Automation with a contactor

A contactor is an electromagnetic switch whose coil can be energised by any small control signal (push-button, timer, sensor, PLC). Because a low-power signal switches a high-power load, contactors are the basic building block of automation of electrical systems.

Example: automatic water pumping system

Assumption: a pump fills an overhead tank. A float/level switch in the tank and a dry-run sensor in the sump control the pump through a contactor.

 Power: L1 L2 L3 --[MCB/fuse]--[K]--[OL]--( Pump M )

 Control circuit:
 L --[F]--[OL NC]--[Sump level OK]--+---------------------+
                                    |                     |
                           [Tank LOW float NO]       [K aux NO]
                                    |                     |
                                    |           [Tank HIGH float NC]
                                    |                     |
                                    +----------+----------+
                                               |
                                          ( K coil )
                                               |
 N --------------------------------------------+

Working

1. When the tank level falls below the LOW float, its contact closes; the contactor coil K is energised and the pump starts.
2. The auxiliary contact of K, in series with the HIGH float (NC), holds the coil, so the pump keeps running as the water rises above the LOW point.
3. When the water reaches the HIGH level, the HIGH float contact opens; the coil drops out and the pump stops.
4. If the sump runs dry, the sump-level contact opens and stops the pump (dry-run protection).
5. The overload relay stops the motor on overload; the contactor gives no-volt protection on supply failure.
6. No operator is needed; the system runs automatically.

Other automation done with contactors

• Star-delta starter: a timer switches the star contactor off and the delta contactor on after the motor speeds up.
• APFC panels: a power factor controller switches capacitor steps through capacitor-duty contactors.
• Street lighting: a photocell or timer energises the lighting contactor at dusk.
• Automatic transfer switch: two interlocked contactors change load from mains to generator on supply failure.
• Conveyors and lifts: limit switches and PLC outputs drive contactors for start, stop and reversal.
• Temperature control: a thermostat switches heater contactors in furnaces.

The contactor thus acts as the power output stage of the control system: sensors and logic decide, and the contactor carries out the switching safely and repeatedly.