Re: weird DC motor

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:

> stevesng@n... wrote:
>
> > You should be able to run it fine as a plain DC motor. There
> > are many variable speed controlers avaiable that can handle
> > the 220 volts at 10 Amps, more than the current expected with
> > these motors. The commutators would be wired in parallel and
> > with the same polarity and operated at 220 volts. Polarity
> > can be check by running one set at a time.
>
> Wiring the two commutators in parallel could cause large
> circulating currents between the two sets of armature windings
> unless the windings (and the field flux that penetrates to them)
> are identical. If the windings are done with one set on top of
> the other, as they almost have to be, then they won't be balanced.
>
> I am concerned because the original writer said the motors ran
> much faster when the two windings were connected in series.
> This doesn't make much sense. Has he really seen two sets
> of brushes, or is this just a 4-wire motor that he doesn't know
> much about, internally? A certain (very early) vintage of

Bridgeport

> CNC machines (CNC control may not have been made by Bridgeport)
> used motors that had both PM and wound fields. They reduced
> the PM field with the wound field to make the motors run faster
> for the rapid feed. this was also done in some computer tape
> drives to get fast rewind speeds.
>
> Jon

Pictures of the motors were posted by Dale at:

http://home.mmcable.com/mediasmith/DCmotor.jpg

The detail is hard to see, but on the rear the brush holders
are flush. I cannot tell if there are brush holders at the
front, but the end bells look the same. Paralleling armature
windings that were not designed to be paralled could be a
problem, however armature windings in single commutator
motors run in parallel all the time because the brushes
make contact with multiple segments of the commutator at
once.

If it is a wound field motor it should not run with
power on only one set of wires unless there are internal
connections putting the field across the armature. This
seems unlikely as field control would not be possible and
because the pictures of the open rear end bell do not seem
to show such connections.

Best bet would be to have a good look at the real motors
and run some measurements or experiments.

Steve Stallings