What is an AC Contactor?

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

Construction of an AC contactor :

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

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 the main power supply generally coming from MCB, and another 3 terminals T1, T2, and T3 as output with Motor, ORL, Timer, and another control circuit.

Additional provision for the auxiliary block is provided with "NO" and "NC" according to the application of the instrument as shown in AC contactor terminal diagram


AC Contactor Construction DiagramAC Contactor Terminal Diagram

Working Principle of AC Contactor

As shown in diagram ©, a current supplied in the form of AC or DC to the coil terminal A1 energizes the coil by generating electro-magnetic field to bring the moving part of the coil with the fixed part in contact. Now, current starts to flow from the “R Y B” input terminal of the contactor to the output of the contactor and acts as a 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 its original position due to the spring force exerted by the return spring. At this time, there will not be any flow of current from the input to the output of the contactor and it will act as a switch-off.

Following is the control wire diagram for DOL

AC Contactor Control Wiring Diagram

AC Contactor Applications

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