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

Abstract

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 papers 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 paper 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.