False Negative Diagnosis of Demagnetization in Direct Drive Permanent Magnet Generators

Demagnetization is a fault occurring in permanent magnet machines, leading to increased vibrations, noise and reduction of efficiency in the short term. However, if not detected early, demagnetization will deteriorate the machine via higher current and temperature increase that will enhance the severity level and lead to a catastrophic failure. Such conditions are extremely costly when they concern direct drive applications, typical in offshore energy harvesting. Prompt diagnosis is required. The stator current analysis has been one of the favorable diagnostic methods due to its non-intrusiveness, low cost and online capabilities. Despite that, this paper proves that the monitoring of the stator current is not always reliable because it directly depends on the geometrical and manufacturing characteristics of the permanent magnet generator. Multiple cases that may lead to false negative diagnostic alarms are revealed in this work for the first time, prompting further research towards alternative diagnostic methods.