Speed-sensorless vector torque control of induction machines using a two-time-scale approach

Conditions for observability of a linearized smooth-airgap induction machine model are examined, and limitations of any speed-sensorless observer scheme based on such a model are discussed. An approach for speed-sensorless flux estimation based on singular perturbation theory is developed. This approach relies on the natural time-scale separation between the electrical and mechanical dynamics of the induction machine. A full-order observer of an induction machine is presented, incorporating a correction term which has an intuitive explanation when one considers steady-state stator currents. Using singular perturbation theory, convergence of the observer is shown for all open-loop stable operating points of the induction machine, with the exception of DC excitation. Sensitivity of the observer to parameter deviations is discussed. Experimental results are presented confirming the validity of the above approach.