Effect of Rotor Type on Open-Set Derating Operations of Multi-Three-Phase Synchronous Machines

Multi-three-phase machines have attracted considerable interest in several fields thanks to the enhanced fault tolerance and the reduced phase current, adopting standard three-phase inverters instead of special custom converters. However, the fault-tolerance and the quality of the derated performance is strongly dependent to the winding topology and the type of rotor adopted. This study compares two 12-phases machines with distinctive rotor structure and magnetic behaviors: one with high anisotropy and low PM content and the other with lower anisotropy and dominant PM flux linkage. Efficiency and torque ripple are included in the comparison, both in healthy and open-set derated conditions. The results shows that derating is not crucial on an average point of view, but can significantly increase the torque ripple and, in some scenarios, to further reduced operating area. Notably, the high-anisotropy machine demonstrates superior field-weakening performance, whereas the motor with greater PM flux linkage achieves better derated output below base speed. These findings highlight the importance of rotor selection in optimizing multi-three-phase drives, according to the specific application.