what is the contactor on an ac unit


What is the Contactor on an AC Unit?

When it comes to understanding the inner workings of an air conditioning unit, it can often feel like diving into a complex web of wires, components, and mechanisms. One such component that plays a crucial role in the functioning of an AC unit is the contactor. But what exactly is a contactor, and what purpose does it serve? In this article, we will delve into the details of the contactor and explore its importance in the overall operation of an AC unit.

The Contactors: Powerhouse of AC Units

A contactor is an electrical switch used in HVAC systems, specifically in air conditioning units, to control the flow of electricity to various components. It is essentially a relay that enables the AC unit to handle the high-voltage current required to power the compressor, condenser fan, and other essential parts of the system. By acting as a switching device, the contactor allows the AC unit to turn on and off, ensuring efficient cooling and temperature control within a building.

Understanding How a Contactor Works

To comprehend the working principle of a contactor, one needs to have some knowledge of electrical circuits and magnetic fields. Contactor coils contain a winding of wire that, when voltage is applied, creates an electromagnetic field. These magnetic forces attract the movable part of the contactor, called the armature, pulling it towards the stationary part, known as the contacts. As the armature moves, it completes an electrical circuit and allows current to flow to the connected components.

The Role of the Contactor in Starting the AC Unit

When the thermostat signals for cooling, the contactor plays a pivotal role in initiating the AC unit's start-up process. As the thermostat sends a low-voltage signal to the contactor coil, it energizes the coil, creating a magnetic field. This magnetic field attracts the armature, causing the contacts to close, thereby allowing the high-voltage current to flow through the compressor and condenser fan motors. Consequently, the AC unit springs to life, and cool air begins to circulate within the space.

During the start-up process, it is essential to note that the contactor also engages the capacitor. The capacitor provides a boost of power to the compressor, aiding in its start-up. Without the contactor's ability to initiate the flow of current, the compressor and the condenser fan motors would not receive the power they need to function, resulting in no cooling.

Ensuring Proper Contactor Operation

Like any mechanical or electrical component, contactors are subject to wear and tear over time. The immense voltage and current they handle can cause contact erosion, pitting, and other damage. To ensure proper operation of the contactor, regular maintenance and inspection are necessary. Here are a few signs that may indicate a faulty contactor:

1. Frequent Clicking Sound: If you notice a repetitive clicking sound coming from your AC unit, the contactor may be at fault. This sound is a result of the contactor rapidly opening and closing due to poor electrical connections.

2. Failure to Start: When the contactor fails to close properly, it may prevent the flow of current to the compressor and condenser fan. As a result, your AC unit may struggle to start or fail altogether.

3. Stuck Contacts: Over time, contacts can become stuck in a closed or open position. If the contacts remain fused together, even when the thermostat is turned off, this may signal a faulty contactor.

4. Burn Marks or Pitting: Visual inspection of the contactor can unveil any burn marks or pitting on the contacts. These signs indicate erosion and are an indication that the contactor requires immediate attention.

Replacing a Faulty Contactor

If you suspect that your contactor is faulty and causing issues with your AC unit's performance, it is crucial to replace it promptly. While replacing a contactor may seem like a challenging task, it is a relatively straightforward process for a trained professional. However, it is essential to remember that working with electricity can be dangerous, and it is always recommended to consult a qualified HVAC technician to handle any electrical replacements.

Upon contacting an HVAC professional, they will first assess the condition of the contactor and diagnose the issue. If the contactor is deemed faulty or beyond repair, the technician will select a suitable replacement based on the specifications of your AC unit. After shutting off the power supply to the unit, the technician will carefully remove the old contactor, ensuring the wires are properly labeled for reconnection with the new component. Once the new contactor is securely installed, the technician will test the system to ensure it functions correctly.


In summary, the contactor plays a vital role in the operation of an AC unit, acting as a switch for the flow of electricity to various components. Its ability to control the high-voltage current required for the compressor and the condenser fan motors is fundamental to the cooling process. Regular maintenance and inspection of the contactor are necessary to ensure its proper functioning. If any signs of damage or malfunction are detected, it is essential to consult a professional technician for replacement. By understanding the importance of the contactor and its role in an AC unit, you can better appreciate the intricate workings of your cooling system and ensure its optimal performance for years to come.


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