Stator Flux Observers for Speed-Controlled PMSMs in Low-Speed Sensorless Applications: Comparative Tests and Hybrid Strategy

In-depth performance analysis of stator flux observers (SFOs) is carried out, in the very low-speed range, for sensorless speed-controlled drives based on permanent magnet synchronous machines (PMSMs), in the presence of no idealities of the voltage source inverter (VSI) and uncertainties in the motor electrical parameters. The original contribution of this brief is twofold. First, it relies on the presentation of experiments, within this framework, which comparatively illustrate the closed-loop performance of: 1) a stator flux (open-loop) estimator with a low-pass filter (LPF), endowed with an additional phase shift and magnitude compensation based on the estimation of machine speed and 2) two adaptive observers constituting the most recent representatives of the class of the theoretically based contributions for PMSMs. While the former can achieve more satisfactory results when speed variations are relatively small, its performance degrades—when speed variations become relevant—when compared to the aforementioned adaptive SFOs which, in turn, still exhibit the advantage of estimating an additional critical parameter (under reliable knowledge of the motor inductance) related to demagnetization effects. Indeed, the crucial role of the adaptation is highlighted throughout the sections, while the conditions underlying the design and the stability proofs of such adaptive SFOs are shown to provide actually effective tools and restrictions under which satisfactory performances for the considered adaptive SFOs can be achieved in practice. Second, the common notation of the brief finally leads to the original formulation of a new comprehensive set of equations that simultaneously covers all the tested solutions and defines a hybrid strategy that might be very effective in practical applications.