| Summary: | Wireless power transfer (WPT) refers to the transmission of electrical energy without a physical contact. WPT based on the resonance principle has been proposed for a wide range of applications, where implementation of physical connectors can be inconvenient, hazardous or impossible. The electromagnetic field generated by a transmitting resonator is captured by a receiving resonator which resonates at the excitation frequency of the power source. Although many researchers have been working on WPT technology, still there are numerous challenges to overcome in the process of wide adoption of commercialization.
Key performance indices of WPT technology are efficiency, transferred power, transfer distance and misalignment tolerance. There are several modelling methods for analyzing the WPT system, namely, equivalent circuit approach, two port network model and coupled mode theory. The performance of a WPT setup relies on number of design parameters such as WPT coils, power converters, transfer distance, frequency and load characteristics. A comprehensive review on the WPT technology and its applications are presented at the beginning of the thesis. WPT coils play an imperative role in WPT performance. With the assumption of optimum load condition, quality factor of the coils and coupling coefficient between coils are the main optimization objectives in improving WPT coils. Decreasing AC resistance of the coils while maintaining a high inductance is useful in high quality factor coils. An optimization guidelines for improving the WPT coils are investigated in this study. A novel toroidal shaped spiral coil has been proposed for high efficiency WPT.
Most WPT applications require wirelessly powered receiver to be moved freely. Therefore dynamic-WPT (D-WPT) which allows receiver movement, has become one of the attracting extensions of WPT technology. D-WPT can be realized either with an array of segmented transmitting coils or using long transmitter track. Misalignment problem becomes an inevitable design challenge for D-WPT. The study of this thesis is extended to investigate D-WPT system and its optimizations. It has been investigated that misalignment tolerance of the D-WPT system can be significantly improved with the use of repeater coils. In addition, the optimum current distribution among transmitter coils has been derived theoretically. A current modulation scheme is explored with the use of an analogy of N-dimensional Cartesian coordinate equivalent of spherical coordinates. The optimal current distribution at arbitrary receiver position is a function of the mutual inductances between transmitter coils and receiver. This theoretical contribution can be equally applied to any kind of multi-transmitter WPT setups.
The study of this thesis is extended to address the misalignment problem. Receiver misalignment can be radial, axial or angular depending on the receiver’s degrees of freedom (DoF). A figure-of-merit is proposed to optimize efficiency, transferred power and six-DoF misalignment tolerances simultaneously. Misalignments in six-DoF are segmented into sub regions, and performance within each region is prioritized based on probability of alignment in deriving the figure-of-merit. A WPT system with tri-spiral-repeater is introduced with the use of proposed figure-of-merit as the optimization objective.
Finally, a tuning method has been investigated to improve the performance against misalignment and load variations. Although, the investigation on the optimal load resistance is useful for the initial design stage, it is not always possible to operate at optimal value during the operational conditions. For example, equivalent load resistance of a battery charging application is dependent on the battery state-of-charge. Therefore, the repeater tuning approach investigated in this research can be applied to improve the performance in dynamic environments.
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