Design, analysis and application of variable reluctance machine

Abstract

Variable reluctance machine (VRM) whose rotor consists of only iron core, has the merits of simple machine structure, mechanical robustness, low manufacturing cost, as well as inherent fault tolerance. However, there are some disadvantages existing in VRMs. Some VRMs suffer from large torque ripple. In addition, torque density and flux regulation capability need to be further improved. Therefore, this thesis mainly contributes to following aspects, 1) Adopting selected slot/pole combination and special winding connection to reduce torque ripple, 2) Exploring novel topologies or new winding design to enhance the torque density, 3) Introducing low coercive force (LCF) PMs to realize flexibly flux regulation. A parallel-excited dual-PM VRM with all excitation sources placing in the stator is proposed in Chapter 2. With special slot/pole combination and winding connection, DC component and even order harmonics of the phase flux are eliminated, resulting in more sinusoidal flux-linkage. The phase self-inductance variation is greatly reduced with odd order harmonics of phase self-inductance removed, leading to small torque ripple. In order to enhance the torque density, a doubly-fed doubly-salient machine (DF-DSM) is proposed in Chapter 3. Bias dc current excitation leads to stator core saturation easily, which limits torque improvement. Therefore, PMs are placing in slot opening to relieve DC saturation. In order to further enhance the torque density, a slot-bottom-PM VRM is proposed in Chapter 4. With auxiliary sub-slot bottom PM excitation, the stator yoke DC saturation can be mitigated to enhance the torque production capability of DC field excitation, meanwhile additional slot-PM flux can be pushed into the airgap, which can build a parallel excitation and generate additional effective torque. In addition, PMs is proposed placing in the slot bottom to save the space for ac armature winding. For torque density enhancement, a novel high-order harmonic winding design (HHWD) is proposed in hybrid-excited VRM with dc field excitation and slot-bottom PMs, as shown in Chapter 5. The key is to break the previous winding design principle using the fundamental field excitation harmonic, but to propose a novel winding design principle with flux modulation effect utilizing high-order field excitation harmonic. In this way, the no-load back-EMF and steady torque can be significantly enhanced comparing with the previous counterpart. Similarly, a high-order harmonic winding design (HHWD) is comparing with previous winding design in dual-PM VRMs, as shown in Chapter 6. In this paper, torque contributions of the main working harmonic with two winding designs are investigated and quantified to offer a comprehensive understanding of their torque production mechanism. Besides enhancing the torque density of VRM, following research work focuses on improving flux regulation capability in Chapter 7. Low coercive force (LCF) PMs are introduced to regulating flux, since which can be freely magnetized and demagnetized through DC current pulses. Also, to maintain relatively high torque density, each pair of yoke PMs is composed with one pole of high coercive force (HCF) PMs and one pole of low coercive force (LCF) PMs.