what does an ac contactor do


What Does an AC Contactor Do?

An air conditioning (AC) contactor is a crucial component in the operation of an AC unit. It plays a significant role in controlling the flow of electricity to the different parts of the system. This article will explore the functions and importance of an AC contactor, as well as delve into its various parts and how they work together to ensure efficient cooling in your home or office.

Understanding an AC Contactor

An AC contactor is an electromechanical switch that controls the flow of electricity to the compressor and fan motor in an AC unit. It is typically located inside the condenser unit of the system. The contactor consists of a coil, contacts, and terminals. When the coil is energized, the contacts close, allowing electricity to flow through the system. When the coil is de-energized, the contacts open, interrupting the flow of electricity.

How Does an AC Contactor Work?

AC contactors are designed to work in conjunction with the thermostat. When the desired temperature is reached, the thermostat signals the contactor to open, turning off the compressor and fan motor. Conversely, when the temperature rises above the set point, the thermostat triggers the contactor to close, starting the cooling cycle. This continuous cycle ensures that the AC unit operates efficiently and maintains your desired indoor temperature.

The Main Parts of an AC Contactor

An AC contactor consists of several key components that work together to ensure proper functionality. Let's take a closer look at each of these parts:

The Coil:

The coil is an electromagnet that activates the contactor. When voltage is applied, the coil generates a magnetic field that attracts the contactor's moving parts, closing the contacts and allowing current to flow. The coil is typically controlled by the thermostat or a control board.

The Contacts:

Contacts in an AC contactor are made of metal and allow the transfer of electrical current. When the contacts close, they form a complete circuit between the power supply and the motor, allowing electricity to flow. The quality and condition of the contacts play a crucial role in the overall performance and longevity of the contactor.

The Terminals:

Terminals serve as connection points for incoming and outgoing electrical wires. They facilitate the transfer of electricity to and from the contactor. It is essential to ensure that the wires are securely connected to prevent any loose connections that may affect the performance of the AC unit or lead to system failure.

The Auxiliary Contacts:

Some contactors have auxiliary contacts that provide additional functions and safety features. These contacts can be usually found on the side of the contactor and are used for various purposes, such as controlling other devices or signaling when the contactor is closed.

Common Issues with AC Contactors

Like any mechanical component, AC contactors can experience problems over time. Some common issues include:

Wear and Tear:

As the contactor is an electromechanical device, its contacts can wear out over time due to frequent opening and closing. When this happens, pitting and burning can occur, hindering the flow of electricity and potentially causing the contactor to fail.

Contactor Buzzing:

A buzzing sound coming from the contactor can indicate an issue with the coil or the contacts. If the coil is faulty, it may not generate the magnetic field required to close the contacts properly. On the other hand, if the contacts are damaged, they may not close fully, causing electrical arcing and a buzzing noise.

Contactor Stuck or Won't Close:

Sometimes, a contactor can become stuck in the closed position, causing the AC unit to run continuously. This can lead to excessive wear on the system and increased energy consumption. Conversely, a contactor that fails to close can prevent the AC unit from turning on at all.

Contactor Replacement and Maintenance

Replacing a faulty contactor requires some electrical knowledge and should ideally be performed by a trained professional. The process involves disconnecting power to the unit, removing the old contactor, connecting the wires to the new contactor, and re-energizing the system.

Regular maintenance can help extend the lifespan and ensure the proper functioning of an AC contactor. Here are a few maintenance tips:

Clean the Contactor:

Dust, debris, and insects can accumulate on the contactor over time, affecting its performance. Cleaning the contactor regularly with a soft brush and compressed air can help maintain its efficiency.

Inspect the Contacts:

Periodically check the contacts for signs of wear, pitting, or burning. If any damage is detected, it is advisable to replace the contactor to prevent further issues.

Test the Coil:

Using a multimeter, test the coil to ensure it is receiving the appropriate voltage. If voltage is present but the coil fails to energize, the contactor may need to be replaced.


AC contactors are essential components of an air conditioning system, controlling the flow of electricity and ensuring efficient cooling. Understanding how contactors work and the various parts involved can help homeowners and HVAC professionals diagnose and address any issues that may arise. Regular maintenance and timely replacement of faulty contactors will contribute to the overall performance and longevity of your AC unit, providing comfortable indoor temperatures for years to come.


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