Coupling coefficient calculation and optimization of positive rectangular series coils in wireless power transfer systems

In wireless power transfer (WPT) systems using magnetic coupling resonance, when there is a registration deviation between the transmitting and receiving coils, there will be significant fluctuations in the coupling coefficient, output power, and transmission efficiency, which will seriously affect the stability of the WPT system. The coupling coefficient is related to the length and width of the rectangular coil, number of turns, mutual geometric position, and space magnetic medium, making it difficult to maintain a constant value when the coil is offset. A single-emitter two-receiver positive-series rectangular coils structure and a method of calculating the structure are proposed. Then an optimization method for mutual inductance was proposed, and the structural parameters that met the design requirements were obtained using the proposed optimization method. The calculation formula for coupling coefficient was verified through simulation and experiments. The results showed that when the offset distance of the receiving coil along the Y-axis (driving direction) and X-axis reaches half of the length of the transmitting coil and 10 cm, the coupling coefficient, transmission efficiency, and output power remain almost unchanged.