| With the advantages of high power density, small geometry size, fast transient response and high efficiency, High-Speed Permanent-Magnet brushless DC motor has broad application prospects. However, in the state of high speed, the frequency of alternating magnetic field is so high that the stator iron loss is large, which leads to the motor overheating. Meanwhile, the torque ripple caused by cogging torque seriously affects the control precision. Considering the characters of high initial magnetic permeability, high electric resistivity and low cost, soft-magnetic ferrite ring is used as stator core to reduce the iron loss effectively. In order to eliminate the cogging torque, slotless brushless Permanent-Magnet DC motor is adopted. This paper focuses on the research of structure, the magnetic field distribution, optimal design and control system of this kind of motor.Firstly, based on the characteristics of high permeability and high resistivity of soft ferrite, a slotless motor structure is constructed with soft ferrite stator core and NdFeB rotor core, and the internal magnetic field of this kind of motor is analyzed especially. On the one hand, it is found that the air-gap magnetic distribution is not homogeneous and the induced electromotive force (EMF) is related to the location of conductor in a winding by analyzing the characteristic of distribution of this kind of motor. Therefore, based on the analytical calculation of the slotless motor air-gap magnetic field, the analytical EMF formula is deduced and verified by finite element method and experiment, which provides the basis for the design and calculation of this kind of motor. On the other hand, the end leakage is serious due to the large effective air gap in slotless motor. So the end magnetic field is analyzed by three-dimensional finite element method. The result indicates that the end leakage affects the air-gap magnetic field near the end seriously, which should be considered in the design of motor.Secondly, the winding inductance has a great effect on the operating characteristics of Permanent-Magnet brushless DC motor. However, the end leakage is serious due to the large air gap of slotless motor, and leakage inductance is relatively great because of the long end coil, which causes the traditional calculation method of winding inductance no longer applicable. In order to calculate the winding inductance, an analytical model of linear part of winding inductance is established by the energy perturbation method, and the approximate calculation of end leakage inductance is derived from the basic theory of electromagnetic fields. Via the comparison of finite element ananlysis results and measured values, the winding inductance calculation is certified correctly, and provides a strong basis for accurately studing the electromagnetic performance of the slotless Permanent-Magnet brushless DC motor.Thirdly, as the saturation magnetization of soft-magnet ferrite is low (0.4T) but that of NdFeB is high, their sizes need to be matched reasonably to reach their optimal operating point. At first, on the basis of EMF caculation. the analytical expression for power density is deduced. The result shows that the sizes of the motor stator and rotor have an important influence on the power density. Then, with optimization variables of the stator inner radius and the permanent magnet outer radius, the optimization model of structural size targeted at power density per unit volume is established. The sizes of stator and rotor are determined by genetic algorithm, and the basic method of size matching design of stator and rotor is obtained. The results show that optimized power density is improved significantly with the verification of MATLAB graphics simulation and the finite element analysis, which has high theoretical value in this kind of actual motor design.Finally, in the basis of the 16-bit enhanced flash digital signal controller dsPIC30F2010, a sinusoidal drive control system of Permanent-Magnet DC motor is designed. Based on space vector modulation, the sinusoidal drive is realized initially by practical debugging. The experimental waveforms are comparatively ideal, and the motor speed and torque become relatively smooth so that the vibration and noise are ameliorated. |
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