Losses And Thermal Field Analysis Of Permanent Magnet Brushless DC Motor Used In Near Space Electric Propulsion System

In recent years there has been an increasing interest in the development of high altitude vehicles operating in near-space environments Near space is the atmosphere which extends from 20 km to an altitude of about 100 km.Permanent magnet brushless DC motors(BLDCM)are very attractive for application in the aerospace field because of their numerous advantages,such as high power density,high efficiency,small size,light weight,high reliability,etc.With the development of near-space vehicle technology,BLDCM has become a vital component of electric propulsion system.Operating in a nearspace environment,however,presents challenges with respect to heat dissipation and thermal management in the BLDCM.It’s a technical difficulty that studying how to reduce the losses and manage the heat of propulsion motors in order to realize the efficient and reliable operation of electric propulsion system.Therefore,it is great significance for studying the loss and temperature rise in BLDCM used for near space vehicles.The main research contents in this dissertation are as follows:1.The accurate calculation of BLDCM stator iron losses helps not only the analysis of the BLDCM performance,but also the calculation of the thermal field in BLDCM.This dissertation presents an iron loss calculation model which considering both alternating and rotating magnetic field.The distribution characteristics of hysteresis loss and eddy current loss are studied;the influence factors and loss variation tendency with the increase of speed are analyzed.The results show that the accurate calculation of iron loss provides the basis for the thermal field analysis.Also it has a guiding significance for the BLDCM optimization design.2.Large amount of eddy current losses will be induced in the rotor PMs and retaining sleeve when the surface mounted BLDCM operates at very high speed.The rotor eddy current loss that results in the heat generation is caused by the time and space harmonics of reluctance and armature winding.The rotor eddy current losses in load condition and no load condition are calculated by time-step FEM method.The influence of retaining sleevre material to loss and thermal field is analysized and compared between stainless steel and carbon fiber.3.The temperature rise of BLDCM not only reduces its operation performance,but also influences the life time and reliability of the motor.This dissertation presents the heat process and heat exchange in BLDCM,the heat transfer parameters and heat sources are analyzed.Based on the temperature field theory and finite element method,the model of the electromagnetic-thermal coupling field is established.The transient thermal field of BLDCM is calculated and the accuracy is verified by the temperature rise experiments.4.The atmospheric parameters and heat dissipation characters for near space environment are prescribed.The high-altitude thermal field of the BLDCM is modeled considering the heat transfer relation between the motor and the high altitude environment.The thermal boundary conditions and the various heat transfer coefficients in the motor at high altitude environment are determined.The influences of altitude,temperature and air density for the thermal field and heat dissipation in BLDCM are analyzed.The temperature field is calculated by finite element method and the simulation results are compared with the experiment data to validate the high-altitude thermal model.5.Combining the experiments on high-altitude adaptation research of the prototype,the variations of thermal fields in BLDCM are compared and verified in ground envirment and high-altitude envirment.The general rules of temperature rise with altitude are discussed when the losses in BLDCM keep constant.A formula of relationship of output power and altitude under the rated temperature rise is put forward.The variation tendency of radiation heat transfer capacity with altitude is analized.The motor temperater rise with altitude related to the change of heat radiation parameters is studied.The calculation results have a guidance function to the thermal radiation design and further study of high altitude thermal field.