Electromagnetic Design And Analysis Of Direct-drive Permanent Magnet Wind Generator

In recent years, air pollution has become a serious problem in China. Gradually substituting coal-fired power generation by renewable energy generation is considered as a practical way to improve air condition. As a mature renewable technology, wind power is rapidly developing in many countries. The direct-drive permanent magnet generators have market potential with the advantage of high power density and efficiency, good reliability, and low maintenance cost. The main contents of this thesis are the electromagnetic design and analysis of a 1.5 megawatt direct-drive permanent magnet wind generator.In the beginning, the design consideration of the slot/pole combination is explained. A slot/pole combination of 12/10 is selected to design a fractional-slot concentrated-windings synchronous permanent magnet wind generator. An approximate equation is used to calculate the effective length of the generator with radial ventilating ducts. The initial electromagnetic plan is designed by the magnetic circuit method. The influences of pole numbers and tooth width on machine performance are analyzed by finite element analysis method. The thickness of the silicon steel sheet in the rotor yoke is carefully designed to minimize the eddy current loss of the structure steel.The operational life of the permanent magnet wind generator is significantly limited by the eddy current loss of permanent magnets. In the next section, a 3-dimensional finite element analysis is employed to analyze the eddy current loss of permanent magnets. The influences of both axial and circumferential segmentation of magnets are investigated by 3-dimensional finite element analysis. The results show that both magnets segmentation methods have evident effect on eddy current loss reduction. In conventional circumferential segmentation ways, the magnets are segmented at the same size. However, since the eddy current losses are uneven in different segments of the same pole due to the armature reaction and slotting effects, uniform magnet segmentations may cause uneven thermal distribution in the magnets, which may further increase the risk of irreversible demagnetization of the magnets. Therefore, the non-uniform circumferential magnet segmentation method is employed in this thesis to balance loss density inside the segmented magnets and reduce the total eddy current losses in the magnets.Combining with the above analysis, a direct-drive permanent magnet wind generator is designed and analyzed. The rated power of the machine is 1.65 MW while its power factor is 0.9. The total harmonic distortion of the back electromotive force is only 0.7%, which implies perfect sinusoidal performance. The torque ripple at rated load is just 1.2%, which means there is no need to apply complicated crafts such as skewing to reduce the torque ripple. For permanent magnets, there is no risk of the irreversible demagnetization in short-circuit condition. The machine performances at different speeds are also analyzed in this thesis.