(ET) field weakening with series-wound motors

["Max Hall" <maxo iname com>]

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