Methods to Reduce Air-Gap Center Region Magnetic and Electric Fields for Large Gap Inductive Wireless Power Transfer Systems

Active, passive, and reactive designs have been developed to reduce the magnetic leakage field. Magnetic shields have been widely used to shield the magnetic field below the transmitter and above the receiver. However, less attention is paid to the effects on the air-gap center region magnetic and electric field distributions, which are often far above safety limits and pose potential threats to human beings and animals. In this paper, the theoretical maximum transferrable power within given magnetic and electric field safety limits is derived for the air-core large gap inductive wireless power transfer system, and the physical limitations are identified. Then, the drawbacks of conventional magnetic shielding structure are presented. Phase manipulation and flux shaping methods are developed to reduce the air-gap center region magnetic field. In particular, an “I” type shielding structure is proposed to simultaneously shape the magnetic flux path to effectively reduce the air-gap center region magnetic field without degrading the coil-to-coil efficiency, and reduce the air-gap electric field by confining the electric field caused by winding excitation within the shielding structure. The proposed methods are evaluated by a 3 kW, 30 cm transfer distance design example using FEA.