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I have a planar coil with inductance L, operating at a given frequency. Now I want to estimate the eddy currents, which are induced in a piece of metal, which is placed in front of the coil, and the change of inductance caused by this eddy currents.

Which would be the best approach to this, by calculating the field equations, or is there any simpler approximation possible?

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  • \$\begingroup\$ How about a sketch with dimensions. \$\endgroup\$ – Andy aka Jul 21 '16 at 7:19
  • \$\begingroup\$ I'd expect that eddy currents not only affect the inductance but maybe even more the Q factor (if the coil is part of a LC circuit). \$\endgroup\$ – Curd Jul 21 '16 at 8:47
  • \$\begingroup\$ @RobertAlpha: What do you mean by "estimate"? Do you need just a qualitative answer of a quantitative? \$\endgroup\$ – Curd Jul 21 '16 at 9:46
  • \$\begingroup\$ What I'd need is a rough estimation, by how many percent the inductance will change, between the metal is present (in a given distance) or not, if such an estimation is possible \$\endgroup\$ – RobertAlpha Jul 21 '16 at 11:23
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'Calculating the field equations' would be one way to do it, but the geometry is far from analytically simple, so you would end up with a huge mess of terms.

If you search the literature for planar coils, you will see that most workers (mostly PhD students) mention complete analytic solutions in passing, then go on to approximate analytical solutions of simpler models like current sheets and short line segments, and FEMM based solutions. So if you take on a complete equation based solution, you are doing PhD level work.

There are numerous 'flat coil' calculators that can be found with your favourite search engine. If you get good agreement between several, you can have some faith in the answer.

Measurement might be another avenue, not least to valid answers from calculators. make a flat coil, resonate it with a capacitor, then note the change in frequency as you bring the piece of metal up to it. This gives you the mutual inductance, from which you can estimate coupling and eddy currents.

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