Critical Aspects of Demagnetization Faults in Multi-Stage Direct Drive Permanent Magnet Generators for Renewables

Demagnetization of permanent magnet machines is a fault that evolves over time leading to an eventual catastrophic failure. This condition is of particular interest in renewables, with emphasis on remote applications such as offshore wind, tidal, and wave energy harvesting. Moreover, the demagnetization fault can be either uniform or non-uniform. The strong majority of research articles are focusing on non-uniform demagnetization and typically study the case of a single faulty magnet. This is because, the uniform demagnetization does not create harmonics that can be detected with spectral methods. Despite the above, the study of single magnet demagnetization studies have been criticized due to being unrealistic to describe most practical cases where permanent magnet machines are subjected to demagnetization due to overloading and/or overheating conditions and leading to uniform demagnetization. This article sheds some light into such theories, while studying a realistically induced in the lab, demagnetization case of a direct-drive C-GEN permanent magnet generator. The goals of this research are to examine the generated harmonic index due to the fault, the distribution of the demagnetization in space and the impact of the fault on the multiple generator stages due to the circulating currents.