On the properties of full-order observers for sensorless induction motor drives

It is shown that a full-order observer for current, flux, and speed estimation of sensorless induction motor drives, under the assumption of fast current dynamics, has an equivalent implementations as a reduced-order observer for flux and speed estimation only. This reduced-order observer is analyzed. All gain selections which give stability for all operating conditions, including the regeneration mode (called complete stability), are found. Consequently, all gain selections of the full-order observer gains for which complete stability is obtained, under the assumption stated, are also found. This allows the stability properties of any candidate full-order observer gain selection to be evaluated.