I am only a technician and really don't know much about motors, but there is this two questions that keeps bothering me and I couldn't find any explanation to it. I will be grateful if any of you can teach me.
Here is the first question :
I was in the electronic timepiece factory and puzzled by the step motor in the watches and small clocks, they have only two poles and for each step, the permanent magnet rotor rotates exactly 180 degrees, there is no mechanical means visible to govern the direction of rotation of the rotor.
The question is, how is that the rotor always rotates in the same desired direction?
I've gone through many books on step motors over some years but they all talk about motors with more than two poles.
Probably the motor has four windings at 90 degree from each other, call them North, West, South and East. North and West are connected in series, the same for East and South. In this way the total numbers of wires out of the stepper are 6 that is 4 + 2 center taps (one where North & West connect + one where the South & East connect). The 2 center taps connect to the +V (power), the other four to 4 switches (transistors for example). It should now be obvious the phasing sequence necessary to force a 180 degree rotation in the desired direction.
IIUC, some of these motors rely on a small permanent magnet, in the rotor, to line them up, say North South or South North. The stator pole pieces are shaped with a very sudden trailing edge and a very long leading edge (which either increases in radius as it increases in distance from N/S - or decreases in size), at the end of which is a small coil.
Applying a short pulse to the coil moves the rotor (both ends) away from the stator pole pieces and towards the coil. At that point, the rotor poles line up with the leading edge of the other stator poles. This attracts the rotor, but the rotor will continue to turn until it reaches N/S orientation, where the air gap is least. The process then repeats.
Pulses are alternately positive then negative going. If the first pulse is "wrong", it simply repels the rotor a little way along the reducing pole pieces. At the end of the pulse, the rotor pulls itself back into line. The next, and following pulses will be "right".
The alternative is the use of asymmetric gear teeth. These have a very high pitch point on one set of tooth faces and a much lower one on the other. The torque of the motor is insufficient to turn the gear wheel the "wrong" way - as the equivalent rolling path is much too steep. This is mostly used on more expensive miniature motor drives. The teeth are so small that it may not be easy to see that their tooth profile is strongly asymmetric.
IANA small motor expert. The above is simply my understanding of how these things work! Mostly obtained by (highly destructive) taking apart things to see...
Thanks for the reply, but not the case for these motors. These small clock or watch motors typically has only one coil mounted to the straight part of a "C" core, and the permanent magnet rotor is mounted between the ends of the "C" core, and these ends (poles) are usually large in a way that they together almost encircles the permanent magnet rotor, leaving a small gap at each end comparable to the spacing between the rotor and the stator poles.
I have used caliper to measure the two poles and they appeared to be of same size, I said "appeared" because the factory bought them as pre-assembled component parts and precise measurement is difficult.
I am puzzled because it seems that as soon as power is applied, these motors always rotates in one single direction, no hesitation, no wrong turns, every step 180 degrees.
I've consulted the engineers of the factory and the few Electrical Engineers and Electronic Engineers I've met, none of them has any idea.
Did you see any evidence of a shading coil (one shorted turn around part of the pole face? Are the poles assymetrical so the gap between rotor and stator is not quite uniform?
That is really unusual. There must be some kind of mechanical/electrical bias somewhere. It cannot be in any other way. Gene P.S. Perhaps Palindr?me is right.
I doubt that a difference in the gap between the rotor pole rims would bias the starting direction (at least not directly). It could cause the rotor to stop in a position that favors staring in a given direction but it would probably throw the rotor out of balance. Try this by stopping the rotor in different positions then starting it.
The stator may have some residual field that causes the rotor to stop in a known position favorable to starting in a given direction. This would require the stator voltage to always be applied with the same phase relation, i.e., the same stator pole is always North first when starting. Again, try positioning the rotor in some other position before starting.
Maybe it has a little mechanical catch that stops rotation in the wrong direction and provides a little kick in the correct direction. I recall seeing such a pall or ratchet in an electric clock about 30 years ago. It intercepted the rotor before it made more than a half turn in the wrong direction and caused a rebound (and start) in the correct direction. It was easy to hold it out of the way and the clock would run in either direction. In fact we could rig the clock to always run backwards. The clock obviously required gravity (and a particular orientation) to start in the correct direction. You can try this by seeing if you can turn the rotor more than a half turn in the wrong direction and maybe by changing the orientation. (I note that this doesn't seem to fit the condition of starting with no hesitation.)
It appears that none of the 'engineers' that you contacted have a very inquisitive nature.
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