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Other ways to do brush timing

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Jeff Shanab wrote:
> What if you could put the fields on the center rotating piece and the
> com bars around the inside of the case.

All these mechanical marvels are interesting. And, they certainly work!
There are lots of examples (many old, a few modern) where people have
done this.

But this is really the "old school" way to do it. Given that we now have
hall effect magnetic field sensors, computers, and powerful simulation
software, perhaps it would be worthwhile to approach the problem in ways
that Tom Edison could only dream of.

The fundamental problem with brush timing on a series motor is that the
rotor (armature) and stator (field) coils each distort the magnetic
fields of the other. The amount of distortion varies with current, shape
saturation, shape of the magnetics, and many other factors.

Compensating windings like interpoles and pole face windings were
attempts to solve these problems. They worked very well, but added cost
to the motor and were tedious to design. In the days of experimental
design, they had to make a guess, build a motor, see how it worked,
change something and try again until it was good enough to ship.

What if you outfitted a motor with a circle of hall effect sensors in
the air gap between rotor and stator? Use a computer to capture the
field strength data as the motor is operated. Then you'll know exactly
where and how the field changes.

Now, add your compensating windings, right where they are needed. I'll
bet they can be much smaller than traditional interpoles, because they
won't need to carry the full motor current -- field power is usually
only 1-2% of the total power anyway, and I'll bet just 10% of that is
all the "warp" power you'll need. The compensating winding could be
driven by a small separate controller, programmed to deliver "just the
right" amount of field warping for the motor's particular operating
conditions at the moment.

Or, take advantage of the fact that the series motor is being driven by
a PWM controller. This means there is AC on the field coil as well as
DC. And this means that the compensating winding can be powered by AC
from the field, transformer-style.

Or, make the compensating windings as shorted turns, like the shading
coils on a single-phase shaded pole AC motor. Shading coils delay or
block the AC magnetic field, in effect creating a second phase.

Or, redesign the shape or materials in the field magnetics, to make the
field shift automatically as a function of field current. For example,
if the armature current is warping the field backward, make the field
poles so the back edge iron saturates first. Then as current goes up,
the field shifts forward because that's where the iron is that is still
out of saturation.

The whole point is to pick up where the "old masters" of series motor
design left off. Use new tools, new materials, and new techniques to
push it forwards. Don't restrict yourself to just copying or
re-inventing what they did before.

There are probably patents lurking in all this. :)
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
Lee A. Hart, 814 8th Ave N, Sartell MN 56377,