AC contactors are widely used for switching and controlling electrical circuits. They utilize main contacts to open and close circuits, and auxiliary contacts to execute control commands. Main contacts typically have only normally open contacts, while auxiliary contacts often have two pairs of contacts with both normally open and normally closed functions. Small contactors are also frequently used as intermediate relays in conjunction with the main circuit. This enables remote control or low-voltage control of high-voltage circuits. The contacts of AC contactors are made of silver-tungsten alloy, which has good conductivity and resistance to high-temperature erosion.
The image above shows an electromagnetic AC contactor. In the diagram, 1 (L1), 2 (L2), 3 (L3), 4 (T1), 5 (T2), and 6 (T3) are the main contacts. 1 and 2 form one pair, 3 and 4 form another, and 5 and 6 form a third pair, for a total of three pairs of main contacts. Since most AC motors are three-phase (although single-phase motors also exist, generally for small power motors), the upper L1, L2, and L3 are connected to the input terminals, and the lower T1, T2, and T3 are connected to the output terminals. 21 and 14 are a pair of auxiliary contacts; the marking "NO" indicates a normally open contact, and "NC" indicates a normally closed contact. The "NO" marking in the diagram indicates that it has its own pair of normally open contacts. A1 and A2 are the two output terminals of the coil. A1 is usually located at the top of the contactor and is not shown in the image. Commonly used coil voltages include 110V, 220V, and 380V. If the contactor is marked as 220V, it cannot be connected to a 380V power supply, otherwise it will burn out. Normally open means that when the contactor coil is not energized, the auxiliary contacts are open; normally closed means that when the auxiliary contacts are closed.
The diagram above shows the schematic of a contactor controlling a motor. Figure 1 shows the main circuit, and Figure 2 shows the control circuit. The yellow boxes indicate the main contacts of the contactor, the blue boxes indicate the auxiliary contacts, and the red boxes indicate the coil. SB2 is the start button, and SB1 is the stop button. When SB2 is pressed, the KM1 coil is energized and closed, and the KM1 auxiliary contact (in the blue box) also closes. Releasing SB2 resumes motor operation, while pressing SB1 stops the motor. The diagram below shows the wiring diagram for an AC contactor; you can compare it with the diagram above.
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