# Research on Three-Phase Wireless Power Transfer System
## 1 Introduction  
With the continuous development of wireless power transfer technology, the wireless power transfer system, as a new type of energy transmission mode, has attracted more and more attention [1]. As a new type of charging method, wireless power transfer technology [2] can transmit electrical energy to electrical equipment without contact and has been widely used in many fields, such as implantable medical equipment, mobile portable terminals, underground exploration facilities, underwater unmanned subernsibles, and electric vehicles [3, 4]. At present, the research on wireless power transfer systems at home and abroad mainly focuses on single-phase-to-single-phase transmission or three-phase to single-phase transmission [5, 6]. However, the single-phase-to-single-phase system has problems of low power and low energy transmission efficiency. With the development of WPT technology, wireless power transfer systems are developing towards large capacity, high power, and high reliability. Therefore, it is necessary to study the three-phase to three-phase wireless power transfer system to improve the efficiency and stability of energy transmission. The power transmission capability of the WPT system is closely related to the size and structure of the emitter and receiver [7]. In the WPT system, the magnetic coupling coefficient of the coil not only affects the power transmission capacity but also directly affects the transmission efficiency of the system. A high magnetic coupling coefficient means better energy transfer efficiency because more magnetic energy is transmitted to the receiving end without being lost in the surrounding environment. Therefore, the design of the coupling mechanism is one of the most critical issues affecting the performance of the wireless power transfer system. Most of the wireless power transfer system coupling mechanisms discussed in the existing literature are composed of single-phase structures with lower loss, similar to the single-phase-to-single-phase wireless power transfer coupling mechanism based on magnetic coupling mentioned in the literature [8]. Professor [PERSON]'s team from Hebei University of Technology proposed aTaiji-type single-phase coil, which can make the magnetic field distribution around the coil more uniform [9]. [PERSON] from the University of Cambridge in the UK designed a single-phase-to-single-phase wireless power transfer coupling mechanism based on multiple coils. The mechanism improves the transmission efficiency and flexibility of the system by reasonably arranging multiple coils [10]. However, these coil structures cannot meet the requirements of high power, and the three-phase system can provide faster and safer high-power transmission when the voltage and current of each bridge arm are small. In addition, compared with the single-phase system, the three-phase system has the advantages of small phase current and more uniform electromagnetic field distribution, so the three-phase wireless power transfer system is considered to be more suitable for high-power applications [11]. In recent years, research on three-phase wireless power transfer systems has been greatly developed in various application fields. At present, the research on three-phase wireless power transfer systems has developed a wireless power transfer system with an asymmetric number of transmitting and receiving coils. For example, the Hong Kong Polytechnic University used three circular transmitting coils in the experiment to enhance the system's power capacity and achieve higher power transmission [12]. The Harbin Institute of Technology team proposed a stable multi-phase receiver including two-phase, three-phase, and four-phase receivers to solve the problem of the induced voltage of the multi-pole power rail coil sinusoidal fluctuation along the driving direction [13]. In Reference 14, a three-phase coil with a double-D quadrature structure was proposed, which effectively increased the anti-translation ability of the system [14].