Do you think this brushless DC motor design will function?


There are four rotor coils -- each wound like the tracks on a vinyl record, then fold the 'record' in half. Now you have four coils, each a half-circle in shape. The red coils will have direct current flowing one direction, and the blue coils will have the current flowing in the opposite direction. The rotor would be made of a material exhibiting high magnetic permeability and low coercivity. The rotor coils would cause a magnetic field to flow through the rotor such that on the half of the rotor sandwiched between the blue coils a magnetic field would flow radially inward toward the center of the rotor, and on the other half of the rotor sandwiched between the red coils the magnetic field would flow radially outward. As the rotor spins, for any given section of the rotor, it would reverse magnetic polarity when it leaves the red coil side and enters the blue coil side (6 o'clock position in picture); likewise, when leaving the blue coil side and entering the red coil side (12 o'clock position in picture), it would reverse magnetic polarity again. The rotor coils are stationary.

Any thoughts?

  • 4
    \$\begingroup\$ "The rotor coils are stationary"....? We use the word stator for the part of the machine that is stationary and rotor for the part of the machine that rotates. If it's a brushless DC motor then the rotor should have a permanent magnetic field, and if it's an induction motor then the (solid) rotor has an induced magnetic field that rotates. It seems like what you're describing might be an induction motor? \$\endgroup\$
    – MarkU
    Jul 8, 2021 at 23:55
  • \$\begingroup\$ The stator is comprised of the magnets above and below the rotor in the picture. I used the term 'rotor coils' because those are the coils that excite the magnetic field in the spinning rotor, even though the coils themselves are stationary. The magnetic field cutting through the rotor doesn't rotate; it is stationary -- only the rotor rotates. \$\endgroup\$
    – Michael
    Jul 9, 2021 at 0:02
  • 1
    \$\begingroup\$ You have a stationary magnetic field going through a non-magnetised rotor. What are you expecting to happen? \$\endgroup\$
    – throx
    Jul 9, 2021 at 0:46
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    \$\begingroup\$ Sorry, non-permanently magnetised rotor. You seem to be trying to induce a magnetic field in the rotor from the stator's field, which you then expect to interact with that same stator field to induce a force. Remember, your entire stator (coils + permanent magnets) is static and resolves to a single magnetic field which then interacts with the rotor. \$\endgroup\$
    – throx
    Jul 9, 2021 at 0:58
  • 2
    \$\begingroup\$ "There are four rotor coils -- each wound like the tracks on a vinyl record, then fold the 'record' in half. Now you have four coils, each a half-circle in shape." - I don't understand this. Are you saying the flat spiral coil is folded in half so the wires in one half run next to the wires in the other half.? Wouldn't this cancel the magnetic field? \$\endgroup\$ Jul 9, 2021 at 1:31

1 Answer 1


No, this will not work

An electrical motor works through the interaction of misaligned magnetic fields, which produce a torque that tends towards their alignment. To achieve this in a non-permanently magnetised rotor, you have to create a moving magnetic field that drags the induced fields in the rotor around.

As presented, this device will have a non-moving field that is the sum of the field in the coils and the permanent magnets at the top and bottom. This will create an aligned field in the rotor, which will either exert no torque if stationary, or will exert a braking torque from eddy effects if it is in motion.

There are also issues with the coils as presented not creating the magnetic field shape you hoped for, but those are ancillary to the core issue here.

I believe in presenting this design there was a false understanding of the magnetic fields, where the field from the permanent magnets was ignored in constructing the induced field inside the rotor and then an assumption made that the induced field would then create a torque when acting against the only the field from the permanent magnets. Magnetic fields don't work that way - there is a single field that acts holistically on the rotor, leaving nothing to torque against.

Consider the thought experiment where you replace the constant fields in the coils with permanent magnets having the same field - this would give a machine that produced energy directly from the magnetic field, something Maxwell's equations forbids.


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