A self-tuning, closed-loop flux observer for sensorless torque control of standard induction machines

This paper proposes a self-tuning, closed-loop flux observer which provides field-oriented torque control for induction machines without a tachometer. The observer algorithm combines the dual methods of calculating flux from the slip relation and from back-EMF estimation in order to provide a closed-loop topology. At low speeds, the observer also depends on a model of the mechanical system. The observer accuracy and robustness is augmented by a parameter-independent, accurate speed detector which analyzes magnetic saliency harmonics in the stator current. The harmonic detection scheme provides accurate rotor speed updates during steady-state operation down to 1 Hz source frequency. This additional speed information is used to tune the mechanical system parameters and the rotor resistance. The tuned observer exhibits improved dynamic performance, accurate steady-state speed control and an extended range of control near zero speed. The algorithm requires no special machine modifications and can be implemented on most existing low and medium performance drives. The closed-loop nature of the flux observer, combined with the harmonic detection scheme, provides flux and speed error feedback which significantly increases the robustness or sensorless control across the entire speed range.