power relay contact closing process

Closing the power relay contacts is not an easy task. Because when the dynamic and static power relay contacts collide, there will be no mechanical wear, and the power relay contacts will bounce or bounce back. When it bounces, a series of short electric or sparks will be generated, which will make the circuit unable to be connected accurately and cause high-frequency interference. In severe cases, the power relay contacts will be burned or even a fusion welding accident will occur. There will be a bounce, which will drive the power relay contact to bounce back, and the distance will increase, and the impact will be more serious. In addition, when some circuits are closed (such as incandescent lamps, electric motors, etc.), a large initial inrush current (up to several times or ten times the current in a steady state or rated state) will appear in the circuit. However, when the contacts of the dynamic and static power relays are just in contact, the contact pressure of the power relay is very small and the actual contact surface is not large. Under the action of a large inrush current, the contacts of the power relay may heat and melt. Then the contacts of the static and dynamic power relay are pressed together, and the molten metal cools and solidifies, causing a serious welding accident.

In order to prevent the occurrence of bouncing when the power relay contacts are closed, or to minimize the duration of the bounce, the usual method is to add a pressure to the contacts of the moving power relay when the contacts of the power relay are just in contact to prevent or weaken the bounce of the contacts of the moving power relay. . This pressure is called the "initial pressure" of the power relay contact (represented by the symbol F.). In some small power relays, in order to simplify the structure, the initial pressure may not be added, but the movable power relay contact reed with certain elasticity should be used to play a buffering role. In order to ensure that the power relay contacts can work normally when connecting a circuit with a large inrush current, it is usually stipulated in the technical conditions that the rated value of the circuit must be reduced in use. For example, it is stipulated in the "General Technical Conditions for Electromagnetic power relays" (Ministry of Four Machines) that when the rated voltage remains unchanged, if the current of the power relay contact connected to the pure resistance load is 100%, the power relay contact is connected to the rated load of the lamp. The current can only be 15%, and the load on the motor is 0%. This point must also be paid attention to in use.