Investigation of the rotor demagnetization characteristics of interior PM synchronous machines during fault conditions

This paper investigates the transient magnetic behavior of an interior permanent magnet (IPM) synchronous machine following a large transient current caused by a fault condition. A combination of finite element (FE) analysis and numerical integration of the machine equations is used to calculate the currents resulting from a symmetrical three-phase short-circuit fault at the machine terminals, including magnetic saturation effects. FE analysis is further used to simulate the magnetic behavior of the machine when subjected to the fault currents, with a particular focus on the propagation of the demagnetizing MMF throughout the conductive laminations as well as the conductive PM material. Eddy currents induced in the laminations and magnet material are shown to impede the progression of the demagnetizing MMF. However, the transient analysis illustrates the manner in which the fault currents ultimately trigger the onset of irreversible demagnetization of significant portions of the rotor magnets in the analyzed machine. The transient demagnetization analysis method is extended to investigate the influence of several key design variables, including machine operating temperature and segmentation of the rotor magnets.