This letter proposes a rotation-free wireless power transfer (WPT) system based on a new coil structure to achieve stable output power and efficiency against rotational misalignments for charging autonomous underwater vehicles. The new coil structure has two decoupled receivers composed of two reversely wound receiver coils and the magnetic flux directions of the two receivers are perpendicular to each other, guaranteeing a relatively constant total mutual inductance and a decoupled characteristic under rotational misalignments. The proposed coil structure is verified via finite element analysis based on ANSYS Maxwell. A rotation-free LCC-LCC compensated WPT prototype is built and the experimental results verify the theoretical analysis and simulations. The system can deliver 664 W with a dc-dc efficiency of 92.26% under the best case and 485 W with a 92.10% dc-dc efficiency under the worst case.

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, 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 for 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.

Due to the conductivity of the seawater, the traditional mutual inductance circuit model in the air cannot be used directly to describe wireless power transfer (WPT) systems in seawater applications. This paper proposes a modified mutual inductance circuit model of an underwater WPT system to analyze the eddy current loss (ECL) and the detuning effect caused by the seawater. The time-harmonic electromagnetic field in the seawater and the air near the coil that carries a sinusoidal alternating current is analyzed. The root-mean-square (rms) value and phase angle of the induced voltage on the secondary coil can be obtained by the integral of the electric field intensity along the coil path. By introducing the equivalent ECL impedance at both the primary and secondary sides, a modified mutual inductance circuit model of an underwater WPT system was obtained. Through adding a compensation inductance to the primary circuit, the detuned system in the seawater is turned back to be resonant at the same frequency as in the air. A seawater WPT prototype was built and the experimental results verified the theoretical analysis.

In the underwater environment, the ocean current will have a great influence on the anti-misalignment performance of the wireless power transfer (WPT) system for the autonomous underwater vehicles (AUVs). In this letter, a free-rotation WPT system with a new magnetic coupler for AUVs is proposed to improve the rotational and axial misalignment tolerance. The magnetic coupler has two decoupled transmitters and one segmented arc solenoid receiver with reversely wound adjacent receiver coils. The mutual inductances between the receiver and the two transmitters can compensate each other. Moreover, cooperated with the phase control between the two transmitters, the system can achieve more stable output power under the rotational and axial misalignment. A free-rotation WPT prototype was set up and the experimental results showed that the output power can reach 700 W and the output power fluctuation is below 5% based on the proposed anti-misalignment method.

This paper presents a rotation-resilient wireless charging system with a reversely wound receiver that supports efficient charging for lightweight autonomous underwater vehicles. By employing a two-part reversely wound receiver coil structure, the total mutual inductance of the proposed coil structure remains relatively constant when rotational misalignment between the transmitter and receiver coils occurs, thereby ensuring wireless charging power is nearly continuous during rotational misalignment. Finite element analysis was performed with ANSYS Maxwell to verify the proposed coil structure. Double-sided LCC compensation topology was applied and circuit analysis was conducted via a mesh current method. A prototype of the rotational-resilient wireless charging system with the proposed coil structure was built and tested. Experimental results are well-matched to simulations, demonstrating that the system can deliver 745 Watts at a DC-DC efficiency of 86.19% when the system is fully aligned, and efficient under worst-case rotational misalignment.

In some applications of Wireless Power Transfer (WPT) technology, such as wireless charging of consumer electronics and electric buses, the coupling between the transmitter and the receiver is strong in order to improve efficiency. With a strong coupling, the compensation on the receiver side can be eliminated to achieve a high-level integration of the receiver device and a smaller receiver-side loss, forming a Series-None (SN) topology, which has not been investigated before. The commonly used first harmonic approximation method for the SN topology has discrepancies with a low-power output and cannot fully reveal the characteristics of the SN topology. Therefore, this paper establishes a model of the SN topology and derives the analytical expressions of the transmitter and receiver currents. The distortion of the receiver current is studied. The coil-to-coil efficiency is obtained and its maximum condition is investigated. The SN topology offers approximately the same efficiency as the SS topology when the coupling is strong and the coil quality factors are large. Theoretical calculations and experimental results confirm the analysis.