Optimizing Motor Thermal Models

The thermal limitations of induction motors are specified by thermal limit curves that are plots of the limiting temperature of the rotor and stator in units of l²t. This paper discusses the thermal protection provided by rotor and stator thermal models defined by the thermal limit curves and supporting motor data. The thermal model is the time-discrete form of the differential equation for temperature rise due to current and is derived from fundamental principles as shown in the appendix. The rotor model derives the slip-dependent l²r watts using voltage and current that permit the safe starting of high-inertia drive motors. The performance of the models is shown in constant and cyclic load tests