Wireless Power Transfer (WPT) technology has been developing rapidly in recent years. Electric Vehicles (EVs), as an emerging mode of transport, also benefit from the WPT technology. Various EV manufacturers have different coupling structures, which can lead to incompatibility problems between the coils. Meanwhile, due to the human parking, misalignment between the transmitting and receiving coils may occur. To solve these problems, this paper proposes a reconfigurable bipolar coil structure with both interoperability and misalignment tolerance. A 200-W experimental prototype is built, which can achieve 84.33% and 84.76% system transmission efficiency with vertical and horizontal bipolar coils. The effectiveness of the proposed reconfigurable bipolar coil is finally verified by comparing the computational and experimental results.

Wireless power transfer (WPT) is an emerging method of energy transfer without direct metal contact. It has the advantages of automation, safety, convenience, and suitability for various conditions (underwater and mining environments). Magnetic induction based WPT technology has received the most extensive research and applications in both academia and industry.1,2 Typical applications include consumer electronics and electric vehicles (EVs).3 

Magnetic couplers, coupling coils consisting primarily of transmitting (Tx) and receiving (Rx) coils, are used in EV wireless charging systems.4 They are an important part of the wireless charging system for EVs. Various problems with the magnetic couplers arise. In terms of the coil design, the coupling coefficient of the magnetic coupler,5 should be high to achieve efficient power transfer. Also, due to parking inaccuracies, misalignment can occur between the Tx and Rx coils, leading to low efficiency and even failures for power output. An anti-offset magnetic coupler design is needed.6 Another issue is the interoperability problem, i.e., compatibility for various types. Three conventional coil types are used, namely the unipolar coils and the bipolar coils along two vertical directions,7 as shown in Figs. 1(a)1(c). When perfectly aligned, these coils are decoupled from each other, indicating that there is no mutual inductance between them. Since the Tx and Rx coils are physically isolated, they can be produced by different manufacturers. If different coil types are used for the Tx and Rx coils, interoperability issues can arise, which are critical and should be addressed.

FIG. 1.

Schematic diagram of coil structure. (a) Unipolar coil. (b) Vertical bipolar coil. (c) Horizontal bipolar coil. (d) Label description.

FIG. 1.

Schematic diagram of coil structure. (a) Unipolar coil. (b) Vertical bipolar coil. (c) Horizontal bipolar coil. (d) Label description.

Close modal

Interoperability study of WPT systems includes compensation networks,8 magnetic couplers, and estimation methods.9 For magnetic couplers, the interoperability issues of square and circular coils were evaluated in Ref. 10. However, these two coil structures have similar flux distribution and interoperability of these two coils is not difficult.11 In practice, the transmitter terminal is fixed at the charging station or on the road surface without easy movement and rotation.12 The whole EV market does not specify a certain kind of wireless charging system as a fixed receiver, which will lead to the coupling device of different EV manufacturers in the face of the same kind of transmitters. Based on the simplicity and advantages of the two common coupling structures, unipolar and bipolar coils are bound to be the preferred solution for many EV manufacturers.13 Comparing the offset resistance and interoperability of the two coupling structures, bipolar coils have a better offset resistance due to the asymmetry of their horizontal and vertical placements.14 Assembly of horizontal bipolar and vertical bipolar coupling structures will show similar effects at the emitting end in the face of the same coupling structure. Based on the above considerations, a fixed reconfigurable bipolar coupling structure is proposed in this paper, which can achieve energy transfer with both the vertical and horizontal bipolar coil structures by simply changing the branch current flow direction, i.e., with all the advantages of the conventional bipolar coupling structures, but with higher interoperability.

To satisfy the energy transfer with both vertical and horizontal bipolar coils, this paper proposes a coupling structure based on the bipolar structure as shown in Fig. 2. The relevant parameters are all represented in Fig. 2(a). The vertical and horizontal bipolar coil structures shown in Figs. 2(b) and 2(c). It can be formed by connecting the conduction of LT1, LT2, LT3 and LT4, respectively.

FIG. 2.

Proposed reconfigurable bipolar coil structure and topology. (a) Simulation models and related parameters. (b) Vertical bipolar coil. (c) Horizontal bipolar coil.

FIG. 2.

Proposed reconfigurable bipolar coil structure and topology. (a) Simulation models and related parameters. (b) Vertical bipolar coil. (c) Horizontal bipolar coil.

Close modal

The vertical structure of the proposed reconfigurable bipolar is shown in Fig. 2(b). The current flows into the “upper” end of LT2 after passing through LT1, and flows out from the “lower” end, and finally forms a circuit with the main circuit after passing through LT3, and the whole current flow path can be equated to the excitation path of the vertical bipolar coil. The magnetic flux of the vertical bipolar coil is formed as a result.

The horizontal structure of the proposed reconfigurable bipolar is shown in Fig. 2(c). The current flows into the “lower” end of LT2 after passing through LT1, and flows out from the “upper” end, and finally forms a loop with the main circuit after passing through LT4, and the whole excitation current flow path can be equated to the excitation path of the horizontal bipolar coil, which forms the magnetic flux of the horizontal bipolar coil.

The inductor-capacitor-capacitor-series (LCC-S) topology is selected to tolerate weak couplings, as shown in Figs. 3(a)3(b). The proposed reconfigurable bipolar coil is selected as the Tx coil and two different directions of coils are chosen as the Rx coil. LF, LT, and LR are the self-inductances of the compensating inductor, the Tx coil, and the Rx coil, respectively. IF (RF), IT (RT), and IR (RR) are their corresponding currents and equivalent series resistances (ESRs). CF, CT, and CR are the compensating capacitances. MTR is the mutual inductance. VINV (VREC) and UT (UR) are the inverter (rectifier) dc and fundamental ac voltages, respectively. REQ is the equivalent load resistance. They can be expressed as
UT=22πVINV,UR=22πVREC,REQ=8π2RL.
(1)
FIG. 3.

WPT system. (a) Topology. (b) Equivalent circuit.

FIG. 3.

WPT system. (a) Topology. (b) Equivalent circuit.

Close modal
The system works at the resonant angular frequency ω
ω=1LFCF=1LTLFCT=1LRCR
(2)
Correspondingly, the mutual inductance can be expressed as
MTR=kTRLTLR
(3)
where kTR is the coupling coefficient.
Using Kirchhoff’s Voltage Law and ignoring the ESRs yield
IF=MTRLF2UTREQ,IT=UTωLF,IR=MTRLFUTREQ,VREC=MTRLFVINV
(4)
The output power and the efficiency can be expressed as
POUT=MTRLF2VINV2RL,η=POUTPOUT+IF2RF+IT2RT+IR2RR
(5)

According to the analyses in Sec. II, a physical model is established, and measurements are recorded for the actual coupling coefficients with the offset process. The recorded results are compared and analyzed with the conventional bipolar coil structure, and then obtained as in Figs. 4(a) and 4(b). The turn numbers of the conventional bipolar coil structure and the proposed reconfigurable bipolar coil structure are both 5. As can be seen from Figs. 4(a) and 4(b), the active offset range (i.e., k < 0.1) of the reconfigurable bipolar coil structure proposed in this paper is (−140 mm, 140 mm).

FIG. 4.

Effect of variation of coupling coefficients with misalignment. (a) Effect of change in coupling coefficient of vertical bipolar type and conventional bipolar type coils. (b) Effect of change in coupling coefficient of horizontal bipolar type and conventional bipolar type coils.

FIG. 4.

Effect of variation of coupling coefficients with misalignment. (a) Effect of change in coupling coefficient of vertical bipolar type and conventional bipolar type coils. (b) Effect of change in coupling coefficient of horizontal bipolar type and conventional bipolar type coils.

Close modal

An experimental prototype is built to verify the advantages of the proposed structure. Figure 5(a) shows the reconfigurable bipolar coil; Fig. 5(b) shows the constructed 200-W experimental prototype. The dimensions of the coupling mechanism of the prototype are 200 × 200 mm. The air gap is 50 mm. Based on the above misalignment boundaries; the interoperability verification experiments of the reconfigurable bipolar coils are carried out at a frequency of f = 85 kHz.

FIG. 5.

Experimental prototype and experimental structure diagram. (a) Reconfigurable bipolar coil structure. (b) Experimental prototype. (c) and (d) Output power and efficiency diagram: Vertical bipolar coil structure with X and Y offset; (e) and (f) Output power and efficiency diagram: Horizontal bipolar coil structure with X and Y offset.

FIG. 5.

Experimental prototype and experimental structure diagram. (a) Reconfigurable bipolar coil structure. (b) Experimental prototype. (c) and (d) Output power and efficiency diagram: Vertical bipolar coil structure with X and Y offset; (e) and (f) Output power and efficiency diagram: Horizontal bipolar coil structure with X and Y offset.

Close modal

During the experiment, the system input power is obtained by measuring the voltage across the DC power supply and the current flowing through it, and then multiplying them together. Correspondingly, the output power of the system is obtained by measuring the voltage across the load and the current flowing through it and multiplying them. Finally, the input and output powers are divided to obtain the actual transmission efficiency of the system. Figures 5(c)5(f) compares the results of the variation of transmission power and efficiency with offset. Figures 5(c) and 5(d) show that both the transmitter and receiver are horizontal bipolar coil structures with a maximum transmission efficiency of 84.33%. The overall trend is consistent with the horizontal bipolar coil structure. Figures 5(e) and 5(f) indicate that both the transmitter and receiver are horizontal bipolar coil structures with a maximum transmission efficiency of 84.76%. The overall trend conforming to the law of the horizontal bipolar coil structure with offset. i.e., the transmission effect decreases from the center position to the sides when offset along the Y-axis. The experimentally measured data have less error with the calculated results and the trend is the same, which verifies the accuracy of the mathematical model. The reconfigurable bipolar coil structure can transmit with both vertical and horizontal bipolar coils, thus demonstrating interoperability. The preceding reconfigurable bipolar coil structure possesses the capability to transmit in both vertical and horizontal bipolar coils, thereby demonstrating interoperability.

Since the reconfigurable coupling structure proposed in this paper is a novel attempted work, it is only used to achieve interoperability performance with vertical and horizontal bipolar coils. Subsequent researchers can optimize and improve upon it. To further verify the reliability of this attempt, a comparative analysis has also been carried out and the results obtained are shown in Table I. As can be seen from the table, the proposed coupling structure in this paper exhibits similar transmission efficiency when confronted with horizontal and vertical bipolar coils.

TABLE I.

Comparisons with existing methods.

System efficiency
at positive position
Ref.Coil structureAirgap (mm)Coil size (mm)Vertical (%)Horizontal
11  Dual-layer 100 300 × 300 ∼88  
12  Single-layer 75 300 × 300 91.6 91.6%  
13  Single-layer 100 300 ×300 ∼87 ∼87%  
14  Dual-layer 150 350 × 200 71.32  
This work Single-layer 50 200 × 200 84.33 84.76%  
System efficiency
at positive position
Ref.Coil structureAirgap (mm)Coil size (mm)Vertical (%)Horizontal
11  Dual-layer 100 300 × 300 ∼88  
12  Single-layer 75 300 × 300 91.6 91.6%  
13  Single-layer 100 300 ×300 ∼87 ∼87%  
14  Dual-layer 150 350 × 200 71.32  
This work Single-layer 50 200 × 200 84.33 84.76%  

In this paper, a reconfigurable bipolar coil structure with both interoperability and misalignment tolerance has been proposed. A 200-W experimental model has been fabricated to verify the proposed system. Upon comparing computational and experimental results, the maximum transmission efficiency of the suggested coil has been 84.76% when structured as a horizontal bipolar coil, and 84.33% when structured as a vertical bipolar coil, which ultimately validates the effectiveness of the proposed reconfigurable bipolar coil.

This work was supported in part by the National Natural Science Foundation of China (52107183).

The authors have no conflicts to disclose.

C.L. and M.Z. contributed equally to this work.

Chao Liu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Mingzhu Zhou: Data curation (equal); Project administration (equal); Resources (equal); Validation (equal). Yizhan Zhuang: Conceptualization (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal). Yiming Zhang: Conceptualization (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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