Electromagnetic Field Modeling for the Charging Pads of a 50 kW IPT System and Mechanical Design for the Pad's Casings

Abstract

Wireless charging technology, in particular, Inductive Power Transfer (IPT), has been evolving during the last decades, and it’s starting to become attractive for electric mobility applications. The working principle of the IPT technology is based on two coils separated by an air gap and magnetically coupled together; since the medium to transfer the energy is the air between the coils, the magnetic circuit of this system becomes remarkably important, as the magnetic flux need to be constrained as much as possible for the sake of securing the magnetic linkage between the coils. In this thesis, the design process of the casing of the coils is presented, such casing will ensure that the coils are always kept fixed according to their design.

From this point, the first goal of this thesis arises: Design an accurate and modular casing for the coils of a 50 kW IPT system that will fix such elements in the correct position, and which will allow for easy
changes/upgrades in the coil if required. The design process of such casing was to first do the CAD model of the coils, and then do the CAD model of the casing, by considering the following criteria: accurate dimensions, easy and fast to manufacture and assemble, modular-designed, light, adequately cooled, and robust. The result of such design was a casing that was actually manufactured and assembled, and that is currently being used to test a 50 kW IPT prototype system.

Besides procuring an efficient coil design for the IPT systems, it’s important to understand the dynamic behavior of the IPT electric system. One of the elements that could potentially influence the electric behavior of such system is known as parasitic capacitance. From here, the second objective of this thesis arises: Derive the parasitic capacitance of the coils in order to perform a better circuit modeling and to predict a possible over-voltage across such capacitance.

The methodology to derive the parasitic capacitance of the coils was first to perform an electromagnetic fields FEM simulation on the coils in order to obtain a set of parameters such as the coils impedance, resonance frequency and inductance. With these parameters and a set of equations, it was possible then to derive the parasitic capacitance of each coil. Finally, with this information, it was possible to build a (virtual) equivalent circuit of the IPT system and evaluate the voltage across the parasitic capacitance. It was concluded from this thesis that the parasitic capacitance at the standard operating frequency (85 kHz) does not represent a risk of over-voltage, but as the frequency goes close to the resonance frequency (1.46 MHz), the electric field across the capacitor could reach values up to 14 kV/mm.