Extending motor life with updated thermal model overload protection

Thermal issues shorten motor life. However, being too conservative on thermal limits decreases motor productivity. An Institute of Electrical and Electronics Engineers 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 resistive temperature detector biasing, resistive temperature detector voting and the role of transducers in the modern motor relay.