(ET) Getting around that shunt

[Chris Zach <czach computer org>]

Was thinking about the shunt problem this evening. RJ is right; one 
can't simply bypass it with wire.

But how about bypassing it with a contactor?

This could be a solution: A 100amp soleniod contactor is wired in 
parallel with the shunt. The rule is the contactor is closed in speeds 
1-4, and open in speeds 5-8. Thus when you're tooling around mowing the 
lawn or getting going the shunt is bypassed. When you engage field 
weakening, the contactor is open and the shunt rules (cut out FW via the 
FW relay and the transistors) applies.

The advantage to this is you get to pull full power in speed 4 till the 
cows come home with nothing in your way. The shunt does nothing till 
speed 5 anyway, so even if you're pulling a trailer the shunt doesn't 
matter as long as your in speeds 4 or less.

Now, how do you power the shunt properly?

Here's the thought: The E20 tractor understands the concept of "cruise 
control" being speed 4 (full power). I spend most of my time there, so 
there is a vested interest in keeping that as efficient as possible. 
Speeds 1-3 arent really a problem for me since there are resistors in 
series with the shunt and the motor anyway, and their resistance (and 
current drop) dwarfs that of the shunt regardless (think current 
limiter). So speeds 1-3 don't matter.

The Elec-trak already has a nice little light that tells you when you're 
in cruise control territory. If I wired the output of that light circuit 
to a power transistor to either turn on a relay or turn on the solenoid 
directly I would have it made: The solenoid would bypass the shunt, and 
the tractor would be able to draw at full power all the way up to motor 
Etemp.

As soon as you move to speeds 5-8. the CC light goes out. No CC light=no 
power to the soleniod, and the shunt is in.

Another thought might be to use the other side of the CC relay (it's a 
DPDT but only one side is used I think) to trigger the solenoid. However 
that would only work if the CC is on, and I like to drive around in 
speed 4 explicitly as well. Shouldn't be penalized for that.

What do people think about this?

Chris