Extending Motor Life With Updated Thermal Model Overload Protection

Thermal issues shorten motor life. Modern motor relays must protect for these effects, which can occur for motors from small to large (50 hp and greater) and for all voltage operational levels. However, being too conservative on thermal limits decreases motor productivity. An ANSI/IEEE 49 thermal element creates a realistic thermal model of the motor because the element takes the load level and negative-sequence currents into account. The 49 thermal element is superior to older methods using inverse-time overcurrent elements (51). This paper discusses updates in the existing 49 thermal model implementation. Also discussed are resistance temperature detector (RTD) biasing, RTD voting, and the role of transducers in the modern motor relay.