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Re: (ET) tractor parts
Cool arrangement, GE really put some thought into these things.
Good thought about bypassing the shunt when not needed.
>>> Chris Zach <czach computer org> 9/12/2004 10:01:26 PM >>>
Right. The trick though is the E20 series has an "overdrive" in field
weakening for speeds 5-8. This is accomplished by putting resistors in
series with the field current, thus causing the tractor to pull more
armature current and "go faster"
Problem is it's like gearing the motor eletrically; if you go up a hill
with field weakening on your current draw will go thru the roof. Enough
to damage your motor.
To prevent this, there is a copper shunt in series with the armature
connections. Connecting to this are two wires that go to a simple pair
of transistors which measure voltage. When the voltage goes above a
certain amount across the shunt, the transistors close and engage a
relay that bypasses the field weakening resistors.
Excellent idea, and something I can rely on. I've been known to floor
the tractor and go up steep hills. The shunt picks up the hill (as a
current increase on the motor) and the FW cuts out. So you slow down
going up the hill. At the top you go back to full speed as the current
draw drops.
The problem is this shunt was built way back in the old days of
electronics, and due to the nature of the transistors there had to be a
rather large voltage drop across the shunt to trigger the transistors. A
large voltage drop (probably in the tenths of volts as opposed to
thousandths for a simple meter) requires a higher resistance shunt.
Resistance=heat in the universe, so this high voltage shunt drops a lot
of heat.
Heat also equals inefficiency, and I hate inefficiency. However RJ is
right: Bypassing the shunt would be suicide for the motor. The brushes
would melt, and the motor would fail.
I'll post my further thoughts on this in reply to his message.
Chris
Bob Murcek wrote:
> I'm not familiar with the E20, but in the smaller tractors the armature
> series resistors are just for getting going, right? I.e., you should
> set the gear so that you can run the motor full on.
>
>
>>>>"RJ Kanary" <rjkanary nauticom net> 9/12/2004 5:29:30 PM >>>
>
> Nothing terrible , IF YOU ARE CAREFUL . That is the
> configuration
> that my CA model E-20 has been living with for fifteen or so years now.
> BUT........all the current limiting features that the Elec-Trak
> engineers provided will now be null and void .Without the control card
> functioning, you will be able to access field weakening at a much higher
> motor load , {and therefore lower armature RPM.} than the design had
> anticipated .
> If you are not alert , you can make big , expensive smoke . The
> only
> protection you will have against meltdown are the large Klixon® limiter
> on
> the motor housing, and the thermal switch in the field windings. Should
> you
> mash the throttle too quickly, or hold it past the fourth speed position
> to
> far for too long, you WILL overtemp the armature, possibly faster that
> the
> Klixon® can react, especially at the beginning of a snow throwing
> expedition
> .
> The symptoms you are describing seem to point to deeper issues .
> You
> would do well to fully assess all the components that are behaving
> badly. It
> will be cheaper in the long run , and will preserve many difficult to
> replace parts .
>
> ----- Original Message -----
> From: "Chris Zach" <czach computer org>
> To: "Elec-trak list" <elec-trak cosmos phy tufts edu>
> Sent: Sunday, September 12, 2004 5:07 PM
> Subject: (ET) tractor parts
>
>
>
>>Well, today I was driving my E20 around on the street and managed to
>>trip
>>the motor temp breaker. Or something; the tractor stopped dead and
>>didn't
>>move for about 5 minutes.
>>
>>I noticed that my disconnect is getting rather hot on one of it's
>>connections; think I will replace it with the evparts.com one. Also
>>getting a new volt meter (the two I bought from Ebay were *BOTH* bad)
>>and
>>a single contactor since one of mine (3A, the top resostor bypass) was
>>getting hot as well.
>>
>>Still, nothing compares to that shunt plate in the center. It is
>>extremely
>>hot after running the motor hard; what kind of bad things might happen
>>if
>>I just jumped around it with some 2/0 cable?
>>
>>Reason I am concerned about power is that winter's coming. Winter means
>>cold which means snowblower, which means I need *full* power in both
>>forward and reverse.
>>
>>Fun fun.
>>
>>Chris
>>
>>
>>_______________________________________________
>>Elec-trak mailing list
>>Elec-trak cosmos phy tufts edu
>>https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak
>>
>
>
>
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