Track it or crack it

Abstract

The article demonstrates that the first-order thermal model is superior in protecting motors with cyclic loads. The first-order thermal model faithfully tracks cyclic load without a premature process interruption due to motor trip, even though the maximum current exceeds the service factor (SF) rating, as long as the motor per unit (p.u.) temperature stays below the design limit of SF. As the period of the cyclic current approaches the motor RTC and the maximum current exceeds the SF rating, the motor temperature will approach the motor design limit p.u. temperature and will result in a motor trip and process interruption. It is imperative when applying a motor to a cyclic-load application that the thermal and electrical characteristics of the motor be scrutinized and compared with the mechanical process characteristics to ensure compatibility.