Numerical Calculations of Wireless Power Transfer Coil Parameters

Abstract: Wireless Power Transfer (WPT) has become a commercially viable technology. The technology uses the phenomenon of electromagnetic induction between coils to transfer electrical power wirelessly. Due to the increasing interest in WPT, the need to understand how the power transfer between coils behaves in different settings, as well as the ability to simulate the process, have become increasingly important for further development of the technology. This thesis investigates some of the existing methods for calculating the mutual inductance between coils. For arbitrarily wound coils the Neumann formula is used. And for taking the presence of ferrite plates into account an expression derived by Hurley and Duffy is used. The methods are implemented in Matlab [1] and compared to physical measurements and full-wave electromagnetic simulations made in the commercial software Ansys Maxwell [2]. The thesis focuses on 2 aspects: 1) The extent to which mutual inductance is affected by the presence of ferrites of varying size. The thesis shows that previously stated finite ferrite margins for expressions derived using the assumption of infinite ferrite plates in a magnetostatic setting, do not seem to hold for a simple two-coil system, and new margins are obtained. 2) The possibility of homogenizing the magnetic field for increased spatial coupling, using a multicoil array. The thesis shows that it is possible to increase the field homogeneity of an initial setup, at the cost of the field strength. It also finds that for a tri-coil system with coil centers forming an equilateral triangle, the optimal separation distance of the coil-centers in terms of providing a homogeneous field, is between 30% to 60% larger than the outer coil radius.