The main function of a thermal relay is to protect the electric motor.
The new electronic thermal relay provides protection against overload, overcurrent, locked rotor, and various phase loss, current imbalance, phase-to-phase short circuit, turn-to-turn short circuit, overvoltage, and undervoltage.
We should pay attention to the following points when using thermal relays.
First, thermal relays may malfunction or burn out electrical components if the motor starts for too long or is operated too frequently. Therefore, thermal relays are generally not used for overload protection in such cases.
Secondly, the connecting wires at the output terminals of the thermal relay should be selected appropriately. If the wires are too thin, the relay may operate prematurely; if the wires are too thick, the thermal relay may operate with a delay.
Third, when a thermal relay is installed together with other electrical appliances, it should be installed below the other electrical appliances to avoid the influence of other electric heating on its working characteristics.
Fourth, the rated current of the thermal relay should be greater than the rated current of the motor.
Fifth, for general light-load starting and short-time operation, a two-phase thermal relay can be selected. When the power supply voltage balance is poor, the working environment is poor, or the power difference between multiple motors is significant, a three-phase thermal relay can be selected.
Sixth, the normally closed contact of a thermal relay is generally used to control the power supply to the contactor coil, while the normally open contact of the thermal relay is used to supply power for fault signals.
Seventh, the setting current should generally be adjusted to be equal to the rated current of the motor. For motors with poor overload capacity, the setting value of the thermal relay can be adjusted to 0.6 to 0.8 times the rated current of the motor. For motors with long starting time, driving impact loads or not allowed to stop, the setting current of the thermal relay should be adjusted to 1.1 to 1.15 times the rated current of the motor.
