Low-frequency signal-injection method for speed sensorless vector control of induction motors

Abstract

A new method for the speed sensorless vector control of induction motors at low speeds is presented. The method overcomes the well-known low-speed limitations of conventional methods based on an ideal motor model, yet without relying on a more detailed motor model or parasitic effects. To overcome the low-speed limitations, the ideal motor model is augmented with mechanical dynamics. The response of the electromechanical system to a low-frequency ac current signal injected into the stator winding depends on the direction of the signal relative to the rotor flux direction. Based on this dependency, a new algorithm for rotor flux angle estimation, applicable when the total moment of inertia is not too high, is developed. The estimation method tolerates any low-speed operation point, including zero stator frequency, which is validated by simulations and experiments. The robustness of the method against errors in motor parameter estimates is shown both theoretically and experimentally. The algorithm is also combined with a full-order flux observer, yielding fast dynamics in addition to steady-state stability at low speeds.