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A rotor temperature estimator is proposed in this paper for grid-connected squirrel-cage induction motors during dynamic or steady-state operations. The method utilizes a pipelined architecture with two superheterodyne receivers that operate on a common complex current vector input. The first superheterodyne receiver detects motor speed by extracting an instantaneous rotor-slot-harmonic frequency from the complex current vector. The second superheterodyne receiver extracts a complex fundamental current vector from the same complex current vector. Given a few known motor parameters and a complex voltage vector, a model-reference adaptive system then produces an estimate of the rotor time constant from the superheterodyne receivers' outputs. Finally, the rotor temperature is derived from the estimated rotor time constant via a correlation between the rotor temperature and its resistance. Experimental results demonstrate that the proposed method is capable of estimating induction-motor rotor temperature on a sample-by-sample basis without using temperature or speed sensors.