FEM-Based Performance Analysis of Circular and Rectangular Planar Spiral Coils for EV Wireless Power Transfer Under Turns Number, Air-Gap, and Misalignment Variation

This paper presents a 3D FEM-based comparative study of circular and rectangular planar spiral coils for electric vehicle (EV) wireless power transfer (WPT) at 85kHz. A geometry-consistent simulation framework is adopted, where both coil types are evaluated under identical conductor material, conductor diameter, inner area, turn spacing, and excitation current, allowing the isolated effect of coil geometry to be analyzed. Three operating conditions are investigated: turn-number variation (10≤N≤30) under perfect alignment, air-gap variation (130–200mm), and lateral misalignment variation (Δx∈[−30,30]mm). The transmitter self-inductance L1, receiver self-inductance L2, mutual inductance M, and coupling coefficient k are evaluated. The main contribution of this work is the identification of a geometry-dependent trade-off between coupling performance and robustness. The results show that increasing the number of turns enhances L1, L2, M, and k for both geometries, while self-inductances remain nearly insensitive to air gap and misalignment. The circular coil demonstrates higher robustness to coil separation and misalignment, whereas the rectangular coil can achieve higher coupling in configurations with a high number of turns under controlled misalignment conditions. These findings provide a solid basis for selecting the most suitable geometry for each wireless charging use case.