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A thermal relay is a protector that uses the thermal effect of current to cut off the circuit. When the thermal overload relay is used for overload protection in the motor, it often work with ac contactor to form an electromagnetic starter, and then to be used in the control and protection circuit of the three-phase asynchronous motor. If the motor is overloaded for a long period of time, under-voltage or phase-failed during operation, the current of the motor will exceed the rated value. At this time, the thermal overload relay will cut off the control circuit of the motor, thus to effectively protecting the motor.