Interface Relay or Contactor - Which Switches the Load?
An interface relay and a contactor both isolate the PLC control circuit from the load circuit, but they are built for very different currents. This guide shows when a slim interface relay is enough and when a contactor rated for AC-3 is required, based on load current, motor load and switching frequency.
View relays and contactorsHow do an interface relay and a contactor differ?
Both devices switch a load circuit from a galvanically isolated control signal - usually the 24 V DC from a PLC. The interface relay (coupling relay) is a compact device for small currents, used mainly for signal matching and isolation. The contactor is a power switching device for high currents and inductive motor loads.
The interface relay sits on a slim base, often only 6.2 mm wide, on the DIN rail and couples the sensitive 24 V PLC output to a consumer such as a valve, signal lamp or a further contactor. The contactor has a powerful coil, main contacts for the two- to three-digit ampere range and arc suppression for switching motors.
Which criteria decide the switching device?
Load current, load type and switching frequency are decisive. The utilisation category to IEC 60947 states what a device is rated for: AC-1 for resistive loads, AC-3 for squirrel-cage motors in normal operation, AC-15 for switching solenoid coils.
- Check load current and inrush: motors draw 6 to 8 times the rated current at start.
- Choose the correct utilisation category - an AC-1 value is not valid for motor loads.
- Match the coil voltage to the PLC, usually 24 V DC.
- Output contact: changeover (CO) on the interface relay, three-pole main contacts on the contactor.
- Account for module width and terminal space in the cabinet.
How does the PLC drive relay and contactor safely?
A digital PLC output typically supplies 24 V DC and only a few hundred milliamps per channel. It must not drive a contactor coil directly if its current or inrush peak overloads the channel. The interface relay decouples the output and switches the stronger load.
Switching inductive coils creates voltage spikes on turn-off. A freewheeling diode or RC snubber across the coil protects the PLC semiconductor output and the relay contacts from wear. Many interface relays already include this protective circuit and a status LED.
- Fit a freewheeling diode or RC snubber on every inductive coil.
- A status LED and manual test button ease commissioning.
- Push-in terminals save wiring time compared with screw terminals.
- For safety functions use force-guided contacts or safety relays.
Frequently asked questions
Can an interface relay switch a motor?
Only very small motors of a few hundred watts, and only if the contact is rated for the inductive load (AC-3 or with proper derating). For standard motors from about 0.75 kW, use a contactor with an AC-3 rating.
Why not switch directly from the PLC?
A digital PLC output supplies only a few hundred milliamps and would be overloaded by the inrush peak of a contactor coil. The interface relay isolates the output galvanically and switches the larger load with low wear.
What does AC-3 mean on a contactor?
AC-3 is the utilisation category to IEC 60947 for switching squirrel-cage motors in normal operation. It accounts for high inrush current and breaking under load, so the AC-3 current is well below the AC-1 current.
Which coil voltage makes sense?
For PLC control, 24 V DC is common because it matches the control level directly. Power contactors often run on 230 V AC or 24 V DC to suit the existing control circuit.
Looking for an interface relay or contactor?
We supply slim interface relays with push-in technology and power contactors in AC-1 and AC-3 - matched to your PLC control and module width.
Standard-compliant
Utilisation categories to IEC 60947 selected correctly.
Correctly sized
Load current, inrush and load type accounted for.
Kind to the PLC
Galvanic isolation and protective circuit included.
Expert advice
Our specialists help with the sizing.