Design, analysis and application of novel slot-opening permanent magnet machines

Abstract

In the past few decades, the stator permanent magnet (PM) machines have attracted much research attention because of their reliable rotor structure, high power density, wide speed range, and strong fault-tolerant ability. Compared with the machines with PM designed on the rotor side, which is referred to as rotor PM machines, the stator PM machines enjoy better thermal conditions because the PM is designed at the stator side, which leads to less demagnetization risk. Besides, as all the excitation sources like PM and excitation current are assigned at the stator side, the stator permanent magnet (PM) machines have simple and robust rotor structures, which consists of iron core only. Therefore, the stator PM machines are promising candidates for various industries like wind power generation, electric vehicles (EV), and aerospace. Among the recently proposed stator PM machine topologies, the stator slot-opening PM (SS-PM) machines are becoming increasingly popular because of their novelty and high stator space utilization factor. According to the SS-PM magnetization direction, the SS-PM machines can be classified into three categories, namely, the tangentially magnetized stator slot-opening PM (TMSS-PM) machines, the radially magnetized stator slot-opening PM (RMSS-PM) machines, and the compound magnetized stator slot-opening PM (CMSS-PM) machines. The design purpose of SS PM and associated working principles are diverse in various SS-PM machines, which leads to the difference in machine electromagnetic performances. In this thesis, the current research state of art of SS-PM machines is reviewed. The SS-PM machines are classified into three types according to the magnetization direction. The basic machine structure and working principle of each type are introduced and the recent development in topology is reviewed. A qualitative performance assessment and comparative study of SS-PM machines are presented, which evaluates the electromagnetic performances like torque density, torque ripple, efficiency, and power factor. Then, the SS-PM assisted complementary-rotor doubly salient machine is proposed. Two groups of TMSS PM, which is parallel-excited with the yoke PM, are employed to improve the torque density. To solve the problem of high torque ripple, which results from the leakage flux of doubly salient structures, the proposed machine adopts the complementary-rotor structure to provide a commutative flux path for PMs so that the leakage flux and associated torque ripple can be largely reduced. Next, the hybrid reluctance machine with skewed PM is proposed. The skewed PM has two magnetization components. The radial component can improve the torque density under the flux modulation effect. The tangential component can relieve the stator core saturation caused by the DC current under the relieving-DC-saturation effect. Therefore, torque promotion and core saturation relieving can be achieved simultaneously. Further, to further improve the efficiency of the hybrid reluctance machine with skewed PM, the armature winding with zero-sequence current is used to replace the DC and AC windings. This is because the copper loss can be reduced when DC and AC windings are combined. The working principle, machine structure, and electromagnetic performance are introduced in detail.