Study Of Coupled Electromagnetic/Thermal Characteristics Of Direct Drive Permanent Magnet Machine For Pure Electric Helicopter

A pure electric helicopter has advantages of low pollution,low noise and high efficiency.Therefore,it has great prospect in the future short distance transportation system.Permanent magnet synchronous machines(PMSM)are widely adopted in propulsion systems of electric vehicles and aircrafts for the characteristics of high efficiency and high power density.Direct-drive has become a promising driving mode,which offers the benefit for elimination of gearbox,lower complexity,less maintanence cost and higher system stability.PM machines are attractive candidates for direct-drive applications.A PMSM for a direct-drive electric helicopter is developed and analyzed in this research.According to operating features of the helicopter and design objectives of the electric machine,two PM machine schemes are proposed and the design features are introduced.Based on differential evolution(DE)algorithm and finite element analysis(FEA),a series of candidate machines are designed and optimized.The characteristics of flux density in air-gap of the machine schemes are analyzed.Losses,efficiency and effective masses of the candidate machines are compared to select the best candidate machine.Experiments are conducted to verify the validity of the machine design method.FEA model of temperature field of the protoype machine is developed.Equivalent thermal models of laminations and windings are developed and heat transfer coefficients in the model are estimated with empirical formulas.Sensitivity analysis of the heat transfer coefficients with highest temperature in winding and magnets is conducted with the method of design of experiments(DOE).Heat transfer coefficients are modified with the results of steady state temperature field experiments.Coupled electromagnetic/thermal analysis is adopted to calculate and predict the temperature field at the rated operating condition.Hence,the power capability of the prototype mahcine within thermal constraints is analyzed based on the modified coupled electromagnetic/thermal model.Variation of dc copper loss,iron loss and eddy current loss in magnets with the increase of output power is depicted.With the improvement of cooling condition,the maximum power output is analyzed with the coupled electromagnetic/thermal calculation and the direction of cooling system optimization is studied.