0
\$\begingroup\$

My goal is to contactlessly power a small generator by rotating a steel plate underneath it.

Specifically, I have mounted 6 permanent magnets (rectangular shaped) on a shaft, which is mounted on ball bearings that make it spin freely (rotor.) All around it are the armature windings.

Now I know that it is possible to make the magnets spin with a conductive metal plate rotating underneath it (the plate is spinning freely at very small distance from the magnets) because of the generated eddy currents. This would work fine with an aluminum plate as I have already tested it.

In my case the spinning plate has to be steel and the main problem is that the magnets on the shaft won't turn because of the magnetic attraction of every single magnet with the steel plate. I tried different rotating speeds and distances between the shaft and the plate but nothing helps. Even with rounded shaped magnets it seems to make no difference

Do you have any suggestions how and if this could work? Maybe some sort of coating for the steel plate?

sketch

\$\endgroup\$
13
  • 1
    \$\begingroup\$ if the attraction of the magnets to the steel plate is causing excess friction in the bearings, that could be stopping it. Can you drill holes in the steel plate so it forms a 'reluctance gear' with the magnets. Then you'd have a very positive drive not just reliant on eddy current. \$\endgroup\$
    – Neil_UK
    Feb 6 at 12:39
  • \$\begingroup\$ @Neil_UK more than the friction on the bearings it seems that the magnets (which are very small but strong N52 magnets) have a "stable" orientation to the plate and therefore won't turn. Also I can't really modify the shape of the plate (like drilling it) altough it would have been an excellent idea. \$\endgroup\$ Feb 6 at 12:56
  • \$\begingroup\$ Why must the spinning plate be steel? \$\endgroup\$ Feb 6 at 13:50
  • \$\begingroup\$ Does the plate have to be whole and nice looking for aesthetic reasons, or can we hack it and make holes in it? \$\endgroup\$
    – bobflux
    Feb 6 at 13:57
  • 1
    \$\begingroup\$ a diagram or photo would be useful, I'm not sure I can understand your comment without one \$\endgroup\$
    – Neil_UK
    Feb 6 at 15:43
2
\$\begingroup\$

The rotor's poles are salient, or 'sticking out' out the rotor. The magnet closest to the steel plate is attracted to it. Any slight rotation of the rotor will move the magnet further away from the steel plate, which is energetically unfavourable, so the rotor stays stuck in that position. This effect is often called 'cogging' of a motor, where permanent magnets attract salient parts of the armature and affect rotation.

This magnetic attraction outweighs the effect of any induced currents due to the motion and conductivity of the plate. With an aluminium plate, the currents are larger, and the cogging effect is absent, so the rotor will turn.

Your configuration will not work with that geometry rotor and a plain plane steel plate. You need to do at least one of

  • Switch to axial rather than radial magnets on your rotor, and make it co-axial with the steel plate. This would eliminate the cogging, but radically change your geoemtry.

  • Make slots or holes in your steel plate to turn the attraction to your advantage, perhaps reducing the number of poles on your rotor to four so there is more of a strong/weak/strong field round the rotor. Position the slots at the same pitch as poles to make magnetic reluctance 'gears'.

  • Place steel shims or pieces of ferrite on the plate to reduce the gap between plate and rotor periodically, to make magnetic reluctance 'gears'. This has the same effect as cutting slots, without the effort of cutting your plate.

  • Place magnets with alternating poles on your steel plate to make magnetic gears.

  • Reduce the cogging by using a helical arrangement of small magnets on your rotor, rather than the long magnets you have. You have illustrated 6 long magnets, at a pitch angle of 60 degrees. Let's say you have room along the length of the rotor for 4 rings of 6 square magnets. Each ring of magnets should be offset by 15 degrees to the previous one. This will reduce the cogging radically, maybe enough for the weak induced current to generate enough torque to overcome it.

  • Can you overlay an aluminium plate on top of the steel one? This moves the steel plate away from the magnets, reducing the cogging. It increases the conductivity of the plate in the magnets' field, increasing the induced current by a few times. Those two changes may be enough for the induced torque to exceed the cogging torque.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.