# A Method to Reduce Eddy Current Loss of Underwater Wireless Power Transmission by Current Control
## 1 Introduction  
Wireless power transmission (WPT) technology through magnetic resonant has been widely studied in recent years because of the number of advantages it has compared with traditional power delivery, such as the convenience of its nonphysical connection between the power supply and the load [1; 2; 3; 4]. Especially in the underwater environment, WPT technology solves the problem of wet plugging and the unplugging of underwater energy supply and provides a safer method for the rapid charging of autonomous underwater vehicles (AUVs) [5; 6; 7; 8] and unmanned underwater vehicles (UUVs). Therefore, in view of the widespread application of underwater WPT technology, it is of great research significance to improve its energy transmission efficiency. The WPT system in the air uses one or more pairs of induction coils. The alternating current (AC) of the primary-side coil generates the induced alternating current (AC) in the coupled secondary-side coil, and then the direct current (DC) is generated through rectification to complete the power supply to the equipment. The process of underwater WPT is similar to that in the air, but the medium in the transmission area changes from air to seawater. Therefore, the coupling characteristics between the coils have changed, and the changed magnetic field will generate eddy current in the seawater, which brings new energy loss, namely, eddy current loss. [PERSON] et al. [9] analyzed various losses in underwater WPT systems, including copper loss, semiconductor loss, core loss, and eddy current loss by using circuit analysis and electromagnetic field (EMF) simulation. This article studies the method of reducing eddy current loss to improve the energy transmission efficiency of underwater WPT systems. In order to reduce the eddy current loss of underwater WPT systems, several researchers have studied it. [PERSON] et al. [10] analyzed some factors that affect the eddy current loss of underwater charging systems, and used EMF simulation software to optimize the number of coil turns on the primary side and the secondary side and the frequency of the AC source, thereby reducing the eddy current loss, and also designed experiments to verify optimization results. [PERSON] et al. [11] presented a new coil structure, taking advantage of two primary-side coils placed symmetrically adjacent to each side of the secondary-side coil to reduce the eddy current loss of WPT systems for underwater vehicles. Additionally, their experimental results show that the method improves the energy transmission efficiency of the system significantly by nearly 10%. [PERSON] et al. [12] established the analytical model of the eddy current loss of the coreless WPT system in seawater. Additionally, the expression of the eddy current loss was deduced. They analyzed the eddy current loss under different conditions and concluded that the effect of eddy current loss in seawater would shift the optimal resonance frequency of the system compared to in the air. Moreover, they calculated the optimal resonance frequency according to the analytical model so as to reduce the eddy current loss and improve the energy transmission efficiency of the system. [PERSON] et al. [13] used Z-parameters to analyze the characteristics of eddy current loss and power transfer efficiency by considering the operating frequency and conductivity of the medium. [PERSON] et al. [14] presented an eddy current loss analysis method in an underwater WPT system and built a coil mutual inductance model with the eddy current loss. The finite simulation and experiment they have conducted proved the effectiveness of the model and method. [PERSON] et al. [15] established an electromagnetic field model of the underwater coil based on the non-axisymmetric model, and developed an analysis method for eddy current loss. [PERSON] et al. [16] analyzed the eddy current loss in seawater under different misalignments and frequencies.