Reasonable selection of thermal relay

The thermal relay is a kind of protective relay made by using the principle of current thermal effect, and is widely used for overload protection of the motor. How to reasonably select and use thermal relays is an old topic. However, at present, accidents in which many units burn electric motors due to unreasonable choices and use of thermal relays still occur. Therefore, the author reminds early contacts that, in addition to observing general regulations, the following points should be noted in the selection and use of thermal relays:

1. The relationship between the specifications of the relay and the mounting position The position of the selected thermal relay varies with the position of the star/triangle control. The selected thermal relay current setting value is equal to the rated current value of the protected motor. The setting value of the selected thermal relay current shall be 1/3 of the rated current of the protected motor.

2. Reset Type Thermal relays generally have two reset types: manual reset and automatic reset. The conversion of these two kinds of reset forms can be accomplished by adjusting the reset screw. When the thermal relay is shipped from the factory, the manufacturer is generally set to the automatic reset state. In use, whether the thermal relay should be set to a manual reset state or an automatic reset state depends on the specific conditions of the control loop. Under normal circumstances, the principle that the protected motor should not be restarted automatically should be followed even if the thermal relay is automatically reset after the thermal relay protection action, otherwise the thermal relay should be set to the manual reset state. This is to prevent the motor from repeatedly restarting and damaging the device after the failure is not eliminated. For example, a button-controlled manual start and manual stop control circuit is generally used, and the thermal relay can be set to an automatic reset mode; the automatic start circuit using the automatic element control should set the thermal relay to the manual reset mode.

3. Selection of thermal relays with different rated current levels but the same adjustment range of thermal elements For example, JR16 series thermal relays have adjustment ranges of 14A~16A for the thermal element setting values ​​in rated currents of 20A and 60A. At this time, the ambient temperature used by the thermal relay and the ambient temperature of the protected motor should be checked. When the ambient temperature of the thermal relay is higher than the ambient temperature of the protected motor by more than 15°C, the thermal relay of the rated current class shall be used; when the ambient temperature of the thermal relay is lower than 15°C of the ambient temperature of the protected motor When using, should use the thermal relay of the rated current rating of the small one. In addition, the load condition of the motor and the adjustment range that the thermal relay may require may also be taken into account.

4. Adjustment of setting current for repeated overload protection of short-time motor In the field, multiple tests and adjustments can obtain more reliable protection. The method is to first adjust the setting current of the thermal relay to be slightly smaller than the rated current of the motor. If it is found that it frequently operates during operation, the temperature of the thermal relay is gradually increased until it meets the operating requirements.

5. Selection of connection conductors The excessively thick or too thin connection wires of the thermal relay will also affect the normal operation of the thermal relay, because the difference in the thickness of the connection conductors will result in different heat dissipation and affect the thermal effect of the thermal relay current. The selection of the connection wires for various types of thermal relays can be made according to the manufacturer's instructions or the manual.