A Contactor and its Working Principles

In this blog, we shall discuss and understand contactors. We'll explain what a contactor is and how it works. Selfguard Automation and Controls is the trusted manufacturer of automatic changeover switch in India and is here to assist you. We will also go over how to wire a contactor and the differences between a contactor and a relay.


What is a Contactor?


A contactor is a device that creates and breaks an electrical power circuit. For example, we typically utilise a contactor to turn on and off an electrical motor.


Why are Contactors Used?


Now you may be wondering why we need to utilise a contactor. Cannot we connect the motor directly to the PLC? So, the short answer is no.


Why? Because you don't want to immediately connect a high-voltage electric motor to your bright, pricey PLC. If any voltage surges occur on the motor side, the PLC cards will be damaged.


So, instead, we utilise a contactor to indirectly and safely connect the PLC to the motor. What exactly do we mean by "indirectly?" you ask. Well, every contactor has a low-voltage coil. We attach the PLC output to this coil. This coil typically operates with a 24-volt DC signal.


When the coil is energised, an electromagnetic field is formed. This electromagnetic field then causes the three contacts to close, allowing the 3-phase electricity to reach the motor and turn it on. Doesn't this look like magic?


Thus, there is no electrical connection between the coil and the contacts. The contacts will open and close according to the electromagnetic field generated by the coil.


When the coil is not energised, the contacts are open; however, when the PLC sends a 24-volt DC signal, the coil is energised, the connections close, and the motor turns on.


With this configuration, there is no direct connection between the PLC and the motor. That is how you may turn on and off a large high-voltage electric motor indirectly and safely, while also ensuring that your PLC card is not damaged by any electrical surges on the motor side. This is why we use a contactor.


Contactor vs. Relay


Now that you understand why we use contactors, you may be wondering how they vary from relays.


You may say... A relay operates in the same way. Can't we use a relay instead of a contactor to start the motor here?


The answer to this question is no! A relay operates in the same manner as a contactor. That indicates a relay has a coil and some contacts. When the coil is energised, the connections close. This is how a contactor works, correct?


But here's the difference... A relay is typically employed on smaller devices with lower current and voltage rates. A contactor is utilised for larger devices with higher current and voltage rates.


So, relays are used to switch on and off tiny devices, whereas contactors are used to turn on and off larger equipment. Very simple!


How to Wire A Contactor


Now, let's talk about the wire terminals on the contactors.


1) Coil Terminals


The front of the contactor has two wire terminals labelled A1 and A2. To energise the coil, connect it to a 24-volt DC power source.


The A1 wire terminal connects the 24-volt DC positive signal wire, whereas the A2 wire terminal connects the 24-volt DC negative signal wire.


We're attaching a 24-volt DC power supply to these terminals because the contactor's coil requires 24-volt DC power.


For some other contactors, this coil may function with different voltages, such as 12-volt DC or 220-volt DC.


Depending on the type of contactor, the coil can also operate with alternating current voltage. For example, the contactor's coil could be powered by 24, 120, or 220 volts of alternating current.


So, before attaching the wires to the coil, you should check the coil voltage. Most contactors, like the one we have here, operate on 24-volt DC power.


2) Contact Terminals.


Six additional wire terminals are located on the other side of the contactor. The top wire terminals are labelled L1, L2, and L3 from left to right, respectively. The bottom wire terminals are labelled T1, T2, and T3, from left to right.


The L1, L2, and L3 terminals link the power lines to the contactor. The T1, T2, and T3 terminals link the device wires to the contactor.


The L1 contact connects to the T1, the L2 contact to the T2, and the L3 contact to the T3.


All of my contactors' contacts are usually open. With the coil de-energized, the device attached to the T1, T2, and T3 terminals loses power. When the coil is energised, the device will gain power.


(3) Auxiliary or Feedback Contact Terminals


As you can see, we have another set of wire terminals on the front end labelled NO (normally open). This is a simple normally open contact, sometimes known as an auxiliary or feedback contact.


How do Feedback contacts work?


This contact is used to transmit a signal to the PLC input regarding the contactor's health. What exactly do we mean by that? When the coil is energised and the three primary contacts are closed, this feedback contact closes as well, sending a signal to the PLC's input.


When the contactor is broken and the coil is energised, the three primary contacts are not closed, nor is the feedback contact closed, and no signal is provided to the PLC input. This way, we can be notified if the contactor fails.


How to Wire a Contactor to the PLC and Motor


Okay, so to operate a motor with a PLC via a contactor, you must connect the PLC output to the coil to energise and de-energize it.


From one end, connect a 3-phase power supply to L1, L2, and L3, and from the other end, connect T1, T2, and T3 to the motor.


To be notified when the contactor fails, connect this auxiliary or feedback contact to the PLC input.


To connect to the PLC input, you also need a start/stop switch. When you press the start switch, the coil is energised, the connections are closed, and the motor is turned on. When this occurs, the feedback contact is also closed, and a signal is transmitted to the PLC input indicating that the contactor is functioning properly.


When you press the stop switch, the coil de-energizes, the contact opens, and the motor turns off.




We utilise a contactor to turn on and off heavy and high voltage electrical devices like motors, fans, and pumps. The reason we use a contactor is to manage these large high-voltage electrical equipment indirectly and safely via a PLC rather than connecting the PLC directly to the output devices.


The primary distinction between a contactor and a relay is that a contactor is used to switch on and off large, high-voltage equipment, whereas a relay is often used to turn on and off smaller, low-voltage devices. Contactors are essential for guaranteeing smooth operations and protecting equipment, from the automatic changeover switch to industrial machinery. As technology improves, developments such as the auto phase sequence corrector enhance the functioning of contactors, increasing the efficiency and dependability of electrical systems.


Contactors continue to be vital components in residential, commercial, and industrial contexts, providing modern infrastructure with smooth control and increased safety. We hope you now understand exactly what a contactor is and how it works. 


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