what is contactor in ac unit


What is a Contactor in AC Unit?

If you've ever wondered how your air conditioner functions to cool down your home or office, it's important to understand the role of various components within the system. One such crucial component is the contactor. The contactor plays a vital role in the operation of an AC unit, allowing it to turn on and off as required. In this article, we will explore the contactor in an AC unit in great detail, providing you with a comprehensive understanding of its function and importance. So, let's dive in!

Understanding the Basics: What is a Contactor?

A contactor, also known as a relay, is an electrical device that controls the flow of electricity within an AC unit. It is essentially an on/off switch for the compressor, the heart of the air conditioning system. Think of the contactor as a gateway that connects the power supply to the compressor. When the system requires cooling, the contactor allows electrical current to flow through it, activating the compressor. When the desired temperature is reached or when the AC unit needs to be turned off, the contactor cuts off the power supply to the compressor, deactivating it. This continuous cycle of operation ensures efficient cooling and keeps the air conditioner functioning optimally.

How Does a Contactor Function?

As mentioned earlier, the contactor acts as an intermediary between the power supply and the compressor. Inside the contactor, there are two sets of contacts – the main contacts and the auxiliary contacts. The main contacts carry the high-voltage current that flows to the compressor, while the auxiliary contacts control the low-voltage circuit that activates the electromagnet.

When the thermostat in the AC unit senses an increase in temperature, it sends a low-voltage signal to the contactor, causing the electromagnet to become magnetized. Once this occurs, the main contacts close, allowing electricity to flow to the compressor, initiating the cooling process. The compressor then begins its work, removing heat from the indoor environment and expelling it outside.

On the other hand, when the desired temperature is reached or when the AC unit needs to be turned off, the thermostat sends a signal to the contactor, causing the electromagnet to demagnetize. As a result, the main contacts open, interrupting the flow of electricity to the compressor, and the system shuts down.

The Importance of a Contactor in an AC Unit

Contactor in an AC unit plays a crucial role in ensuring the smooth operation and efficiency of the air conditioning system. Here are a few key reasons why the contactor is so important:

1. Control and Regulation of Electricity: The contactor allows for the precise control of electrical current flow to the compressor, which is essential for maintaining optimal cooling conditions. By turning the compressor on and off, it helps regulate the amount of cool air produced, preventing unnecessary energy consumption and potential overcooling.

2. Protection against Power Surges: During power surges or electrical fluctuations, which commonly occur during storms or fluctuations in the power grid, the contactor acts as a safeguard for the compressor. It prevents excessive voltage or current from reaching the compressor, protecting it from damage and ensuring its longevity.

3. Prevention of Motor Burnout: The contactor helps prevent motor burnout by providing a controlled supply of electricity. Without a properly functioning contactor, the compressor may experience overload due to continuously running or faulty ignition. This can result in motor burnout, leading to costly repairs or even a complete unit replacement.

4. Efficiency and Energy Savings: By accurately controlling the power supply to the compressor, the contactor helps maintain the desired temperature level with minimal energy consumption. It ensures that the compressor operates only when necessary, reducing unnecessary power usage and potentially lowering energy bills.

5. Enhanced System Lifespan: A well-maintained and functional contactor can significantly extend the lifespan of the air conditioning system. By preserving the compressor and preventing excessive usage, the contactor helps avoid premature wear and tear, reducing the need for frequent repairs or replacements.

Common Issues with Contactors

While contactors are durable and built to withstand frequent usage, like any other component, they may encounter issues over time. Some common problems associated with contactors include:

1. Wear and Tear: Due to the constant switching on and off, the contacts within the contactor can experience wear and tear. Over time, the contacts may become pitted or burnt, leading to decreased functionality or failure.

2. Contamination and Corrosion: Contaminants such as dirt, debris, or moisture can accumulate on the contacts, hindering their proper functioning. This can result in incomplete circuits, erratic operation, or sticking contacts.

3. Electromagnet Issues: The electromagnet within the contactor can also encounter issues, such as coil failure or demagnetization problems. These issues can prevent the main contacts from closing properly, leading to a lack of power supply to the compressor.

4. Electrical Problems: Faulty electrical connections, loose wires, or problems with the control circuit can also affect the operation of the contactor. In such cases, the contactor may not receive the necessary signals or fail to activate the compressor.

Maintaining and Repairing Contactors

To ensure the proper functioning of the contactor in an AC unit and avoid potential issues, regular maintenance and periodic inspections are essential. Here are a few tips to help you maintain and repair the contactor:

1. Cleaning: Regularly clean the contactor to remove dirt, debris, or any other contaminants. Use a non-conductive cleaner and a soft-bristle brush to gently clean the contacts. Be sure to disconnect the power supply before attempting any cleaning or maintenance.

2. Inspecting for Damage: Regularly inspect the contactor for signs of damage, such as pitted or burnt contacts, loose wires, or visible corrosion. If any issues are detected, it is recommended to seek professional assistance for repairs or replacements.

3. Testing: Periodically test the contactor to ensure its proper functioning. This can be done by manually activating the contactor and checking if the main contacts close and open as required. If there are any abnormalities in the operation, it is advisable to contact a qualified technician for further inspection.

4. Professional Assistance: If you encounter any significant issues with the contactor or if you lack the necessary expertise, it is always best to seek professional help from a licensed HVAC technician. They have the knowledge and experience to diagnose and rectify contactor-related problems effectively.


In conclusion, the contactor is a critical component within an AC unit that enables the proper functioning of the compressor. By controlling the flow of electricity, it allows the compressor to turn on and off as required, ensuring efficient cooling and preventing unnecessary energy consumption. The contactor also safeguards the compressor against power surges, protects it from motor burnout, and contributes to overall system longevity. Regular maintenance and periodic inspections are crucial to keep the contactor in optimal condition. By understanding the importance of the contactor and addressing any issues promptly, you can ensure the reliable and effective operation of your air conditioning unit for years to come.


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