Re: weird DC motor

--- In CAD_CAM_EDM_DRO@y..., "Dale Smith" <mediasmith@m...> wrote:

> OK guys!
>
> Photos are at http://home.mmcable.com/mediasmith/
>
> Look for DCmotor.jpg for the motor photos <snip>
>
> A motor with two sets of high current (16 gage wire) brushes
> running on two separate commutators is new to me. Multiple
> sets of brushes running on one commutator is common for
> high quality servo motors designed for low torque ripple.
> Many servo setups have a motor and tach on a common shaft
> with two separate sets of everything including brushes, but
> the tach brushes and wiring are typically much smaller.
>
> +++Everything appears to be equal and the exit points of the wires
>indicate the brushes may be 45 or 90 degrees offset from each other.
>

OK, looks like a servo motor to me. The current rating could
be explained by the "continious duty". Servos will have a
much higher "peak" power and current drain, and that may
not have been put on the name plate. This also would be
consistent with the fan port on the motors. Servos often
have to put out a lot of power while not actually turning
very fast, making a conventional shaft mounted fan useless.

<snip>

>
> +++ Grab a tight hold! The resistance is about the same.
>One set of brushes reads from 0.4 to 0.7 ohms. the other set
>reads from 0.6 to 1.0 ohms on both of my incredible Radio
>Shack DMMs. Something must be wrong with Ohm's law. I keep
>winding up with somewhere between 220 and 440 amps at that
>voltage/resistance; 100 amps if wired in series.

Motors do not react in the normal Ohm's law fashion. Remember
that a spinning motor generates a back EMF due to the windings
cutting through a magnetic field.

>
> wdsmith
> (I love a mystery too but I like the ones with a possible
> solution much better)
>

This mystery seems mostly solved. We are left with wondering
why they used two commutators instead of the more usual
arrangement of multiple brush pairs on one commutator.

Cheers,
Steve