Re: (ET) field weakening with series-wound motors

["Bob Murcek" <RMURCEK geisinger edu>]

I'm a little surprised that there haven't been horrified responses to the 
concept of turning off the series field on motor with only a series field. 
 When you do this you're relying entirely on the residual magnetism in the 
motor which can vary a lot.

In the case of the compound motors used in the tractors, though, there'd 
be no harm since the shunt field is there, but shorting out the series 
field probably wouldn't have much effect on speed since I've learned from 
others that this series field is for speed compensation.

>>> "Max Hall" <maxo iname com> 3/31/2002 9:51:34 PM >>>
Paul, all,

Minding that we're close to the border of relevant... (some of the ETs use
series wound, no?!)

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

Yessir. It's not much, but can be a help. I was running a 36v (nominal)
series motor with its differential and axle with a 72v traction pack last
summer. No trouble from that alone. Add to that that I did this field shunt
trick. No trouble then, either. And interestingly, no unusual heat. (Since
there was no trouble, I haven't looked at the commutator. Just in the
interest of disclosure.) Shouldn't have been, really. Same upper current
limit, though twice the EMF.

> 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?

Someone on this list may offer up some horror stories or sober wisdom, but 
I
have done this on three different EVs all with golf cart series-wound
motors, and it's always worked without trouble. It should, really... the
overall current goes up a bit, yes, but the current through the field is
less when the shunt is engaged. The current through the brushes is higher,
though, and I suppose that's a concern. But my use of it was always, as you
say, for lower torque times.... when you're already chugging along. I would
be drawing something like 100A before engaging this "overdrive," and 120A 
or
so after; nothing like the 275A through the motor when accelerating up
hills.

> Thanks for the idea,
'welcome, but it's not new with me. I first got it from my father-in-law, a
cagey electrical theoretician for sure.

I look forward to hearing other thoughts on it from the veterans in ET 
land,
ever mindful of topic, topic and topic.

-Maxo