The protection object of the thermal relay is the motor, so the technical performance, starting situation, load nature of the motor and the allowable overload capacity of the motor should be understood when selecting it.

(1) A motor that works stably for a long time

The thermal relay can be selected according to the rated current of the motor. Take 0.95～1.05 times of the set current of the thermal relay or the intermediate value equal to the rated current of the motor. When using, adjust the setting current of the magnetic latching relay to the rated current value of the motor.

(2) The insulation class and structure of the motor should be considered
Due to the different insulation levels of motors, their allowable temperature rise and ability to withstand overloads are also different. Under the same conditions, the higher the insulation level, the stronger the overload capacity. Even if the insulating materials are the same, but the motor structure is different, there should be differences when selecting thermal relays. For example, the heat dissipation of a closed motor is worse than that of an open motor, and its overload capacity is lower than that of an open motor. The setting current of the thermal relay should be selected as 60-80% of the rated current of the motor.

(3) The starting current and starting time of the motor should be considered

The starting current of the motor is generally 5 to 7 times the rated current. For motors that start infrequently and run continuously, if the starting time does not exceed 6s, thermal relays can be selected according to the rated current of the motor.

(4) If a thermal relay is used for motor phase loss protection, the connection method of the motor should be considered

For a Y-connected motor, when a certain phase is disconnected, the current of the remaining unbroken phase windings increases in proportion to the current flowing through the thermal relay. The general three-phase thermal relay, as long as the setting current is adjusted reasonably, can realize the phase-failure protection for the Y-connected motor. For a delta-connected motor, when the phase is disconnected, the increase ratio of the current flowing through the unbroken phase winding and the current flowing through the thermal relay is different.