The French engineer Marius Lavet invented this kind of drive and described it in 1936 in his patent application FR823395.
In a clock, a circuit generates the bipolar pulse train, which alternately delivers a positive and a negative voltage to the coil for short periods (providing a correct mechanical output to move a second hand).
However, unlike a shaded-pole motor, the grooves are at backward positions, and the positions where the rotor settles after each cycle are well determined, which is not the case for induction motors in general, where slip and load affect the angle that the rotor turns each cycle.
Essential for the movement of the Lavet motor are the cogging points of the rotor, which differ depending on whether the stator coil is energized or unenergized.
Movement of the common two step Lavet motor: To make a Lavet motor turn, the current through its stator coil must change direction each step (bipolar) followed by an interval without current while the rotor moves to its reluctant position.
Lavet-type stepping motor of a quartz clock. A black
rotor sprocket
provides the mechanical output.
The rotor (circle) is a permanent magnet (with red and green poles). In figure (a) it turns towards the stator (xx' cogging point) so as to minimize reluctance. (It does so by turning away from the small semicircular air gaps.) When the electromagnet is switched on in figure (b) its horizontal magnetic field attracts the rotor's poles (yy' cogging point). This cycle repeats through figures (c) and (d) with the current and magnetic poles reversed.
Slow motion video (960 fps to 30 fps, 32-fold) of the motor wheel of a clockwork with modified actuation of the coil (1:1:1:1 duty of the phases N-S polarity, off, S-N polarity, off). Normally the active time of the coil is about 31 milliseconds to save battery energy. This time is chosen so short that the wheel just accelerates but the current through the coil is switched off before the wheel reaches the 0 or 180 degrees position, so the inertia moves it directly further to the reluctant position. To be able to see the 4 stop positions of the motor it is necessary to prolong the phases with switched-on current to about 200 milliseconds.
