Demagnetization Characteristics of a Permanent Magnet Embedded Salient Pole Wind Generator

Abstract

Offshore wind power has become an important direction in the field of wind electricity generation. The commonly used surface-mounted PM wind generator cannot resist the corrosion due to humidity and salt spray. In order to improve the reliability of wind generator, the interior PM structure is adopted in some wind power generators. An improved flux intensifying PM embedded salient pole wind generator (FI-PMESPWG) is proposed in our previous work. As other PM machines, the FI-PMESPWG also faces the demagnetization problems, especially under short-circuit and high temperature conditions. Some important research works about demagnetization performance of interior PM machines are carried out, for example, the transient magnetic behavior of an interior PM machine during fault conditions is investigated in [1], and the partial demagnetization performance of a 7MW interior PM wind generator is investigated in [2]. But these investigations are focus on the normal interior PM machine, and their conclusions are not suitable for the salient pole ones. Therefore, in this paper, the demagnetization characteristics of FI-PMESPWG are investigated to find out some rules suitable for this kind of machines. Fig.1 shows the configuration of FI-PMESPWG. Its rotor core upper surface is designed to a bias arc for yielding a sinusoidal waveform of air-gap flux density. And in each magnetic pole, two PMs with mirror symmetrical magnetizing directions are embedded. In the salient pole, some non-magnetic material is added to guide the magnetic flux path. The magnetic field in the radial direction is intensified owing to the PM's placement. To modify the angle between the two PM pieces, the magnetic field in the magnetic pole will be changed and the demagnetization performance will varies consequently. So, the magnetic field distributions with different PM and non-magnetic material placements and thickness are all calculated and investigated. The variations of magnetic field distributions with different short-circuit currents and operating temperatures are also analyzed based on finite element method. Fig. 2 shows the partially demagnetized regions of selected magnets under 3-phase symmetrical short-circuit faults under rated speed. The demagnetized area, shown in color contour map, is covered by the nodes with flux density <0.6T in the magnetization direction. As shown in the picture, different from the normal interior PM machine, the PM region near the bottom of pole shoe has the highest demagnetization. That's because the side frame of the pole shoe has some leakage flux and it changed the magnetic flux path and hence affects the demagnetization performance. The demagnetization in the two PMs are not symmetrical due to the armature reaction. Due to its special structure, FI-PMESPWG has some other different demagnetization characteristics from the normal interior PM machines, which will be analyzed and compared with the normal ones detailed in the full paper. The effect of demagnetization on the electromagnetic parameters will be investigated. Based on the analysis results in this paper, the demagnetization rules of interior PM salient pole machines will be concluded and it can supply an important theoretical basis for design of this kind of machine.