Coupling factor Estimation for Wireless Charging

MSc thesis of Hongjie Song

With the popularity of electrical vehicle (EV), the demand of EV charging is also increased. Compared to traditional wired charging, wireless charging can avoid many safety issues. The efficiency of wireless power transfer (WPT) system is determined by many factors, coupling factor is one of the most important factors. Therefore, estimating the coupling factor of WPT system is necessary. The research objectives of this work are to review and benchmark different methodology of coupling factor estimation. Besides, a comprehensive method is selected and simulated.

This work is focus on magnetic inductive WPT system. Five different methodologies of coupling factor estimation for static WPT system and one methodology for dynamic WPT system is reviewed and analyzed. The methodology that uses alternative capacitors to estimate coupling factor of static WPT system is analyzed in detail. By using the zero crossing unit, the primary side and secondary side of WPT system is under full resonant. The WPT system will work in two different modes by changing the primary side capacitance. The two sets of circuit parameters will be recorded and calculated by a PLL unit. Based on theoretical analysis and the calculated parameter, the load and mutual inductance of WPT system can be identified. Therefore, the value of coupling factor can be determined. The accuracy of coupling factor estimation of this methodology is higher than 97 %.

Compared to the literature that proposed this methodology, this work made a sensitivity study. The circuit parameters are made to fluctuate in a certain range. The accuracy of estimation of mutual inductance are still higher than 95 % when the primary side resistance, secondary side resistance and load fluctuate. However, the system become unstable when the primary side capacitance, inductance and secondary side inductance fluctuate. The reason is that when the capacitance and inductance change, the operating frequency will deviate from the resonant frequency. However, the mutual inductance identification unit is not able to detect this deviation. If the deviation of circuit parameters is updated, the accuracy of coupling factor estimation will remain about 97 %.