Study On Rotor Loss And Ventilation Cooling Of High-Speed PM Machine

The high speed permanent magnet (PM) machine has been widely investigated in the field of electrical engineering since it has small volume, high power density and high efficiency. Compared with the convertional low-speed machine, the rotor speed can reach up to or even above 100000r/min, so that the air friction loss and high frequency eddy current loss of its rotor are much greater than that of the convertional one. Due to the small size and difficulty of heat dissipation, the PM rotor is easier to be overheated and irreversible demagnetized. Therefore, Anti-demagnetization of PM rotor, accurate calculation of rotor loss and effective design of ventilation cooling system are key techniques for the design of the high-speed PM machine.The research work of this thesis is a part of the project-"The distributed high speed generator system driven by micro-turbines and its energy conversion system", which is supported by the National Natural Science Foundation of China (No.50437010). The study of the thesis is concentrated to the anti-demagnetization technology of PM rotor, calculation of air friction loss and high frequency eddy current loss on the rotor surface, and ventilation cooling of high speed PM machine. The main contents are as follows:(1) Based on the analysis of magnetic field, stress field and temperature field,-the demagnetization possibility of PM rotor caused by armature reaction and overheating is investigated. The design methods to reduce rotor losses, enhance heat dissipation and prevent the PM rotor from demagnetization are proposed through the choice of the material and structure of the PM and its enclosure, air gap and stator core structure, ventilation and heat dissipation structure, etc.(2) The influence of the number of stator slots on machine performance and eddy current loss of the rotor is analyzed using FEM. Based on the fluid field analysis combined with experiment, the temperature rises of the 6-slot high speed machine without inner vents and 24-slot machine with inner vents are comparatively studied, and the influences of axial ventilation structure and parameter on the temperature rise are investigated. (3) The influences of rotor speed, length of air gap, surface roughness of rotor and stator and axial wind speed on the air friction loss of the rotor are studied. Based on fluid field analysis, the calculation model of the air friction loss of rotor is proposed. Through finite element analysis, the relationship between eddy-current loss and rotor speed is established. The validity of the proposed calculation methods of air friction and eddy-current losses of the rotor is verified by the experiment of losses segregation.(4) On the basis of fluid-solid coupled analysis, the calculation model of temperature rise of high speed PM machine with air cooling and oil cooling systems is established. Through the comparison of the calculated temperature rises with tested results of the 24-slot machine with air cooling and 12-slot machine with oil cooling, the effctivity of the temperature calculation method is validated.(5) Based on the high-speed characteristics of the high-speed machine, a new structure of axial fan which is directly mounted on the rotor of high-speed machine is investigated. Through the design of internal ventilation, a self-cooling system is established to simplify the ventilation and cooling structure of high-speed PM machines.