Re: (ET) e-15 eats reverse relays

["harry landis" <hlandis hotmail com>]

It seems to me that the reason the reverse relay burns contacts is that the 
field is just a big electromagnet, ie inductor. When you open the contacts, 
all the energy stored in the field has to go somewhere. That somewhere is an 
arc across the contacts.
You could put a snubber circuit across the contacts to absorb the surge, get 
a beefier relay, or not reverse direction while the motor is running.

Harry Landis

From: "Bob Murcek" <RMURCEK geisinger edu>
To: <elec-trak cosmos phy tufts edu>
Subject: Re: (ET) e-15 eats reverse relays
Date: Sat, 08 Jun 2002 15:23:11 -0400

This deal about the reverse relay being damaged by changing while moving has 
had me wondering since I first started following this group.  Theoretically, 
I think, the shunt field is just an electromagnet, so there really shouldn't 
be any change in its electrical characteristics due to the motor being 
energized, moving, etc.

I've heard lots of evidence to the contrary here, so theory must not quite 
apply!  I could imagine that maybe the construction of the motor isn't 
perfect so there's some interaction with the armature's field.  Or maybe the 
series field is muddled up in this.

Any serious DC motor people out there know what's going on?!

>>> "Pieter Litchfield" <plitch attglobal net> 6/7/2002 8:24:37 PM >>>
Gang:

My e-15 w/bucket loader has a nasty habit of eating the reverse relay - it
welds or fries the contacts while in reverse, or when switching from 
reverse
to forward modes.  Over the past several years it has eaten 3 or 4.

I am now using the improved Bill Gunn relay, and while definitely stronger,
the tractor's demon is stronger still.

I had assumed that perhaps the motor of a rolling  tractor acts as a
generator and that shifting too quickly from reverse to forward might allow
the motor to weld contacts (or something).  I do notice a spark when the
rely breaks contacts with the reverse contacts if I shift it without the
motor coming to a standstill, even with the tractor in neutral.  No spark
seems to occur when the motor is completely stopped.  However, this could
just be coincidental with something else - like a component that looses a
charge over a short time.

I do have the complete "homeowner's repair manual", but I ain't no 
engineer,
so I look at the pretty pictures of wires and remain more or less clueless.

I'll ask Bill when I order more relays, but does anyone else want to 
venture
a suggestion as to why I eat relays?

Thanks!


_________________________________________________________________
Send and receive Hotmail on your mobile device: http://mobile.msn.com