Detection of magnet demagnetization in five-phase surface-mounted permanent magnet generators

Multiphase generators are attractive particularly for medium and high power applications, because this technology allows reducing the current per phase and consequently the requirements of the inverter switches. The interest in multiphase machines is motivated also by their greater number of degrees of freedom compared to three-phase machines. This characteristic is often used to improve the machine performance, to increase the power density or for diagnostic purposes. In particular, the condition monitoring of the status of the magnets in permanent magnet synchronous generators is critical and is receiving an increasing attention. In this paper, the demagnetization of the rotor magnet trailing edges due to abnormal stator currents in five-phase surface-mounted permanent magnet synchronous generators is analyzed. A new non-invasive on-line detection method, which emphasizes the emf variation due to the demagnetization, exploiting the additional degrees of freedom of a multiphase generator, is presented. The proposed approach is validated by means of finite element analysis and numerical simulations.