Analysis of dynamic characteristics of foreign metal objects under electromagnetic force in high-power wireless power transfer

Because of the noncontact structure of wireless power transfer (WPT) systems, foreign metal objects can easily enter into the coupling region—and often move under the action of electromagnetic force (EMF), instead of staying relatively static, which brings a difficult problem for foreign object detection technology. In this paper, we investigate the motion state of foreign metal objects with different properties under the action of electromagnetic force in the coupling space of WPT system. The equivalent model of the circuit parameters with the intervention of foreign metal objects and the differential equations for the motion of foreign metal objects are derived. Combined with finite-element simulation calculations, the motion characteristic of ferromagnetic and non-ferromagnetic metals under EMF was analyzed. The results show that, due to the size and properties of the metal, non-ferromagnetic foreign metal objects have four states: vibration, suspension, static and flying out. The ferromagnetic foreign metal objects will adsorb on the coil surface and rapidly heat up. By establishing an experimental prototype, the analysis uses high-speed acquisition equipment to obtain the movement of foreign metal objects which verified the correctness of the simulation. This research is also beneficial to the operational safety and reliability of the WPT.