Magnetic Characterization of Unsymmetrical Coil Pairs Using Archimedean Spirals for Wider Misalignment Tolerance in IPT Systems

Resonant inductive power transfer (RIPT) is gaining popularity for wireless charging applications of future electric transportation. A fundamental challenge of implementing an RIPT system for wireless power transfer is the coupling variation between the primary and secondary coils, due to misalignment, while parking the car over the primary coil. Thus, it is desirable to have a coil pair, which is least sensitive to misalignments, to ensure efficient power transfer over a wide operating area, and to enable ease of use. The operating area of a coil pair is limited by the existence of a magnetic null in its coupling profile. For circular charging pads, employing a symmetrical Archimedean spiral, a magnetic null occurs at a misalignment distance equal to approximately 40% of the pad diameter (or about 70% of the coil diameter), irrespective of the air gap. In this paper, five diverse unsymmetrical coil pairs have been investigated, to find the coil pair least sensitive to misalignments. For unsymmetrical coil pairs, the position of the magnetic null shifts with the change in separation between the coils. By operating the primary and secondary pads at a distinct separation, the null position can be shifted further away from the center of the coils. This, in turn, increases the effective area, over which a given amount of power can be transferred to the load. After investigating the unsymmetrical coil pairs, it was found that the outer radius of the secondary coil should be kept equal to the outer radius of the primary coil, and inner radius of the secondary coil should be maintained greater than the inner radius of the primary coil, to obtain the best coupling profile.