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Models are of interest to application engineers because they identify important electrical, thermal, and mechanical interactions in an induction motor. They provide a format for the parameters which must be obtained. The loading and terminal conditions can then be specified and the model state equations used to obtain the thermal limit curves and, what is more important, the time solution for current, torque, speed, and the temperatures of the rotor and stator. This paper presents a set of electrical, thermal and mechanical models which provides an accurate prediction of rotor and stator temperatures. Included is the change in rotor resistance due to the skin effect of the slip frequency. The authors introduce an algorithm for speed estimation using measured voltage and current which makes this possible. With the incorporation of this algorithm, the electrical and thermal models provide a major advance in the protection of induction motors.