Frequency control, modeling, alignment adaptation, and safety concerns for the wireless charging of electric vehicles
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Wireless power transfer (WPT) is a promising way to reduce the inconvenience of electric vehicle (EV) charging, which has been considered one of the major issues hindering widespread adoption of EVs. This dissertation includes four parts aiming to solve key issues associated with control, modeling, misalignment and safety concerns of wireless EV charging. Part 1 of this dissertation addresses the output voltage variation and efficiency drop caused by misalignment; a uniform voltage gain control for a series–parallel resonant converter is implemented to improve the alignment robustness of wireless chargers. The uniform gain control is achieved by analyzing the voltage gain and impedance across the frequency domain, and an embedded program is developed to tune the switching frequency for a stable output voltage based on phase angle feedback of the primary inverter. Experimental results show that the efficiency of fixed frequency control dropped from 82% to 34% when the misalignment was changed from 0 to 200 mm, while the efficiency of uniform gain control dropped from 81% to 60%. Part 2 of this dissertation describes modeling a series–series topology WPT system for maximum power transmission efficiency. A frequency-spacing model is proposed to calculate the unity-gain switching frequency when the coil geometry and displacements are known. The experimental results show that the unity-gain frequency can be predicted with varying loads and air gaps (75mm to 250mm), and a demonstrated unity-gain error of <6%. Part 3 of this dissertation introduces a parking alignment system for wireless EV chargers. The alignment detection system is low cost because it uses the existing charging facility and only four auxiliary coils to determine the direction and magnitude of required parking adjustments. An alignment system for wireless EV chargers should be a fundamental approach to solve the misalignment issues in EV wireless charging by making it easy for the EV driver to park properly. The coil positioning accuracy of the system is <3cm. Part 4 of the dissertation investigates safety concerns regarding exposure to strong electromagnetic (EM) fields present during WPT. The body of work examines that a metal debris placed on the primary pad have a temperature rise of 81℃ under 3.1kW WPT, and the EM field between the two charging pads can reach 1.656 mT, which largely exceeds the safety guideline imposed by the ICNIRP standard. The methodology of this work can provide a fundamental theory that allows for efficient, robust, and safe wireless EV charging.