Thermal Protection of Induction Motors Enhanced by Interactive Electrical and Thermal Models

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. Adequate protection of induction motors is routinely achieved via locked-rotor, phase unbalance, and overload protection. However, conditions of changing load torque, frequent starts, temporary phase unbalance and high inertia loading must often be tolerated. Designing protective schemes for these conditions using conventional relays is difficult and sometimes impossible. Motor protection is primarily a temperature estimation problem. Recognizing that good electrical and thermal models exist leads to their adoption for motor protection so that the heating and cooling process is well represented for virtually all loading and terminal conditions. This paper first reviews electrical, thermal, and mechanical models. Their parameters are discussed and a novel technique is introduced for accommodating the frequency dependent skin effect of the rotor resistance using a simple speed estimation algorithm. The correlation with protection practice is established by using the models to obtain the thermal limit curves for a motor including the relatively recent rotor accelerating time limit. The models are then used to analyze conditions of locked rotor, high inertia starting and overload torque.