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

["Paul Heinzerling" <pheinzerling uakron edu>]

Hi Max,

Wow!  Do you mean I can just shunt the field winding on my series-wound 
golf
cart and get some extra speed?

What are the rules?  I'm guessing that you would only do this in a
low-torque application such as when already travelling at top speed.  Any
limits on how long you can run it this way?  Is there any penalty to the
life of the motor?  Anything else I should know?

Thanks for the idea,

Paul Heinzerling
Hudson, OH

----- Original Message -----
From: "Max Hall" <maxo iname com>
To: <elec-trak cosmos phy tufts edu>
Sent: Sunday, March 31, 2002 11:05 AM
Subject: Re: (ET) field weakening on E15 and moving dirt


> 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/
>