What is Contactor?

A contactor is the extremely important part of the electrical circuit with wide application. It is used for switching of electrical circuit on and off by a circuit which has much lower power than the switched circuit such as 24V DC, 120V AC, 220 V of AC. The operation of a contactor is similar to the relay switch. The difference between two is that the contactor can carry more load while relay is operated at lower voltage and current. Contactors are generally designed as normally open “NO” with optional auxiliary contacts block with "NO" and "NC".

Construction of a contactor :

AC Contactor consists of major three components such as Coil, Enclosure and Contacts. Coil compartment with one fixed and another movable iron core. The copper coil is attached to the fixed coil while movable iron core is supported with return spring whose one end is fixed to the contactor wall as shown in diagram (a).

There are 6 power terminals and 2 coil terminals (A1 & A2) for 3 Phase AC supply. Out of 6 terminals, 3 terminals L1, L2 and L3 are connected as input with main power supply generally coming from MCB and another 3 terminals T1, T2 and T3 as output with Motor, ORL, Timer and other control circuit.

Additional provision for auxiliary block is provided with "NO" and "NC" according to the application of instrument as shown in diagram (b).

                                           Diagram-(a)                                                                                          Diagram-(b)

Working Principle of Contactor

As shown in the diagram ©, a current supplied in form of AC or DC to the coil terminal A1 to energized the coil by generating electromagnetic field to bring the moving part of the coil with the fixed part in contact. Now, current starts to flow from “R Y B” input terminal of the contactor to the output of contactor and acts as switch on. While another terminal of coil A2 is connected to the neutral to complete the circuit.

When the supply to the contactor is stopped, the contactor movable coil returns to the original position due to spring force exert by return spring. At this time, there will not be any flow of current from input to the output of the contactor and it will act as switch off.

Following is control wire diagram for DOL © .

                                                                               Diagram-(c)

Contactor Applications

  • Motor starter as DOL and star delta.
  • It controls the lighting.
  • Magnetic starter.
  • Vacuum contactor.
  • It is used as AC conditioner contactor.
  • Coil energized with AC and DC supply.