A procedure is described for discrete-time evaluation of the speed of an inverter-fed induction motor without using a speed sensor. The motor's rotor flux-oriented control contains a discrete-time operating flux observer, simulating the continuous-time mathematical state model of the motor. The estimate of stator current at each of the discrete time instants is compared with the measured stator current and the discrepancy is back-coupled through the coupling matrix to the flux observer for flux determination. The discrepancy is also passed back to the speed observer which augments the flux observer and where a speed estimate for the immediately-preceding time instant is corrected.
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