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The use of permanent magnet-assisted synchronous reluctance motor (PMA-SynRM) is increasing due to its advantages and superiority over induction motors. The undesirable effects of the encoder measurement error on the drive systems especially during low-range speeds may cause disorder in drive performance and sometimes its instability. For the low-cost application that requires a low-resolution encoder, this problem is felt obviously. This article presents a mathematical solution in order to compensate for the instability problem of a PMA-SynRM control at low speeds based on an augmented state unscented Kalman filter (ASUKF). First, a mathematical analysis for the effect of encoder error on the speed control instability is performed, and then an augmented nonlinear dynamical model for the PMA-SynRM is derived. Using an ASUKF on the presented augmented model of the PMA-SynRM, the error of the encoder is detected and accurately estimated. Using the estimated errors and pure motor state variables, the accuracy and robustness of the control are increased and its stability is guaranteed. The experiments performed for a 0.35 kW PMA-SynRM confirm the performance validity of the proposed method. [ABSTRACT FROM AUTHOR]