Re: (ET) Fine Control

["Christopher Zach" <czach computer org>]

Sure. Only issue might be compatibility with the motor. I don't know if the
E12 used a series motor or a shunt; my guess is it's a simple shunt with 
the
field wired to +36v.

You could add a simple set of resistors that cut in when the switches open
(think NC switches) and put the field in series with these
switches/resistors. Then when you move your speeds, the field will weaken 
in
stages, and the tractor will go more. A lot more.

The electronics on the ET's FW system is basically a current sensor off the
shunt. The problem with FW is that as you increase the load on the motor
with the field in a weakened state, the amount of current going to the
armature goes up. A *lot*. This can cause the motor to burn up and out if
say you went up a hill in speed 8.

So the circuitry is there to sense the current going to the armature, and 
if
it's too high it will turn off the FW circuit and restore full power to the
field. Which will bring the motor out of overdrive and the current back to 
a
safe level.

I recommend you build something like that, or never forget to overload the
motor.

Chris

----- Original Message -----
From: "Bob Murcek" <rmurcek geisinger edu>
To: <czach computer org>; <elec-trak cosmos phy tufts edu>;
<wilbett earthlink net>
Sent: Monday, July 29, 2002 2:28 PM
Subject: Re: (ET) Fine Control


> Interesting.  If the upper positions are just for field-weakening, 
> there's
probably a good chance that a clever person could add the weakening 
controls
from a 20 to a 12.
>
> >>> "Christopher Zach" <czach computer org> 7/29/2002 2:07:00 PM >>>
> Well, they used the same physical tractor parts, but the speed control is
a
> truncated version of the one used on the E20. Unlike the E20, there is no
FW
> circuit, the field is basically wired into the main power (full field all
> the time) and all of the extra relays just aren't in there. Result is a
> simple system with no high-end speed avail, and thus the 30 cent welded
> plate as opposed to building a whole new part.
>
> Chris
>
> ----- Original Message -----
> From: "Bob Murcek" <RMURCEK geisinger edu>
> To: <elec-trak cosmos phy tufts edu>; <wilbett earthlink net>
> Sent: Monday, July 29, 2002 1:56 PM
> Subject: Re: (ET) Fine Control
>
>
> I'm fairly certain the plate had been welded on at the factory to reduce
the
> number of speed positions to the three that an E12 has.  Like perhaps GE
> used the same control assembly for all the tractors and just blocked out
the
> upper positions when the assembly was used on an E12.  Or perhaps you're
> right and someone replaced a bad E12 assembly with one from some other
> tractor.  That may well be the case since the plate is very obvious when
> looking down at the slots the lever moves in, and I would think that
someone
> would have jumped into this thread and volunteered that their E12 had a
> plate.
>
> I was too lazy to use a real pot.  I would have had to find an 
> appropriate
> gear to connect the pot to the big gear on the assembly.  Worse, since 
> the
> big gear continues to turn in the same direction as you go from full
forward
> to zero to full reverse, you'd actually have to use two pots and switch
> between them or do some really tricky op-amp work.  And then you'd need a
> mounting bracket for the pot(s).  It just sounded daunting.
>
> >>> Bill Alburty <wilbett earthlink net> 7/29/2002 12:35:42 PM >>>
> Bob,
> I assume your E12 came that way, modified by someone else, since you
> chiseled off
> the plate.  Apparently you have a neat solid state controller fed by
> discrete
> speed commands. Why couldn't you put in a real potentiometer ( disconnect
> the
> micro switches) on the speed lever and get smooth speed changes out of
your
> SS
> Controller. I'd like to have one. The pot apparently only has to handle
1/2
> watt.
>
>     I haven't opened up my speed lever yet to count my micro switches.
>
> Bill A.
>
>
>
>