Improved stator flux estimator based on a Programmable LPF for sensorless control of induction motors

In this paper, the improved stator flux estimator has been proposed to ensure a Programmable Low Pass Filter (PLPF) to act as a pure integrator for sensorless control of induction motors. In a conventional PLPF, the estimated synchronous angular speed is used as a cutoff frequency of a PLPF whose gain and phase compensator is determined on the assumption of the estimated synchronous angular speed being coincident with the actual one. However, if the synchronous angular speed is not exactly estimated, the gain and phase compensation will not be appropriate. To overcome the mentioned problem, this paper analyzes the relation between the synchronous speed error and the phase lag error of the stator flux. Based on the analysis, the synchronous speed error compensation scheme is proposed. For a sensorless start-up, the current model is used as the stator flux estimator at the standstill. Therefore, a stable transition between the voltage model and the current model is required. This paper proposes the simple transition method which determines the initial values of the voltage model and the current model at the transition moment. The validity of the proposed schemes is proved through simulation results and experimental results.