摘要
随着高速轴承技术和电机控制技术的发展,高速永磁电机以其低转子损耗、高功率密度的特点,应用越来越广泛。然而,在高速状态下,永磁电机铁心内磁场交变频率很高,定子铁耗很大,发热严重。为了减少定子铁耗,本文结合软磁铁氧体高初始磁导率、高电阻率、低成本的特点,提出了一种基于软磁铁氧体的高速永磁无刷直流电机结构,并对这种电机进行了深入地分析,主要内容如下：
首先,基于软磁铁氧体高磁导率、高电阻率的特点,将软磁铁氧体磁环作为电机定子铁心,转子采用钕铁硼磁环,构成无齿槽电机结构,通过定转子尺寸的合理匹配可以使软磁铁氧体和转子钕铁硼各自运行于理想工作点,提高了电机功率密度的同时使电机铁耗大大降低,效率明显提升。通过样机实验证明本文提出的电机设计方案是正确可行的。
其次,对基于软磁铁氧体的高速永磁无刷直流电机内部磁场进行了深入分析研究。一方面,通过分析计算无齿槽结构永磁电机内的气隙磁场分布特点,指出电机气隙内磁场分布不均匀,即同一根绕组导体处于不同的位置感生旋转电动势不同。同时转子磁钢充磁方式对电机气隙磁场影响较大,通过计算比较发现转子磁钢采用平行充磁可以有效提高电机内气隙磁场的大小,适用于高速永磁电机。另一方面,基于无齿槽电机气隙磁场的解析计算结果,进一步推导了该类电机绕组感生电动势解析计算公式,其计算结果与有限元对比,吻合较好,证明该计算方法准确可行,为电机的设计计算提供依据。另外,由于无齿槽结构电机有效气隙很大,电机的端部漏磁严重,本文利用三维有限元分析了电机的端部漏磁,指出端部漏磁对于靠近端部的气隙磁场影响严重,电机设计时必须考虑这一因素的影响。基于此本文提出了一种改进的转子励磁结构,以改善电机端部漏磁,通过三维有限元计算结果证明该转子励磁结构是有效的、可行的。
再次,结合软磁铁氧体材料饱和磁感应强度较低以及软磁铁氧体为各向同性软磁材料的特点,提出了一种新的高速永磁电机弱磁结构,电机的直流励磁绕组通过外部旁路铁轭给电机定子铁心加一轴磁场使其饱和,从而增大转子永磁磁路的磁阻,达到弱磁的目的。通过三维有限元分析计算,证明此弱磁结构的正确性、有效性。为了简化有限元计算,对上述弱磁结构电机的三维磁场,提出了一种等效的二维有限元模型,通过与三维有限元计算结果的对比证明该二维等效模型是准确可靠的。
最后,基于无位置传感器无刷电机专用芯片ML4425,本文设计了一套高速永磁无刷直流电机控制系统,并进行了样机实际运行试验,证明了此控制系统的可行性。
With the development of high-speed bearings and motor control technique, the High-speed permanent-magnet motors, which have advantage of low rotor loss and high power density, have been applied more and more extensively. 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. In order to reduce the stator iron loss, A permanent magnet brushless DC motor is designed, the material of which the stator is soft ferrite, which has the characteristics of high initial permeability, high resistivity and low cost. And then a depth study of this motor is carried out. The main contents are as following:
Firstly, based on the characteristics of high permeability and high resistivity of soft ferrite, a slotless motor structure is constructed, which the stator core is made up of soft ferrite and the rotor of NdFeB magnetic ring. The sizes of stator and rotor are matched appropriately, which makes the soft ferrite and NdFeB run at the ideal operating point and the power density is increased while the motor iron loss is reduced greatly and efficiency improved significantly. Prototype experiments indicate that the design proposed in this paper is feasible.
Secondly, the internal magnetic field of ferrite-based high-speed permanent magnet brushless DC motor is analyzed especially. On the one hand, by analyzing the characteristic of the distribution of slotless permanent magnet motor, the conclusion that the distribution of gap magnetic field is not homogeneous, i.e.the same conductor in a winding generates different electromotive force in different location. Besides, the motor air gap magnetic field is influenced greatly by direction of the magnetization of rotor magnets. Via calculation, the conclusion that the parallel magnetized rotor magnet can enhance the air gap magnetic field is proposed, and parallel magnetization is fit to the high-speed permanent magnet motor. On the other hand, based on the analytical results of the slotless motor air gap magnetic field, analytical emf formula of that kind of motor is deduced and verified by the result of finite element analysis, and then the formula can provide the basis for the design and calculation of motor. Additionally, due to the big air gap of slotless motor, the magnetic leakage of motor end is serious. By means of three-dimensional finite element analysis, the flux leakage of motor ends are analyzed, and the result indicates that magnetic flux leakage influences the air gap magnetic field seriously, which should be considered in the design of motor. In order to reduce the magnetic leakage of the motor ends, an improved structure of the rotor excitation is proposed and verified by three-dimension finite element analysis.
Thirdly, with the combination of the soft ferrite's characteristics of low saturation magnetic flux density and isotropy, a novel weak magnetic structure of high-speed permanent magnet motor is proposed, the DC excitation winding of which generates an axial magnetic field and enhances the saturation of the stator yoke iron core and increases the magnetic reluctance of permanent magnet circuit, and the goal of weakening magnetic is achieved. The correctness and feasibility are verified by finite element analysis. In order to simplify the calculation of finite element analysis, a two-dimension finite element model of the weakening magnetic structure equivalent to three-dimension counterpart is proposed and compared with the result of three-dimension finite element analysis, and reach the conclusion that the two-dimension finite element model is accurate and reliable.
Finally, based on sensorless brushless motor ASIC, a control system of high-speed permanent magnet brushless DC motor is designed and prototype testing is carried out, which verifies the feasibility of the control system.
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