Anisotropy-based Sensorless Control at Excessive Load

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By avoiding position sensors in electric drives, it is possible, inter alia, to save cost and space, and achieve improved reliability. Methods for sensorless control in the lower speed range employ the so called anisotropy of the inductance for the position estimation. Due to magnetic saturation, the anisotropy is generally current-/load-dependent. This results in a load limit for sensorless control at low speeds, above which anisotropy-based method become unstable in a closed loop opration.The exact value of the load limit mainly depends on the machine design and could therefore be influenced or optimized in the design process. But for this it is essential to be able to correctly calculate the value of the load limit already in the design process. Such approaches have been proposed sporadically in the literature which use the anisotropy ratio and the anisotropy angle as stability indicators. However, since actually both criteria have no direct connection with the instability, those predictions lead to incorrect optimization results.This application is based on a first complete theoretical derivation of the instability problem, wherein the anisotropy-displdacement rate turned out to be the critical measure. The theory in simplified form (2D) has been supported by simulations and must now be experimentally verifified. An extension of the theory (3D) shall improve the accuracy of the prediction of the load limit and also will be verified experimentally in the project.After verification of the 3D prediction rule it will be integrated into the machine design process in order to manufacture machines optimized with respect to the anisotropy-load limit. It will herein finally turn out, to which extent the sensorless overload capacity can be increased and whether even hitherto excluded applications, such as electromobility, can be realized even without encoder.