Types and Construction of Contractor


What is Contactor?

The contactor is an electrically controlled switching device. It is used for electrical switching. The basic operation of this is often almost like a relay, but the sole difference is that contractors can carry large current compared to relay up to 12500A. They cannot provide short or overload protection but can break the contact when coil excites.

Circuit Diagram of Contractor

A contactor is one among the most circuit parts, which may stand on its own power control device or a neighborhood of a starter. They are wont to connect and break power supply lines running through power lines or repeatedly establish and interrupt electric power circuits. These are utilized in light loads, complex machine control. They are used with motors, transformers, heaters. It are often considered as an intersection between the negative feedback circuit and power circuit because it's controlled by the negative feedback circuit , it also controls the circuit between power and loads. This article focuses on the importance of contactor in and field .  

Construction of a Contactor

The contactor consists of midiron cores, where one is fixed and therefore the other one is that the movable coil and it's an insulated copper coil. Where the copper coil is found on the fixed core. There are six main contacts for power connection, where three are fixed cores and therefore the other three are movable cores. These contacts are made up of pure copper, and therefore the contact points are made up of special alloy to face up to high starting current and temperature. A spring which is found between coil and therefore the movable core, auxiliary contacts it might be normally open or closed. The main contacts cut on and off the sunshine current loads like contactors coil, relays, timers, and lots of other negative feedback circuit parts are linked to contact mechanism. This three-phase AC supply is provided to the circuit as shown below.    


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


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


The main function of this is often that it carries the present to varied parts of the circuit. They are classified into three-part contact springs, axial contacts, power contacts. Where each of the contacts has its own functions, which is explained in theory of operation of the contactor.  

Block Diagram of Contractor

Working Principle of Contactors

  • An electromagnetic field is generated whenever current flows where the moving coils attract one another . A large amount of current is drawn by electromagnet initially. 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, alittle portion of the coil is that the shaded coil, where the magnetic flux within the core is slightly delayed. This effect is just too average because it prevents the core from buzzing at twice line frequency. There are internal tipping point processes to make sure rapid action in order that contactors can open and closed very rapidly.
  • From the figure the availability is given employing a switch, that's 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 therefore the motor starts running. When the switch is switched on. So the electromagnetic energy is activated. The spring arrangement then moves the moving coil back to its initial location. And the power supply to the motor stops.

How to Chooses an accurate Replacement for a Contactor?

A correct replacement for this will be chosen as follows  

  • Firstly, one should check the coil voltage, which may be a voltage wont to energize the contactor.
  • Checking for auxiliary contacts available, that's what percentage open and closed nodes are utilized in the contactor.
  • Checking the rating which is mentioned in a table format on it.

Whenever contacts are open or closed. Then ARC suppression is derived. If there's an important load breakdown, an arc that's formed damages the contacts. Along with it if the temperature is high the arc causes harmful gases like carbon monoxide gas which results in a discount of the lifetime of motors.  

Types of Contactors

These are classified supported three factors they're  

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

Knife Blade Switch

It is the primary contactor wont to control an electrical motor within the late 1800s. There is also a metal strip. Which acts as a switch to connect and break the connection. But the disadvantage of this method is that it switching process if in no time , thanks to which corrosion occurs within the copper material, counting on the capacity of current the size of the motor is increased which results in high physical damage.  

Knife Type Contactor

Manual Contactor

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

  • The operation performed is safe
  • They are properly encased so as to guard 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's it are often operated remotely, less amount of current is enough to form a connection and take away the connection. It is the most advanced contactor.

Difference between AC Contactors and DC Contactors

The difference between AC and DC contactor is as follows.  

AC Contactors

  • They are designed for the contactors with self-extinguishing arc is drawn whenever the contact opens.
  • They don’t use freewheel diode.
  • Separation time is less.

DC Contactors

  • They are specially designed to suppress electrical arching when there is switching in the DC circuit.
  • They use freewheeldiode.
  • 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  

  • When a coil does not have a magnetic field, it burns.
  • Aging of components causes corrosion of materials when exposed to moister.

Applications of Contactors

The following are the application of contactors

  1. It controls lighting
  2. Magnetic starter
  3. Vacuum contactor
  4. Mercury relay
  5. Mercury-wetted relay.


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