MCSA versus Flux Monitoring for Demagnetization Diagnosis in Axial-Flux PM Generators

Partial demagnetization is a condition occurring under elevated thermal operation or overloading of a Permanent Magnet (PM) machine. It may lead to a breakdown, if undetected. This paper compares the conventional monitoring of the stator current versus the flux monitoring for steady state and transient operation of demagnetized axial flux PM generators. The flux is captured with a stationary flux sensor, which is spanned in two pole pitches so that under healthy conditions there is zero rotor magnetic flux. On the other hand, when a demagnetization fault occurs there is voltage generated in the sensor. Due to the high poles number in this type of PM generators, a family of fault sidebands around the fundamental appears with significant amplitudes. However, the fault signatures in the stator current are limited. The results suggest that the stray flux monitoring has significant advantages over the stator current for transient operation of the generator.