AC Loss Behavior of Wireless Power Transfer Coils

Wireless power transfer (WPT) systems, based on near field magnetic induction, typically operate in the frequency range of a few hundreds of $\mathbf{kHz}$. Although, most commercially available transmission coils are made of high frequency litz wire to suppress skin and proximity effects, the power losses under AC conditions are significantly bigger than under DC conditions. In the design process of a WPT system it is essential to know the power losses and the resulting self heating of the transmission coils under operating conditions. In this paper we demonstrate the analytic calculation of power losses and we present a new setup to measure the self heating of transmission coils under AC conditions with large current amplitudes. We compare the results to measurements in the small signal regime and we evaluate the frequency dependent derating of the rated current over a broad spectrum of coils. Finally, we investigate how the choice of coil geometry and litz wire influence the AC loss.