Contactless transformer modeling and optimization based on “null coupling position” analysis under coil misalignment

In the inductive wireless power transfer system, the specified operating condition such as coil misalignment will cause the coupling coefficient of the contactless transformer to vary in a large range, further affecting the system operating characteristics. To build the coupling coefficient model under large coil misalignment, this paper gives an effective method for calculating the null coupling position based on the method of combining the field concept with circuit. According to the change law of the coupling coefficient, a model for finding the coupling coefficient under coil misalignment is developed. Then a novel contactless transformer with multiple receiving coils is proposed based on the characteristics of magnetic field distribution. A 60 W contactless energy transmission system is designed with the optimized contactless transformer structure. The calculation results of the coupling coefficients are in agreement with actual test results, which proves the correctness of the proposed method and model. The novel coil set has achieved good tolerance to coil misalignment.