A Study of Effective Coupling Coefficient and Its Application to Evaluate the WPT Pads

Magnetic coil is a major part of a wireless power transfer (WPT) system, therefore, it is important to evaluate the performance of different coils in the WPT system design, particularly when considering the misalignment. Magnetic coupling coefficient is a generally used metric to evaluate the coils misalignment; however, in multi-coil systems, such as BPP, the coupling coefficient is not enough to describe the coupling condition because there are more than two coils in the system. In this paper, a term called effective coupling coefficient (k
_eff
) is defined based on the calculation of output power of WPT system and the misalignment tolerance of BPP is analyzed by using the effective coupling coefficient concept. Firstly, a definition of effective coupling coefficient is derived based on the circuit analysis, after that, the calculation of k
_eff
when using BPP as primary pad is given. Finally, a comparison of BPP and DD pad (DDP) using k
_eff
is made. By assuming an approximate coupling coefficient curve, the effective coupling coefficient of the two coils varying with misalignment is depicted. The results show that BPP achieves a better misalignment tolerance than DDP by adjusting the current phase of its two branches. And from the comparison, it can be concluded that k
_eff
makes up for the deficiency of k when comparing the misalignment tolerance characteristic of multi-coil pads and single-coil pads.