Tutorial: From the Steinmetz model to the protection of high inertia drives

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 I²t. 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 Annex. In the rotor model, voltage and current are used to derive the slip-dependent I²r watts that permit the safe starting of high-inertia drive motors.