The basic function of an AC contactor is as follows: When an AC contactor is energized, the main contacts close to connect the circuit, and the normally open and normally closed auxiliary contacts close to cause certain dynamic changes in the circuit and control the normal operation of the circuit.
1. Function of main contacts.
The main contacts are connected in series in the main circuit (mostly for motors) to connect or disconnect the circuit, thereby controlling the circuit (for motor operation). The function of the main contacts is relatively simple.
2. Functions of auxiliary contacts. Auxiliary contacts generally have three functions:
(1) Forming a self-locking circuit. The self-locking circuit is an essential and widely used basic circuit in power control, as shown in the figure:
When the start button SB2 is pressed, the coil of the AC contactor KM is energized and engaged. The main contact KM of the AC contactor in the corresponding main circuit closes to connect the circuit, and the auxiliary normally open contact closes to form a self-locking mechanism, ensuring that the circuit continues to operate even after the start button SB2 is released.
In practical applications: the self-locking circuit uses the normally open contact of the contactor, connected in parallel with the start button.
(2) Form an interlocked circuit.
Contactor interlock circuits are also widely used basic circuits. The interlock principle is shown in the figure:
The normally closed contact of AC contactor KM1 is connected in series in the coil circuit of AC contactor KM2. Correspondingly, the normally closed contact of AC contactor KM2 is also connected in series in the coil circuit of AC contactor KM1. In short, these two AC contactors cannot work at the same time.
In practical applications: In the interlocking circuit of the contactor, the normally closed contact of the contactor is connected in series with the interlocking contactor coil.
(3) To transmit signals in the circuit.
Generally, switching signals can be transmitted through the auxiliary normally open or normally closed contacts of a contactor. For example, a contactor controls the operation of a motor; the motor's stop and start signals are used.
Generally, we use the normally open contact of the contactor as the motor running signal indicator and the normally closed contact as the motor stopping signal indicator. Under normal circumstances, the motor is in standby mode, and the normally closed contact of the contactor is used to connect the stop indicator light signal to tell us that the motor is in the stopped position. When the motor is running, the contactor is energized, the normally open contact closes, and the normally closed contact opens. The corresponding "stop" indicator circuit is de-energized, and the "run" indicator circuit is energized. At this time, the signal transmitted is: the stop indicator light is off, and the run indicator light is on.
