Mutual inductance is a key parameter in a wireless power transfer (WPT) system that determines transmission power and transmission efficiency. Mutual inductance can be increased, and magnetic leakage can be reduced by adding electromagnetic (EM) shielding materials to the coil structure. However, the calculation of the mutual inductance between two arbitrarily positioned circular coils, including the effect of EM shielding, has not been resolved. In this article, an analytical formula for calculating the mutual inductance between two circular coils with arbitrary relative positions on a double-layer EM shielding is obtained based on the coordinate transformation method and electric field intensity. A series of experiments are carried out to validate the analytic calculations. The experimental results for each of the misalignment cases are compared with analytical predictions and finite-element method (FEM) solutions. The theoretical results and FEM simulation results are in good agreement with the experimental data, which verifies the validity of the proposed method.

To improve the power level of a diode rectifier, a multi-phase diode rectifier can be employed to increase the current rating. This letter proposes a passive current balancing method for a multi-phase diode rectifier in a series-series compensated wireless power transfer system. For an N-phase diode rectifier, the receiver-side compensating capacitance is split into N branches, forming a parallel resonant network with the newly added parallel resonant inductors. The circulating currents can be greatly suppressed by the parallel resonant network. The resonant inductors only withstand differential-mode currents, which are small. Thus, these inductors are small and low-cost. Experimental results reveal that the proposed method can well balance the phase currents of the three-phase diode rectifier.

Lateral misalignment along the driving direction may achieve half the length of a transmission coil in a dynamic wireless charging system for electric vehicles, which is called limit misalignment. This limit misalignment may cause the dynamic wireless charging system to malfunction because of the large fluctuation in mutual inductance. In this paper, an asymmetric and reverse series coil (ARSC) structure is proposed. In the ARSC structure, the transmission coil is smaller than the receiving coil in size to smoothen the fluctuation in mutual inductance. The receiving coil consists of a large coil ( $\boldsymbol{R}_{\mathrm {x1}}$ ) and a small coil ( $\boldsymbol{R}_{\mathrm {x2}}$ ) and $\boldsymbol{R}_{\mathrm {x1}}$ is reversely connected to $\boldsymbol{R}_{\mathrm {x2}}$ in series. The purpose of this design is to further reduce the fluctuation in mutual inductance. A calculation method of mutual inductance is then proposed based on the vector potential to calculate the mutual inductance of the ARSC structure. The law of mutual inductance is analyzed by the proposed calculation method for the ARSC structure. Moreover, an optimization method of coil parameters is proposed to obtain quasi-constant mutual inductance. The ARSC structure and optimization method are not only simple but can also make the mutual inductance nearly constant. Calculation, simulation, and experimental results validating the proposed method and structure are shown.

Dynamic wireless power transmission (DWPT) is a promising technology as it reduces the battery capacity carried by automated guided vehicles (AGVs) and lowers its cost and weight. However, there are large mutual inductance and power fluctuation in the dynamic charging process. In order to solve this problem, this paper proposes a low-cost and low fluctuation dual-receiver magnetic coupler, which are connected in parallel by two half-bridge rectifiers. The transmitter employs an I-type solenoid coil to enhance the magnetic field and improve the coupling. The optimized design of the coupler structure enables the system to have smooth mutual inductance fluctuation by switching the two receiving coils with each other during the dynamic charging process. The experimental results show that the measured equivalent mutual inductance fluctuation is 3.6%, which verifies the feasibility of the optimized design of the magnetic coupling structure and the effectiveness of the proposed DWPT system.

The rapid development of electric vehicles (EVs) calls for an efficient and convenient charging method. Wireless power transfer (WPT) is favored by the academia and industry because of its convenience, security, and automation. However, there are still many problems to be solved, one of which is the interoperability issue between different coil types. In this article, an EV wireless charging system based on three decoupled non-overlapping unipolar transmitting coils is proposed. The inductor-capacitor-capacitor-series topology is adopted. The current directions of the three transmitting coils can be altered by adjusting the outputs of the three-phase independently controlled half-bridge inverter. In this way, the proposed transmitting coils can be tolerant with the unipolar coil and the two perpendicular bipolar coils, achieving interoperability and certain anti-misalignment capability with different receiving coils. The experimental results verify that the proposed system can achieve interoperability for both unipolar and bipolar coils and has misalignment tolerance in specific directions with a maximum efficiency of 93.67%.

Mutual inductance can be increased, and magnetic leakage can be reduced by adding finite magnetic core materials to the coil structure. However, the problem of the calculation of the mutual inductance between two arbitrarily positioned circular coils with a finite magnetic core has not been resolved. In this article, an analytical formula for calculating the mutual inductance between two circular coaxial coils on a finite magnetic core is obtained based on regional hierarchical expansion analysis (RHEA), the Maxwell equation, the Bessel function, and the boundary conditions for electromagnetic fields. The mutual inductance between two arbitrarily positioned coils is obtained by using the analytical geometry method. A series of experiments are carried out to verify the analytic calculations. The experimental results for each misalignment case are compared with the calculated results and simulated results. The theoretical results and simulated results are in good agreement with the experimental data, which verifies the validity of the proposed method.