Some emerging aspects on wireless power transfer

Abstract

Wireless power transfer (WPT) owns several benefits, which include elimination of various charging adapter types, avoidance of cord clutter, electric isolation, and easier maintenance. Its development has undergone significant progress in the past few decades due to the advancement in high frequency switching devices as well as growing application areas such as portable devices, medical implants, and electric vehicles. However, the rapid evolution of existing wireless charging standards and on-going studies of WPT show that there are uncovered areas and improvements to be made. Three emerging aspects of WPT are investigated in this thesis: 1. It is expected that there will be more applications in which power has to be transferred wirelessly with metal objects present in the future. One such emerging application is powering online monitoring systems on high voltage (HV) transmission towers with domino WPT systems in the form of insulation rods; a metal ring is present on the insulation rod to avoid corona discharge. Existing WPT studies focus primarily on foreign or unintended metallic objects. Owing to the lack of studies toward intended metal objects in WPT systems, a systematic evaluation of the behavior of WPT link with metal ring is carried out based on a reduced-order model. The conduction loss of the metal ring was found not to be the reason for efficiency or output power drop, but its alteration of the self- and mutual- inductances of the coils. Suggestions for minimizing the effects of metal ring were given and experimental results have proven them effective in enhancing the WPT link performance. 2. Different coupling structures have been studied recently for different purposes and applications such as increasing the coupling factor or achieving omnidirectional charging. A new ball-joint WPT structure aiming to achieve constant mutual coupling under a range of rotational angles for the application of movable mechanical parts is presented in this work. This structure is based on capacitive coupling and comprises four plates embedded in a rotatable ball-joint structure. Detailed design procedures and equivalent circuit model were presented. It is implemented and verified to possess constant coupling under different rotating angles. 3. One aspect of battery charging rarely studied by power electronic researchers is the thermal issue. The study of relationship between charging current profile and heat generation of battery is initiated in this work based on coupled electric and thermal models of a Li-ion battery. Through judging the average, RMS, and form factor of a charging current waveform, optimal charging methods with faster charging speed under the same heat constraints are established. These methods apply to both wired and wireless charging. A widely adopted guideline in industry proposed by Japan Electronics and Information Technology Industries Association (JEITA) for the prevention of extreme temperature charging is studied and found to charge batteries without optimal charging speed. Two new charging methods are therefore proposed to improve upon without violating this guideline. Experimental results have shown significant improvement in charging speed with the two new charging methods.