High speed motors: A comparison between synchronous PM and reluctance machines

In high speed systems, the recent trend is to couple the electric machine directly to the mechanical shaft, in order to avoid the use of a gear-box and to increase the overall efficiency and reliability. Surface Permanent Magnet mounted (SPM) machines are usually adopted in such systems for their high torque density thanks to rare-earth PMs. The cost of rare-earth PMs together with the risk of their demagnetization induces to consider other alternatives such as the Synchronous Reluctance (REL) machine. The design of REL machines for high speed applications has to be carried out considering both magnetic and mechanical aspects. In particular, the iron ribs must be accurately designed with the aim of achieving the minimum flux leakage in the ribs and ensuring the structural integrity. The present paper deals with the design and comparison of an SPM and a REL motors with fixed stator geometry and winding. Such an approach allows to investigate the potential of the synchronous reluctance machine in comparison to the SPM one. It is shown that the synchronous reluctance machine represents an interesting alternative to SPM machines for its low cost and insignificant susceptibility to demagnetization. In fact the lower torque density achieved by the reluctance motor can be compensated with a limited increase of the stack length or with a higher electric loading.