The following views may not be comprehensive, and only represent the project experience summarized by individuals
1. As shown in the above two pictures, thermal relays are generally divided into two types, independent and non-independent;
2. In most cases, the thermal relay is used in conjunction with the AC contactor and placed behind the AC contactor. As shown in the figure below, the left side is an independent thermal relay. The input end of the thermal relay is connected to the output of the main contact of the AC relay. The only difference between the non-independent type and the independent type is that the non-independent input does not need to be wired, and it comes with a guide post, which can be directly inserted into the output terminal of the AC contact main contact;
3. Thermal relays and AC contactors appear at the same time, usually in the three-phase motor control circuit, which plays a role of overheating protection;
4. The structure of the thermal relay is shown in the figure below. The left is the action part, and the three red wires on the right are wound on three metal sheets. Each red wire has two connectors, corresponding to the input and output of the thermal relay;
5. When there is alternating current passing through the red wire, due to the principle of electromagnetic induction, (in alternating current, the metal sheet cuts the magnetic induction wire to generate heat), heat is generated. When the current is too large, the heat increases, and the heat reaches the metal sheet. After a large deformation, the connecting rod will be pushed to disconnect the control circuit, so as to protect the motor from overload protection;
6. After the overload protection occurs, you need to wait for a while before pressing the reset button to be effective, because you need to wait for the temperature to drop before the metal sheet can recover from the deformation to the previous state;
7. Thermal relays generally only have main contacts and auxiliary contacts, which are similar in structure to AC contacts. The difference is that the AC contact action relies on the coil to turn on and off the power, while the thermal relay does not have a coil and relies on the internal metal sheet. Deform to move;
8. The main contact of the thermal relay is connected to the main circuit, and the basic function of the auxiliary contact is the same as that of the AC contactor. It can give PLC signals to monitor the status of the thermal relay; it can also be used to disconnect the control circuit; Generally, the auxiliary contacts of thermal relays also appear in pairs. Normally open and normally closed share a common terminal. Normally open is closed, and normally closed is open;
9. The symbol of the thermal relay is as shown in the figure below. The one with a square frame is the symbol of the main contact, and the one without the frame is the symbol of auxiliary normally open and auxiliary normally closed points;
10. The analysis circuit is shown below
The figure below is also mentioned in the article on AC contactor. It is a three-phase motor start-stop circuit with self-locking. Here we mainly talk about the function of the thermal relay. When the thermal relay is disconnected, the main circuit is disconnected and the control circuit When the thermal relay is activated, the auxiliary normally open point is turned on and the auxiliary normally closed point is disconnected. Here, the normally closed point of FR is equivalent to the function of the stop button. The circuit is disconnected, which protects the motor and stops the entire action;
