Output Relay Welding in Roll Forming Machines – Causes, Stuck Outputs, Inspection & Repair Guide

Output Relay Welding

Roll Forming Machine Electrical & PLC Failure Guide

Output relay welding is an electrical failure in roll forming machine control systems where the contacts inside an electrical relay become permanently stuck together due to excessive electrical current or arcing.

Relays are commonly used in PLC-controlled roll forming machines to switch electrical loads such as motors, solenoids, contactors, and other actuators.

An output relay works by opening and closing electrical contacts when triggered by a control signal from the PLC.

When functioning normally, the relay contacts close to activate a device and then open again when the PLC removes the command signal.

However, if excessive current flows through the relay contacts, the electrical arc created during switching may cause the contacts to overheat and fuse together.

This condition is known as relay welding.

Output relay welding commonly affects roll forming machines producing:

metal roofing panels
metal wall cladding panels
standing seam roofing systems
structural deck profiles
C and Z purlins
light gauge steel framing components

Typical production symptoms associated with output relay welding include:

machine devices remaining permanently activated
solenoid valves staying energized after commands stop
motors continuing to run unexpectedly
machine outputs not responding to PLC commands
control system faults or safety alarms
unexpected machine movement

If a relay becomes welded, the PLC may no longer be able to turn off the affected device, which can create safety risks and disrupt machine operation.

Maintaining proper electrical load management helps prevent this issue.

Causes of Wear or Failure

Output relay welding typically occurs due to electrical overload or excessive switching stress.

Several factors may contribute to this condition.

Excessive Electrical Current

Loads exceeding relay ratings may overheat contacts.

Inductive Load Switching

Motors or solenoids may create high voltage spikes.

Lack of Arc Suppression

Absence of snubber circuits may increase contact arcing.

Frequent Switching Cycles

High switching frequency may accelerate contact wear.

Poor Relay Quality

Low-quality relays may fail prematurely.

Electrical Short Circuits

Short circuits may generate extremely high current.

Why It Happened and What Caused It

From an electrical engineering perspective, relays switch electrical circuits by opening and closing metal contacts.

When a relay switches a load, especially an inductive load such as a motor or solenoid valve, an electrical arc may occur between the contacts as they separate.

This arc is caused by the sudden interruption of current flow.

If the electrical load is large or if arc suppression circuits are not installed, the arc may generate intense heat at the contact surfaces.

Over time, repeated arcing may damage the contact material.

In severe cases, the heat generated during a switching event may melt the contact surfaces.

When the contacts cool, they may fuse together and remain permanently closed.

Once welded, the relay can no longer interrupt the electrical circuit.

This means the connected device may remain energized even when the PLC command is removed.

Proper electrical protection and load management help prevent relay welding.

How to Inspect the Problem

Inspection Procedure

Diagnosing output relay welding requires inspecting the control system and relay components.

Step 1 – Observe Device Behavior

Check whether a device remains active after the control signal stops.

Step 2 – Inspect PLC Output Status

Verify whether the PLC output indicator shows the device should be off.

Step 3 – Inspect Relay Operation

Listen for relay switching sounds when the output signal changes.

Step 4 – Test Relay Contacts

Measure whether the relay contacts remain closed.

Step 5 – Inspect Electrical Load

Verify that the connected device does not exceed relay ratings.

Step-by-Step Technician Guide – How to Fix

Correcting output relay welding typically requires replacing the damaged relay and correcting the electrical load conditions.

Method 1 – Replace the Welded Relay

Install a new relay with the correct electrical rating.

Method 2 – Inspect Connected Devices

Ensure loads do not exceed relay capacity.

Method 3 – Install Arc Suppression Devices

Add snubber circuits or flyback diodes.

Method 4 – Inspect Electrical Wiring

Ensure no short circuits or overload conditions exist.

Method 5 – Test Control System Operation

Verify that outputs respond correctly to PLC commands.

Preventative Maintenance Tips

Preventing output relay welding requires maintaining proper electrical design and load control.

Use Relays with Proper Ratings

Choose relays designed for the electrical load.

Install Arc Suppression Circuits

Protect relay contacts from voltage spikes.

Monitor Electrical Loads

Ensure loads remain within relay specifications.

Inspect Control Components Regularly

Routine checks help detect early wear.

Use Industrial-Grade Relays

High-quality relays offer greater durability.

FAQ Section

What is output relay welding in roll forming machines?

It occurs when relay contacts fuse together due to electrical overload or arcing.

What happens when a relay becomes welded?

The connected device may remain permanently activated.

Can relay welding create safety risks?

Yes. Devices may remain energized even when they should be off.

How can relay welding be detected?

If a device stays on while the PLC output is off, relay welding may be the cause.

Should welded relays be repaired or replaced?

They should be replaced with new relays.

How can output relay welding be prevented?

Using properly rated relays and arc suppression circuits helps prevent this issue.