Thermal modeling protects motors without overtripping

Today’s modern microprocessor-based protection relays offer much more than the standard motor protection provided by electromechanical relays. Advanced features include motor-heating, time-to-trip and countdown-to-restart calculations — plus real-time diagnostics. Even with the calculating power of microprocessor-based protection, most relays today still attempt to provide motor protection just by measuring current. The various manufacturers’ models calculate motor heating by thermal capacity or thermal register, where 0% is completely cooled and 100% is the trip threshold. This thermal capacity is accumulated based on the measured current such that during motor starting, protection is essentially a function of the current squared multiplied by time (an I2t element), with maximum starting time dictated by the hot motor safe-stall time. Limits of accuracy While this type of protection dates back to electromechanical relays, the I2t model does not result in the most accurate motor protection, and limits true available horsepower.