基于保角变换与等效磁路法的永磁直驱发电机气隙磁场计算

Air Gap Magnetic Field Calculation of Permanent Magnet Direct Drive Generator Based on Conformal Mapping and Magnetic Equivalent Circuit Method

该文开展了负载工况下永磁直驱发电机气隙磁场研究。基于精确域求解和保角变换,计算考虑齿槽效应的气隙磁场标量磁势;采用离散线电流元,定量模拟发电机定转子局部出现的磁饱和现象;基于考虑几何尺寸的定转子非线性磁阻计算方法,建立包含发电机定转子结构以及气隙长度的等效磁路,给出相应的迭代求解流程。以实际在役兆瓦级永磁直驱风力发电机为对象,计算考虑齿槽效应的气隙磁场分布,分析了空载和负载工况下磁饱和效应对气隙磁场分布曲线波形和谐波成分的影响,并与有限元计算结果对比,验证了所提出的分析模型和求解方法的有效性。

Owing to its outstanding advantages such as good low-speed performance, high power generation efficiency, low maintenance cost and reliable operation, megawatt permanent magnet direct drive wind turbine is an ideal choice for large-scale onshore and offshore wind power generation. In the performance analysis and optimal design of permanent magnet direct drive wind turbines, accurate and efficient air gap magnetic field distribution is an important premise.In this paper, the air gap magnetic field of permanent magnet direct drive generator under load condition is studied. Based on the exact domain solution and conformal mapping, the scalar magnetic potential of the air gap magnetic field considering the cogging effect is calculated. Discrete line current element is used to quantitatively simulate the local magnetic saturation of generator stator and rotor. Based on the calculation method of the nonlinear reluctance of the stator and rotor considering the geometric size, the magnetic equivalent circuit model including the structure of the stator and rotor and the length of the air gap is established, and the corresponding iterative solution process is presented. Taking the actual megawatt permanent magnet direct drive generator in service as an example, the air gap magnetic field distribution considering the cogging effect is calculated, and the influence of magnetic saturation effect on the air gap magnetic field distribution curve waveform and harmonic components under no-load and load conditions is analyzed. The proposed analysis model and solution method are verified by comparing with the finite element results.Under the no-load condition, the air gap magnetic field intensity is basically unchanged whether the saturation effect is considered or not. The harmonic component includes the fundamental frequency and its odd frequency multiples, and the combination of the tooth slot frequency and the fundamental frequency. Under the load condition,the relative difference between the magnetic field intensity with and without saturation effects can exceed 15%,indicating that the influence of magnetic saturation effect is significantly enhanced and cannot be ignored in the load condition. Compared with the result in the no-load condition, the variation amplitude of the air gap magnetic field intensity increases obviously. The harmonic component is basically the same as that in no-load condition, but the amplitude is higher than that in no-load condition, especially for the increase of the amplitude of fundamental wave frequency.The analysis model and solution method proposed in this paper provide important preconditions for the magneto-solid coupling analysis and performance optimization design of megawatt permanent magnet direct drive wind turbines.