We all know what the cogging problem is for magnet motors, it's the major bugaboo in getting them to work.

In thinking about it, has anyone tried making a shorted coil that generates its own magnetic field as the rotor magnet approaches, counteracting the repulsive effect between rotor and stator permanent magnets. Once the rotor magnet moves past the shorted coil (and thus past the stator permanent magnet), the coil field collapses, allowing the stator and rotor magnets to repulse each other.

Not quite the same, actually more simple.  Have a look:

https://www.youtube.com/watch?v=oUlDMY1iE5A

Just had another idea... rather than permanent magnets on the stator, have coils that can be shorted at the appropriate time. Make them of a ferromagnetic material so as the rotor magnet approaches, they're attracted and they're building up a voltage differential. Short that voltage differential, and the magnetic field generated should oppose the permanent magnet in the rotor, pushing the rotor away.

Well, what you will likely find is the coil will begin to act like a Leedskalnin PMH, trying to control the relative motion of the rotor.  One has to keep the association of magnetic force and current in mind.  When you short a coil it begins to act like a copper tube and you remember what that does when you drop a magnet through it.  It resists change of motion.  Only current does this.  Voltage alone has no influence on motion.  So...

Unless you put current back into those coils of the proper polarity, I can't see how it would ever push the rotor away.

Just like that video shown, an open coil will not repel a magnet, but will instead build up a voltage differential. A shorted coil (to drain off that developed voltage differential), however, will repel the magnet, as shown in the video. Now the only task is to short the coil at just the right time such that the rotor is just past the point where that repulsion will give the rotor a kick to keep it moving. The ferromagnetic wire in the coil would act to help attract the rotor magnet toward the coil, until that coil was shorted and repelled the rotor magnet.

In further searching, the Flying Dutchman has a "broken flux generator" that operates on that principle.

Quote

When the coil is shorted a widely known and understood phenomena caused by eddy currents as displayed in eddy current tube causes a braking effect. The braking effect actually causes the core with the shorted coil to actually be repelled away from a changing number of magnetic lines.

An old style traction motor for trains used a similar concept, but used electromagnets for the stator, and magnetic induction to generate a voltage in the rotor windings, which was shorted at the appropriate time via a commutator and shorted brushes.

So I guess this would be a sort of "delayed Lenz"... it's delayed until that bEMF can be used to repulse the rotor magnet in the direction we want.

Quote from Cycle on February 14th, 2017, 10:54 PM

In further searching, the Flying Dutchman has a "broken flux generator" that operates on that principle.

This gizmo is actually pretty interesting:



Note it uses permanent magnets to keep the core in saturation.  That's the trick to developing a repulsive force at the proper time.  Without the core being in saturation, all you would get is drag when you short the coil.

Ideally, you wouldn't want the rotor to slow down on approach though.  You would want acceleration on approach and departure.  The only point where you would expect no acceleration is at TDC.


I still have a hunch what is needed is a typical LC setup:  Coil and capacitor.  What you do differently though is connect a switching mechanism between the coil and capacitor so that when TDC is reached, you flip the charge/polarity stored in the capacitor and send that back to the coil.  The idea being you don't want the force to return from whence it came, like a pendulum.  Instead, you want the force to continue in the same vector so as to continually accelerate the rotor.  If this could be done while only paying for the switching while the magnets do all the work, seems to me COP > 1 should be quite possible.