what tells ac contactor to energize


What Tells AC Contactor to Energize?


The AC contactor is a critical component in any air conditioning system. It plays a crucial role in controlling the flow of electrical current to the compressor and fan motor. But how does the AC contactor know when to energize and start the cooling process? In this article, we will explore the various factors that determine when the AC contactor is signaled to energize and what happens behind the scenes.

The Thermostat:

The thermostat is the primary device responsible for signaling the AC contactor to energize. It monitors the temperature in the room or space being cooled and sends appropriate signals to the contactor based on user-defined settings. When the temperature exceeds the set point, the thermostat sends a signal to the contactor, allowing it to close and complete the electrical circuit.

The thermostat achieves this by utilizing a sensor called a thermistor, which measures changes in temperature. The thermistor is connected to the thermostat, and when the temperature rises above the desired level, it triggers the contactor through a low-voltage signal. Once the contactor receives the signal, it quickly closes, allowing the high-voltage current to flow and power the compressor and fan motor.

It's important to note that the thermostat's signal is dependent on the user's settings. If the thermostat is set to a specific temperature, it will only send a signal to the contactor when that set temperature is reached or exceeded. This ensures that the AC system operates efficiently and only cools the space when necessary.

Control Wiring:

While the thermostat signals the AC contactor to energize, there is a network of control wiring that allows this communication to happen seamlessly. The control wiring consists of low-voltage cables that connect the thermostat to the contactor, ensuring the proper transmission of signals.

Typically, the control wiring consists of color-coded cables that facilitate easy installation and troubleshooting. The thermostat and contactor are connected through specific terminals on each device, creating a closed loop between the two. When the thermostat sends the signal, it travels through the control wiring and activates the contactor, allowing the flow of electricity.

To ensure the control wiring remains intact and efficient, it is recommended to use high-quality cables and connectors. Loose connections or damaged wiring can lead to intermittent contactor operation or complete failure. Regular inspections and maintenance of the control wiring are essential to avoid any issues with the AC contactor's energizing process.

Contactor Coils and Magnetic Fields:

The AC contactor's energizing process is facilitated by magnetic fields generated within the contactor coils, which leads us to electromagnetic theory. AC voltage powers the primary coil, creating a magnetic field that surrounds it. This magnetic field then induces voltage in the secondary coil positioned within the contactor.

When the thermostat sends a signal to the contactor, it triggers the secondary coil, creating an opposing magnetic field. This opposing magnetic field attracts a plunger or armature within the contactor, causing it to move. The movement of the plunger or armature closes the contact points, completing the electrical circuit and allowing the high-voltage current to flow to the compressor and fan motor.

It's worth mentioning that the contactor's design and construction play a crucial role in ensuring the efficiency and reliability of the energizing process. High-quality contactors are engineered to minimize contact resistance, prevent arcing, and provide consistent operation over an extended period.

Power Supply and Circuit Breakers:

Before the AC contactor can energize, it requires a stable power supply and protection mechanisms in the form of circuit breakers. The power supply is needed to provide the necessary voltage to the contactor coils and enable them to generate magnetic fields.

AC systems are typically connected to the main power supply of a building or residence. Voltage fluctuation or sudden power surges can disrupt the energizing process and potentially damage the contactor. Thus, it is crucial to have proper electrical grounding and surge protection systems in place.

Circuit breakers act as protective devices, ensuring that the contactor is not overwhelmed by excessive current flow. If there is a fault or circuit overload, the circuit breaker will trip, disrupting the electrical circuit and preventing any damage to the contactor or other components. Additionally, circuit breakers also facilitate routine maintenance and troubleshooting by allowing easy disconnection of power to the contactor.

Electrical Safety and Interlocks:

In any electrical system, safety is of utmost importance. The energizing process of the AC contactor is no exception. Several safety devices and interlocks are incorporated to ensure that the contactor operates within safe parameters and minimizes the risk of electrical hazards.

One such safety feature is the auxiliary contact, which is built into the contactor mechanism. The auxiliary contact is responsible for enforcing safety interlocks and preventing contactor operation under certain conditions. For example, if the system's power supply is unstable or interrupted, the auxiliary contact will prevent the contactor from closing, ensuring the multiple safeguards remain intact.

In addition to auxiliary contacts, AC systems may also include other safety measures such as pressure switches, flow switches, and temperature sensors. These devices monitor critical parameters in the cooling system and can prevent the contactor from energizing if any abnormal conditions are detected. Such safety interlocks provide an added layer of protection and reliability to the overall AC system.


The AC contactor's energizing process is a sophisticated and orchestrated sequence of events. It relies on the thermostat's signal, control wiring, contactor coils, power supply, circuit breakers, and several safety interlocks to ensure proper and safe operation. Understanding these factors can help users and technicians troubleshoot any issues with the contactor's energizing process and keep their AC systems running efficiently.

In conclusion, the AC contactor's energizing process is heavily dependent on various components and mechanisms working harmoniously. From the thermostat to the contactor coils and safety interlocks, every element plays a significant role in ensuring the smooth operation of an air conditioning system. By comprehending how these factors interact, users can maintain and troubleshoot their AC systems more effectively, ensuring comfort and reliability in their living or working spaces.


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