Demagnetization Monitoring and Identification in PM Generators With Concentrated Windings During Transient Conditions

Direct drive permanent magnet machines used in renewables typically have high numbers of poles leading to low speeds. This poses a challenge for detection of faults during transients, while few periods of the operating waveforms can be captured. Moreover, it has been recently shown that the numbers of poles and stator coils determine the demagnetization harmonic generation mechanism and consequently the location of the fault signatures. This leads to different signatures in every permanent magnet machine. Additionally, the traditional signatures may not be clear in the spectrograms due to the low rotor speed. Under such conditions, the monitoring of specific higher harmonics can lead to reliable detection and identification of the demagnetization fault. The authors prove that the fault can be reliably distinguished from other rotor-related faulty conditions with the use of specific higher signatures in the stator current. Finally, the computed harmonics are experimentally verified in the lab, while the detection of demagnetization under transient conditions is achieved in this paper, that studies a double rotor axial flux PM generator as a case subject.