Low Air Gap Magnetic Flux Density, Low Air Gap Electric Field Intensity, Low Loss Coil Design Methodologies for Multi-kW, Large Distance Wireless Power Transfer Systems
Inductive wireless power transfer (WPT) is being actively investigated as a promising technology to conveniently charge electric vehicles. Existing WPT systems generate large air-gap region magnetic (B) and electric (E) fields to transfer multi-kW of power, which violate typical safety standards. This research focuses on developing general design methodologies to meet the safety requirements by maintaining inherently low air-gap B and E fields while achieving multi-kW, large distance inductive WPT with high power transfer efficiency. General design variables are identified, and their effects on the air-gap B and E fields, and power transfer efficiency are investigated. A general design methodology based on fast converging analytical models for the estimation of the air-gap B and E fields, and transfer efficiency is developed. Particular winding configurations such as the surface spiral parallel and anti-parallel windings are proposed to achieve high efficiency, low dielectric losses, and reduced spatial voltage stress. A design example is analyzed in detail using both FEA and experiments. Besides the mutual coupled magnetic flux, the power transfer capability within the safety standard was found to be fundamentally limited by the leakage flux. A combination of new active and passive techniques is developed to mitigate these limitations. In particular, an “I” type shielding design is developed to shape the magnetic flux paths as desired to achieve low air-gap B field and shield the E field without affecting the transfer efficiency. The shielding design is optimized with reduced mass. In addition, the power-scaling law within the field safety limits are developed. Thermal modeling is developed based on the loss distribution analysis. At the end, the impact of coil misalignment is investigated. Unavoidable misalignment leads to reactive power and change of field distributions. Low loss capacitor and inductor online active tuning techniques are developed to reduce the reactive power, and improve the output power capability. Alternative compensation techniques are investigated to mitigate the variations of the field distributions, and provide an access to tune the field distributions actively.