# Design and Performance Analysis of Misalignment Tolerant Charging Coils for Wireless Electric Vehicle Charging Systems

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Inductive Link Modeling

## 3. Charging Coil Geometries

- Non-polarized geometries, such as circular and rectangular coils, in which the perpendicular component of the magnetic flux is responsible for the wireless power transfer process.
- Polarized geometries, such as DD, DDQ and bipolar coils, in which the parallel component of the magnetic flux dominates and becomes responsible for the wireless power transfer process.

- In non-polarized coil geometries, perpendicular magnetic field lines provide satisfactory coupling at perfect alignment conditions, and the coupling strength is impacted by the vertical air gap between the primary and secondary coils.
- For effective coupling between circular charging coils, the vertical separation between the coils should be proportional to a quarter of the coil diameter [34]. Hence, significantly large circular coil diameters are required to improve the coupling performance in WEVC systems due to the large EV-to-ground clearance distance, i.e., the large air gap between the primary and secondary charging pads.
- Parallel field patterns, on the other hand, are not as significantly affected by increasing the air gap, thereby they are expected to provide better coupling performance for large air gap applications such as WEVC systems, in comparison with non-polarized charging pads of equal dimensions.
- For similar-sized charging pads, the flux path height of DD coils is double that of circular coils, which significantly reduces the flux leakage and enhances the coupling factor.
- For symmetric DD-DD inductive link structures, as the secondary coil is displaced from the perfect alignment position, the flux linkage of the parallel fields experiences a significant degradation [35]. This motivates the addition of a quadrature coil, forming a DDQ charging pad, that utilizes parallel fields in perfect alignment conditions and benefits from the perpendicular field lines coupling with the quadrature coil during misalignments [26].
- The key drawback of using DDQ coils is the added system weight, cost and complexity, due to the need to connect the DD and the quadrature portions of the charging pad to separate power management circuitry. This, on the receiver’s side, means that two separate compensation and rectification circuits are required to capture the maximum amount of transferred power during perfect alignment as well as when misalignments occur.
- Bipolar charging pads leverage on the advantages of DDQ coils with 25–30% less copper in comparison with DDQ charging pads [36]. However, separate compensation, rectification and control circuits are also required for each coil constituting the bipolar charging pad.

## 4. Design Strategy

#### 4.1. Definition of Efficiency Thresholds

#### 4.2. Definition of Constant Coil Parameters

- Outer coil surface area, $l\times w$: Acknowledging the space restrictions in the placement of the charging coils at the bottom of EVs, the outer coil area is determined given a set of considerations and is eliminated from the conducted parametric optimization process. According to [37], a reasonable secondary coil occupancy is around 480,000 mm${}^{2}$ for a typical sedan EV, while the outer charging pad dimensions need to be larger to ensure effective shielding. In addition, according to [38], the optimal value of the width of a rectangular coil is three times the vertical separation between the primary and the secondary coils. The average vehicle-to-ground clearance of a typical EV is estimated to be ∼200 mm, based on which the outer coil width is selected to be 600 mm. Accordingly, for a total coil area of 480,000 mm${}^{2}$, the coil dimensions are selected to be $800\times 600$ mm${}^{2}$.
- Wire diameter, d: Stranded copper wires are utilized for all the simulations conducted in this work to reduce eddy current losses, effectively replicating the performance of Litz wires. According to [39], the current carrying capacity of a Litz wire is 4 A/mm${}^{2}$. Assuming the current through the coils is around 50 A for a 25 kW/500 V wireless EV charging system, the wire diameter is selected to 4 mm with a cross-sectional area of ∼12.5 mm${}^{2}$.
- Ferrite and shielding layers: Since this work focuses on analyzing the performance of different coil geometries, identical ferrite and shielding layers are used for all the FEM simulations, as shown in Figure 4. The specifications of these layers are detailed in the authors’ earlier works in [11,23,40] and are summarized in Table 1.

#### 4.3. Identification of Design Variables

- The number of turns of the coils, N.
- The corresponding spacing between adjacent turns, s.

#### 4.4. Parametric Optimization Using FEM Simulations

## 5. Simulation Results and Experimental Verification

## 6. Rectangular, DD and DDQ Charging Coils

- The coupling performance at 200 mm lateral misalignment.
- The magnetic null point.

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 4.**Different charging coil geometries: (

**a**) Rectangular, (

**b**) DD, (

**c**) DDQ, and (

**d**) bipolar coils.

**Figure 5.**Theoretical relationship between the S-S compensated power transfer efficiency and FoM, showing desired efficiency thresholds.

**Figure 7.**FoM at perfect alignment for different ${A}_{ratio}$ using different turn-to-turn spacing, s.

**Figure 8.**FoM at $\pm 200$ mm lateral misalignment for different ${A}_{ratio}$ using different turn-to-turn spacing, s.

**Figure 9.**Relationship between N, s and ${A}_{ratio}$ for $800\times 600$ mm${}^{2}$ DD charging coils, with optimal design point identified.

**Figure 11.**Magnetic field intensity plot of the inductive link with no ferrite and shielding layers.

**Figure 14.**Simulation vs. experimental results of coupling factor, k, at different lateral misalignments.

**Figure 15.**Comparison between rectangular and DD coils in terms of the coupling factor, k, at different lateral misalignments.

**Figure 16.**Extended positive coupling range as a result of adding a quadrature coil to the secondary side.

Parameters | Value |
---|---|

Gross ferrite surface area | $900\times 700$ mm${}^{2}$ |

Bar dimensions | $900\times 37.5$ mm${}^{2}$ |

No. of ferrite bars | 9 |

Ferrite thickness | 16 mm |

Relative permeability | ${\mu}_{r}=3000$ |

Ferrite material | T-type ferrite |

Gross aluminum surface area | $1000\times 800$ mm${}^{2}$ |

Aluminum shield thickness | $1.8$ mm |

Aluminum 1050A alloy Conductivity | $\sigma =33.9$ MS/m |

Parameters | Value |
---|---|

Coil surface area | $800\times 600$ mm${}^{2}$ |

Number of turns, N | 13 |

Wire diameter, d | 4 mm |

Edge-to-edge spacing, s | 6 mm |

Equipment | Part No. | Rating |
---|---|---|

AC supply | BK PRECISION 4017A | 1 Hz–10 MHz, up to 250 V |

Oscilloscope | TBS1052B-EDU | Up to 50 MHz |

LCR meter | GW Instek LCR-916 | $1/10/100$ kHz, 20 μH–20 kH |

Tunable capacitor | Cropico CM5-N | 100 pF–10 F |

Tunable resistor load | AEMC BR07 | 1 $\mathsf{\Omega}$–1 M$\mathsf{\Omega}$ |

PVC Insulated Cables | – | BS6231, $600/1000$ V |

**Table 4.**Comparison between experimental and simulation results for self inductances, ESRs and quality factors.

Parameters | Prim. Coil Prototype | Sec. Coil Prototype | FEM Simulations |
---|---|---|---|

Self inductance, L | $169.5$ μH | $161.4$ μH | $198.1$ μH |

Series resistance, R | 605 m$\mathsf{\Omega}$ | 574 m$\mathsf{\Omega}$ | $56.0$ m$\mathsf{\Omega}$ |

Quality factor, Q | 190 | $176.8$ | 2220 |

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## Share and Cite

**MDPI and ACS Style**

ElGhanam, E.; Hassan, M.; Osman, A.; Kabalan, H.
Design and Performance Analysis of Misalignment Tolerant Charging Coils for Wireless Electric Vehicle Charging Systems. *World Electr. Veh. J.* **2021**, *12*, 89.
https://doi.org/10.3390/wevj12030089

**AMA Style**

ElGhanam E, Hassan M, Osman A, Kabalan H.
Design and Performance Analysis of Misalignment Tolerant Charging Coils for Wireless Electric Vehicle Charging Systems. *World Electric Vehicle Journal*. 2021; 12(3):89.
https://doi.org/10.3390/wevj12030089

**Chicago/Turabian Style**

ElGhanam, Eiman, Mohamed Hassan, Ahmed Osman, and Hanin Kabalan.
2021. "Design and Performance Analysis of Misalignment Tolerant Charging Coils for Wireless Electric Vehicle Charging Systems" *World Electric Vehicle Journal* 12, no. 3: 89.
https://doi.org/10.3390/wevj12030089