Contactor construction and remote control circuit

A contactor is an electrically operated switch in which a magnetic coil closes the main contacts and a spring opens them when the coil is de-energised. It is used for frequent on-load switching and remote control of motors and other loads.

Construction

• Coil and laminated core (electromagnet), AC cores with a shading ring.
• Armature carrying the moving contacts through an insulated carrier.
• Main contacts (normally 3 NO, silver alloy) for the power circuit.
• Auxiliary contacts (NO/NC) for latching, interlocking and indication.
• Return spring, arc chutes/splitters, moulded housing and terminals.

Operating principle

1. Control voltage on the coil produces flux; the armature is attracted against the spring.
2. Main contacts close and the load is energised; auxiliary contacts change state.
3. Removing coil voltage lets the spring open the contacts; the arc is extinguished in the arc chute at current zero.
4. Since the coil drops out at low voltage, the contactor also gives no-volt protection.

Circuit for remote control of a contactor

START buttons are connected in parallel and STOP buttons in series, so the motor can be started or stopped from the local panel or from a distant point. The hold-in auxiliary contact K is connected in parallel with the START buttons. Only thin control wires go to the remote station.

 L --[Fuse]--[STOP local]--[STOP remote]--+--------------+--------------+
                 (NC)           (NC)      |              |              |
                                    [START local]  [START remote]   [K aux NO]
                                         (NO)           (NO)          (hold)
                                          |              |              |
                                          +--------------+--------------+
                                                         |
                                                   [OL relay NC]
                                                         |
                                                    ( K coil )
                                                         |
 N ------------------------------------------------------+

 Power: L1,L2,L3 -> fuses -> K main -> OL -> Motor

Working of the remote scheme

1. Pressing any START button energises coil K; the main contacts close and the motor runs.
2. The auxiliary contact K (in parallel with the START buttons) closes and holds the coil after the button is released.
3. Pressing any STOP button opens the series path; the coil drops out and the motor stops.
4. The overload relay NC contact also opens the coil path on overload.
5. Indicating lamps through auxiliary contacts show ON/OFF status at the remote point.

This arrangement allows control of large motors (pumps, fans, cranes) from a control room using low-power control wiring, and safe automatic disconnection on overload or supply failure.