The primary function of a contact device is to connect or disconnect the main circuit. The so-called main circuit refers to a circuit whose operation is marked by whether the circuit is connected or not. The concept of main circuit corresponds to the control circuit. The current passing through the main circuit is generally greater than that of the control circuit. Therefore, as my friend on the first floor said, high-capacity contact devices usually come with an arc extinguishing cover (due to high current interruption, an arc will occur, and not using an arc extinguishing cover to extinguish the arc will burn out the contact).

The characteristic of a relay is that its output will only undergo a step change when the change in its input reaches the relative program. It is suitable for long-distance connection and disconnection of AC and DC small capacity control circuits, and is used for control, maintenance, and signal conversion in electric drive systems. The input quantity of a control relay is usually electrical quantities such as current and voltage, and can also be non electrical quantities such as temperature, pressure, and speed. The output quantity is the electrical signal emitted when the contact is activated or the parameter change of the output circuit.

Relays generally consist of three fundamental components: sensing organization, central organization, and executing organization. The sensing organization transmits the sensed electrical quantity to the time organization, compares it with additional setting values, and when the setting value (excessive or insufficient) is reached, the central organization performs organizational actions and then turns on or off the controlled circuit.

If the operating current of a main circuit is very high, so that the capacity of the contact device used to turn on and off the main circuit is very large, the current flowing through its control circuit is also very high, which means that it is difficult to turn on and off its control circuit with ordinary relays. At this point, it is possible to choose an electrical appliance that was originally used as a contact to be used for the on/off of the control circuit, and the effect of this contact in this situation is a relay.

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