Effect of the closed-loop control on the diagnosis of rotor demagnetization in five-phase surface-mounted permanent magnet generators

Abstract

Multiphase permanent-magnet generators are an attractive alternative for a variety of emerging applications owing to their reliability and dynamic performance such as hybrid traction systems. In this context, diagnosing the status of the rotor magnets is crucial for improving the reliability of the generators. The effect of the closed-loop control system cannot be neglected when the fault detection process is based on the signature analysis of electrical variables. Therefore, investigating the relevance of the bandwidth of the control system on the induced fault component is mandatory.In this paper, the detectability of the demagnetization of the rotor magnet trailing edges, due to large stator currents, in five-phase surface-mounted permanent magnet synchronous generators, is investigated. The theoretical analysis is validated by means of numerical simulations.Index Terms--Five-phase permanent magnet generators, rotor demagnetization, FOC, fault diagnosis.

A Comprehensive Survey on Fault Tolerance in Multiphase AC Drives, Part 1: General Overview Considering Multiple Fault Types

“…The former (uniform) implies a reduction in the amplitude of the back-EMF waveform, and, hence, a decrease in the output torque for given stator current [74,129,177]. The latter (local) additionally leads to a distortion of the back-EMF waveform and to torque ripple [68,74,77]. The most relevant causes, detection methods and tolerance approaches for magnet demagnetization are illustrated in Figure 39.…”

“…Since local demagnetization can give rise to back-EMF harmonics, these may be used for diagnosis of this kind of fault. The specific components to be monitored depend to a great extent of the machine design [74,77,78]. The methods available in the literature for detection of local demagnetization in multiphase drives, based on this principle, are summarized in Table 23.…”

“…If a certain space harmonic is not contained in the stator winding function, the respective back-EMF harmonic does not arise after demagnetization. Conversely, if a given back-EMF harmonic is minimized in healthy conditions by instead selecting adequately the magnet pole arc, this harmonic is expected to increase as a consequence of demagnetization, and it could, thus, be used for demagnetization detection [77,78]. Furthermore, even if a back-EMF harmonic is not reduced by design in any manner, demagnetization could be detected by checking potential changes in its magnitude and phase [77].…”

“…Conversely, if a given back-EMF harmonic is minimized in healthy conditions by instead selecting adequately the magnet pole arc, this harmonic is expected to increase as a consequence of demagnetization, and it could, thus, be used for demagnetization detection [77,78]. Furthermore, even if a back-EMF harmonic is not reduced by design in any manner, demagnetization could be detected by checking potential changes in its magnitude and phase [77]. In any case, if it is decided to avoid minimizing a certain harmonic during the design of the winding function (for allowing demagnetization diagnosis), it is advisable to do it for a back-EMF harmonic in a secondary subspace where significant current reference is not expected, so that torque pulsation does not occur when demagnetization arises [78].…”

“…This recommendation is respected in all the approaches displayed in Table 23, where the monitored harmonics belong to the (only) x-y plane and the x-y current reference is set to zero. Similarly to other faults, such as high-resistance connections, it is shown in [77] by Gritli et al that detection of local demagnetization should preferably be based on currentmeasurement or voltage-reference (control-output) harmonics depending on whether there is small or large impact of closed-loop current control on the corresponding frequencies, respectively. In agreement with this fact, it is proposed by Casadei et al in [76] to detect local demagnetization of a five-phase SPMSM by monitoring the output of PI controllers rotating synchronously with the back-EMF seventh-order harmonic in the x-y plane.…”