Capacitive power transfer to car through wheel - is it possible?
Abstract: Introduction: This work aims to evaluate the possibility to transfer electrical power from a metal plate to the steel-cord within a car tire. If the tire stands on the plate, a connecting surface occurs which can be interpreted as a capacitor, where the rubber compound acts as a dielectric. Method: First the impedance of the tire is evaluated: An analytical model is set-up to describe the system; a nite element simulation is made using the FEMM software and nally measurements using a Hewlett Packard 4194A impedance analyzer is made on a Dunlop SP Sport Maxx Tire. Secondly, the possible power transfer is analyzed, both by creating an analytical model including the wheel impedance and a load, and by doing experiments on a real tire. The experiments and simulations are conducted both with and without a resonant inductor. LTspice models are made to support the practical results as far as possible and to investigate the limits for the capacitive power transfer. Finally, a discussion is held regarding which parameters that can be improved to enhance the technique. Results: The impedance measurements show that the impedance coupling has a capacitance of around 200 pF, and an equivalent series resistance which is inversely dependent on the frequency and has a value of 4400 at a frequency of 10 kHz. The LTspice simulations show that the imperfections in the transformer and the inductance that are used in the practical experiments give rise to a lot of losses. This makes the results from the analytical model and the practical experiments dier, especially when a resonant inductance is introduced. For a square wave source voltage of 1600 V and 10 kHz, and a load of 50 k , the analytical model give a load power of 20.1 W with an efficiency of 89 %. The practical experiments give a load power of 19.5 W with an efficiency of 81 % when a DC-link voltage of 30 V which results in a voltage about 1600 V on the secondary side of the transformer - is used. For the same source voltage and load conditions, but with a resonant inductor introduced, the simulations give a load power of 34 W with an efficiency of 89 %. The practical experiments give a load power of 25 W with an efficiency of 76% (33 V out from the DC link). Conclusion: This thesis shows that the impedance feature of the tire that is investigated makes it inappropriate for a big electrical power transfer. The capacitive element of the impedance can be compensated for with a resonant inductor, but the resistive part of the tire impedance is much too big to pass a lot of electrical energy. With a dierent rubber compound though, that would have a smaller dielectric loss angle, the technique could be worth evaluating more. For example, a tire with silica filler instead of carbon black filler - which is used in the tire of this project - could be worth analyzing.
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