Contactors and relays: understand their roles in a circuit

1. What is a contactor, and what is an interposing relay?

A contactor is an electrically controlled switching device. In a motor panel, it commonly opens and closes the motor supply path according to control-circuit commands.

An interposing relay transfers signals, provides additional contacts, or creates a logic step between a command and an actuator. For example, a signal energizes KA1, whose contact then allows power to the KM1 coil.

This article compares conventional electromagnetic contactors and interposing relays. Relay describes a whole family; overload relays, timers, solid-state relays, and latching relays are not all the same device.

2. Look at two electrical paths to see the difference

Suppose a run-permission signal energizes KA1. The control path may be: control supply → KA1 contact → KM1 contactor coil → supply return. Current through KA1’s contact in this branch feeds the KM1 coil.

In the power circuit, the supply passes through protection and KM1’s main contacts to the motor as designed. Energizing KM1 closes the main contacts. Motor load current passes through those contacts, not through the coil.

The coil and contacts belong to one device but do different jobs. The coil receives a command; the contacts switch another path. Coil voltage alone does not tell you what load the contacts can switch.

3. Compare the job, not just the size

An interposing relay and contactor may share an electromagnetic principle but are designed for different tasks. A small relay is not automatically suitable for motor starting current; a large contactor is not an economical way to add a logic signal.

4. Can a relay replace a contactor?

Do not answer yes merely because the current printed on the case appears sufficient. Carrying current and switching a particular load are different requirements. A resistive-load rating does not prove suitability for motor switching.

Nor should you assume every relay is limited to small currents. Power relays exist; for a specific load, check the manufacturer’s load ratings and conditions. What the device is rated to do matters more than its generic name.

Motor-contactor documentation includes utilization categories such as AC-3. Select using the table for the voltage and operating duty; do not use an AC-1 rating for a less inductive load as the motor rating.

The contactor switches; protection requires suitable devices and coordination. In a motor starter, an overload relay commonly acts on the control circuit to release the contactor; short-circuit protection must be considered separately.

• Read about interlocking two contactors

5. Ask five questions before selection or a material takeoff

If the customer says only “control a pump,” you still lack enough data to choose a reference. Gather the information below before consulting the catalogue. This article does not prescribe one reference for every power rating.

For devices with suppressors, electronics, or polarized terminals, follow the coil wiring diagram. Do not assume every A1–A2 connection can be reversed.

1. What is being switched: a motor, coil, resistive load, or signal?
2. What are its voltage, current, or power and operating conditions? For motors, obtain the nameplate and starting method.
3. Is the control supply AC or DC, and what coil voltage is needed?
4. Are holding, interlocking, or status contacts required?
5. Are there special switching-frequency, environmental, or protection-coordination requirements?

6. Try relay and contactor roles in a small circuit

In EleRun, test the command chain first without building the three-phase power circuit. Use a 24 VDC control supply and symbols with suitable simulation behavior, all on one page.

This shows which device commands which. It does not prove that real devices can switch the load or replace protection selection.

1. Connect +24 V → maintained switch S1 → KA1 coil → 0 V.
2. Connect +24 V → KA1 NO contact → KM1 coil → 0 V.
3. Connect +24 V → KM1 NO auxiliary contact → H1 lamp → 0 V.
4. Assign KA1 to the relay coil/contact pair and KM1 to the contactor coil/contact pair.
5. Turn on S1 and trace KA1 picks up → KA1 NO closes → KM1 picks up → KM1 NO closes → H1 lights. Turn S1 off and compare the return states.