An overload protection device in the motor control circuit that disconnects the drive when sustained overcurrent causes the bimetallic element to reach trip temperature.
Full Definition
The thermal relay (or thermal overload relay) protects the elevator drive motor from damage caused by sustained overcurrents during overloading, locked-rotor conditions, or phase imbalance. It works on the bimetallic strip principle: the strip bends when heated by the motor current and trips a normally-closed contact in the safety circuit when the deflection exceeds the set point. Trip class (Class 10, 20, or 30) defines the maximum tripping time at 7.2 × rated current; Class 10 is standard for elevator motors. The relay is set to the motor nameplate full-load current; a typical setting range is 0.1–38 A. In variable-frequency drive (VFD) installations, the thermal relay function is usually incorporated into the drive electronics with programmable I²t thermal models, eliminating the need for a separate bimetallic relay. Manual or automatic reset must comply with EN 81-20 requirements for safety circuit design. Differential protection against phase failure is an integral feature on three-phase relays.
