Null-Coupled Electromagnetic Field Canceling Coil for Wireless Power Transfer System

In this paper, we present a kind of null-coupled canceling (NCC) coil to shield electromagnetic fields (EMFs) for wireless power transfer (WPT) systems. The proposed NCC coil is based on the particular zero-mutual-inductance characteristic between two general coils. The working principles, shielding effectiveness, and practical expansions of the proposed NCC coil are illustrated. Furthermore, locating of the zero-mutualinductance position and determination of the canceling coil current are elaborated, and effects that an NCC coil exerts on the output power, efficiency, and misalignment tolerance of the applied WPT system are theoretically studied. Finally, targeted at our predesigned electric vehicle wireless charging-oriented WPT prototype, a practical NCC coil is built and tested with comparative experiments with the prevalent aluminum (Al) plate shielding. The experimental results show that, with a properly chosen canceling current, the proposed NCC coil could not only shield the EMF more effectively, but also cause a lower efficiency reduction to the applied WPT system than an Al-plate does.