On the nature of the electromagnetic field in the immediate vicinity of inductive power transfer system

Inductive power transfer (IPT) is a subset of contactless power transfer appropriate for high-power applications such as the charging of electric vehicle batteries. Previous work has considered primarily the magnetostatic far field, that is, the region well away from the couplers in terms of coupler dimensions, but still close to the couplers in terms of the wavelength of the radiated field. The nature of the electromagnetic field in the high-flux region in between and in the immediate vicinity of the couplers is complex and merits close examination. Because of the paramount importance of electrical efficiency, it is necessary to tune an IPT system for maximum efficiency, as opposed to tuning for minimum extraneous field. This causes the primary and secondary currents to be 90° out of phase with one another. Therefore, the fields can be thought of as being due to two displaced solenoids or magnetic dipoles operating in phase quadrature. The resultant electromagnetic field exhibits time-domain rotation of field direction. The rotating electromagnetic field is not strictly elliptical polarization in the traditional sense as it is not a propagating wave but rather a superposition of two quasi-static magnetic field distributions in phase quadrature. Because of the time-changing direction of the magnetic field, isotropic magnetic field probes employing rectification or detection cannot be used to accurately characterize such a system. Finally, coupling into devices inadvertently or intentionally introduced into the high field region is quite complicated necessitating numerical simulation.