Accurate finite-element modeling and experimental verification of inductive power transfer coil design

Inductive Power Transfer (IPT) is a promising technology for the charging of the traction batteries of electric and hybrid electric vehicles due to the significant simplification of the charging process implied by the contactless transmission of the charging energy. Typically, Finite-Element (FE) methods are used to predict the power loss in the IPT coils, to calculate equivalent circuit parameters, and to estimate the magnetic stray field of IPT coils. This paper gives insight into the FE modeling of the IPT coils of a 5kW prototype IPT system with a dc-to-dc efficiency of more than 96% (52mm air gap, coil diameter 210 mm). In the paper, modeling of IPT coils in the frequency domain using the FE tools FEMM and Ansys Maxwell is discussed and the calculation methods to predict power loss, equivalent circuit values, and stray fields are presented. The main part of the paper is a comprehensive experimental verification of the obtained FE results, which includes dc-to-dc conversion power loss, circuit parameter measurements, and measurements of the stray field. For the verification of the calculated stray fields the design of a highly compact, high-bandwidth, low-cost magnetic field probe is presented. Experimental data obtained from the designed field probe shows that the FE tools used in the design of the IPT coils deliver field data with a calculation error of less than 5%.