ET) tractor no worky

[Michael S Briggs <msbriggs unh edu>]

On Wed, 21 May 2014, Konstanty, Walter (GE Energy Management) wrote:

> Mike,
>
>   Check the varistor on card #3 – little round disc.  It may be blown.  It
> suppresses voltage spikes from the SCR’s across the motor field.
>
> Also, all connections to PC card.
>
> And, relay contacts.

Damn, that's a lot of stuff to check. :)
	Would a blown varistor actually cause this? My understanding is 
that the varistor is to absorb back EMF from switching forward/reverse 
while the motor is spinning, causing a big back EMF. But, the motor isn't 
even spinning yet. Of course, starting it spinning will cause an EMF, but 
not nearly as big. But perhaps it could still be enough to throw the relay 
back and forth...?

Thanks Walt!

Mike