Today, I will talk about the adhesion detection of relays. It is a very important part of the BMS function. Let's first introduce the related concepts.
Regarding the adhesion concept of the relay, its main scenario is that the fixed contact and the movable contact of the relay are melted and welded together, and then the disconnection cannot be controlled normally; the adhesion phenomenon is that the relay is in an open state and cannot be closed normally .
There are two reasons for contact welding: static welding and dynamic welding.
Static welding is when the relay is in the closed state, due to the abnormally large current, the Joule heat generated by the contact resistance causes the metal in the contact part of the two contacts to melt, weld, and combine, but cannot be disconnected; static welding occurs in the fixed contact connection or contact In the closed state contact with sufficient force.
Dynamic welding refers to the fact that the arc releases a large amount of heat in a short period of time when the relay is closed or opened, which causes the contacts to melt or vaporize, and the welding phenomenon occurs.
When the welding is not serious, sometimes under a certain vibration, the adhered contacts will be separated again, leaking the traces of melting. (The picture comes from the Internet)
Two reasons for arc drawn in dynamic welding are as follows:
1. At the moment the relay is closed, there is a large short-circuit current in the circuit caused by the capacitive load, and the arc is caused by the jitter of the relay, which generates a lot of heat and leads to welding;
2. Use a relay to cut off the high current load, causing arcing, and also generating a lot of heat to cause welding;
There is also a special case called the electromagnetic repulsion scenario; it is when the relay is closed, due to a very large short-circuit current, the contacts bounce off, causing arcing and generating heat to cause welding. The details are as follows:
The following figure is a schematic diagram of the current flow after the closed contact between the fixed contact and the movable contact, because the contact surface is not flat; the black circle indicates that the directions of the two current paths are parallel and opposite; due to the formation of the two The magnetic field causes a repulsive force between them, which is applied to the two contacts, forcing the contacts to separate.
When the current passing through the contact is very large, the repulsive force is greater than the suction and holding force of the contact, and the two contacts are separated, resulting in arcing, and the released heat causes the contact to weld.
