What is Contactor : Construction, Types and Applications

A contactor is one of the main electrical circuit parts, which can stand on its own power control device or a part of a starter. They are used to connect and break power supply lines running through power lines or repeatedly establish and interrupt electrical power circuits. These are used in light loads, complex machine control. They are used with motors, transformers, heaters. It can be considered as an intersection point between the control circuit and power circuit because it is controlled by the control circuit, it also controls the circuit between power and loads. This article focuses on the importance of contactor in and electric field.

What is Contactor?

Definition: Contactors are electrically controlled switching devices which are used for switching electrically. The basic operation of this is similar to a relay, but the only difference is that contractors can carry large current compared to relay up to 12500A. They cannot provide short circuit or overload protection but can break the contact when coil excites.

Construction of a Contactor

The contactor consists of two iron cores, where one is fixed and the other one is the movable coil and it is an insulated copper coil. Where the copper coil is located on the fixed core. There are six main contacts for power connection, where three are fixed cores and the other three are movable cores. These contacts are made from pure copper, and the contact points are made from special alloy to withstand high starting current and temperature. A spring which is located between coil and the movable core, auxiliary contacts it could be normally open or closed. The main contacts cut on and off the light current loads such as contactors coil, relays, timers, and many other control circuit parts are linked to contact mechanism. A three-phase AC power supply provided to the circuit which is shown below,


It consists of three main parts they are


It provides a force which is required to close the contact. The coil is also named as an electromagnet. An enclosure is used to safeguard the coil and contactor.


It acts like an insulator and protector, which protects the circuit form any electrical contact, dust, oil, etc. They are made up of different materials like Nylon 6, Bakelite, Thermosetting plastic, etc.



The main function of this is that it carries the current to various parts of the circuit. There are classified into contact springs, axillary contacts, and power contacts. Where each of the contacts has its own functions, which is explained in principle of operation of the contactor.



Working Principle of Contactors

An electromagnetic field is generated whenever current flows where the moving coils attract each other. A large amount of current is drawn initially by an electromagnetic coil. The moving contact is pushed forward by moving core, as a result, the force created by the electromagnet holds the moving and fixed contacts together.

  • On de-energizing, the contactor coil gravity or spring moves back the electro-magnetic coil to its initial position and there is no flow of current in the circuit.
  • If contactors are energized with AC current, a small portion of the coil is the shaded coil, where the magnetic flux in the core is slightly delayed. This effect is too average as it prevents the core from buzzing at twice line frequency. There are internal tipping point processes to ensure rapid action so that contactors can open and closed very rapidly.
  • From the figure the supply is given using a switch, that is when the switch is closed current flows through the contactor coil and attaches the moving core. The contactor attached to the moving core closes and the motor starts running. When the switch is released the electromagnetic energizes spring arrangement pause the moving coil back to its initial position and power supply to the motor is cut off.

How to Chooses a Correct Replacement for a Contactor?

A correct replacement for this can be chosen as follows

  • Firstly, one should check the coil voltage, which is a voltage used to energize the contactor.
  • Checking for auxiliary contacts available, that is how many open and closed nodes are used in the contactor.
  • Checking the rating which is mentioned in a table format on it.

ARC suppression concept arises whenever the contacts are open or closed. If there is a heavy load breakdown, an arc that is formed damages the contacts. Along with it if the temperature is high the arc causes harmful gases like carbon monoxide which leads to a reduction of the lifetime of motors.

Types of Contactors

These are classified based on three factors they are

  • The load being used
  • The current capacity and
  • The power rating.

Knife Blade Switch

It is the first contactor used to control an electric motor in the late 1800s. It consists of a metal strip, which acts as a switch in connecting and disconnecting the connection. But the disadvantage of this method is that it switching process if very fast, due to which corrosion occurs in the copper material, depending on the capacity of current the size of the motor is increased which leads to high physical damage.


Manual Contactor

The disadvantages of knife blade contractors are overcome using a manual contactor. Some of the features of these are,

  • The operation performed is safe
  • They are properly encased in order to protect from external environment problem
  • The size of the manual connector is small
  • Only a single break is used
  • The switches are controlled using a contactor.


Magnetic Contactor

It operates electromagnetically that is it can be operated remotely, less amount of current is enough to make a connection and remove the connection. It is the most advanced contactor.

Difference between AC Contactors and DC Contactors

The difference between AC and DC contactor are as follows,

              AC Contactors                DC Contactors
They are designed for the contactors with self-extinguishing arc is drawn whenever the contact opens They are specially designed to suppress electrical arching when there is switching in the DC circuit.
They don’t use freewheel diode They use freewheel diode
 Separation time is less Separation time is higher if the load is heavy a shunt load is attached to the main contact.


The following are the advantages of the contactor

  • Fast switching operation
  • Suitable for both AC and DC devices
  • Simple in construction.


The following are the disadvantages of the contactor

  • In the absence of magnetic-filed, the coil may burn
  • Aging of components causes corrosion of materials when exposed to moister.

Applications of Contactors

The following are the application of contactors


1). What is the difference between a relay and a contactor?

The main difference between a relay and a contractor is that,



A relay is used for low voltage switching purpose It is used for high voltage switching purpose


The relay contactor is similar to the axillary contactor.


There are two types of contactors auxiliary and power


The size of the relay is small The size of the contactor is large
Cannot be repaired Can be repaired

2). What is a contactor used for?

It is a switch used to switch high power load and protect the motor from external damage.

3). What is a normally closed contactor?

A normally closed contactor can be represented as NC, which means the connection is made and the circuit is normally on.

4). How do you wire a 3 phase contactor?

The connection of three-phase contactor is made as follows

  • Disconnect the power supply
  • The three colored phase wires are connected to three terminals T1, T2, T3 of machine
  • Connect the power supply and allow the current to flow.

5). How do you size a contactor?

The size of this is a product of 100% and full load current.

Thus, this all about an overview of the contactor, it is an electrical switch used in electrical circuits like electrical motors switching circuits or capacitive switching circuits. They carry the high current to various parts of the circuit. They are operated by energizing electromagnetic coil inside it when the coil is energized the movable contacts move towards fixed contacts and close the circuit. Here is a question for you, what is the function of a contactor?