Impact of Surrounding Objects on Inductive Wireless Power Transfer

Sammanfattning: Many new models of smartphones and small electronic devices are using their wireless charging feature as a key selling point. This is due to the convenience of cable-free charging as well as not requiring multiple power adapters for charging different devices. However, with the increasing use of devices having wireless charging capability, safety and efficiency issues arise. Safety concerns are critical for manufacturers of these devices. In inductive wireless charging systems, knowledge on the impact of surrounding objects on the system is imperative in designing safer and more efficient systems. This thesis gives a quantitative analysis of the influence of foreign objects to an inductively coupled Wireless Power Transfer (WPT) system. This is done by measuring carefully chosen square metal plates of different dimensions and material properties, and quantifying their impact on the coil inductance, equivalent coil resistance and quality factor of the transmitter coil. Low-frequency electromagnetic simulations are performed in Ansys Maxwell to verify the experimental measurements. The results obtained by experiments and simulations are consistent with each other. These results are analyzed, and conclusions drawn pertaining to how the materials impact the system. In particular, the tested metal plates increase the equivalent coil resistance as compared to that of the empty coil, due to the induced eddy currents on the plates. Simultaneously, eddy currents tend to decrease the coil inductance as compared to that of the empty coil. However, at frequencies lower than 100 kHz, the presence of ferromagnetic metal plates can actually increase the coil inductance, as these plates are strongly magnetized by the magnetic field generated by the coil currents. In general, the quality factor is decreased in the presence of the test metals. Further, the thickness of the metals influences the power loss due to eddy currents only if the metal thickness is below the skin depth of the metal. In addition, the square metal plates with side lengths equal to or greater than the coil diameter have similar impact on the coil parameters, indicating highly localized magnetic fields around the coil. The metal plates with side lengths smaller than the coil diameter have more unpredictable impacts on coil parameters. Investigations into the impact of foreign objects on both the transmitter and receiver coils are proposed for future work.