A general and unified theory of flux-modulated electric machines and its application to innovation of machine topologies

Abstract

Electric machines (EMs) play a main role to convert mechanical and electrical energy interchangeably. As there are many EMs, small increment in the efficiency of energy conversion will save huge energy. Exploring novel EMs with high efficiency, high torque density, and low cost is always an interesting research topic. In recent decades many different types of new EMs have emerged. Among others, these machines include a basic flux-modulated motor which uses the variation of magnetic reluctance to produce constant torque, a Vernier motor which has special slot combinations, a doubly-fed magnetic reluctance machine which has two sets of windings on its stator, a flux-switching motor which has permanent magnets (PMs) on its stator, a dual-PM-excited motor with PMs on both rotor and stator. These machines have their own merits in specific applications. All these EMs follow a basic operating principle and it is possible to establish a general and unified theory for such types of machines. As the operating principle of these machines is relatively new and the combinations of variation of different topologies are large, there is a great possibility to invent more novel machines. In this thesis, a general theory, which can explain the operating principle of all these machines, is introduced. The unified design guidelines are employed to design and construct new motors. The thesis mainly includes the following contents. a) A literature review on the development of different types of magnetic gears and flux-modulated (FM) machines is present. FM machines include magnetic-geared machines, basic FM machines, Vernier machines, doubly-fed magnetic reluctance machines, stator flux-switching machine, and dual-permanent-magnet machines. b) The internal relationships and operating mechanisms of general flux-modulated machines (GFMMs), which all employ non-uniform magnetic reluctance to modulate their magnetic flux to produce constant torque, are studied. A general theory to explain the operating principle of all these FM machines is proposed. c) Based on the proposed general theory, a doubly-fed dual-stator (DFDS) motor is presented. The performances of the machines are evaluated by finite-element method (FEM) of magnetic field. A prototype is made, and the test bench is established. Experimental results are presented to showcase the performance of the proposed DFDS motor. d) To improve the torque capability, different PM arrangements of a dual-layer PM-excited (DPME) synchronous motor are compared. The configurations of the PM arrangement in the motors are designed in general patterns and optimized by a multi-objective genetic algorithm. A novel triple-layer PM-excited (TPME) synchronous motor is further presented, which has tactfully integrated the three parts of flux-modulated motors inside one frame with one shared set of armature windings. e) A dual-electrical-port synthetic-slot permanent-magnet (SSPM) motor is proposed. With dual-airgap configuration the torque capability of the SSPM motor can be improved by virtue of the increased total area of air-gap surface. A new general pattern of a synthetic-slot structure, containing copper, permanent magnets, ferromagnetic materials, and insulators, is put forward and ingeniously employed to boost the torque density of the proposed SSPM motor. The simulation results using FEM are discussed and verified by experimental results of a prototype and vehicle-test results.