Determination and analysis of compensation capacitor for a robust distance-variable wireless power transfer system

Wireless power transfer (WPT) systems have been widely adopted for full autonomy in various fields due to their convenience. However, changes in the air gap between the Tx and Rx coils significantly affect efficiency. To overcome this challenge, this paper introduces the determination of a compensation capacitor for a distance-variable WPT system that is robust in varying air gap conditions. The proposed method was verified using theoretical analysis, simulation, and experimental measurement. The electrical circuit was modeled using a T-equivalent model in a series–series (SS) topology to calculate power transfer efficiency (PTE). Specifically, compensation capacitors were analyzed at distances of 10, 30, and 50 mm, considering different self-inductance values. These results are compared against varying load resistances to demonstrate the effectiveness of the proposed approach. Additionally, the PTE drop ratio was defined to facilitate comparison. The results show that the PTE drop ratio for the compensation capacitor at the farthest distance was consistently smaller than that for the capacitor at the nearest distance under varying air gaps and load resistances. In this research, the difference in the PTE drop ratio between 10 and 50 mm was measured, demonstrating that determining the capacitor at the farthest distance reduces the PTE drop ratio across a range of operational conditions.