Robust sensorless sliding mode flux observer with speed estimate for induction motor drives

This article discusses the problem of state and rotor flux angle estimation for the induction motor drive and presents a sensorless sliding-mode observer that is developed based on the induction motor model in the stationary reference frame. The observer is realized using the measured motor voltages and currents; it can be used to provide field orientation in a sensorless control scheme. The observer estimates the stationary frame fluxes of the induction motor and is developed using switching (sliding-mode) terms based on the current mismatches. The novelty is the design method used: first, a sensored observer is constructed; second, the observer is made sensorless by replacing the speed with a speed estimate; last, a gain design is found that allows accurate estimation of the rotor flux angle. The observer is analyzed with Lyapunov-based methods. It is found that with the proposed design, when an inaccurate speed signal is used, the resulting flux estimates are in phase with the real fluxesthis yields the correct rotor flux angle. The magnitudes of the estimated fluxes are inversely proportional with the speed signal used. The theoretical development is supported by simulation results and experimental tests. The advantages of the method are explained.