Research On Magnetic System Of High Torque Permanent In-Wheel Motor

The in-wheel motor driving system has been an important research direction in the field of electric driving system because of its advantages of high efficiency and compact structure. With the increasing demand for the perfomance of electric drive system, the further improvement of design method and the intensive study of external characteristics for the magnetic system of in-wheel motor are required such as structure design of high torque density motors, accurate calculation of parameters, inhibition of torque ripple and research on vibration characteristic, etc. The permanent synchronous in-wheel motor of high torque is taken as the research object in this paper, which delves into the problems mentioned above, and provides the theoretical basis for improving the performance of in-wheel motors.Firstly, the design principle of in-wheel motor is presented from the aspect of electromagnetic performance and the selection method of rotor structure and winding type is also discussed. From the angle of maximizing torque density, the mathematical model of unit torque density is built with end winding taken into account, and the curves of torque density maximum with stator split ratio and magnetic density are also obtained. Consequently, a primary selection region of the main dimensions for the in-wheel motor can be confirmed. The flux weakening capability of in-wheel motor is analyzed based on general flux weakening control. The variation trend of electromagnetic torque and structure strength with magnetic bridge width and rib width of magnetic bridge is discussed in detail based on the unidirectional coupling FEM method. The optimization principle of rotor magnetic bridge is given and the mechanical strength of stator and rotor for the in-wheel motor is calculated. The prototype motor is made and the experiments are conducted to verify the design method.The exact evaluating for d- and q-axis inductances is very important for the design and control of in-wheel motors. The influence of winding magnetic potential harmonic and saturation on inductance is analyzed in this paper. The calculation method for d- and q-axis inductances based on assuming magnetic pole is proposed, and the principle of the assuming magnetic pole method (AMPM) is also introduced. The variation curve of self- and manual-inductance is rebuilt. Compared with traditional way, this method can exactly reflect the influence of saturation. The principle of AC standstill test method for inductance parameters is given and experiment on inductance of the prototype motor is conducted. A research method of inductance parameters is proposed based on steady-state operation of high torque permanent magnet in-wheel motor. The steady-state phasor diagram of PMSM is built considering iron losses and the mathematical model of inductance parameters is derived from the phasor diagram. The inductance measurement method in the steady-state operation is introduced, and the sensitivity analysis for this inductances model is performed under disturbance of phase voltage, no-load EMF and winding current.In the in-wheel motor drive mode, the unbalance state of the in-wheel motor magnetic system complicatedly influences the torque ripple. The pulsating torque and the cogging torque when magnetic system is under unbalance state are analyzed through Lorentz force and energy method, respectively, and the variation law of the torque ripple is given. The influence of slot width, tooth tip height, pole-arc coefficient and air-gap length on torque ripple is analyzed by the FEM. The actual model is built according to the measured data, by which the torque ripple and cogging torque is studied respectively when there are elliptical stator, offset-center rotor and magnet offset. And vibration frequency is also given which caused by torque ripple. The method and measures for restraining torque ripple are given. A static measurement system for cogging torque is proposed in order to verify the accuracy of the calculation, which is composed of stepping motor, etc. The prototype experiments for cogging torque and torque ripple are carried out.Furthermore, vibration of the in-wheel motor is an important excitation source which results in the vibration of powertrain and vehicle's interior noise. Based on the distribution of air-gap magnetic field, the characteristics of electromagnetic force wave are shown under the condition of different power supply. The electromagnetic force is analyzed while vibration displacement of in-wheel motor is calculated with different load using FEM. The double-ring analytical method and the FEM are adopted to calculate the natural frequency of the in-wheel motor. The modal of the prototype motor is measured by the hammering method and major vibration source is analyzed. The experimental research on vibration and noise of the prototype motor is carried out based on frequency spectrum analysis method and the experimental results are in agreement with the calculated results. The proportion of electromagnetic and mechanical noise is distinguished by using the method of suddenly cutting down power supply, and the method and measure for inhibition of vibration are given.