State of the art of controlled AC drives without speed sensor

The operation of speed controlled AC motor drives without mechanical speed or position sensors requires the estimation of internal state variables of the machine. The assessment is based exclusively on measured terminal voltages and currents. Low cost, medium performance sensorless drives can be designed using simple algebraic speed estimators. High-performance systems rely on dynamic models for the estimation of the magnitude and spatial orientation of magnetic flux waves in the stator or in the rotor. Open loop estimators and closed loop observers differ with respect to accuracy, robustness, and limits of applicability. The overview in this paper uses signal flow graphs of complex space vector quantities to give an insightful description of the physical and mathematical systems used in sensorless control