Equalizing the radial attraction forces in direct-drive PM generators

Large scale PM generators are subject to very large radial attraction forces between the rotor and stator. Combined with unequal deformation of the airgap clearance, this can lead to serious challenges for the generator structure. To equalise the radial forces in different parts of the machine, stator modules with variable flux-weakening and flux-strengthening are used in this paper. A case study of a permanent magnet generator for a direct-drive wind turbine is analysed magneto-statically. The results show that by shifting the current angle in the modules, the imbalance in radial forces (1.1×10^5N) can be equalized. Simplified analytical modelling is shown – by 2D finite element analysis – to over-predict the radial forces, so the required variation in current angle shift will be less than predicted analytical (i.e. +15.8° in one module and -12.2° in another). This motivates the development of the analytical model to include slotting, saturation and higher spatial airgap flux density harmonics.