An investigation into wireless power transfer systems for mid-range applications

Abstract

Wireless power transfer, as a reemerging technology, has become a preferable solution for charging wirelessly to a variety of applications due to its unique features of safety, flexibility and convenience, especially for portable electronic devices, which has reached commercialization stage. In these applications, the transmitter is normally very close to the receiver that the transmission distance is short. However, in some situations such as in industry automation or Internet of Things scenario, short-range wireless power transfer technology would restrict the spatial flexibility of the devices. This thesis presents the investigation of wireless power transfer systems for mid-range applications. Maximum power transfer theorem is usually adopted by this kind of system. The objective of optimization is to maintain high power transfer efficiency under various distance or misalignment conditions. Both of near-field wireless power transfer systems and far-field wireless power transfer systems are investigated to show the performance in the mid-range in terms of various distance and misalignment conditions. Differences between maximum power transfer and maximum energy efficiency in the wireless power transfer system are introduced. Detailed analysis and comparisons of near-field inductive power transfer system and far-field radio frequency power transfer system are reported. Three different inductive power transfer systems based on the number of coils in the system are presented. The voltage gains of each system, which can represent power transfer efficiency of the system, are calculated. The frequency splitting phenomenon is explained in mathematic forms. The features of three different systems for mid-range applications are compared. A modular framework of radio frequency power transfer system is analyzed for investigating the efficiency. The distance and misalignment adaption of this system and magnetic-resonance coupled four-coil system are compared. The capability of power transmission of these two technologies are described clearly. This system is a promising technology to power millions of sensor nodes in the Internet of Things scenario. High power transfer efficiency is only happened in a fixed condition (distance or misalignment) for conventional mid-range inductive power transfer systems. A novel reconfigurable three-coil inductive power transfer system using zone impedance matching technique is proposed to overcome this boundary. This technique does pre-matching of the transmitter coil to the desired coupling coefficients. Thus, the power transfer capability of the inductive charger could be maintained reasonably high over a long transmission distance and under a wide receiver misalignment. The proposed system equips a multi-tap transmitter coil to provide flexibility in selecting different number of turns in the transmitter coil. The corresponding control method without any direct measurement on the load side is introduced. A mathematical analysis is conducted to formulate the design procedures for the proposed system. Under a wide range of distance and misalignment conditions, practical measurement results verified that the proposed system achieves higher power delivery than ordinary design. In summary, this thesis investigates the potentiality of wireless power transfer for mid-range applications. Near-field inductive power transfer systems are analyzed in detail from coupled model to circuit model. The modular framework of far-field radio frequency power transfer system is analyzed and the comparisons between these two systems are introduced. One novel inductive power transfer system is proposed as an alternative solution to deliver power wirelessly for some moving devices in many outdoor inductive charging applications.