High speed permanent magnet synchronous machines (PMSMs) have attracted much attention due to their high power density, high efficiency, and compact size for directdrive applications. However, the consequent power loss density is high, and hence heat dissipation is a major technical challenge. This is particularly the case for high-speed operation. In this paper, a MW level high speed PMSM is designed and its electromagnetic and mechanical power losses comprehensively investigated using finite

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... lement analysis. The transient machine demagnetization performance is studied, and a composite rotor structure is proposed to improve machine anti-demagnetization capability. The temperature distribution of the proposed high speed PMSM is also analyzed using a fluid-thermal coupling method with calculated power loss. Experiments conducted on a prototype of the high speed PMSM demonstrate the effectiveness of the numerical models developed and validate the results obtained. Index Terms--Demagnetization, finite element method, high speed PM machine, magnetic field, power loss, thermal analysis.