Thermal overload relay, or simply thermal relay, is an electrical protection device that operates based on the principle of the thermal effect of current, primarily used for overload protection of electric motors.
Core Definition: It is a non-instantaneous protective device. When the current of a motor exceeds its rated value for an extended period due to excessive load, the thermal overload relay will delay and disconnect the control circuit, thus protecting the motor windings from burnout.
It cannot be used for short-circuit protection (short-circuit currents are extremely large and require rapid disconnection by fuses or circuit breakers), but it can accurately handle prolonged overload, the most common motor fault.
Main Functions:
Overload Protection: Prevents insulation aging and burnout caused by prolonged overload operation of the motor.
Phase Loss Protection (some models): When a three-phase motor operates with a phase loss, the current is unbalanced; the thermal relay can detect and activate this.
Temperature Compensation: Advanced models can automatically adapt to changes in ambient temperature to avoid false tripping.
Manual/Automatic Reset: The circuit can be manually or automatically restored after fault diagnosis.
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