Research on Characteristics of Wireless Power Transfer System Based on U-Type Coupling Mechanism

Aiming at the power supply problem of high-speed rotating equipment, a wireless power transfer system based on U-shaped core coupling mechanismwith a primary coil is proposed. Firstly, the transfer model of the U-type coupling mechanismwireless power transfer system is established.*e expressions of transfer power and transfer efficiency are obtained by theoretical calculation, and the factors affecting the transfer characteristics of the system are analyzed. At the same time, the magnetic field distribution of the system and the coupling parameters change when the relative position of the primary and secondary coils changes through simulation analysis. Finally, an experimental platform is established for experimental verification.*e results show that the system can obtain 1.72w output power with 51.19% transfer efficiency when the distance between the secondary coil and U-core is 15mm and 30mm, respectively. *e transmission efficiency and power of the primary coil and secondary coil under different misalignments are tested and compared. It is proved that the wireless power transfer system based on U-type couplingmechanism can effectively realize the stable power supply of the rotating equipment monitoring system.


Introduction
As a new type of power transmission mode, wireless power transfer (WPT) technology has been widely developed in different working environment and application fields with the deepening of its research. In particular, dynamic WPT technology provides wireless power for relative motion equipment, which effectively solves the problems of friction loss, contact arc, and inconvenient installation. WPT technology is an effective and feasible power supply scheme to realize safe, long-term, and stable operation of relative moving equipment [1][2][3].
For the dynamic wireless power supply of relative moving equipment, the most widely used fields are the dynamic wireless charging of electric vehicles and rail trains and the wireless power supply of rotating equipment. In 2015, Korea Academy of Science and Technology proposed an I-type structure bipolar core track and S-type bipolar core track for wireless power supply of trams. I-core track has the advantages of narrow width, large air gap, higher output power, and low construction cost. e U-core track is smaller than the I-type track, with the maximum efficiency reaching 91%, and the power is 9.5 kW [4,5]. In 2016, Seoul University of South Korea proposed a WPT system with SStype coupling structure and achieved 300 kW power transfer with 96% efficiency under 7 cm air gap through the built experimental platform [6]. In 2016, German Fraunhofer Institute of Integrated Systems and Device Technology proposed an inductive wireless power supply technology for rotating equipment.
e system consists of three parts: bearing, rotating shaft, and rotary transformer and sensor load fixed on the shaft. It is small in size and easy to install. It can realize 20 W wireless power transmission under 89.7% transfer efficiency [7]. In 2017, Tianjin University of Technology proposed an asymmetric coupling wireless power supply system with single rectangular coil at the transmitter and multisquare coil at the receiving end. A 10 : 1 reduced experimental model was established, and it was verified that the transfer efficiency of the system could reach 91.4% and the maximum load power was 139 W under different coil offset degrees when the transfer distance was 5 cm [8]. In 2018, Harbin Institute of Technology proposed a wireless power supply system for rotating equipment. e system uses a solenoid nested coupling mechanism with a skeleton magnetic core. rough the experimental system, the 3 kW power level wireless power transfer is realized with 87.1% transmission efficiency under 1 cm gap [9]. In 2019, the Institute of Electrical Engineering of Chinese Academy of Sciences designed an omnidirectional three-dimensional receiving resonator in view of the situation that the load end will have angle deflection and offset under special environment in 2019. It can realize all-angle, multiattitude, and high-power wireless power transmission.
rough the simulation experiment, when the system offset distance is within the radius, the load power attenuation is less than 25%, which proves that the designed three-dimensional resonance device has a certain antioffset ability [10]. In 2020, Auckland University of New Zealand proposed an inductive WPT system based on active boost bridge inverter for wireless charging of electric vehicles. Compared with the traditional full-bridge system, the system does not need additional switching equipment, reduces switching loss, and can track the maximum efficiency under a wide range of working conditions. e feasibility of the proposed system is verified by the developed 3.5 kW experimental prototype [11]. In 2020, in order to solve the problem of wireless power supply for equipment on rotating shaft, University of Science and Technology Beijing designed a high-frequency induction power supply system with the primary coil composed of aluminum ring and secondary coil made of PCB. e eddy current loss caused by the central drive shaft of the coil was studied and analyzed. e system has been applied stably in the field with good operation effect [12,13]. In 2018, the University of Malaya designed a 1 kW WPT system, which can transmit 1 kW of power with 90.5% transmission efficiency and can still guarantee 72% transmission efficiency under the condition of 40% misalignments [14]. In 2021, the University of Malaya proposed a three-coil WPTsystem with S-S-LCLCC compensation structure. When the load resistance of the system is 222 Ω, the power transmission efficiency of the designed system is 10% higher than that of the traditional WPT system [15]. e important experimental parameters of the above research results are listed and compared as shown in Table 1.
Aiming at the problem of wireless power supply for rotating equipment, this paper proposes a WPT system based on U-core coupling mechanism in the primary side. Compared with the traditional space planar coil and spiral coil, the primary side U-shaped coupling mechanism can reduce the size of the primary side coil structure and facilitate the installation in a narrow space. e U-shaped core structure can enhance the coupling coefficient and the magnetic field distribution of the power transmission channel. Based on the circuit topology of U-type coupling mechanism WPT system, the expressions of system transmission efficiency and power are derived, the system simulation model is established, the spatial magnetic field distribution of the system is analyzed, and the feasibility of the system is verified by setting up an experimental platform.

WPT System Analysis of U-Type
Coupling Mechanism e mutual inductance and coupling coefficient between the primary and secondary coils are required in the power transfer process of WPT system, and the mutual inductance and coupling coefficient are related to the relative position and spacing between the primary side and the secondary side. In order to improve the efficiency and output power transfer, it is necessary to design appropriate coupling mechanism to meet the requirements of mutual inductance and coupling coefficient. e structure diagram of WPT system based on U-core coupling structure is shown in Figure 1, in which the primary coil is wound around the U-shaped magnetic core and remains stationary, while the secondary side coil is fixed on the rotating axis and rotates with the shaft. e U-type coupling mechanism WPT system adopts a series-series (SsS) reactive power compensation circuit topology. e S-S topology structure has simple circuit structure and convenient parameter design. Moreover, the output characteristics of the system are independent of the inductance values of the primary and secondary coils and can output constant current [16,17]. e circuit topology of the system is shown in Figure 2. e AC voltage at the input side of the system is transmitted to the inverter after rectification to generate high-frequency AC voltage as the input voltage of the primary coil. e secondary coil generates high-frequency AC voltage through induction and outputs it to the load side through the rectifier circuit. In Figure 2, US represents AC voltage source, S1, S2, S3, and S4 represent switch tube of inverter circuit, C1 and C2 represent resonance compensation capacitance of the primary side and secondary side, respectively, L1 and L2 represent coil selfinductance of the primary side and secondary side, respectively, M represents mutual inductance of the primary side and secondary side coil, and RL represents equivalent resistance value of load.
In order to calculate the transfer efficiency and power of the system, let R 1 � R S + R Tx , X 1 � L 1 + 1/ωC 1 , R 2 � R L + R Rx , and X 2 � L 2 + 1/ωC 2 , where R S is the internal resistance of the voltage source, R TX and R RX are the equivalent resistance of the primary side and the secondary side, respectively, and ω is the working angular frequency of the system. e primary and secondary loop impedances can be expressed as follows: (1) e reflection impedance of the primary and secondary circuits can be expressed as follows: Among them, From formula (1) and equation (3), the total impedance of the primary and secondary circuits can be obtained as follows: According to KVL and KCL, the primary and secondary loop current expressions are as follows: e output power and transfer efficiency of the system can be obtained as follows: It can be seen from equation (6) that when the system parameters have been determined, the main factors affecting the output power and transfer efficiency of the system are the resonance frequency and mutual inductance of the system, and the mutual inductance is closely related to the relative distance between the primary and secondary coils. erefore, the change trend of transfer efficiency and power can be found by studying the change of relative distance between the primary coil and secondary coil.

Simulation and Analysis of Electromagnetic Parameters of U-Type Coupling Mechanism WPT System
In order to study the influence of the relative position of the system on the mutual inductance and coupling coefficient, it is necessary to understand the distribution of the magnetic induction intensity of the system. e parameters in Table 2 were used for modeling and analysis. e distribution diagram of magnetic induction intensity around the primary side coil based on U-shaped magnetic core is shown in Figure 3. It can be seen from   Figure 3 that the magnetic field around the primary side coil based on the U-core is symmetrically distributed. e magnetic induction intensity inside the core is the highest, and the opening of the magnetic core decreases, and finally a closed magnetic field loop is formed. e maximum magnetic induction intensity inside the core can reach 390 mT, the magnetic induction intensity at the opening of the core is mainly about 10 mT, and the maximum magnetic induction intensity at the opening can reach 100 mT.
When studying the influence of the relative position of the coils on the mutual inductance and coupling coefficient of the system, the variation law of the mutual inductance and coupling coefficient is explored by changing the transverse distance d and the longitudinal distance h between the primary coil and the secondary coil, as shown in Figure 4. In order to avoid the interference between the primary and secondary coils caused by high-speed rotation, d � 15 mm and h � 15 mm are taken as the minimum reference spacing, and the parameters shown in Table 2 are used for modeling analysis and physical production.
Set d � 30 mm and h � 30 mm as the initial standard spacing; the electromagnetic parameters of coupling mechanism are measured by Maxwell electromagnetic simulation and RLC bridge meter under the initial standard spacing, as shown in Table 3. From the data in Table 3, it can be seen that the simulation results and the actual measured values are basically the same, and the maximum error is only 3.03%. When the system operating frequency changes from 100 kHz to 300 kHz, the self-inductance and mutual inductance of the coil have little difference. erefore, in the later analysis, f � 200 kHz can be used as the system operating frequency for analysis and research.
When the working frequency of the system is 200 kHz, d and h are fixed, respectively. According to the simulation calculation, with the distance d and h changing, respectively, the mutual inductance between the coils changes, and the change trend is shown in Figure 5. It can be seen from Figure 5 that when the secondary side coil moves to the right, the change of mutual inductance M is not obvious. When the coil moves to the right for 15 mm, the mutual inductance increases by 0.2 μH and 1.21%. When the secondary coil moves downward, the mutual inductance changes obviously. When the coil moves 15 mm, the mutual inductance increases by 4.28 μH and 25.90%. erefore, the maximum efficiency and output power of the system can be improved by moving the secondary coil up and down to make the system mutual inductance to the maximum value.
It can be concluded from the above analysis that the change of d has little effect on mutual inductance. In order to keep a large distance between the primary and secondary coils in the horizontal direction, d � 30 mm can always be maintained, and the system can obtain the maximum mutual inductance M � 20.81 μH at h � 15 mm under the condition of minimum reference distance in vertical direction. erefore, d � 30 mm and h � 15 mm are selected as the maximum mutual inductance position of the system. At this time, the distribution of magnetic induction intensity of the system is shown in Figure 6. It can be seen from Figure 6 that the magnetic induction intensity around the primary coil is the largest near the opening of the U-shaped core, which is about 10 mT.

Simulation Analysis and Experimental Research
Based on the above analysis results, this paper selects d � 30 mm and h � 15 mm as the relative position of the primary side and secondary side and uses Maxwell and Simplorer software to establish the model based on the structural parameters in Table 1 for joint simulation analysis. e joint simulation model is shown in Figure 7. e specific parameters of the system circuit in the simulation model are shown in Table 4.
e input voltage, current, output voltage, and current waveforms of the system are shown in Figure 8. Figure 8(a) shows the input voltage and current waveforms of the system simulation. It can be seen that the voltage and current are basically in phase. After several cycles, the current reaches a stable state, and the peak value can be stabilized at 560 mA. erefore, the input power can be estimated as 4.20 W. Figure 8(b) shows the output voltage and current waveforms of the system before rectification. It can be seen that the output voltage and current are in inverse phase with the input voltage and current waveform. After several cycles, the voltage and current reach a stable state.
e peak values of voltage and current are, respectively, stabilized at 10.4 V and 530 mA, and the output power is  2.76 W. In conclusion, the simulation output efficiency of the system is 65.71%. According to the above simulation results, an experimental platform of the U-shaped coupling mechanism radio power transmission system is built as shown in Figure 9. e size parameters of system coupling mechanism are shown in Table 1, and the parameters of system circuit components are shown in Table 4. In the experiment, UTP3705 S is used as DC power supply, UTD2112CEX oscilloscope is used for waveform detection, and VC4092D digital bridge meter is used to measure the inductance, capacitance, impedance, and other parameters of the system. e self-inductance and mutual inductance values of the primary and secondary coils measured by VC4092D digital bridge meter are shown in Table 3. e primary side input voltage waveform and secondary side output voltage waveform are measured by oscilloscope, as shown in Figure 10.
e amplitude of primary side input voltage   Journal of Electrical and Computer Engineering   waveform is about 12.2 V, and the amplitude of secondary side output voltage waveform is about 6.14 V. e input power and output power of the system can be obtained by measuring the input power of the DC power supply at the primary side and the power consumed by the load at the secondary side. e input power of the system is the input power of the inverter circuit of the primary side circuit, and the output power of the system is the power input to the load after the secondary side rectification. e input power and output power of the system are 3.36 W and 1.72 W, respectively. e transfer efficiency of the system is 51.19%. Compared with the results of simulation and experiment, the difference of transfer efficiency between simulation and experiment is 14.52%. Because the transfer efficiency measured by experiment includes the power loss caused by inverter and rectifier, while the transfer efficiency    of simulation part does not calculate the loss of these two parts, the transfer efficiency of experimental results is smaller than that of simulation analysis. When the relative distance between the primary and secondary coils of the system is d � 25 mm, d � 20 mm, d � 15 mm, and d � 10 mm, the comparison results are shown in the figure.
It can be seen from Figure 11 that when the relative position between the primary coil and the secondary coil changes, the transmission efficiency and transmission power of the system change. With the decrease of the distance d, the transmission efficiency and transmission power gradually increase, but the decrease of the distance increases the risk of collision between the primary coil and the secondary coil. erefore, the distance d between the primary coil and the secondary coil is still set as 30 mm; it is the optimal relative position between the primary and secondary coils.
In summary, although there are some differences between the experimental results and the simulation results, through the simulation analysis and experimental verification, it can be seen that the U-type coupling mechanism wireless power transfer system can effectively realize the wireless transfer of electric energy, which provides a good solution for wireless power supply of high-speed rotating shaft equipment.

Conclusion
In this paper, a WPT system based on U-type magnetic core coupling mechanism is proposed. rough theoretical analysis, simulation analysis, and experimental verification, the following conclusions are obtained: (1) the relative distance between the primary U-core coupling mechanism and the secondary coil affects the mutual inductance and coupling coefficient of the system, thus affecting the transfer efficiency and output power of the system; (2) electromagnetic simulation shows that the magnetic field of the system is mainly distributed near the opening of the U-shaped core; (3) the experimental results show that the transfer efficiency of the system can reach 51.19%, and the output power is 1.72 W, which verifies the feasibility of the U-shaped magnetic core coupling structure to realize wireless power transfer. e conclusion of this paper provides theoretical basis and application guidance for the application of U-core coupling mechanism wireless power transfer. ere is still room for improvement and optimization. How to improve the transfer efficiency and output power of the system is the direction of further research in the follow-up work.
Data Availability e datasets supporting the conclusions of this article are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.