High overall efficiency and wide constant power speed range (CPSR) enabled by flux regulation (FR) capability of a variable flux memory (VFM) machine make it a promising candidate for electric vehicles (EV) applications. This thesis focuses on novel asymmetric rotor topologies of VFM machines with hybrid permanent magnets (PMs), comprising both constant PMs (CPMs, e.g., NdFeB magnets) and variable PMs (VPMs, e.g., Alnico magnets). The synergy of magnetic-field-shifting (MFS) and FR effects are exploited, with particular emphasis on improving torque density and FR range.
Five existing symmetrical V-type-based VFM machines are optimized and analyzed. Their electromagnetic performance is compared to establish guidelines for the proposed novel topologies. The results indicate that the iron layers between CPMs and VPMs are crucial to the PM interaction effect, thereby influencing both torque and FR capabilities. A relatively wider iron layer can effectively enhance FR capability, with only an acceptable reduction in torque.
Five novel asymmetric VFM machine topologies, incorporating both asymmetric PM arrangements and asymmetric rotor core structures, are proposed and investigated. The electromagnetic performance under both flux-enhancing and flux-weakening magnetization states, including open-circuit characteristics, torque and torque components, CPSR and efficiency maps, magnetization characteristics, and unintentional demagnetization withstand capability, are analyzed and compared with those of the existing benchmark machines employing the same stator, rotor diameter, stack length, and PM volume. The comparison confirms that the asymmetric designs can enhance torque through MFS effect and by mitigating the demagnetization cross-coupling effect of CPMs on VPMs. Furthermore, the asymmetric designs can improve FR capability and thereby increase overall efficiency by enlarging the volume of VPM. All proposed topologies are fabricated and tested for validation.
Although the investigation is conducted on small-scale machines, the design guidelines for VFM machines can been established from this thesis, and the proposed topologies are considered promising candidates for EV applications.
