Comparison of Heat Transfer Characteristics of the Hollow-Shaft Oil Cooling System for High-Speed Permanent Magnet Synchronous Machines

The rotor cooling poses a significant challenge for high-speed permanent magnet synchronous machines. This article presents a recirculating hollow-shaft oil cooling structure. Through the computational fluid dynamics (CFD) method, its convective heat transfer characteristics are studied in depth. Besides, the recirculating hollow-shaft cooling (RHSC) is compared with the direct-through hollow-shaft cooling (DHSC). Based on the CFD analysis, it is found that the convective heat transfer coefficient (CHTC) of the RHSC is higher than that of the DHSC, which can be as high as 58%. Furthermore, the advantage of the RHSC is most prominent at low rotational speed and small flow rate due to its relatively large oil friction loss at a high rotational speed. Moreover, the explanation for the fluctuation of the CHTC before the critical rotational speed, as well as the law of its variation with rotational speed and flow rate, are revealed in this study. The correctness of the simulation analysis is verified by experiments implemented on a rotor setup. In order to summarize the convective heat transfer law of the RHSC better, a dimensionless empirical equation is fitted finally.