RE: (ET) e-15 eats reverse relays

["Pieter Litchfield" <plitch attglobal net>]

Right you are - on page 5-3 of my repair manual, it says "Since switching 
is
possible when drive motor is rotating at a slower speed  (than is detected
and prevented by CC7), spikes of energy may be generated as the switching
occurs.  If unsuppressed, these spikes may be capable of closing SCR's by
supplying gate current.  The varistor, VDR-1, effectively suppresses these
spikes."

So it appears that the circuit associated with CC7 senses back EMF and
prevents field switching until it is reduced to a tolerable level.  VDR-1
absorbs that.  So I have 2 possible points of failure.  Your discussion
below of the varistor is very helpful.  Could it be that a failure of CC7
allows switching at speed and therefore the generation of a surge that 
VDR-1
couldn't handle?  I can't find a component labelled CC7 in the diagrams, 
and
think that the title may mean those circuits relating to pad #7 of the main
printed circuit card as shown in my manual on page 5-24.  I note that the
circuit at pad#7 has some influence on the circuit at pad#8 (remember - I'm
not an engineer) and in fact there is an scr at pad#8 which has some impact
on the reversing function at pads#8 and #9 (to reverse coil, field) of the
printed circuit card (figure 5-3, page 5-5 and figure 5-8, page 5-24). Any
ideas about this?

Is there an easy, low tech (ie - multimeter) way to check the integrity of
the components and/or circuits described above?

Again, many thanks to all for helping me through these questions.

Pieter

-----Original Message-----
From: Tom G [mailto:wavetech superlink com]
Sent: Saturday, June 08, 2002 8:18 AM
To: Pieter Litchfield
Subject: RE: (ET) e-15 eats reverse relays


Pieter,

        I have an E-12S which is similar (uses the E-15 motor) I'll have 
to take a
look today to see if there is any arcing on the reverse relay, but I don't
seem to be burning any up. Maybe your varistor is bad, there is no real way
to check them but they are cheap enought to replace. It is shown as VR1 on
the schematic wire number 49 and 10 go to it on card #3.
The way I understand it the varistor is so posed to take any excessive
reverse emf from the motor field saving your contacts. When I ordered a new
reverse relay from Bill (the  old one had a blown coil) I got a spare
varistor just in case. The numbers listed on it  are ZNR V20680

tom

At 07:52 AM 6/8/2002 -0400, you wrote:
>As I mentioned in another post, your observation about the current flow
>seems borne out by my experience - unless I allow the motor to come to a
>complete standstill (even in neutral), I can see quite an arc on the
reverse
>relay points.  I can read enough of a schematic to confirm that you are
>correct in your recollection of e-15 technology; the reversing relay does
>indeed reverse the field.
>
>Of course my problem is heightened by use - a loader spends a lot of time
>reversing direction compared to a trac used to just mow the lawn, allowing
>lots of opportunity to burn those relay points.
>
>Many thanks for your reply.
>Pieter
>
>-----Original Message-----
>From: owner-elec-trak cosmos phy tufts edu
>[mailto:owner-elec-trak cosmos phy tufts edu]On Behalf Of Christopher
>Zach
>Sent: Friday, June 07, 2002 10:50 PM
>To: Pieter Litchfield; Discussion list Electrac tractor
>Subject: Re: (ET) e-15 eats reverse relays
>
>
>Well, if I recall correctly, the E15 reverses the direction of current in
>the stator field when you go to reverse. The E20 reverses the contactors
>that flow power to the armature.
>
>When you toss an E20 into reverse, the relay basically only has to switch
>the current for two contactors. Not much at all. The contactors take the
>surge from the armature, but they are really big, so this is not too much
of
>a problem.
>
>On the E15 though, going through to reverse while the motor is going
forward
>will set up a significant amount of current which will fry the very small
>pads on the relay.
>
>My guess is they went to the E15 method of reversal (stator vs. armature)
>because it's cheaper. Going this way removes four contactors from the
>circuit (each direction requires two contactors). The "right" way to do it
>would be to put a stronger relay in there, or a solenoid, however if this
>were done my quess would be that the back-current would blow up the field
>weakening logic pretty quickly.
>
>Chris
>
>
>
>
>----- Original Message -----
>From: "Pieter Litchfield" <plitch attglobal net>
>To: "Discussion list Electrac tractor" <elec-trak cosmos phy tufts edu>
>Sent: Friday, June 07, 2002 8:24 PM
>Subject: (ET) e-15 eats reverse relays
>
>
>> 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!
>>
>>
>>
>
>
>