Analysis And Design Of PM Brushless DC Motor Based On Airgap Magnetic Field Modulation Theory

With the increasingly serious energy crisis and environmental pollution,the motor drive mode with high efficiency,excellent dynamic performances and flexible control has become one of the research hotspots in the world.The permanent magnet motor uses high-performance permanent magnets as its excitation source,so it has the advantages of high power density and high efficiency.According to no-load electromotive force(EMF)waveforms and the drive control strategy,permanent magnet motor can be divided into two categories: Brushless AC(BLAC)motor with sinusoidal wave EMF and Brushless DC(BLDC)motor with trapezoidal wave EMF.Compared with BLAC motor,BLDC motor systems have the advantages of structural simplicity,easy control,low cost,high reliability and high torque density,so BLDC motor systems are used in many fields such as industrial applications.However,the no-load EMF waveform of BLDC motors is often not ideal,which leads to the problem of large torque ripples.Therefore,from the perspective of the airgap magnetic field,this paper studies reasons that affect the no-load EMF waveform of BLDC motors,and proposes effective methods to improve the torque density and reduce torque ripples of this kind of motor.The main content of this article includes:1.Based on the analysis of the topology and electromagnetic performance of the rotor permanent magnet BLDC motor,the complexity of the airgap magnetic density of the motor is revealed,thus pointing out the deficiency that airgap flux density harmonics with the same number of pole-pole and different speeds cannot be separated based on the finite element analysis.Based on the airgap magnetic field modulation theory,a method combining the undetermined coefficients method and the finite element method to analyze the airgap magnetic density of BLDC motors is proposed.Taking a 24 slot 4 pole BLDC motor as an example,the airgap magnetic density is analyzed and calculated.The correctness of the calculation method is verified by the finite element simulation.So the theoretical basis is established for quantitative analysis of electromagnetic characteristics and motor optimization by the airgap magnetic density.2.Based on the proposed undetermined coefficients method,the permanent magnet shaping of the 24 slot 4-pole BLDC motor is carried out,which effectively reduces the torque ripple of the motor and improves the torque output capacity.On this basis,the inner diameter of the permanent magnet,dummy slots of stator tooth and shaping parameters of the permanent magnet are scanned with the individual parameter method to further improve performances of the motor,and the final design parameters are obtained.3.In order to further improve the analytical accuracy of the airgap magnetic field,an analytical calculation method of the airgap flux density based on the virtual winding is proposed.The mathematical expressions of each flux density harmonic are obtained,and the formation mechanism of the trapezoidal wave no-load EMF is revealed.4.In order to fully verify the effect of the motor design,the loss of BLDC motor before and after optimization is calculated based on the finite element software.Combined with influences on the airgap flux density due to the optimization,the motor loss is analyzed.