กระบวนการตัดการเชื่อมต่อของรีเลย์กำลัง

In production or daily life, we often use certain electrical appliances such as knife switch power relays, contactors, etc. to disconnect the circuit, and we often observe such a phenomenon when disconnecting the circuit: a dazzling spark appears between the contacts, Sometimes it is fleeting, sometimes it lasts for a certain period of time. This is what we usually call a spark or arc. Its appearance will seriously affect the normal breaking of contacts, reduce its service life, and even cause more serious accidents. Therefore, we must carefully analyze the characteristics and laws of its production and development in order to try to eliminate it.

Everyone has seen an electric welding machine. When it is used for welding, an incandescent dazzling fire will be generated between the electrode and the workpiece, and dazzling sparks will be splashed. This is the arc. The arc is actually a kind of incandescent giant heat that is dissociated from a gas in a neutral state and becomes a large number of charged particles (positive and negative ions and electrons) under the action of circuit voltage and current.” torrent. This phenomenon is also called arc discharge. Arc discharge can generate extremely high temperatures (the center part can reach several dry degrees), and electric welding machines are specially used to generate electric arcs and use their high temperatures to melt metals for welding. However, between electrical contacts, the appearance of arcs is a great scourge. We don't often see: a complete new knife switch, when used for a period of time, is burned by the arc with a large gap: a contact on a power relay or contactor, after many operations, is The arc burns burntly; in some individual cases, if the arc between the contacts cannot be quickly extinguished, the circuit cannot be broken, and the contacts and even the entire electrical appliance, and even other equipment in the circuit will be burned. Therefore, how to quickly extinguish the arc has become a primary problem in the process of contact disconnection. The combustion of the arc—that is, the dissociation of the gas—is only one aspect of the contradiction in the arc. On the other hand, when the gas is dissociated, the negative electrons and positive ions will also collide and recombine into neutral molecules: many electrons and ions will also be far away from the contact gap due to diffusion; the contact and the surrounding cold air will also spread Part of the heat is taken out and taken away, which makes the arc weaken and weakens the dissociation. The disappearance process of electrons and ions is called the dissociation process. Dissociation and dissociation are two contradictory aspects, and the nature of things is mainly determined by the main aspect of the contradiction that takes the dominant position. When the contact is just disconnected, the gap is very small and the electric field strength is very high. The heat is very concentrated, and dissociation becomes the main aspect of the contradiction, and the arc becomes more and more blazing. If the conditions are changed, such as continuing to open the contacts to increase the gap, or blowing in with cold air, the arc will be well cooled, the temperature will decrease, the dissociation process will be weakened, the contradiction will be transformed to the opposite direction, and the dissociation process will be intensified. As the main aspect of the spear eyebrow, the arc tends to go out.

It can be seen that, to extinguish the arc, we can start from two aspects: on the one hand, weaken and limit the dissociation process, and on the other hand, increase and accelerate the dissociation process. For the former, we usually take the following measures: choose contact materials that are not easy to emit electrons under the action of voltage, current and high temperature; fill the contacts with gas with good arc extinguishing performance or insulation performance (such as nitrogen, nitrogen, sulfur hexafluoride) etc.) or evacuated. For the latter, the following measures can be taken to select contact materials and gas components with good thermal conductivity: ensure a certain breaking distance and breaking speed; adopt some special cooling methods and arc extinguishing devices formed thereby.For electrical appliances such as contactors and automatic switches, since their main task is to switch large-capacity circuits, how to quickly and reliably extinguish the strong arc generated between their contacts has become a key issue. In this type of electrical appliances, in addition to using a certain contact material, contact size and disconnection distance, almost all of them need to use a special arc extinguishing device to strengthen their performance functions. However, in the power relay, its main function is only to respond to the signal. Generally speaking, the capacity of the circuit switched by the specified contact is not large, so the arc is also small. Usually, no special arc extinguishing device is required, and the contact is mainly disconnected. A certain distance to extinguish the arc.Then, when the line voltage and current are constant, how far should the contacts be disconnected to extinguish the arc (without the arc extinguishing device)? Or, what is the line voltage and current that can be cut off by a certain contact opening distance (referred to as contact opening distance)? The solution to this problem mainly relies on experiments.

A large number of experimental studies have shown that for contacts with a certain distance, under certain conditions (that is, when the contact material, atmospheric pressure and composition are constant), the maximum voltage and current values of the circuit that the power relay contacts can break have such a value. Rule: The higher the circuit voltage, the smaller the circuit current that can be interrupted; the lower the circuit voltage, the larger the current that can be interrupted; however, the product of these voltages and currents (that is, power) is basically identical. That is to say, for the contact under the above conditions, the maximum power of the circuit it can open is basically certain. Therefore, people use this power to represent the maximum possible breaking capacity of the contact, and it is called limit breaking capacity. Of course, in order to ensure the reliability and safety of the contact, the circuit capacity to be interrupted during normal operation should be smaller than the limit breaking capacity that the contact can achieve by a certain multiple (usually 3 to 5 times), this capacity is called The rated breaking capacity of the contacts. That is to say, for a certain contact, the circuit capacity it cuts off under normal conditions cannot be greater than the rated breaking capacity specified by the contact, and the maximum fault capacity that may occur on the circuit cannot be greater than the limit breaking capacity of the contact . It is not difficult to understand that the larger the distance between the contacts, or the higher the melting point of the material, the better the arc extinguishing and insulation performance of the gas composition in the gap, and the greater its breaking capacity. On the contrary, the breaking capacity is smaller.

In addition, the nature of the controlled circuit also has a great influence on the arc extinguishing of the point. Especially when switching inductive loads, the difficulty of arc extinguishing increases because the inductance can prevent the current from decreasing and generate overvoltage. Therefore, the breaking capacity of the same contact when breaking an inductive load is smaller than that when breaking a resistive load. In the technical conditions, the circuit properties and breaking capacity that can be interrupted by the docking point are clearly determined. For example, the Ministry of Four Machines Department's General Technical Conditions for Electromagnetic power relays stipulates that when the rated breaking voltage remains unchanged, the current when breaking the inductive load can only be 30% of the resistive load. When using, it is necessary to pay full attention to the difficulty of this arc extinguishing, which is also related to the type of circuit current. Obviously, AC circuits are easier to break than DC ones. Because alternating current varies with time, it has an automatic process of decreasing to zero and then increasing again. When AC crosses zeroIf the contacts have been opened to a certain distance, the arc is automatically extinguished.