Contactor Overview

What is a Contactor?

Contactors are electromechanical switches that allow you to start and stop the flow of current in one circuit. A key feature is that the control circuit powering a contactor is electrically isolated from the circuit(s) being switched. Having electrically isolated circuits means you can switch heavily loaded, high-power circuits with relatively low-power signals and prevent damage from overcurrent or voltage spikes.

Contactors can be used wherever turning a machine on or off remotely (i.e. from a distance) with a smaller input signal is preferred or needed. Electric motor control, lighting installations, and ignition systems in cars are a few examples of contactor applications.

WEG Contactor CWM25

How do contactors work?

Relay

A relay is essentially any electrically powered switch that is isolated from the circuit(s) being switched. A contactor is a type of "heavy-duty" relay that can handle switching higher currents through its contacts.

WEG Contactor Relay R117D

Switches

Contactors and relays are electrical switching devices whose core function is to open and close electrical circuits, so to better understand contactors you need to understand switches.


Electromagnets

Any coil of wire can be made into an electromagnet. Electromagnetism is the phenomenon where any moving electric charge (i.e. proton or electron) also produces a magnetic field, with the direction of the magnetic field lines being perpendicular to the direction of current. Thus, all electrical conductors produce a small magnetic field when conducting a current, and we can control the direction and concentrate the force of a magnetic field by winding conductive wire into coils and running an electric current through them.


Solid-State (SSR) vs. Electromechanical Relays (EMR)

EMRs use magnetic fields to move armatures that in turn open and close switches, while SSRs are optically-coupled. Opto-coupling just means an LED is used to activate a light sensor, and in SSRs the light sensors are connected to transistors.

Since SSRs have no moving parts on the output circuit, the terms "pole" and "throw" do not apply.

For a look at the advantages and disadvantages of EMRs see "Solid state vs. Electromechanical Relays"


How do I select a contactor?

The content we provide is meant to inform you and help support the proper selection and use of contactors. As always, we recommend you consult a licensed and competent electrician to help you with the sizing and selection of parts for your particular application.

First, remember that contactors are switching devices, so you will need to familiarize yourself with some of the selection factors for electrical switches. Additionally, there are three general things to consider for selecting contactors in particular:

  1. Contact ratings have to do with the main circuit being switched by the contactor.

  2. Coil ratings have to do with the control circuit powering the contactor.

  3. What kind of auxiliary contacts you need.

You can choose what coil voltages you want for a given contactor for AC or DC control circuits. For example, for AC coil voltages we have 24V, 120V, 208-240V, or 480V contactor coils available, and for DC coil voltages we have 24V or 110V coils. Thus, it is more helpful to select a contactor based on the switching needs of the main circuit, and then select a contactor coil based on power supplied by the control circuit.


1. Contact (switch) ratings

Maximum Switching Voltage

The maximum voltage contacts can handle. For example, a voltage spike will occur whenever you open an inductive current, and contactors use inductors.

Maximum Switching Current

The maximum inrush current contacts can handle during startup.


2. Coil ratings

Rated Coil Voltage

The voltage of the control circuit powering the coil must match the rated coil voltage.

Pick-Up Voltage

As the voltage is increased on the coil, the voltage above which all contacts will switch to their operating position.

Drop-Out Voltage

As the voltage is decreased on the coil, the voltage below which all contacts will switch to their un-operated position.

Operating Current

Current of the coil running at the rated coil voltage.


3. Auxiliary Contacts

Primary switches connect to and control the main circuit, while secondary switches have no affect on the main circuit’s operation. Auxiliary contacts are secondary switches that connect to the control circuit (the circuit that powers a contactor) but NOT the main circuit that powers the machine or device being used.

Auxiliary contacts are used to monitor or protect primary switching devices (e.g. power indicator lights that tell you if a machine is turned on or not), or to power accessories (e.g. cooling fans).

WEG Auxillary Contacts Power button light

Power indicator lights are powered by auxiliary contacts