Feasibility of High-Pole-Number Synchronous Reluctance Machines for Low-Speed, High-Torque Direct-Drive Applications

Abstract

Abstract

Direct-drive, low-speed, high-torque (LSHT) applications operating at a fixed rated frequency require electrical machines with high pole numbers. However, synchronous reluctance machines (SynRMs) are often considered unsuitable in this range because increasing the pole number can reduce saliency and power factor. This paper investigates the feasible pole-number range of magnet-free SynRMs for direct-drive operation and compares inner- and outer-rotor configurations. A systematic optimisation campaign, comprising 93,650 two-dimensional finite-element-evaluated designs, was performed for 12- to 24-pole machines operating at 50 Hz. The designs were evaluated under fixed constraints on machine envelope, terminal voltage, and total losses, while efficiency, torque density, power factor, and torque ripple were considered as performance objectives. Inner-rotor designs satisfied all targets up to 18-pole; beyond this point, the reduction in saliency required higher stator current, thereby increasing copper losses. By contrast, the outer-rotor configuration enabled a larger air-gap diameter within the same envelope, preserving the saliency ratio and satisfying all targets up to 24-pole at the 20 °C reference condition adopted for the screening, the best 24-pole design reaching 91.2% efficiency and 17.1 kNm/m3 at 250 rpm. The results demonstrate the potential of outer-rotor SynRMs as magnet-free candidates for low-speed, high-torque direct-drive applications.