Design, analysis and applications of low-frequency electromagnetic resonance-based structures

Abstract

Resonance is a pivotal and fundamental phenomenon in the realm of electric/magnetic circuits, garnering much research and industrial attention in past decades. Prominent electromagnetic resonance structures (ERSs), i.e. metamaterial, metasurface, and antenna array, have found widespread use in fields ranging from molecular physics, communication, and military application, with extraordinary magnetic properties. However, those solutions have three aspects that need to be enhanced in low-frequency applications. First, the power level of the ERS in those applications is always at milliwatts, impeding their scalability to medium-power low-frequency applications. Secondly, the negative permeability model is not suitable to model the ERS-based applications, given that the permeability is not directly associated with the existing electric or magnetic circuit model. Thirdly, the property and applications of ERS have not been fully investigated. The existing solutions merely employ the negative permeability of ERS at the designed frequency. In this thesis, based on the negative permeability model of the metamaterial, the ERS is named and redefined as negative equivalent magnetic (NEMR) reluctance. The research on NEMR structure is divided into two major aspects, including the formulation and establishment of the NEMR model as well as its applications in wireless power transfer and transformers. The framework of the thesis is organized as follows. The current state-of-art ERS and ERS-based applications, including the design of intermediate metamaterial, and metasurface as well as their applications in wireless power transfer (WPT) are reviewed. Then, a transmitter-embedded metasurface-based WPT system for extend-distance applications is proposed. This design incorporates the metasurface slab into the transmitter coil without requiring any additional space, enhancing the portability and practicability compared to the generalized metamaterial/ metasurface-based WPT systems. Then, the NEMR model and a dual-coil-embedded negative equivalent magnetic reluctance (NEMR) structure for efficiency enhancement of weak coupling WPT system is proposed. The proposed NEMR model revealed the relationship between magnetic reluctance and the parameters of the NEMR structure, the feasibility of negative equivalent magnetic reluctance, as well as how the negative equivalent magnetic reluctance enhances the mutual inductance and efficiency of the WPT system. Next, apart from the negative equivalent magnetic reluctance of the NEMR structure, the flux suppression property for the components beyond the design frequency is explored and applied in the kHz transformer. The NEMR structure is installed in the air gap of the open-core topology, serving as a magnetic band-pass filter based on the frequency-variation magnetic reluctance property. It indicates negative equivalent magnetic reluctance for the fundamental component of flux and high magnetic reluctance for the DC component. Therefore, strong withstanding DC bias and high efficiency can be achieved with the proposed design. Further, to extend the application of the NEMR structure, a pole-wrapped negative equivalent magnetic reluctance (PNEMR) structure-based line-frequency transformer is proposed. The key merit of the proposed design is to modify and verify the magnetic circuit of the transformer iron core and PNEMR from series connection to parallel connection, to improve the quality factor, accordingly, enhance the efficiency, withstanding DC bias capability and power factor of the transformer.