what is the contactor on ac



Contactor is an essential component in an air conditioning (AC) unit that plays a crucial role in its operation. It is responsible for controlling the flow of electrical current in the system, allowing the AC to function efficiently. Understanding the contactor and its functions is vital for anyone who wants to have a better grasp of how their AC works and maintain it properly. In this article, we will delve deeper into what the contactor is and its significance in an AC system.

The Purpose of a Contactor

A contactor is an electromagnetic switch that enables the flow of electrical current in an AC unit. Its role is similar to a relay but designed specifically for higher voltage circuits. When the thermostat detects a signal to cool the air, it sends a signal to the contactor, which then engages to close the circuit and allows the electrical current flow to the compressor and condenser fan motor. Effectively, the contactor switches on the AC unit, initiating the cooling process.

The contactor consists of a coil and contacts. The coil is an electromagnetic component that energizes when the thermostat's signal is received, generating a magnetic field that pulls the contacts together, closing the circuit. Once the contacts are closed, the electrical current flows through the contactor and powers the compressor and the condenser fan motor.

The Components of a Contactor

1. Coil: The coil is the core component of a contactor. It is made up of copper wire wound around an iron core. When the thermostat sends a signal to the contactor, the coil is energized, producing a magnetic field that attracts the contacts, closing the circuit. The coil requires low voltage to operate, typically around 24 volts in residential AC systems.

2. Contacts: The contacts are crucial elements of a contactor. They are made of sturdy materials that can handle high electrical currents repeatedly. When the coil generates a magnetic field, it attracts the contacts and pulls them together, connecting the circuit. The main contacts are responsible for carrying the current between the power source, compressor, and condenser fan motor.

3. Auxiliary Contacts: Apart from the main contacts, some contactors also have auxiliary contacts. These contacts are essential for controlling other components in the system, such as the outdoor fan motor, reversing valves, and electric heaters. Auxiliary contacts are typically designed to handle lower voltages compared to the main contacts.

4. Enclosure: The contactor is enclosed in a protective housing, which shields it from external factors like moisture, dust, and physical damage. The enclosure ensures the contactor's durability, extending its lifespan and ensuring its performance even in harsh environments.

5. Contact Springs: Contact springs are located within the contactor and provide the necessary tension to ensure reliable contact between the contacts. They help maintain a tight connection between the contacts, allowing for efficient flow of electrical current without any loose connections or arcing.

How the Contactor Works

The contactor is a crucial component in the functioning of an AC unit. When the thermostat sends a signal to the contactor, the coil is energized, creating a magnetic field that attracts the contacts together. This closes the circuit, allowing the electrical current to flow through the contactor.

The electrical current then powers the compressor, responsible for circulating refrigerant and removing heat from the indoor air. Simultaneously, the current also energizes the condenser fan motor, which helps dissipate the heat absorbed by the refrigerant. The contactor ensures that these components receive the necessary electrical power to perform their functions effectively.

Signs of a Faulty Contactor

Over time, the contactor may experience wear and tear or develop faults. It is important to recognize the signs of a faulty contactor to prevent further damage to the AC unit. Here are some common signs to watch out for:

1. Failure to Start: If the contactor fails to close the circuit and initiate the AC unit, it may be a symptom of a faulty coil or worn-out contacts. This can be indicated by a lack of response from the AC unit when the thermostat signals for cooling.

2. Intermittent Cooling: If the AC unit turns on and off irregularly or provides inconsistent cooling, it may be due to a faulty contactor. The contacts might not be closing properly, resulting in intermittent power flow to the compressor and condenser fan motor.

3. Clicking Sounds: Unusual clicking sounds coming from the AC unit during startup or operation can suggest a faulty contactor. The sounds often indicate that the contacts are not engaging correctly, leading to disruption in the electrical circuit.

4. Burn Marks or Pitting: Inspect the contactor for any burn marks or pitting on the contacts. These can be signs of electrical arcing, which can occur due to worn-out contacts. Arcing can further damage the contactor and other components if not addressed promptly.

5. High Energy Bills: A faulty contactor can cause increased energy consumption as it may fail to maintain an efficient flow of electrical current. This can result in a higher electricity bill without providing the expected cooling performance.

Maintaining and Replacing the Contactor

To ensure the longevity and efficiency of the contactor, regular maintenance is essential. Here are some maintenance tips to keep in mind:

- Regularly clean the contactor's enclosure to remove any dirt, debris, or dust that may hinder its performance.

- Inspect the contacts for any signs of wear, burn marks, or pitting. If these signs are present, the contactor should be replaced.

- Lubricate the contact springs to maintain their functionality and prevent any sticking or seizing.

- Ensure proper electrical connections to the contactor, including tightened screws and secure wiring.

When it comes to replacing a faulty contactor, seeking professional help is recommended. An HVAC technician can accurately diagnose the issue and ensure a suitable replacement is installed. It is crucial to use a contactor with specifications precisely matched to the AC unit's requirements to ensure compatibility and optimal performance.


The contactor is an indispensable component in an AC unit, responsible for initiating the cooling process by allowing the flow of electrical current. Understanding its functionality, components, and signs of failure is essential for proper maintenance and troubleshooting. Regular maintenance and periodic inspections can help identify any contactor issues early on, preventing further damage to the AC unit. Remember, if any signs of a faulty contactor are present, seeking professional assistance is crucial to ensure the AC unit's reliable operation.


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