Re: Stepping motors that never go backwards -- why?

On Mon, 27 Apr 2009 10:56:05 +1000, dAz wrote a reply.

[dAz]
basically that is it, the stator is offset or asymmetrical, the coil is
reversed on the alternate pulses.

Next time, I'll have to look very carefully!

[dAz]
excuse the crude drawing,

Gosh; no sweat, as we say. It's quite decent enough, and extremely clear.
I just spent several hours marking up an image of the innards of a Magic
Domino, like a Rubik's Cube, but 3x3x2. Been wondering for years how the
innards were done. Was using Krita, the Linux KDE image editor, and it
had me ready to screech in fury; it can be very buggy.

Very interesting that with the coil power off, the rotor simply "seeks"
the nearest "iron", which is where the minimum air gap is. Coil power
forces it to line up. (Wouldn't surprise me if the coil power cuts at
about the time it lines up, and it coasts the rest of the way.)

[dAz]
in some designs of stators it is possible to misalign them and cause the
watch or clock to run backwards, better designs have the stator fixed to
the plate in one piece and a saw cut placed at an angle straight through
where the rotor sits.

Interesting; thanks!

A few decades ago, Haydon Switch and Instrument made a two-pole stepping
motor (not a micropower type) that made one complete revolution for one
pulse. Pulses were always the same polarity, too. A permanent magnet in
the field (stator) structure and a two-pole rotor ensured that the rotor
always returned to home position. The stator pulse overcame the flux of
the magnet, and actually reversed the polarity at the poles, so the rotor
made half a turn. At the end of the pulse, the magnet took over again.

To ensure that the rotor always turned forward, each pole of the stator
was split, and had shading coils (thick copper rings) like modern single-
coil induction motors, such as those in table fans. The shading coils
slowed down the magnetic flux buildup, and delayed its decay. That
ensured that the rotor always turned the right way.

Regards, and thanks!

nb

.

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