Self-ventilated variable-speed induction motors cannot be protected using standard overload relays. At reduced operating speeds, the heat-loss dissipation capability is deteriorated, and the motor could be subjected to overheating, even with operating currents below the rated value. The practical solution generally employed is to oversize the motor in order to avoid overheating at reduced speeds. This paper proposes and evaluates a third-order thermal model capable of giving good temperature-rise predictions for any arbitrary variable-load variable-speed operating cycle. The key issues addressed are the estimation of the thermal capacitances of the model and the variation of the frame-to-ambient thermal resistance with the operating frequency (speed). Evaluation is done in an experimental totally enclosed fan-cooled induction motor fitted with thermocouples in the stator winding and frame. The model parameters are first obtained using the identification algorithm proposed, and then, the model is used to predict the temperature rise in the stator and frame during variable-speed operation.