Comparative Study on Hybrid-Variable-Flux Machines with Integrated- and Separated-Magnetic-Circuit Configurations

Hybrid-variable-flux machines (HVFMs) can achieve more pronounced efficiency improvement compared to the single-variable-flux machine by integrating both variable magnetization state (MS) and variable leakage flux (VLF) properties. However, the optimal magnetic-circuit configuration for arranging these two variable-flux units remains unclear, yet is critical for practical HVFM design. To fill this gap, this paper presents a comparative study of HVFMs with integrated- and separated-magnetic-circuit designs, highlighting the advantages, limitations, and trade-offs of each configuration. Firstly, the machine topologies and operating principles are introduced, with magnetic equivalent circuits (MEC) developed to identify the differences in magnetic-circuit coupling between two variable-flux units, and provide physical insights into how the leakage bridges influence flux regulation behaviors. Then, to ensure a fair comparison, a design optimization is performed to obtain the optimal topologies for each machine design. Afterwards, a comparative study of the electromagnetic performance of the two HVFMs is conducted, including torque capability, efficiency maps, and other metrics. Moreover, mechanical stress are also performed. Finally, a prototype with a separated-magnetic-circuit configuration is manufactured and experimentally tested, which validates the theoretical analyses.