Variable Flux Machines: A Review

Generally, Permanent Magnet (PM) synchronous machines are common for applications where a higher torque density is required because they use rare-earth magnets with high coercive force (HCF) and high remnant flux density. However, for applications that require variable speed, they exhibit some issues since the magnet flux is fixed. Variable Flux Machines (VFMs) are investigated to address such issues and they have potential due to their capability of varying the flux especially at high speeds by demagnetizing the low coercive field (LCF) magnets. This paper reviews the VFMs and hybrid VFMs (HVFMs) to obtain an overview of the current state in the designs and applications. This paper mainly focuses on recent VFM and HVFM literature and the HVFM machine designs were placed in three categories (series, parallel and hybrid designs). Magnetic materials used in these machines were also discussed. The literature survey shows that torque and power densities for PM machines can be achieved by VFMs particularly for electric vehicle (EV) applications where driving conditions differ based on the driving cycle pattern. Hybrid Variable Flux Machines (HVFMs) are found to address challenges faced by VFMs which only use LCF magnets. One of the challenges is the low reluctance torque output which limits the variable flux regulation range and results in potential demagnetization, especially under load. However, more work is needed to determine the appropriate magnet dimensions. In the case of hybrid VFM, the split ratio between the HCF PM volume and the LCF PM volume is still a hot research area.