An Efficiency Improvement Method for the Small Air Gap Wireless Power Transfer System with Variable Parameters

The magnetic coupling wireless power transfer (MC-WPT) system for automatic guided vehicles is characterized by a small and unfixed air gap, which leads to significant changes in self- and mutual inductance of the magnetic coupler. In this article, an efficiency improvement method is proposed for the MC-WPT system with variable self- and mutual inductance parameters. By optimizing the design of magnetic couplers and compensation parameters, a wide range of zero-voltage switching (ZVS) operation and minimization of the reactive current are achieved. The self-inductance variation is utilized to provide a variable system input impedance, which expands the ZVS operation range with phase-shift modulation. Meanwhile, the reactive current of the resonant tank in the entire air gap range can be reduced to the minimum when the tuning parameters are designed properly. Compared with the traditional hybrid control strategies, the proposed method offers a simpler control scheme and eliminates the adverse effects caused by the self-inductance variation. A laboratory prototype is built to verify the theoretical analysis. The experimental results show that the system achieves ZVS operation and minimum reactive current within an air gap range of 20–50 mm, and efficiency reaches 91.2%.