Analytical Optimization of a Litz Wire Spiral Coil Based Underwater IPT System

Inductive power transfer (IPT) is an important technology in the design of roadway and underwater vehicle charging systems. For underwater IPT (UWIPT) systems, especially for the ones with small coils and higher operating frequencies, it is important to get estimate of the seawater eddy current losses, which exponentially increase with coil size and frequency. This paper proposes an analytical approach to achieve it in terms of the coil eddy loss resistance for the widely used spiral coils made of litz wire. It also presents a method to optimize a UWIPT system using the analytical equations, so that time-consuming optimization through electromagnetic simulation can be avoided. Comparison between calculated, finite element simulation and experimental results of eddy loss resistance is used to validate the analysis. The optimization is presented through the example of a 1 kW system capable of achieving 96% magnetic coupling efficiency with 8 mm seawater gap. The design is validated through experiments with a dc voltage-input 1 kW IPT system, with 48 V input and 48 V output; achieving a magnetic coupling efficiency of 96.4%.