A Universal Diagnostic Strategy for Direct-Drive Permanent Magnet Generator Faults by Novel Sensing of Stator and Rotor Magnetic Fields

This paper presents a new approach for reliable diagnostics in permanent magnet machines by novel sensing of magnetic field quantities, notably the rotor airgap magnetic flux. The proposed diagnostic strategy utilises a closed-circuited search-coil mounted on the rotor of the machine. This sensing coil renders non-zero signals with unique features during fault conditions, in terms of the search-coil induced voltage and current, due to the non-stationary magnetic flux induced by fault asymmetries with distinct pattern both in the time and the frequency domains. The amplitude of the induced harmonic signatures is severity dependant with a distinctive pattern depending on the type of fault. The proposed method is validated for a direct-drive permanent magnet generator by extensive finite element simulations with several fault cases of eccentricity and inter-turn short circuits. The results show that the stationary unbalanced magnetic flux needs conversion into a rotating one to achieve induction of harmonic components from the machine's stator.