New approach for stability improvement of speedsensorless induction-motor controls at zero frequency using multirate adaptive observer

The paper describes a new approach to improve the stability of speed-sensorless vector-controlled induction motors at zero-frequency operation. In the speed-sensorless vector control of induction motors, speed identification based on a fundamental-component model becomes unstable at zero-frequency operation. At zero frequency, the stator voltage and the stator current of induction motors are both direct current, and speed information cannot be obtained from them. The speed identification thus becomes unstable at zero frequency if disturbances such as parameter mismatches, noises and measurement errors affect the speed identification system. The paper proposes a method of improving the stability in the speed identification of speed-sensorless vector-controlled induction motors at zero frequency by means of a new configuration of the adaptive flux observer. A review of this problem is carried out, namely that speed identification becomes unstable at zero frequency, and it is shown that this problem originates in an unstable pole-zero cancellation in the speed-identification system. A new observer configuration with a multirate input control is proposed and the results of experiments carried out to confirm the effectiveness of the proposed method are given