New contributions for speed observation of asynchronous motor fed by multilevel inverter

Control fulfilment of asynchronous motors requires accurate feedback of speed information. Previously, this function was attributed to mechanical sensors, however, due to the limited use restrictions, scientists have developed variety of speed observation algorithms. These latter are distinguished by their simplicity and sensitivity to machine parameter variations as well as their robustness even in a wide speed range. This paper proposes three new structures of observers in order to achieve an efficient behaviour at low and high speed. The first is a combined scheme of sliding mode observer and model reference adaptive system, the second consists in switching between the two aforementioned observers according to the operating speed, while the third consists in switching from sliding mode observer to the combined estimator. Moreover, the overall control scheme performance is enhanced by using sliding mode principle in a direct torque structure to control the motor fed by NPC multilevel inverter. Besides, a sliding mode controller is inserted in the external loop to regulate speed and provide a stable torque. To shed light on the efficiency of the proposed contributions, a critical and comparative study of simulation results has been conducted.