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

["Max Hall" <maxo iname com>]

Bob, David, (all),

Fear NOT! No one is turning off the field! This is all about field
*weakening*. David is right. (And read my post carefully.) By shunting, I 
do
indeed mean putting a "bypass" *in*PARALLEL*with* the field windings.

No horror. Please, no horror.

-Maxo

http://www.maxmatic.com/


----- Original Message -----
From: "David Roden (Akron OH USA)" <roden ald net>
To: <elec-trak cosmos phy tufts edu>
Sent: Tuesday, April 02, 2002 1:37 AM
Subject: Re: (ET) field weakening with series-wound motors


> On 2 Apr 2002 at 0:29, Bob Murcek wrote:
>
> > 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.
>
> I assume that everybody involved here knows that when we say "shunt the
> series field," we mean "connect a low-value resistor in parallel with the
> series field."  That's the usual way field-weakening is performed on
series
> motors used in forklifts and the like.
>
> Maybe we shouldn't assume that -- but just in case, now you know.
>
> Bob is correct, completely bypassing the field in a series motor will 
> have
> undesirable effects.  What happens will depend on how much residual
> magnetism the field has and what the load is.  With a very light load the
> speed may increase greatly, perhaps enough to damage the motor.  Or the
> speed may drop to almost nothing in spite of monstrous currents flowing 
> in
> the armature, which will also wreak havoc inside the motor (crisp
armature,
> burnt brushes, fried commutator, lots of smoke).
>
> There may be other possibilities I don't know about; I'm sure other folks
> more well-versed than I in motor theory can predict the possible carnage.
>
> So if you experiment with this, be careful out there.
>
>
> David Roden - Akron, Ohio, USA
> 1991 Solectria Force 144vac
> 1991 Ford Escort Green/EV 128vdc
> 1970 GE Elec-trak E15 36vdc
> 1974 Avco New Idea 36vdc
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