Rotor design considerations for a variable-flux flux-intensifying interior permanent magnet machine with improved torque quality and reduced magnetization current

This paper presents design process of the rotor to obtain a variable-flux flux-intensifying interior permanent magnet machine (VFI-IPM) with improved torque quality and reduced magnetization current. The VFI-IPMs have attracted much attention because the magnetization state of the permanent magnets can be adjusted according to load conditions. By applying a d-axis pulse current to magnetize the PMs to a high state at low speeds and decease the magnetization state of the PMs at high speeds, the VFI-IPM can achieve high efficiency at a wide speed range. However, since low coercive-force magnets, such as Alnico, are always used and the remnant flux density is always lower than rare-earth magnets, methods to achieve high torque density should be explored. Moreover, to re-magnetize the PMs completely when the rotor slows down, the magnetization current is usually several times of the rated current. Therefore, it is essential to reduce the magnetization current and thus reduce the inverter rating. In this paper, tangential magnetization orientated Alnico magnet with suitable magnetic energy (BH_max) and perfect squareness is selected to obtain high torque density. However, torque ripple is serious in tangentially magnetized PM machines, so methods to reduce torque ripple are also proposed in the following study. To further improve the torque density and reduce the magnetization current, four different topologies are built in finite-element (FE) software and then an optimal topology is confirmed by comparing these four models. Based on the optimal topology, prototype has been manufactured.