Speed control of induction motors requires the accurate estimation of the fluxes in the motor. But the flux estimate, when estimated from the stator circuit variables, is dependent on the stator resistance of the induction motor. As a consequence, the flux estimate is prone to errors due to variations in the stator resistance, especially at low stator frequencies. A scheme is presented in this paper for an online estimation of the stator resistance under steady state operating conditions, using variables that can be measured from the terminals of the motor alone. The scheme is based on estimating the steady-state magnitudes of the stator and rotor flux space phasors using the reactive power. An analysis of the effect of the stator resistance variations on the flux estimate is presented. A simulation of a rotor field oriented speed control of a VSI-fed induction motor using stator circuit variables is performed incorporating the online stator resistance estimation strategy.
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