Research on Mutual Inductance Identification and Efficiency Optimization of the Three-Coil Wireless Power Transfer Systems With Switchable Relay Coil

One of the most difficult problems of the three-coil wireless power transfer (WPT) systems is that the receiving coil is hardly fully aligned with the transmitting and relay coils, which affects the efficiency of the practical system seriously as the coupling exists among the transmitting, relay and receiving coils simultaneously. Therefore, this paper presents a new mutual inductance identification and efficiency optimization method for the three-coil WPT system with a switchable relay coil. Firstly, the mathematical model of the three-coil WPT system with and without the relay coil is constructed. Secondly, the mutual inductance identification method is proposed to analyze the mutual inductances among the three coils by the switchable relay coil, which does not need complicated mathematical calculation strategies and the operating frequency of the system can be held constant. Then the coil-to-coil efficiency of the system is compared with and without the relay coil when the load resistance or position varies. Finally, a coil-to-coil efficiency improvement method is proposed to ensure that the system can always operate under high-efficiency conditions according to the identified mutual inductances of the three-coil WPT system.