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Field-oriented control methodology requires knowledge of the machine parameters and, in particular, the rotor time constant. The interest here is in tracking the value of TR as it changes due to ohmic heating so that an accurate value is available to estimate the rotor flux linkages for a field-oriented controller. The approach presented here uses a nonlinear least-squares approach using measurements of the stator currents and voltages along with the rotor speed. The nature of this technique lends itself directly to an online implementation and therefore can be used to track the rotor time constant. Experimental results are presented to demonstrate the validity of the approach.