The speed-sensorless control of induction motors at zero frequency

For most speed sensorless controlled induction motors, even those based on state observers, unstable phenomena occur at very low or zero frequency. In this paper, the potential unstable reason of adaptive rotor flux observer based speed sensorless control is examined and the nonlinear observability of induction motor is analyzed. The limitations of any speed estimation scheme based on observers and effective techniques for sensorless induction motor control at zero frequency are also discussed.