Frequency Optimization for Inductive Power Transfer Based on AC Resistance Evaluation in Litz-Wire Coil

A coil with a high quality factor $Q$ is desired to obtain a high efficiency for inductive power transfer (IPT). $Q$ is proportional to the coil inductance and operating frequency, while it is inversely proportional to the coil resistance, which increases with frequency. An optimized frequency exists to achieve the maximum efficiency. Eddy currents and resulting ac resistance in Litz-wire coils are attributed to magnetic field. Especially, the induction component of the ac resistance is approximately proportional to the squared magnetic field to which the coil is exposed to. FEA simulations are conducted and surface integral method is employed to determine the squared field. Additionally, the volume integral method is proposed to evaluate the overall effect of the field on the induction resistance. The optimized frequency for the maximum efficiency is obtained, using the squared field calculation and resulting ac resistance evaluation. Sample prototype coils are manufactured to verify the resistance analysis methods. An IPT system is built employing these coils. Experiments show that the IPT system achieves the highest efficiency at frequencies closed to the predicted optimized ones.