Efficiency improvement of wireless power transfer via magnetic resonance using the third coil

A wireless power transfer system via magnetic resonance has been reported by several groups [1-3]. In those reports, two resonance coils work as a transmission coil and a reception coil, respectively. The transfer efficiency between two coils is a function of the distance and the orientation between the coils. When the distance between the two coils is long, the transfer efficiency is small. When two coils share a single axis, the transfer efficiency is maximal, but otherwise the efficiency becomes lower. If the axes of them are orthogonal and the center of one coil lies on the axis of another, the transfer efficiency becomes minimal. In some applications where the distance and the orientation of the coils are difficult to keep fixed, an appropriate countermeasure against the decline of transfer efficiency is desired. In this paper, a wireless power transfer system via magnetic resonance using the third coil is proposed to improve the transfer efficiency. The third coil, which is implemented in the wireless power transfer system, does not need to carry any electrical device such as an amplifier circuit or a rectifier circuit. The proposed system can improve the transfer efficiency when the distance between two coils is large. Also, the proposed system can improve the transfer efficiency when one of the coils rotates and their axes become separated. The effectiveness of the proposed system is confirmed by measurement using a vector network analyzer.