After the motor’s disconnect contactor, a “thermal relay” (often referred to as a “thermal overcurrent relay” or simply “thermal relay”) is connected. In the thermal relay, each of the three-phase conductors passes through a bimetallic strip for electrical conduction. If the motor is overloaded or operated with a phase missing, the current increases, causing the bimetallic strips in the thermal relay to gradually bend. When the bimetallic strips bend to a certain degree, they activate a lever, interrupting the circuit and protecting the motor. Even if only one phase is activated among the three phases, it acts on a common lever and can provide protection. The thermal relay has a reset button, but if it activates, it’s important to identify the cause before rushing to reset and restart.

The control current of the thermal relay can be adjusted. If adjusted properly, it can tolerate short-term starting currents without activating, while effectively controlling excessive working currents. Depending on the nature of the load, the control current is set at around 1.05 to 1.25 times the rated current, slightly lower for no-load startup and a bit more lenient for heavy loads. The thermal relay has become an essential component in motor control circuits.