A model for coupling under coil misalignment for DD pads and circular pads of WPT system

The coupling coefficient of a transformer is a key parameter affecting the performance of a wireless power transmission system. Based on the magnetic reluctance model under aligned condition, this paper gives an effective method for calculating the null coupling position, as well as a practical and accurate model for finding the coupling coefficient under coil misalignment and varying air gap. The null coupling position is determined by the horizontal offset when the flux of opposite direction that is intersected with the receiving coil is cancelled out. An empirical formula is given to correct the null coupling position. Based on the physics of coupling under misaligned conditions, a practical model for coupling is constructed. Transformers with DD structure and typical circular structure are fabricated for verification. Experimental measurement validates the model.