Enhanced low-speed operations of back EMF-based sensorless anisotropic PMSM drives

Under the thrust of the increasing demand of energy-efficiency and reliability, sensorless permanent magnet synchronous motor drives are showing compelling advances. The presence of a salient magnetic structure (anisotropic rotor) reduces the presence of magnets and it is fundamental for operations at zero and very low speed, obtained by injecting and processing high frequency signals. But the excitation signals cause unwanted torque ripple, noise and vibrations. This work deals with the lowering of the lower speed limit, above which the estimation techniques can work without any signal injection. Specifically, the proposed method mitigates the instability problems of back electromotive-based observers when the drive operates in generating mode. The speed and position estimates are based on a modified phase locked loop observer. It trusts on direct phase voltage measurements, even if it works also with standard compensated reference voltage signals. An accurate laboratory measurements campaign, focused on showing the injection-free speed range improvements, is included in the paper.