RE: (ET) blown relay...

["Rick Barnes" <barnesr nwlink com>]

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
>