Stator thermal time constant

The thermal model providing motor overload protection is derived from the first order differential equation for heat rise due to current in a conductor. Only the stator thermal time constant and the service factor are the required settings. The thermal model utilizes the full thermal capacity of the motor and allows current swings and cyclic overloads that would trip conventional overcurrent protection but do not actually overheat the motor. Four examples of thermal limit curves and their equations are used to discuss the varying plotting practices in use. The paper also includes a method to calculate the stator thermal time constant using two points read from the overload curve when not available from motor data.