Replacing Electromechanical Relays With Microprocessor Relays to Provide Thermal Protection for Motors

Thermal overload protection is a critical part of any motor protection scheme. Older electromechanical relays provide thermal protection with an inverse-time overcurrent element. Replacing electromechanical relays with modern microprocessor-based relays requires the development of settings for a more sophisticated thermal protection element, such as the first-order thermal model. Additionally, this requires a planned outage to install the new relay and commission the settings. To correctly set the thermal element in a microprocessor-based relay, motor manufacturer data (such as a nameplate, data sheet, and system data) are needed. However, these data are not always available for older motors. This paper describes the settings required for the first-order thermal model and provides case studies from various end users illustrating how settings from existing electromechanical relays were used to develop settings for a microprocessor-based thermal element.