The interesting thing to check would be whether the metal
band on the outer, rotating piece has a permanent magnetic
field or not, thus differentiating it from an induction motor,
and making it a permanent magnet, synchronous motor (although synchronous motors tend to not be self starting, especially on single phase excitation, thus usually requiring an induction motor component to get them up near synchronous speed, where they can then lock into synchronous speed; but, I see no real reason why they’d need a synchronous motor with the design of the device). I can’t really tell what that band is composed of from the photographs.

The part I’m not sure about, though, is that I’m not sure what all of the fingers do. My initial thoughts are that they function similar to the shaded poles on a shaded pole induction motor, but I can’t make out enough detail to be sure.

As you may remember, an induction motor produces a rotating magnetic field, which induces a field into the opposite part, which then causes it to be dragged around. However, with a single phase excitation, the field produced just pulsates without rotating, unless some trick is used.

There are, of course, several tricks which can be used. One is to use an auxiliary winding to excite a set of poles, and to provide excitation to this auxiliary winding by a phase shift of the current (either a leading component, produced by a
capacitive phase shift, or a lagging component, produced by an inductive phase shift). The auxiliary winding can only be used when starting the motor (e.g., start winding), or can be used all the time when the motor is in operation (e.g., run winding).

But, since this example only shows two wires going to the coil (and, one presumes, only one coil), then this technique isn’t being used. Another trick involves “shading” a portion of some of the poles, usually by wrapping a piece of Copper or other highly electrically conductive metal around the portion of the pole, which produces a slight phase shift to the magnetic field, giving the illusion of a rotating magnetic field. That might be what’s being done here, but it’s hard to tell, since I can’t see where/what all of the fingers are connecting to.

In any case, with this being an 8 pole motor, the rotation speed should be a little less than 3600/8 RPM (if I’m remembering my induction motor theory correctly [1]), when used with a 60 Hz excitation. That gives a figure of a little less than 450 RPM for the rotor, with the difference being due to the “slip”, which should be fairly small, given that the motor is only lightly loaded.

[1] It’s been a LONG time since I studied induction motors, so take what I’ve said with a grain of salt.

Dave

–Philip.