I hate to leave a good puzzle unsolved. Last week I was writing about a cheap quartz mechanism I got from an old clock that stopped working. I said that I could not figure out why its rotor only turns in one direction given a seemingly symmetrical construction of the coil that drives it.
There is quite a number of tear downs and descriptions of how such mechanisms work on the web. However, very few seem to address this issue of direction of rotation and those that do don't give a very convincing argument. Some mention that the direction has something to do with the asymmetric shape of the coil's core. This forum post mentions that the direction can be reversed if a different pulse width is used.
So, first of all I had a closer look at the core. It's made of three identical iron sheets, each 0.4 mm thick. Here is one of them on the scanner with the coil and the rotor locations drawn over it:
It turns out there is in fact a slight asymmetry. The edges of the cut-out for the rotor are 0.4 mm closer together on one diagonal than on the other. It's hard to make that out with unaided eye. It's possible that the curved edge on the other side makes it less error prone to construct the core with all three sheets in same orientation.
The forum post about pulse lengths and my initial thought about shaded pole motors made me think that there is some subtle transient effect in play that would make the rotor prefer one direction over the other. Using just a single coil, core asymmetry cannot result in a rotating magnetic field if you assume linear conditions (e.g. no part of the core gets saturated) and no delay due to eddy currents. Shaded pole motors overcome this by delaying magnetization of one part of the core through a shorted auxiliary winding, but no such arrangement is present here.
I did some measurements and back-of-the-envelope calculations. The coil has approximately 5000 turns and resistance of 215 Ω. The field strength is nowhere near saturation for iron. The current through the coil settles somewhere on the range of milliseconds (I measured a time constant of 250 μs without the core in place). It seems unlikely any transients in magnetization can affect the movements of the rotor.
After a bit more research, I found out that this type of a motor is called a Lavet type stepping motor. In fact, its operation can be explained completely using static fields and transients don't play any significant role. The rotor has four stable points: two when the coil drives the rotor in one or the other direction and two when the rotor's own permanent magnetization attracts it to the ferromagnetic core. The core asymmetry creates a slight offset between the former and the latter two points. Wikipedia describes the principle quite nicely.
(Click to watch Stepping a small clock motor with Arduino video)
To test this principle, I connected the coil to an Arduino and slowly stepped this clockwork motor through it's four states. The LED on the Arduino board above shows when the coil is energized. The black dot on the rotor roughly marks the position of one of its poles. You can see that when the coil turns off, the rotor turns slightly forward as its permanent magnet aligns it with the diagonal on the core that has a smaller air gap (one step is a bit more pronounced than the other on the video above). This slight forward advancement from the neutral position then makes the rotor prefer the forward over the backward motion when the coil is energized in the other direction.
It's always fascinating to see how a mundane thing like a clock still manages to have parts in it whose principle of operation is very much not obvious from the first glance.