What is the working principle of an AC contactor?

The working principle of an AC contactor is based on electromagnetic induction and mechanical linkage,

 indirectly controlling the on-off of the main circuit by controlling the current of the coil. The following are

 its core working processes and key mechanisms:

### I. Core Working Principle

1. **Drive of the Electromagnetic System**

   - **Coil Energization**: When the control circuit (such as buttons, relays) applies the rated voltage to the 

contactor coil, the coil generates an alternating magnetic field, magnetizing the iron core.

   - **Armature Suction**: Under the action of the magnetic field, an electromagnetic suction force is generated 

between the iron core and the armature (movable iron core), overcoming the resistance of the spring and driving

 the armature to move towards the iron core.

   - **Contact Action**: The armature pushes the main contacts and auxiliary contacts to act through the linkage

 mechanism:

     - **Main Contact Closing**: Connects the large current of the main circuit (such as motors, loads).

     - **Auxiliary Contact Switching**: The normally open contacts close (such as the self-locking function), and the 

normally closed contacts open (such as the interlocking function).

2. **Power-off Reset**

   - After the coil is powered off, the magnetic field disappears, and the spring force makes the armature return to 

its original position. The main contacts and auxiliary contacts return to their initial states, and the main circuit is 

disconnected.

### II. Functions of Key Components

1. **Electromagnetic System**

   - **Coil**: Passes the control current to generate a magnetic field.

   - **Iron Core and Armature**: Transmits the electromagnetic force and drives the contacts to act.

   - **Short-circuit Ring (Only for AC Contactors)**:

     - A copper ring is embedded in the end face of the iron core. The induced current forms a lagging magnetic 

field, avoiding the disappearance of the suction force when the alternating current passes through zero, and 

eliminating the vibration and noise of the armature.

2. **Contact System**

   - **Main Contacts**: Bear the large current and control the on-off of the main circuit.

   - **Auxiliary Contacts**: Used for the control circuit (such as self-locking, interlocking, signal feedback).

3. **Arc Extinguishing Device**

   - The arc generated when the main contacts are opened is quickly extinguished by the arc extinguishing cover

 (such as ceramic sheets, metal grids) to prevent the contacts from being eroded or causing a short circuit.

### III. Differences from DC Contactors

- **AC Contactors**:

  - The iron core is stacked with silicon steel sheets to reduce eddy current losses; the short-circuit ring is 

standard equipment.

  - The coil has fewer turns and a low resistance, adapting to the alternating voltage.

- **DC Contactors**:

  - The iron core is mostly made of a whole piece of steel, and there is no need for a short-circuit ring (there is 

no zero-crossing problem in the DC magnetic field).

  - The coil has more turns and a high resistance, adapting to the stable DC voltage.

### IV. Working Characteristics

1. **Self-locking Function**: By connecting the auxiliary normally open contacts in parallel with the start button, 

the coil is kept continuously energized (it still remains attracted after releasing the button).

2. **Interlocking Protection**: Using the auxiliary normally closed contacts to prevent two contactors from being

 attracted simultaneously (such as the control of the forward and reverse rotation of the motor).

3. **Voltage Sensitivity**: The coil voltage needs to match the rated value (such as 380V/220V). Too low a voltage 

will lead to insufficient suction force, and too high a voltage will easily burn out the coil.

### V. Application Scenarios

- **Motor Control**: Start-stop, forward and reverse rotation, star-delta step-down starting.

- **Industrial Equipment**: On-off of the power supply of heating devices, fans, water pumps.

- **Automation Systems**: Cooperate with PLCs and relays to achieve remote control and logical linkage.

**Conclusion**: The AC contactor controls the strong current with weak current, converting the control signals 

of low voltage and small current into the on-off of the main circuit with large current. At the same time, it has

 the advantages of rapid response, multiple protections, and modular expansion, and is a core component of

 power dragging and automation control.