基于磁-热双向耦合的电动拖拉机轮边电机电磁性能分析与结构优化

Electromagnetic performance analysis and structural optimization of the wheel-side motors for electric tractors based on magneto-thermal bidirectional coupling

为预测和评估电动拖拉机轮边驱动电机的电磁性能同时优化电磁转换装置结构,该研究以一种减速式12槽7对极永磁无刷直流轮边驱动电机为对象,建立电机电磁有限元模型并通过试验验证模型的正确性,分析了空载、半载和额定负载工况下电机的动态特性。在此基础上,基于热损耗载荷构建电机的磁-热双向耦合模型,模拟分析额定负载工况下电机的温度场分布规律。结果表明,3种工况下电机最高温度均发生在永磁体部位,温升分别为54.61、77.63和87.1℃。以气隙宽度、定子槽口宽度、永磁体间距和宽度为变量,以齿槽转矩、转矩密度和永磁体损耗为优化目标,提出一种田口法-响应面法双层多目标优化方法,确定电机最佳结构参数为:永磁体间距为0.4 mm、永磁体宽度为3.5 mm、气隙宽度为1.1 mm、定子槽口宽度为4.75 mm。仿真结果表明,优化后的电机转矩密度较优化前增加了8%,永磁体损耗降低了18.6%。样机台架试验结果表明,优化后电机齿槽转矩较优化前降低了20.9%,验证了所提优化方法的有效性,对电机性能研究具有一定的参考价值。

The wheel motor drive system is one of the structure transmission schemes in an electric tractor.The coordinated control of electronic differential speed and torque can significantly improve the energy utilization of tractors.The motor of the electric tractor can also be urgently required for the energy conversion device for higher energy,efficiency,and reliability,compared with the traditional fuel tractors.Among them,the finite element(FE)method can be used to accurately predict and evaluate the electromagnetic performance of the wheel drive motor in electric tractors and then to optimize the structure of the electromagnetic converter.However,traditional models cannot consider the bidirectional magnetic-thermal coupling between the electromagnetic and temperature field,leading to the low prediction accuracy of the model.In this work,the FE model was developed and then verified by a series of experiments.A kind of 12-slot,7-pole brushless DC wheel drive motor with the reducer was also taken as the research object.Dynamic characteristics of the motor were then analyzed under no-,half-and rated loading.Simulation results show that there was no magnetic leakage,where the flux density was in the unsaturated state under no-load conditions.The loss of copper,iron,and eddy current in the motor rose dramatically with the increase of the load,where the copper loss increased the most.Specifically,the copper losses of the motor tended to be 0,60,and 340 W,respectively,under no-,half-and rated loading.The average core losses were 9,15,and 22 W,respectively,whereas,the average eddy current losses were 11,32,and 58 W,respectively,under the three load conditions.The magnetothermal bidirectional coupling model of the motor was then constructed to simulate the distribution of the temperature field.The results demonstrated that the highest temperature of the motor occurred on the permanent magnets under three working conditions.Temperatures of permanent magnets were elevated by 54.61,77.63,and 87.10℃,respectively.In addition,the air-gap width,stator-notch width,spacing,and the width of the permanent magnet were set as the variables,while the groove torque,torque density,and permanent magnet loss were taken as the optimization objectives.A two-layer multi-objective optimization was proposed to improve the optimization efficiency and accuracy using Taguchi and response surface.The optimal structural parameters were determined as follows:The optimized spacing between permanent magnets,the width of permanent magnets,the air-gap width,and stator-notch width were 0.4,3.5,1.1,and 4.75 mm,respectively.Simulation results showed that the motor torque density increased by 8%compared to that before optimization,whereas,the permanent magnet loss decreased by 18.6%.The bench test of the prototype showed that the groove torque of the motor was reduced by 20.9%after optimization,indicating the effectiveness of the optimization.This finding can provide a strong reference to promote motor performance.