What is An AC Contactor? Working and Construction

The AC contactor is a useful device in many high-voltage AC systems. These can be heavy-duty motors, industrial equipment, large lighting installations, and so on. Here, we explain what this type of contactor is and how it works. Additionally, we will be comparing it with the DC type, both in terms of construction and working principle.

What is an AC Contactor

The AC contactor is a type of electrical device that, using a low voltage circuit, can switch high-power systems on and off. It essentially uses an electromagnetic mechanism that, when energized, causes high-energy contacts to close.

AC means the contactor works with an alternating current circuit. This is in contrast with DC contactors that are meant for use with direct current electrical systems. As you can tell, that makes it a common switching device when using the grid to power high-voltage electrical loads.

What Does an AC Contactor Do?

The main AC contactor function is to isolate high energy equipment or systems from their control circuits. That way, the switching action happens safely without damage to the delicate control components.

For example, when used with an electric motor, the switch protects the PLC and its parts against high energy supply that powers the motor. It also allows the operator to safely monitor the motor operation.

AC Contactor Parts

Insides the alternating current contactor are parts that make its working possible. These mainly include the following: coil and its core (electromagnet), power and auxiliary contacts, terminals, and the enclosure or housing.

Contactor Coil

The AC contactor coil is responsible for creating the magnetic field that causes movement and the closing of power contacts. Wound around an E-shaped magnetic core, it receives power from the control circuit.

When energized, the coil generates a magnetic flux. The flux is then amplified by the magnetic core. That, in turn, creates a magnetic force that pulls an armature to close the main contacts. The coil is usually resin bonded to protect it from the effects of moisture and other damage.

Contactor Contacts

These are parts that close or open to switch or break the flow of current, and categorized as either static or moving. Moving contacts attach to the armature and move when the coil is energized. Static contacts remain stationary.

There’s also usually an auxiliary contact included. This serves as the feedback contact. Its function is to help the control hardware monitor the contactor’s health and working. In other words, it ensures the switching action happens as required.

Contactor Terminals

Your AC contactor switch comes with terminals or connection points where you attach the wires that will go into it. These are usually labeled to make them identifiable based on their function. A typical contactor of this will have these terminals:

• A1 and A2: to connect the power supply, also called coil terminals.
• LI, L2, and L3: you connect the high power supply to these terminals.
• T1, T2, and T3: here is where you connect the device that’s being powered.

Contactor Housing

The housing is the contactor’s enclosure. It forms the protective covering that shields the mentioned parts, and must be strong enough to prevent damage.

Since the device works with high voltages and high currents, the housing is also made using an insulating material and securely sealed. This is commonly nylon 6, thermosetting plastic, polycarbonate, and other similar materials.

How Does an AC Contactor Work?

The AC contactor working principle is based on the action of an electromagnet, or the magnetic flux that a coil creates when it conducts current. This allows the device to be used as a mechanical switch that turns heavy-duty circuits to the on or off position.

AC Contactor Working

Now that we understand its principle of operation, here are the contactor’s working steps in more detail, from when the power button is pressed to when it disconnects the high energy circuit.

• To energize the contactor, a button is pressed. This sends power to the coil.
• The coil energizes, creating a field that magnetizes the core.
• The magnetized core attracts an armature.
• The armature is connected to a set of moving contacts.
• Its movement causes the moving and stationary contacts to come together
• The connection closes a high-energy circuit
• A high-energy device or system receives power and starts working.
• When the current to the contactor is switched off, the opposite happens.
• The closed contacts open, and the device or system that was being powered stops working.

Can AC Contactor be Used for DC?

Although you could, it’s not advisable to practically use AC contactors in electrical systems that use direct current. AC based contactors are specifically rated for alternating currents, and do not require elaborate arc extinguishing features since the AC current does that.

Their contacts and coil are not rated for DC currents either, and are mostly rated higher than those of direct current contactors. With these differences in mind, you can expect a contactor that is meant for AC not to work properly in a direct current or DC system. It could even cause safety concerns or get damaged.

DC Contactor vs. AC Contactor

AC contactors operate with AC electrical systems, while DC contactors are used in DC systems. This is the main feature that distinguishes the two types of devices. Other differences between the two types of the device include the following;

• Since it operates using alternating current, the AC based contactor is prone to energy losses by Eddy currents. In order to prevent that, its core is normally shaped like an E and composed of single steel sheets,
• On the other hand, the DC contactor core is a solid U shaped electromagnet. That’s because no eddy currents are generated when the device is in use.
• An AC electrical contactor can naturally extinguish arcs and doesn’t require a pronounced mechanism for arc suppression.
• In contrast, the DC type has an elaborate arc extinguishing components consisting of a blowout coil and arc chute.
• Direct current contactors are also usually rated lower for similarly sized contactors of the AC type, both in terms of voltage and current.

Conclusion

The AC contactor is the most common type of contactor, seeing that may high voltage equipment and systems use AC power. As we have seen, it features an electromagnetic mechanism to work, which makes it rugged and reliable. These devices are mostly used to turn on and control the running or large motors, fans, and any high energy system such as those used in large lighting and industrial circuits.