Current References for Optimized Loss and Torque Ripple Per Operating Point in the Full Torque-Speed Range for Symmetrical Six-Phase PMSMs With Arbitrary Back-EMF

Abstract

Six-phase permanent-magnet synchronous machines (PMSMs) offer multiple benefits compared with three-phase ones, e.g., increased torque density and reduced phase-current rating. A method has been recently proposed to generate current references ensuring minimum stator copper loss (SCL) per torque for nonsalient PMSM drives with sinusoidal or nonsinusoidal back-electromotive force (back-EMF), while the peak currents were suitably limited (with harmonics) to the maximum instantaneous currents of the inverter even during transient overloads. Furthermore, even greater mean torque could be attained by permitting a certain (limited) torque ripple when needed. However, such technique did not take into account the voltage limits imposed by the dc-link voltage. This paper proposes an alternative method based on look-up tables (LUTs) that provides these functionalities for symmetrical six-phase nonsalient PMSMs, while effectively considering the voltage constraints in the optimization. This makes it possible to minimize the losses and torque ripple per mean torque in not only the entire torque range, but also in the full speed range. To decrease the computational requirements, the offline LUT-generation process is designed so that it identifies the range of operating points where the solution changes, and then the optimization problem is solved only within such range.