Programmable LPF-based stator flux estimator for sensorless induction motor control

In this paper, the improved stator flux estimator is proposed to ensure a programmable low pass filter (PLPF) to act as a pure integrator for sensorless control of induction motor. The estimated synchronous angular speed is used as a pole 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, this paper utilizes the current model as a flux estimator at the standstill. When the current model is switched to the voltage model, large fluctuations in the stator flux and the motor speed can be caused. Therefore, the simple method for a smooth transition between two models is proposed. Validity of the proposed schemes is verified through both simulation results and experimental results.