A Unipolar Coil Arrangement Method for Improving the Coupling Coefficient Without Ferrite Material in Wireless Power Transfer Systems

In recent years, wireless power transfer (WPT) is extensively researched because it is a reliable and safest way of electric vehicle charging. Generally, rectangular-, square-, and circular-shaped unipolar coils are used in the static and dynamic WPT systems. However, these unipolar coils suffer from reduced coupling coefficient (k) due to large leakage flux, which decreases power transfer and transmission efficiency. In this context, this article proposes a unipolar coil arrangement method (UCAM) for improving k in WPT systems compared with conventional coils having the same self-inductances and outer dimensions. Furthermore, an analytical model is developed and used to calculate k of the coil structures (having different coil parameters) obtained using the proposed UCAM. For the rectangular geometry of 400 mm x 300 mm, 6.78%-27.04% improvement in k is achieved at the 150-mm air gap for the case 3 coil system compared with the different conventional coil systems. The interoperahility between the proposed and conventional coils is also studied. Moreover, this article examines the impact of increased k on power transfer capability and transmission efficiency. Prototypes of various coil systems are built. Experimental results confirm improvement in k, even for different misalignments between the transmitter and the receiver coils, of the designed coil system using the proposed UCAM.