New characteristics analysis considering transmission distance and load variation in wireless power transfer via magnetic resonant coupling

Wireless power transfer (WPT) via magnetic resonant coupling has been attracting research attention for various applications. Conventionally, load is assumed to be constant and only transfer efficiency is studied. In actual WPT applications, the load and transfer distance change frequently. Furthermore, information such as ratio of input voltage to output voltage, ratio of input current to output current, and input impedance are needed for understanding and constructing the power transfer system. In this paper, not only the transfer efficiency but also the three parameters mentioned are studied. These parameters are then analyzed for changing load and changing transfer distance conditions using actual antennas' parameters. From the analysis results, the load resistance value for maximum efficiency exists. Secondly, improving efficiency by changing load resistance for small mutual inductance case has larger effect. The optimum load resistance also changes according to transmission distance and also the consumed power peak may not correspond to maximum efficiency. Finally, fault protection may also be necessary for the cases when the load resistance is extremely high and when the receiver antenna is not present causing high supply current.