Coil Comparison and Downscaling Principles of Inductive Wireless Power Transfer Systems

High-power wireless charging for electric vehicles (EVs) is an essential technology for the development of EVs. This paper compares four coil types: square, circular, rectangular, and bipolar, in terms of coupling coefficients, single-turn self-inductances, and maximum power capability varying with the coil width, airgap and misalignment. The coupling coefficients are only determined by the ratio of coil width and airgap over coil length and the coil type. The single-turn inductance increases linearly with the increasing coil length. For a small coil width and airgap, the bipolar coil has the largest coupling coefficient; for a large coil width and airgap, the square coil has the largest coupling coefficient. The maximum power capability of each coil is studied. High-power capability is normally unavailable in research laboratories of universities, so downscaled prototypes are implemented to verify the design. The downscaling principles for high-power wireless charging systems are investigated and discussed.