Firstly some background:

I would like to make a magnetic stirrer, like the ones they use in chemistry labs:

Magnetic Stirrer

A small bar magnet sits inside the glass jar, while another bar magnet, attached to a motor is spun underneath the jar. This causes the first magnet to spin too, thus stirring the liquid.

But I'm sure it must be possible to do this without the motor underneath, simply using coils. Either a brushless motor or a stepper motor.

And now my question:

Is there a name for such a motor, where there are coils on one side of a sheet of glass (or similar), but the rotating / permanent magnet part is on the other side?

  • \$\begingroup\$ Couldnt you just energise coils via a uC. Possibly you would need a minimum of four coils (or two with four transistors) maybe with ferrite cores to strengthen the mag field...? \$\endgroup\$ – Paul Sullivan Aug 29 '12 at 21:33
  • \$\begingroup\$ @PaulSullivan - Yes, obviously that's what I want to do. My question is "What is this type of motor called?" \$\endgroup\$ – Rocketmagnet Aug 29 '12 at 21:38
  • \$\begingroup\$ Possibly 'switched reluctance motor'. Dyson re-invented the brush motor removing the brush and it does so in a manner as described i.e. energising coils to push the magnet (no doubt patented) \$\endgroup\$ – Paul Sullivan Aug 29 '12 at 21:41
  • \$\begingroup\$ en.wikipedia.org/wiki/Stir_bar \$\endgroup\$ – Paul Sullivan Aug 29 '12 at 21:46
  • \$\begingroup\$ There is no certainty that the magnet ois being driven by an EM induced field. It may be, BUT you could use any motor whatsoever in the base to rotate an external magnet to drive the stirrer. \$\endgroup\$ – Russell McMahon Aug 29 '12 at 21:47

Finally found the closest thing to an answer, thanks to hints from David Kessner and Ignacio Vazquez-Abrams.

A pancake motor (AKA Floppy Drive motor) has a planar separation between the rotor and stator, and so is most suitable for working through a sheet of glass.

Floppy drive pancake motor


My college roommate built exactly what you are talking about. This would have been back in 1978-79 or so. It worked quite well, and we used it mainly to mix up our TangTM. IIRC, he didn't use a microprocessor — just a 555-based oscillator and a ring counter to generate the waveforms to drive the coils.

I don't think this type of motor has a specific name; the closest thing would be a degenerate form of stepper motor.

There are a few drawbacks to doing it this way. At slow speeds, the motion is very jerky, and sometimes the magnet would just jerk back and forth rather than rotating, especially if it was sitting in a pile of undissolved Tang.

Also, I'm not sure if this is related to your question or not, but there is a class of pumps called "magnetic coupled pump" that are used in situations where rotary seals would be a problem. Sometimes they use a separate motor and magnetic coupling, and sometimes the rotor of the motor is part of the impeller.

  • \$\begingroup\$ I believe this is a kind of PMSM - permanent magnet synchronous motor. I wound one once as a ultra-low-maintainence motor for a project of mine but it looked like a conventional DC brushed motor, but with 6 pins connected to a simple VFD and 6 coils inside. \$\endgroup\$ – Maxthon Chan Feb 5 '15 at 15:47

Maybe this is called "induction motor"? This is what some turntables use to spin the platter. http://en.wikipedia.org/wiki/Induction_motor


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