Hi
I've seen such a stirrer in action. The one I saw had no such thing as a "rotor". It had a circular magnetic base, on which you could place a flask/container on any non-magnetic material. The stirrer was actually a steel ball (or some other magnetic alloy I assume), around 2 cm in diameter, which was placed in the liquid to be stirred. The moment you switched the thing on, the ball started running around in a circular motion within the circumference of the magnetic base beneath. It had a knob with which you could control the speed of the stirrer.

From the operation I kind of assumed a rotating magnetic field, which caused the steel ball above to follow it. I cannot design a magnetic circuit to achieve that, but I can visualize a different scheme: Have a number of independent electromagnets arranged radially beneath a non-magnetic metal base. When the stirrer is turned on, you can start energizing and de-energizing each electromagnet sequentially, creating a magnetic field that "runs" around the circumference of the base. If the field is strong enough, and if you time the energizing and de-energizing sequence properly, a steel ball should start "running" behind the magnetic field.

I hope you get what I mean! :-))

Kundi