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I am looking to incorporate a inductive charging mechanism into a project I’m working on. On the receiving side I only have room for a single layer coil design. For my transfer side I have more spatial freedom. What I’ve found online thus far is that by aligning my coils (Same ID, OD, # of turns) and adding ferrite reflector to both sides I will have the greatest efficiency. If I continue to add layers of coils ( one article mentioned having a layer of coil separated by a soft iron core then the second coil layer) will I be able to increase my magnetic field strength and thus increase efficiency?, granted I keep the resonant frequency the same for each coil?

All the studies I've seen have had identical double layer coil designs. Will the efficiency still increase if the transfer side is a double layer but the receiver is a single layer? What are risks associated with this mismatch?

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  • \$\begingroup\$ I believe ferromagnetic shields under the charging pad and above receiving coil are added mostly to prevent eddy currents and heating of metal objects nearby. The slightly increased efficiency is merely a bonus side effect. Also, I think you are using "efficiency" where you mean the rate of power transfer, not as a measure of losses in a system that it actually means. \$\endgroup\$ – Maple Aug 28 '18 at 19:16
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will I be able to increase my magnetic field strength and thus increase efficiency?

Yes, magnetic field strength will be increase if you keep same currents in the coils. However, increased field strength does not guarantee higher efficiency. For the purpose of maximum efficiency you should look for kQ product as follows - This assumes your load is at its optimal value. $$\eta = k^2Q_1Q_2/(1+\sqrt(1+k^2Q_1Q_2))$$ $$k = M/\sqrt(L_1L_2), Q = \omega L/R$$ Having multiple layers in Tx side will increase the self inductance(L_1) mutual inductance (M) and - in most cases, coupling will also increases. However, AC resistance of Tx will also increases with the increase of number of turns. Therefore, your quality factor may be affected. You should carefully examine how the quality factor varies. Usually, if your operating frequency is not very high and coupling coefficient is very small, adding an extra layer helps to improve the efficiency.

What are risks associated with this mismatch?

Having different Tx and Rx coils does not mean low performance. If you carefully redesign the system with modified parameters, you can get the best out of it. without actual specifications such as power level, coils size, distance, frequency etc, it is hard to say how to optimize and what will happen.

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