Sensorless control of induction motors-performance and limitations

Controlled induction motor drives without mechanical sensors for speed or motor shaft position have the attraction of low cost and high reliability. The identification of rotor speed is generally based on measured terminal voltages and currents. Dynamic models are used to estimate the magnitude and spatial orientation of the fundamental magnetic flux waves in the stator or in the rotor. Open loop estimators and closed loop observers differ with respect to accuracy, robustness, and sensitivity against model parameter variations. Dynamic performance and steady-state speed accuracy in the low speed range is achieved by exploiting parasitic effects of the machine. The overview in this paper uses signal flow graphs of complex space vector quantities to provide an insightful description of the systems used in sensorless control of induction motors