Re: (ET) blown relay...

[dave barden <daveb speakeasy org>]

Rick,

    Don't know if you can get the rating from this but the mov that I got 
from
Bill Gunn has these markings on it:  ZNR V20680 S7N.


Rick Barnes wrote:

> Dave:
>
> My E15 schematic shows a MOV (metal oxide varister) across the field. The
> E15 reverses the field polarity to change direction. I do not currently 
> have
> the MOV installed (anybody know the rating on this?). I have never had 
> any
> welded contacts, and I am not particularly careful about waiting for the
> tractor to stop before changing directions. I use the forward reverse as 
> a
> brake, I also move quickly back and forth to mow close to bushes, etc.
>
> The control card has diodes to deal with the collapsing relay coil back
> voltage. I don't think one of these failing would cause your contacts to
> weld.
>
> Rick Barnes
> Aloha, OR
>
> > -----Original Message-----
> > From: owner-elec-trak cosmos5 phy tufts edu
> > [mailto:owner-elec-trak cosmos5 phy tufts edu]On Behalf Of dave barden
> > Sent: Friday, August 25, 2000 5:23 PM
> > Cc: elec-trak cosmos5 phy tufts edu
> > Subject: (ET) blown relay...
> >
> >
> > So we were trying to figure out why the reversing realay keeps
> > getting frie
> > on the E12.  Looking at the blue print schematic we think (my
> > friend is the
> > electrician, not me) determined that when the motor stops and the
> > mag field
> > collaspes there is a surge of current that finds it way to ground thru 
> > the
> > relay and that there should be a diode across the armature to shunt 
> > this
> > surge to ground.
> >
> >     I don't know anything about electric motors but as explained to me 
> > it
> > makes sense.  What really puzzles me though is why there isn't a diode
> > already there?    Or is it?  We've noticed that the schematic
> > doesn't quite
> > match the real thing in several spots.
> >
> >     Or is this the varistor's purpose and could my varistor be bad?
> >
> >     Any other ideas what to look for?
> >
> > thanks,
> >     dave barden
> >