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

["David Roden (Akron OH USA)" <roden ald net>]

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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