Comparative analysis of eddy current loss in permanent magnet synchronous generator considering PM shape and skew effect for wind power generation

Summary form only given. This paper analyzes the eddy current loss in permanent magnets (PMs) caused by the armature reaction field of a PM synchronous generator (PMSG) on the basis of three-dimensional finite element (3D FE) analysis. The eddy current loss in PMs is the main source of thermal issues. It can result in the irreversible demagnetization of PMs because of direct thermal conduction. Furthermore, because the eddy current loss significantly increases with increasing output power, accurate analysis is an important issue. The analysis models used and the machine manufactured for the experiment is shown. The machine consists of a stator and PM rotor. The stator is of a slotted type employing concentrated winding, and 1 slot-pitch skew is applied. The PM rotor has multi poles, and chamfered PMs are used. The rotor and stator have identical stack lengths of 101 mm; as a result, no overhang exists. In addition, we applied 3D FE analysis for the eddy current loss analysis of the PMSG. 3D FE analysis was adopted to consider the magnetic field distribution resulting from the armature reaction field in PMs. Because the analysis model applied the stator skew structure mentioned previously, it has a different magnetic field distribution along with axial length in comparison with machines without skew structure. The chamfered PM rotor influences the magnetic distribution along the tangential direction. In particular, edge effects should be thoroughly investigated as the PM demagnetizes relatively easily owing to the concentrated magnetic field at both edges of the PM. The edge effects are due to complicated electromagnetic field phenomena such as fringing flux and leakage flux in coil end turns; therefore, 3D analysis is required to accurately take them into account in the analysis. There are 3 categories of the armature reaction field according to the operation mode. They are the current of motoring mode and that of generating mode in both AC and DC load conditions. As the back-to-back test is generally applied for evaluating the performance of PM machines, the current in motoring mode is also considered. In this paper, for better understanding, the back-to-back test uses two identical machines: one operated as a motor and the other as a generator. Furthermore, the electrical input power and output power are subtracted to measure the power losses in both machines. Owing to the different harmonic components in each phase current according to the operation modes, their electromagnetic losses have difference, so the losses are also required to be thoroughly investigated as performed. The experimentally measured current according to the operation mode is shown. As the eddy current loss is difficult to measure, it was calculated by FE analysis on the basis of the measured current. Each current characteristic analyzed by FFT is compared. The motoring mode has the highest THD because the employed SVPWM inverter produces a high chopping frequency. Owing to the influence of the harmonic components, the motoring mode has the highest eddy current loss. It can also be confirmed that the skew structure is useful in reducing the eddy current losses. It is clear that the overall value as well as the distribution of eddy current loss requires thorough investigation. In the full paper, the eddy current distribution derived from 3D FE analysis will be presented in detail.