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I'm struggling to understand why a homopolar motor still operates when the field magnet is corotating with the current carrier as shown in the first image (the blue and red representing poles on a nickel coated magnet which acts as the current carrier), but not in others such as my design in the second.

Homopolar motor

non functioning homopolar motor

This second image is a representation of a motor design I attempted but the prototype did not develop any rotation (the prototype had several hundred wire turns and carried 4 amps, the field magnets were a stack of N42 neo magnets with a 20mm square face).

Edit -> The current carrying loop and rectangular magnet are both meant to co-rotate in unison at the axis

Since the Lorentz force happens even when the magnetic field rotates with the conductor for the first design in my mind this should be equivalent for the second.

Where is the flaw in my design - or is this a case of experimental error?

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An important feature of a homopolar motor is that the orientation of its magnetic field relative to the current does not change. Because of that, the magnetic field keeps pushing the current in the same direction.

The magnetic field of a cylinder magnet on your first diagram is symmetric relative to the axis of rotation, so the orientation of its field relative to the current does not change as the magnet spins. It would also not change, if the magnet was stationary and the current was spinning, as is often the case.

On the second diagram, the orientation between the magnetic field and the current changes every $180$ degrees, which means that the magnetic field will alternatively push and pull the current, resulting in no continuous motion. For this configuration to work, the direction of the current in the loop has to change every half cycle, which would require a commutator and would turn it into a regular DC motor.

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  • $\begingroup$ Sorry my diagram wasn't clearer but I meant to imply that magnet and conductor are rotating together at the same speed in the second diagram. $\endgroup$
    – user263399
    Oct 21, 2018 at 22:08
  • $\begingroup$ @user263399 For the motor to work, either current should be able to move relative to the magnet or the magnet should be able to move relative to the current. In the first diagram, the current loop which matters is stationary (it is not shown on the picture), while the magnet is rotating. If the magnet and the current on the second diagram are rigidly connected, they wound not move: the forces between them would be internal to the magnet-current system and the internal forces cancel each other and, therefore, cannot make the system move. $\endgroup$
    – V.F.
    Oct 22, 2018 at 0:39
  • $\begingroup$ The patent at patents.google.com/patent/US8288910 shows a configuration where the current loops move with the magnet and I assume that since the patent was granted that it works. $\endgroup$
    – user263399
    Oct 22, 2018 at 3:17
  • $\begingroup$ @user263399 To be sure, the arrangements of magnets in both of the two presented embodiments are different than the arrangement shown on your second drawing, but the wire is indeed wound around the magnets and spins with them. I would like to be proved wrong, but I don't understand how this could work. In general, awarding of a patent is not a proof that the invention works and many patent applications are filed for things that have never been built. Of course, this is not a proof that this invention does not work. Perhaps you could contact the inventor and ask him for advice. $\endgroup$
    – V.F.
    Oct 22, 2018 at 12:22
  • $\begingroup$ So I constructed a prototype of the linked patent and you are correct it does not work (used around 25 turns on a 'toroid' with two Neo ring magnets - 30mm diameter, 5mm height with a 12mm hole repeling each other and three steel washers between to reduce the repulsion - hooking it up to 12v produced nothing but heat - no apparent torque whatsoever). $\endgroup$
    – user263399
    Oct 24, 2018 at 8:21

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