Online Updating of Rotor Time Constant Based on Combined Voltage and Current Mode Flux Observer for Speed-Sensorless AC Drives

The simultaneous online estimation algorithm of rotor time constant and rotor speed for a speed-sensorless induction machine is proposed. With a small signal injected to the flux command, the method can accurately complete the online tuning of T-r over a wide range of speed estimation, including the rated speed and the very low speed of 1 Hz or even lower speeds under both no-load and loaded operations. Sensitivity of stator resistance error is studied, and the impact of stator resistance error on this method is severely reduced by the closed-loop structure of combined voltage and current (UI) mode flux observer. The identification process only needs the ac component amplitude without considering ac component phase drifts; thus, the division by zero at the zero-crossing point is avoided, which greatly increases the accuracy. Furthermore, any differential integral operations or even a filter is not needed; hence, the computation is rather simple and time saving. Moreover, unlike other signal injection methods, there are no special requirements on the injected signal frequency. The accuracy and validity of the method has been verified by Matrix Laboratory simulation model. Further experimental results on a 2.2-kW 50-Hz 4-pole induction motor also show effectiveness of this proposed method.