The choice of thermal relay is mainly based on the rated current of the motor, and also takes into account the type, operating characteristics and working system of the motor. The following points should be considered when selecting the heat relay:
(1) In principle, the thermal relay current is selected according to the rated current of the motor at 90-110%, and the operating characteristics are verified. However, pay attention to the insulation grade of the motor, because different insulation materials have different allowable temperature and overload capabilities.
(2) It is necessary to ensure that the heat does not malfunction during the normal starting of the motor. If the motor starts infrequently and the starting time is not long, the heat can be selected according to the rated current of the motor. The level of CLASS 10/20 is determined according to the length of the starting time (IEC947-4-1 standard designation: the current is the setting current) The CLASS 10 action time is 4-10 seconds and the CLASS 20 action time is 6-20 seconds. If the start time is too long, the thermal relay should not be used. The electronic overcurrent relay should be used.
(3) Since the heat has thermal inertia and cannot be short-circuit protected, consideration should be given to the problem of short-circuit protection with the circuit breaker or fuse.
(4) Pay attention to the working system of the motor. If the operating frequency is high, thermal relay protection should not be used, but other protective measures should be taken, such as pre-embedded thermal resistance/galvanic temperature measurement in the motor for temperature protection.
(5) Pay attention to the normal working temperature of the heat. After the normal working range of the heat is -15 °C--+55 °C, the ambient temperature compensation is invalid, and there may be thermal failure or no action.
(6) The terminal wiring should be secure after the installation, and the wire cross-section should be selected within the current range. Otherwise, the temperature rise will raise the temperature of the bimetal and cause malfunction.