what does the contactor do in ac unit



Air conditioning units are complex systems that rely on various components to function properly. One such crucial component is the contactor. This article delves into the workings of an AC unit's contactor and its significance in the overall operation of the system. By understanding the role of the contactor, you can better grasp the inner workings of your air conditioning unit and troubleshoot potential issues more effectively.

What is a Contactor?

A contactor is an electrical switch that controls the flow of electricity to different parts of an air conditioning unit. It is typically composed of a coil, contacts, and terminals. When an AC unit is turned on, the contactor is responsible for establishing an electrical connection between the incoming power source and various components of the system, such as the compressor and condenser fan motor. By controlling the flow of electricity, the contactor ensures that the AC unit operates efficiently and effectively.

How Does the Contactor Work?

The Coil

The coil is a crucial part of the contactor and is responsible for initiating the switch between open and closed positions. When an electrical current passes through the coil, it generates a magnetic field, which causes the contacts to move. This movement establishes or breaks the electrical connection in the contacts, controlling the flow of electricity in the AC unit. The coil serves as the driving force behind the contactor's operation.

The Contacts

The contacts in a contactor refer to the points where the switch is made or broken. In an AC unit's contactor, there are typically at least two sets of contacts: the main contacts and the auxiliary contacts. The main contacts handle the flow of high voltage electricity between the power source and the various components of the AC unit. The auxiliary contacts, on the other hand, are used for control purposes. They are typically connected to the thermostat and provide feedback to the system regarding the current state.

The Terminals

The terminals in a contactor are the connection points where the electrical wires from the power source and the AC unit are attached. These terminals ensure a secure and stable connection between the electrical components. It is essential to ensure that the wires are properly connected and tightened to the terminals to prevent any loose connections or electrical hazards.

The Significance of the Contactor in an AC Unit

The contactor plays a crucial role in the overall operation and efficiency of an AC unit. Here are some key reasons why the contactor is significant:

Control of Power Flow

As mentioned earlier, the contactor controls the flow of electricity within an AC unit. By acting as a switch, it allows power to be directed to different components as needed. For example, when the thermostat signals a cooling requirement, the contactor closes the switch, allowing electricity to flow to the compressor and condenser fan motor. Without the contactor, the power flow would be uncontrolled, leading to improper functionality or even potential damage to the AC unit.

Prevention of Electrical Overload

The contactor also plays a vital role in preventing electrical overload. Whenever the compressor or any other component requires power, the contactor will engage to provide the necessary electricity. However, if the system becomes overloaded, the contactor will disengage, breaking the electrical connection and protecting the components from damage. This feature ensures the safe and efficient operation of an AC unit.

Integration with the Thermostat

The contactor works in conjunction with the thermostat to maintain the desired temperature in a space. When the thermostat detects that the temperature has exceeded the set limit, it sends a signal to the contactor, initiating the cooling cycle. The contactor then closes the electrical circuit, allowing the necessary components to start functioning. This integration ensures that an AC unit only operates when needed, helping to conserve energy and reduce overall utility costs.

The Importance of Regular Contactor Maintenance

To ensure the smooth functioning of an AC unit, regular maintenance of its contactor is crucial. Here are a few maintenance tasks that can help keep the contactor in optimal condition:


Over time, dust, dirt, and debris can accumulate on the contactor, interfering with its operation. Regularly cleaning the contactor can prevent such buildup and ensure smooth electrical connections. Use a soft brush or compressed air to remove any particles from the coil, contacts, and terminals. This simple maintenance task can go a long way in preserving the contactor's functionality.

Tightening Connections

Regularly inspect the contactor for any loose wires or connections. Loose connections can lead to voltage drops, uneven power distribution, and even equipment failure. Carefully tighten any loose connections using a screwdriver or pliers. Always ensure that the power to the AC unit is turned off before performing any maintenance tasks.

Visual Inspection

Perform a visual inspection of the contactor to check for signs of wear, corrosion, or damage. Any abnormalities should be addressed promptly to prevent further issues. If you notice significant damage or signs of wear and tear, it may be necessary to replace the contactor.

Replacing a Faulty Contactor

Over time, contactors can deteriorate or fail due to extensive use, age, or environmental factors. When a contactor becomes faulty, it is important to replace it promptly to avoid further complications. Here's a step-by-step guide on how to replace a faulty contactor:

1. Turn Off the Power: Before working on any electrical component, always turn off the power supply to the AC unit. This ensures your safety and prevents any accidental electrical shocks.

2. Locate the Contactor: The contactor is usually located near the electrical panel of the AC unit. It is often enclosed in a metal box or housing for protection.

3. Disconnect the Wires: Carefully disconnect the wires connected to the faulty contactor. Take note of the wire placement or take a picture for reference.

4. Remove the Old Contactor: Unscrew or unclip the old contactor from its mounting bracket. Carefully remove it from the AC unit.

5. Install the New Contactor: Position the new contactor in place, aligning it with the mounting bracket. Secure it in place using screws or clips.

6. Reconnect the Wires: Attach the wires to the corresponding terminals on the new contactor. Double-check that the wiring is correct to avoid any electrical issues.

7. Test the New Contactor: Turn on the power supply to the AC unit and test the new contactor. Ensure that it engages and disengages properly when the thermostat signals a cooling requirement.

8. Close and Secure: Once everything is functioning correctly, carefully close any panels or covers that were removed during the replacement process. Ensure they are securely fastened.


The contactor is an essential component in an air conditioning unit, responsible for controlling the flow of electricity and ensuring the proper functioning of various components. Understanding the role of the contactor can help you troubleshoot issues, perform regular maintenance, and even replace a faulty contactor if necessary. By proactively caring for your contactor, you can enhance the efficiency and longevity of your AC unit, providing you with optimal comfort and cooling during hot summer days.


Just tell us your requirements, we can do more than you can imagine.
Send your inquiry

Send your inquiry

Choose a different language
Current language:English