Design, analysis and application of advanced hybrid-excited brushless doubly salient machines

Abstract

With the ever-increasing interest in energy efficiency and environmental protection, recent decades have witnessed a boost in low-carbon transportation systems such as electric vehicles (EVs). As a critical component for the conversion of electromechanical energy in EVs, electrical machines are expected to possess the characteristics of high torque and power densities, high efficiency over a wide speed range, high reliability and robustness for harsh environment. Although permanent magnet (PM) machines are perceived as good candidates for EVs due to their high torque density and high efficiency, their inherent poor flux regulation ability leads to possible narrow speed range and uncontrolled overvoltage fault at high speed, which limit their direct applications for EVs. Thus, this thesis aims to develop advanced hybrid-excited machines (HEMs) as an alternative solution for EVs so that the proposed HEMs can offer high torque density, high efficiency and good flux regulation ability.

Firstly, this thesis reviews the development of HEMs with an emphasis on machine topologies and characteristics. On the basis of the investigation of previous research works, the challenges of existing machines and the upcoming trends of development are also introduced. Secondly, to improve the flux regulation ability of HEMs and reduce the structural complexity, a harmonic-shift (HS) structure is proposed in the HEM, forming a new breed of the parallel-hybrid-excited machine (PHEM). Thirdly, the PHEM with harmonic-differential (HD) effect is proposed in order to improve the machine performances further, such as torque density, power density, and efficiency. Next, the idea of integrating the traction motor and differential gear into one single motor, forming a new proposed HEM with magnetic differential (MagD), is covered. Finally, the performances of the proposed hybrid-excited brushless doubly salient machines are comprehensively studied and validated by using the finite element method, while the prototypes of the proposed machines are also manufactured to verify the proposed concepts experimentally.