Answer: A "thermal relay" (often called "thermal couple") is connected behind the disconnecting contactor of the motor. In the thermal relay, the three-phase wires are each conductive through a bimetallic strip. If the motor is overloaded or running in phase loss, the current will inevitably increase, and the bimetallic strip of the thermal relay will gradually bend. When the bimetallic strip bends to a certain extent, it pushes a lever to cut off the circuit and protect the motor. As long as one of the three phases is activated, it acts on a common lever and can play a protective role. The thermal relay has a reset button, but if the thermal relay is activated, the cause should be found out and do not rush to reset it.
The control current of the thermal relay can be adjusted. If it is adjusted properly, it can withstand a short-term starting current without actuation, but it can control the excess working current. According to the nature of the load, the control current is 1.05~1.25 times the rated current. The no-load start-up is smaller and the load is larger. The thermal relay has become a necessary component of the motor control circuit.
