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I sometimes need air core transformers for contactless energy transfer of small powers (usually less then 1 watt). However, on rotating machines, sometimes a bearing is needed in proximity of the energy transformer. The bearing than gets some coupling to the transformers field and as it's steel just forms a shorted winding, consumes some energy.

Now I have the case of a larger bearing, spanning about half of the transformer coils plane. Obviously, it will short out much of the energy transmitted.

I wonder if it would be possible to remove the inner part of the field, by adding some reversed windings of the bearing diameter to the primary coil. The primary field would then be constricted to the outer part of the primaries core area I guess, leaving the bearing uncoupled. But how many windings would that take? The same amount as there are forward windings in the outer coil?

The situation is most likely this: induction transfer with metal nearby red: coils, grey: interfering metal, non-metal parts omitted.

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I have to tackle similar problems on rotating machines - sending power and receiving data. Do you use power resonant coils? That's the first question. What sort of power transmission frequency do you use? Steel doesn't have to act like a shorted turn and if it does, it won't completely kill the mag field - with resonance you can re-tune and use the metal as part of the tuned circuit.

Trying to cancel parts of the magnetic field is a fruitless task - Sometimes I use ferrites when the metal (such as stainless with poor conductivity) is heating up too much. Normal steel is better but it still has losses and ferrite does the job of masking areas that would otherwise drain all the energy.

Some diagrams/pictures would help but the above are my thoughts to start off with.

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  • \$\begingroup\$ It's similar to Charles Joubert drawing, except the bearing is inside the coil plane. \$\endgroup\$ – dronus Nov 3 '13 at 14:12
  • \$\begingroup\$ So the larger diameter coil sourrounds the bearing. \$\endgroup\$ – dronus Nov 3 '13 at 14:13
  • \$\begingroup\$ @dronus if you really want help a half decent sketch of what mechanical parts you have will work wonders. Please also add what operating frequency you have trialled before and what sort of coil arrangements you have tried. As I said above, the metal (even though it may seem like a shorted turn) can be used as part of the resonant circuit for transmitting power. I've had to do this on several jobs and a good sketch will help me understand what you are up against and what you have tried. My aim is always to avoid ferrite because it's never the right shape and temeperature can affect performance. \$\endgroup\$ – Andy aka Nov 3 '13 at 14:37
  • \$\begingroup\$ Added a sketch to the question. Operating fequency was avout 15kHz, but may be changed. \$\endgroup\$ – dronus Nov 4 '13 at 10:24
  • \$\begingroup\$ But the image is only one example, there are several likewise made devices, with for example different heights between the coils and the bottom metal disk. \$\endgroup\$ – dronus Nov 4 '13 at 10:26
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I second Andy aka for the idea of ferrite. It can both shield the magnetic field and increase the coupling between the two circuits (static and rotating). In the figure below, a magnetic field line is given in red.

enter image description here

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  • \$\begingroup\$ Interesting, I will check out ferrite. In my construction, the bearing would be placed in the winding plane with a smaller diameter as the windings, so a ferrite ring or cylinder of a diameter between the winding and bearing ones would be placed in the plane, but this should work too i guess. \$\endgroup\$ – dronus Oct 17 '13 at 21:26
  • \$\begingroup\$ I have more and more different designs to handle, and custom ferrite is hard to get. Even flat disks like the one in your drawing seem quite unusual.. A wire-only solution still would be cool. \$\endgroup\$ – dronus Nov 3 '13 at 14:11

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