What is a contactor and what is it used for?

A contactor is a switch operated by a coil. With a small control current (for example, 24 V DC from a PLC output) it switches a power load on and off, such as a three-phase motor, heaters or a pump. It is the part that separates the control side from the power side.

How it works

Inside there is an electromagnet. When the coil is energised, it pulls in an armature that closes the main contacts (and changes over the auxiliary ones). When the coil loses power, a spring returns everything to its rest position and the motor stops.

Its terminals

Terminals	What they are
A1 – A2	The coil. Its voltage is marked on the contactor (24 V DC, 230 V AC, 120 V AC…)
1-2, 3-4, 5-6 (L1-T1, L2-T2, L3-T3)	Main contacts, for the three motor phases
13-14 (and others ending in 3-4)	Normally open auxiliary contact
21-22 (and others ending in 1-2)	Normally closed auxiliary contact

Contactor or relay

They work the same way, but a relay is designed for signals and small loads, and a contactor for power: its contacts are larger, break at two points and have arc chutes to put out the arc that forms when opening under load. To drive the coil of a large contactor from a PLC, an interposing relay is often used.

Utilisation categories: AC-1 and AC-3

The same contactor can carry more or less current depending on what it switches. The most used IEC categories:

AC-1: resistive loads, such as ovens and heating elements. The highest current in the catalogue.
AC-3: squirrel-cage motors that start and stop normally. This is the one to check for a motor, and it is always lower than AC-1.
AC-4: motors that are plugged (reversed to stop) or inched. The most demanding.

In North America contactors are also rated by NEMA size (00, 0, 1, 2…), each with a maximum motor horsepower.

The seal-in contact

If the motor is started with a push button, how does it keep running when you let go? With a normally open auxiliary contact of the contactor itself (13-14) in parallel with the start button: once the contactor pulls in, it holds itself in until someone presses stop. Try it here:

Try it right here. Open it full screen, with the explanation.

A contactor does not protectA contactor switches the motor on and off, but it does not protect the motor or the cable. You need an MPCB, or an overload relay with its fuses: see the guide overload relay vs MPCB.

Typical mistakeFitting a contactor with a coil for a different voltage. A 230 V AC coil fed with 24 V DC will not pull in; a 24 V DC coil fed with 230 V AC burns out in seconds. Always check what is printed next to A1-A2.

Other guides

Overload relay or motor protection circuit breaker: which one? · PNP or NPN sensor: the difference and how to tell which you have · What 4–20 mA means and why it does not start at zero

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