Development and optimization of printed spiral coils have significant impacts on the efficiency and operating range for magnetic resonant wireless power transfer (WPT) applications. In this paper, the effects of different material losses (substrate and conducting coating) of printed coils are considered and experimentally studied in this paper. For the purposes of comparison and finding the dominated losses, lossy loaded capacitors with equivalent series resistances have also been investigated. A four-coil system with an external capacitor-loaded (ECL) magnetic resonant WPT system is considered, and a self-resonant coil is designed and compared. Results show that the ECL resonant coil has higher efficiency than the self-resonant coil with the same size and distance between the transmitting and receiving coils. Through observing the simulated results and analyzing experimental data, it can be concluded that the dominant cause of the decrease in efficiency of this ECL-WPT system is the strip resistive loss of coil of 57% and the ohmic loss in ECL of 37%. Meanwhile, the substrate loss significantly impacts on the efficiency of the self resonant coil. The overall measured efficiency is about 66% of the ECL coil at a distance of 50 mm when the above loss factors are considered. The measured results are in good agreement with the analysis and simulations.
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