Contactor wiring: start-stop latching and forward-reverse motor control

A contactor is a heavy-duty relay: its coil (A1-A2) closes the main contacts (1-2, 3-4, 5-6; odd numbers from the supply, even to the motor) and auxiliary contacts (for example 13-14 NO, 21-22 NC). For forward-reverse you use two contactors, the second one with two phases swapped. An interlock is mandatory: each contactor's NC contact sits in the other's coil circuit, so they can never close together.

Contactor wiring: start-stop and forward-reverse motor control with interlock

For 230/400 V installations to IEC standards (Europe, UK and most of the world). North America uses different voltages, colours and rules: see the “Around the world” notes.

Contactor markings (EN 50005 / IEC 60947)

Terminals	What it is
A1-A2	the coil (for example 230 V AC: line to A1, neutral to A2)
1-2, 3-4, 5-6 (L1-T1, L2-T2, L3-T3)	main contacts: odd from the supply, even to the motor
13-14	auxiliary contact NO (normally open): for example the hold-in
21-22	auxiliary contact NC (normally closed): for example the interlock

On auxiliary contacts the second digit gives the type: 1-2 = NC, 3-4 = NO. Always check the print on the actual device.

Start-stop with latching

The basic control circuit: a START button (NO) energises the coil, and the contactor's own 13-14 contact, wired in parallel with START, holds the coil in after you release the button. A STOP button (NC) in series with the coil breaks the circuit and the motor stops.

Start-stop contactor diagram: main circuit through a motor protection breaker and contacts 1-6 to the motor; control circuit STOP NC, START NO, hold-in 13-14, coil A1-A2 — Start-stop with latching, shown at rest. Main circuit left, control circuit right.

A nice side effect: after a power cut the motor does not restart on its own, because the latch drops out. That is a safety feature.

What else sits in the coil circuit

Any device whose contact is in series with the coil can stop or start the motor:

a thermostat, switching a fan or heater when the temperature leaves its setting,
a phase failure and phase sequence relay, which opens its contact if a phase is lost or the order is wrong, stopping the motor before it is damaged.

Forward-reverse (reversing) control

A three-phase motor reverses when you swap any two phases (for example L1 with L3). Two contactors do it:

K1: phases straight → forward,
K2: two phases swapped → reverse.

The interlock is mandatory: K1's NC contact goes into K2's coil circuit and K2's NC contact into K1's. While one is on, the other physically cannot close. Without it, both closing at once means a phase-to-phase short circuit. Reversing contactor sets often add a mechanical interlock as well.

Forward-reverse diagram: K1 phases straight, K2 with L1 and L3 swapped; control with STOP, START forward and reverse, 13-14 latching, NC 21-22 interlock — Forward-reverse on two interlocked contactors, at rest.

A single-phase motor behaves differently: swapping L and N changes nothing. See reversing a single-phase motor.

The motor protection breaker

In front of the contactor there is usually a motor protection circuit breaker with two trips:
- thermal: for sustained overload (jammed shaft, lost phase),
- magnetic: instant trip on a short circuit.

Set it to the rated current on the motor's rating plate. Bigger motors are started more gently: star-delta starting.

Around the world: IEC contactors (Europe, UK, most of the world) use the A1-A2, 1-6, 13-14 markings above. North American NEMA starters often mark main terminals L1-L3 / T1-T3 and call the latch a seal-in or holding contact. The circuit logic is the same.

Important: motor control circuits run at 400 V and high currents. Installation, protection sizing and commissioning are done by a qualified person, with the supply isolated, following the diagram and the device documentation.