Design, analysis and application of novel hybrid excitation flux modulation machines

Abstract

Hybrid excitation machine, which combines the PM excitation and wound field excitation, has the potential to achieve both good torque density and flux regulation ability, and thus can be a potential candidate for electric or hybrid electric vehicle applications. For traditional PM machines in which PMs are arranged at rotor side, realizing a hybrid excitation structure usually needs brush and slip rings, which decreases system reliability. Although some design techniques featured by radial and axial combined magnetic circuits can eliminate brush and slip rings, they increase the complexity of mechanical structure, and worse, degrade torque density due to redundant air gap. For emerging stator PM machines with PMs arranged at stator side, the corresponding hybrid excitation structures can naturally achieve good flux regulation with a brushless structure. However, their torque density is poor than that of rotor PM counterparts, due to low PM utilization, extra DC saturation in stator core, and the space conflict between the AC field coils and DC field coils. Therefore, there is need of new hybrid excitation technologies, which realizes a brushless structure and integrates both good torque density and flux regulation ability. In the recent decades, with the development of flux modulation theory, a variety of new design technologies have been proposed to increase machine torque density by utilizing modulated air gap harmonics. Moreover, the flux modulation principle allows more flexible arrangement of excitation sources and more possible slot pole combinations. This thesis aims to give a synthesis and investigation of new brushless hybrid excitation flux modulation machines with both good torque density and flux regulation ability. A new hybrid excitation Vernier reluctance machine is proposed, in which consequent-pole PMs are artificially introduced into stator slots to generate flux modulation effect and interact redundant armature harmonics, so that the machine torque density is greatly improved compared to its non-PM counterpart. Moreover, the introduced slot PMs share a parallel magnetic circuit with armature excitation, thus providing bidirectional flux regulation ability and demagnetization withstand ability. Besides, to address the space conflict between AC armature coils and DC field coils in the proposed topology, the zero-sequence current control is applied to generate DC current component in AC armature coils, which can function as virtual DC field coils at design stage and contributes to improve torque density and efficiency. The theoretical analysis, finite element simulation and protype experiments are conducted to demonstrate the proposed topology and its improved design based on zero-sequence current control. A new hybrid excitation dual-PM Vernier machine is proposed. This machine adopts dual consequent-pole PMs arranged in both stator slots and rotor slots, thus contributes to boosted air gap flux density and increased torque density. Moreover, DC field excitation is introduced in stator to provide an extra flux weakening variable, which can be coordinated with the traditional d-axis vector flux weakening to further extend the torque speed range. Further, to address the issue of parasitic stator DC saturation effect in the proposed topology, an improved design is proposed, namely relieving-DC-saturation hybrid excitation dual-PM Vernier machine. Especially, a constant flux bias is constructed in stator core by use of tangentially-magnetized stator slots PMs, thus to cancel the DC flux component excited by DC field excitation. In this way, the DC saturation effect in stator core can be eliminated, which improves machine torque density and flux regulation ability. The theoretical analysis, finite element modeling and prototype experiments are performed to verify the proposed topology and its improved design with eliminated DC saturation.