Construction and principle of contactor

A contactor is an electromagnetically operated switch used to make and break a power circuit many times, under normal load conditions, by a small control signal. It is used for motors, heaters, lighting and capacitor banks.

Construction

        +------------------------+
        |  fixed contacts (3)    |
        |  ==  ==  ==            |
        |  moving contacts       |
        |  --  --  --  <- bridge |
        |  [ armature ]          |
        |      ^  spring         |
        |  [  coil on core ]     |
        +------------------------+
   A1  A2 = coil terminals

1. Electromagnet (coil and core): a laminated E- or U-shaped iron core with a coil. The coil works on AC or DC (24 V, 110 V, 230 V, 415 V). AC cores carry a shading ring to stop chatter.
2. Armature: the movable iron part attracted by the magnet; the moving contacts are fixed to it through an insulating carrier.
3. Main contacts (power contacts): usually three NO poles, made of silver alloy (silver–cadmium oxide or silver–tin oxide) for low resistance and resistance to welding. Double-break bridge contacts are common.
4. Auxiliary contacts: small NO and NC contacts for holding, interlocking and signalling.
5. Return spring: opens the contacts when the coil is de-energised.
6. Arc chutes / arc splitters: splitter plates or blow-out coils in larger contactors cool and lengthen the arc.
7. Enclosure and frame: moulded insulating housing with terminals.

Operating principle

1. When the control voltage is applied to the coil, current in the coil sets up flux in the core.
2. The magnetic pull exceeds the spring force; the armature moves and closes the main contacts (and changes over the auxiliary contacts). The load is connected.
3. The coil current is small compared to the load current, so a small switch or PLC output can control a large load.
4. When the coil is de-energised (STOP button, overload relay or supply failure), flux collapses and the spring opens the contacts. The arc formed is quenched in the arc chute at the next current zero.
5. Because pick-up needs about 85% and drop-out happens below about 60–70% of coil voltage, a contactor gives built-in no-volt protection.

Main features

• Designed for very high mechanical life (millions of operations) and frequent switching.
• Breaking capacity is limited to a few times rated current (overload/motor stall current), not short-circuit current; fuses or MCCBs give short-circuit back-up.
• Utilisation categories (IEC 60947-4-1) such as AC-1 (resistive), AC-3 (squirrel-cage motor start and run), AC-4 (inching, plugging) define the duty.