A Combined Speed Observer Applied in Sensorless Control at Low Speed

Signal injection estimators are effective for sensor less AC motor drives operated in low speed region, but still have some shortcomings in practical use due to power losses, slow dynamics, torque ripple and acoustic noise. So a general speed observer, from low speed to maximum speed without signal injection, need to be further studied. In this paper, an improved sensor less control speed observer combining the direct speed estimator and a phase locked loop (PLL) speed estimator is proposed. The presented speed observer employs a standalone speed estimated directly from the q-axis voltage model as a reference, while the rotor position is calculated with the flux linkage estimators. A PLL is applied to estimate speed from the estimated rotor position. The output of the PLL is subtracted from the estimated speed above-mentioned. The simulation results are given to compare the performance of the proposed speed observer with the original PLL speed observer. The combined speed observer achieved more accurate speed and better performance than the original PLL observer.