Re: (ET) field weakening on E15 and moving dirt

["Max Hall" <maxo iname com>]

Hey Andy, hey ETers,

You can get "overdrive" on your ET-15, with its series motor, (er, um, if 
it
has one), with field weakening.

I have built a bunch of EVs with series-wound motors, and taken advantage 
of
the ability to get a little extra speed out of them by shorting the field
winding. (I usually just put a contactor between S1 and S2 with a switch on
the dashboard.) All this does is reduce counter EMF, kick up the current
overall, and give about a small (10% to 20% in my experience) boost in
speed. (No free lunch, of course; power consumed is, as ever, VI.)

And Andy, though a FET can be used, for all intents and purposes, as a
variable resistor, you shouldn't in this case. Whoever suggested "pulsing"
it was right: When it is "closed," no current passes through it, and thus
there's no I-R drop and no Joule heating; when it is "open," or has very
little resistance, then the I-R drop is tiny so there, too, Joule heating 
is
minimized. It's when it is acting as a resistor, it has to dissipate heat,
and that's tough stuff. They do well as two-state devices.

But just use a contactor! I mean, just *TRY* a contactor to shunt the field
windings. (Again, this is a short in PARALLEL with the field winding.) I
don't think you'd get the nasty "jump" you are worried about, but try it.
You can also take an old golf cart resistive controller, or make a
FET-enable PWM switch, to ramp the field shunt up and down.

Take care, all of you. Seems like winter's over here 'round Boston. The 
last
little bit of my snowpile melted yesterday. (That's a big annual 
milestone.)
I am thinking about taking the thrower off the tractor, but that usually
makes it snow one more time. (The gods of weather take it as a taunt, I
think.)

-Maxo
http://www.maxmatic.com/electrak.htm

http://www.maxmatic.com/