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Re: (ET) tractor parts

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


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