Frequency optimization of a loosely coupled underwater wireless power transfer system considering eddy current loss

Wireless power transfer (WPT) has attracted much attention in recent years. In an underwater WPT system, the eddy current loss tends to be non-negligible as the frequency or the coil current increases. Thus, it is crucial to analyze the eddy current loss in an underwater WPT system. The analytical model of the eddy current loss of a coreless WPT system in the seawater is established with Maxwell's equations. The expressions of the electric field intensity and the eddy current loss are derived. The eddy current loss is analyzed in different circumstances to illustrate the impacts of related factors. For a WPT system in the air, there is an optimum resonant frequency, for a higher frequency leads to a larger induced voltage, but will result in larger coil losses simultaneously. However, the optimum resonant frequency will be shifted because of the eddy current loss in the seawater. Then, the optimum operating frequency is obtained based on the analytical model. It is found that the optimum operating frequency is supposed to be larger than the resonant frequency to achieve the maximum dc-dc efficiency in the seawater. An underwater WPT prototype was built and the experimental results verified the theoretical analysis.