Non-linear direct torque control of sensorless induction motor drives with parameter identification and capable for very low speeds

The performance of sensorless induction motor drives is generally poor at extremely low speeds. The main recognized reasons are the limited accuracy of pulse-width modulation stator voltage acquisition, unwanted offset and drift components superimposed on the acquired measured signals, voltage distortions of induction motor caused by non-linear behavior of switching inverter, and finally, increased sensitivity against motor parameters mismatch. A sensorless induction drive capable of very low speed operation is presented in this article in which the principles of direct torque control and integrator backstepping non-linear control are combined to ensure high performance operation both in transient and steady state conditions. Stator flux vector is determined using a pure integrator in the stationary reference frame and is employed for rotor flux vector identification. An improved method is introduced for unavoidable offset and drift components identification and compensation. The average of stator phase voltages in each switching interval is estimated and employed in order to eliminate the AC voltage sensors. Feasible solutions are also proposed for on line stator and rotor resistances identification to ensure further improvement of drive performance at very low speeds. Effectiveness of the proposed control scheme is studied through simulation and experimental results.