# A Method to Reduce Eddy Current Loss of Underwater Wireless Power Transmission by Current Control
## 4 Electromagnetic Field Simulation Analysis and Discussion
### Change the Current Modulus Ratio  
According to Equation (45), for the underwater WPT systems with different resonant frequencies, there is an optimal current modulus ratio \(\alpha_{0}\) between the primary-side coil and the secondary-side coil, which can reduce the eddy current loss of the system and improve the energy transmission efficiency. In order to verify this conclusion, we kept the primary side's compensation structure unchanged, changed the parameters of the compensation components on the secondary side to control the current modulus ratio in the two coils, and then adjusted the input voltage to make the output current on the equivalent load resistance equal to 1A. The electric field intensity distribution and energy loss density distribution when  
Figure 12: Electric field intensity, energy loss density, and AC-AC efficiency under different modulus ratios when f = 200 kHz, \(\alpha_{0}\) = 1.1068. (**a**) \(\alpha/\alpha_{0}\) = 0.60, \(\eta\) = 0.7837; (**b**) \(\alpha/\alpha_{0}\) = 0.99, \(\eta\) = 0.8343; (**c**) \(\alpha/\alpha_{0}\) = 1.33, \(\eta\) = 0.8114.  
Figure 13: Electric field intensity, energy loss density, and AC-AC efficiency under different modulus ratios when f = 300 kHz, \(\alpha_{0}\) = 1.1103. (**a**) \(\alpha/\alpha_{0}\) = 0.65, \(\eta\) = 0.7563; (**b**) \(\alpha/\alpha_{0}\) = 1.01, \(\eta\) = 0.7832; (**c**) \(\alpha/\alpha_{0}\) = 1.33, \(\eta\) = 0.8114.  
In Figures 12 and 13, \(\alpha_{0}\) represents the optimal modulus ratio calculated according to Equations (43) and (45), and \(\alpha\) represents the modulus ratio of the current through the two coils in the simulation software. Additionally, it can be obtained that the energy transmission efficiency of the system changes with the dimensionless quantity \(\alpha/\alpha_{0}\), as shown in the Figure 14 below.  
It can be seen from the simulation results that as the dimensionless coefficient \(\alpha/\alpha_{0}\) deviates from 1, the asymmetry of the electric field excited by the two coils increases, and the energy loss in the area between the two coils also increases, that is, the eddy current loss increases. According to Figure 14, it can be seen that at the resonant frequencies of 200 kHz and 300 kHz, when the current modulus ratio of the two coils is equal to the optimal modulus ratio calculated by Equation (45), the energy transmission efficiency of the system is the highest, and is increased by 5.06% and 3.93%, respectively, compared with the minimum efficiency at the two frequencies. At this time, the field strength distribution around the primary and secondary side coils is evenly distributed, and the field strength maximum value in the adjacent space is small, so the energy loss due to eddy current loss is reduced, which improves the energy transmission efficiency of the system. Therefore, for a two-coil underwater wireless energy transmission system, when the modulus ratio of the current in the primary and secondary coils is controlled to ensure that it is the optimal modulus ratio, the eddy current loss of the system can be effectively reduced, and the energy transmission efficiency of the system can be improved.