Design of Static Wireless Charging System for Electric Vehicles with Focus on Magnetic Coupling and Emissions

Static wireless charging using resonant inductive principle offers environmental friendly, comfortable and automatic charging solution for electric vehicles. This technology as of now is nascent with few products on the market and leading companies and universities actively engaged in research. On similar lines, a pioneer PhD project was undertaken in 2012 at Department of Energy Technology, Aalborg University with objectives of improving system design and developing high power density (weight and dimensions), low cost and low magnetic emissions power inductors for this application. This thesis summarizes the research findings of the study. Wireless charging system as per state of art design approach consists of four major blocks: primary power electronics, inductors, secondary power electronics including load and resonant circuits (capacitors). The first contribution of this project is addition of fifth block named reflected quality factor in the system design. The fifth block similar to the resonant circuits block is dependent on the other blocks but its addition is highly beneficial in understanding and simplification of the system design in two main ways. Firstly, the system can be represented as an equivalent power source and transmission system including the load similar to other electric system like grids. Secondly, design parameters of output power, circuit efficiency and voltage or current stress across resonant components can be expressed as simple functions of the five blocks. Inductors of wireless charging systems mostly consist of the coils and passive shielding materials and are different from inductors used in other industrial and consumer applications as majority of magnetic flux passes through air. The first objective in the inductor design is to explain effect of the shielding materials on the magnetic field path. The objective serves as base for the second objective dealing with minimization of passive shielding usage with respect to power transfer capability, weight, dimensions. There have been two major results obtained during the optimization process. The first set of results show that additional ferrite should be added in center of the inductors above base ferrite as this provides maximum increase in the power transfer per unit weight added. In the second investigation, it is shown that reducing passive shielding (ferrite and aluminum) thickness reflects comparatively lower decrease in the power transfer and efficiency in comparison to high reduction achieved in weight and dimensions of the inductors. Additionally, the same comparative analysis has been shown to be true when commonly used high ferrite grade is replaced by comparatively lower and cheaper ferrite grade. The last objective of this project has been minimization of the magnetic emissions. For this, a semianalytical method has been proposed for calculating ratio of the magnetic emissions at different values of the coil currents for given inductor setup. This method will help in including the emissions as a design parameter for the primary power electronics, secondary power electronics with load and capacitors in addition to the inductor design. For development of the analytical method, space variation of the magnetic emissions is studied first in the project and results show that ratio of secondary coil emission to primary coil emission is constant in the surroundings. This is utilized in introducing the analytical method and three applications have been addressed during the project. In the first application, it is shown that higher load quality factor is favorable for given inverter current and switching frequency as it provides comparatively lower increase in the emissions compared to the output power. In the second application, a novel active shielding method of generating cancellation current in the secondary coil without using additional third coil has been proposed. This is implemented by designing the secondary capacitor bigger than its resonant value and making the secondary circuit inductive. On the negative side, the reduced emissions require higher inverter current and bigger primary capacitor to deliver the same output power. At last, two resonant topologies series-series and series-parallel are compared in term of the emissions for similar power rating. Series-parallel topology has slight advantage over its series-series counterpart on account of additional inductive secondary current component as advised by the results. At the end, a wireless charging system has been designed and constructed as part of the project. The setup delivers output power of approximately 2 kW and 1.2 kW for vertical distance of 10 cm and 20 cm respectively. Measured resonant circuit efficiencies (primary inverter AC terminals to secondary rectifier AC terminals) for the two cases are 89% and 82% respectively. The setup has capability to deliver much higher output power subject to availability of higher current rating input power source.