Servo drive alarms: where to start

It's 5am on a Saturday. The case packer's pick-and-place has stopped. The drive display shows a three-letter code and a number. The HMI just says "Axis 2 fault". Nobody on shift has seen this one before.

Someone resets it. The axis homes, runs a dozen cycles and stops again with the same code. Someone else suggests swapping the drive with the one on axis 3 to "see if the fault follows".

Stop there. Swapping drives between axes without copying the parameters can leave you with two axes that don't work. A servo alarm is a clue about where to look. Treat it like one.

Sort the alarm before you chase it

Every drive maker numbers its alarms differently. But nearly all of them fall into six families. Work out which family you're in and half the job is done.

1. Supply and power. Undervoltage, DC bus faults, phase loss, a blown fuse or a tripped breaker upstream.
2. Current and overload. Overcurrent, short circuit, motor overload, drive overload or I²t.
3. Feedback. Encoder lost, encoder comms error, position data mismatch.
4. Motion. Following error, position deviation, overspeed, software limit reached.
5. Temperature. Drive heatsink too hot, motor thermistor tripped.
6. Comms and safety. Fieldbus lost, PLC watchdog, safe torque off active.

If the manual's gone, our guide to reading drive fault codes shows how to find the meaning from the drive itself or the maker's website.

When did it go?

The alarm tells you what. The timing tells you where.

• At power-up or enable. Think supply, feedback cable, a brake that hasn't released or a parameter that's been changed.
• During a fast move. Think current limits, a ramp that's too steep for the load, or mechanical binding.
• At steady speed. Think the load. A jam, a tight bearing, a product pile-up or a belt that's lost tension.
• Only when the machine is warm. Think heat. Panel fans, filters, motor ventilation and a duty cycle that's grown since the machine was set up.
• At random. Think noise, a loose plug or a cable breaking up in a drag chain.

Photograph the alarm and the drive's fault history before anyone resets it. Many drives log the current, speed and bus voltage at the moment of the trip. That's the best evidence you'll get, and some drives lose it on a power cycle.

Following error: look at the mechanics first

Following error is the alarm you'll see most on servo axes. It means the axis didn't keep up with where it was told to be. The drive is usually fine. Something is stopping the axis.

Start with the simple things. Is the motor brake releasing? You can often hear it click, and many drives show the brake output. Will the axis move freely by hand with the power off and the load safe? Is there a jam, a dry linear guide or a coupling that's slipped? Has someone fitted a heavier tool or changed the product?

Then ask whether anyone's changed the tuning. Gains, speed limits and following error windows get tweaked to squeeze out more speed. Compare the current parameters with your last backup before you change anything else. Write down the old values.

Encoder problems can show up as following error too. Our guide to encoder faults on servo systems covers the cable, screen and coupling checks.

Overcurrent and overload

These two get confused. Overcurrent is a sudden spike, often a short or a hard jam. Overload builds up over time, usually because the axis is working harder than it was sized for.

For an instant overcurrent at enable, don't keep resetting. Isolate, then test the motor and the motor cable. Our guide to inverter trips covers the insulation test, and the rule about disconnecting the drive first applies to servo drives as well.

For an overload that comes on after an hour, look at the duty. Faster cycle times, a heavier product or a guide rail that's lost its lubrication all add load. The drive is doing its job by tripping.

Safe torque off is a safety function

An STO alarm means the safety circuit has taken away the drive's ability to make torque. A guard is open, an e-stop is pressed, a light curtain is broken, or the safety relay has seen a fault.

Never bridge it to get the line running. Work back through the safety circuit to find out why it opened. Our guide to safety relays and light curtains covers how.

STO also isn't a way to make the machine safe for work. The PUWER code of practice says interlocked guards and emergency stop controls are not suitable as a means of isolation (L22, paragraph 195)*. A drive with STO active is still connected to the supply. Isolate and lock off before you go in. Then wait. The guidance on the Electricity at Work Regulations warns that energy may be stored in capacitors within equipment (HSR25, paragraph 195)*. Wait for the time on the drive's label and prove it dead.

Live fault-finding is live work

Some servo faults can only be found with the drive powered. Watching the diagnostics on a laptop is one thing. Putting meter probes inside the panel is another. HSE counts live testing, such as using a test instrument to measure voltage on a live control system, as live work (HSG85, paragraph 3)*.

The same guidance says to avoid live working during fault-finding where possible, by using equipment with built-in test facilities and diagnostic aids (HSG85, paragraph 17)*. Most modern servo drives have exactly that. Use the drive's own software and scope first. Open the panel live only when you've planned it and it's justified.

The cost of guessing

Here's our view. Servo faults are where sites lose the most time to the wrong person. A good mechanical fitter can free a jammed axis. Reading a drive's trace, checking the tuning and re-homing an axis after a motor change takes someone who does it often.

Every swap that doesn't fix it costs you twice. Once in downtime, and again in the part. Drives and motors swapped without the right parameters can take hours to bring back. If it isn't clear in half an hour, make the call. Our post on what to have ready before the callout engineer arrives shows how to save the first hour once they're on site.

What to do on Monday

1. Back up the parameters for every servo drive, and keep a copy off the machine with the date on it.
2. Stick a note in each servo panel: photo the alarm and fault history before reset.
3. Find the manual or software for every drive model on site, and keep it where nights can get to it.
4. Agree who you'll call for servo faults, and give nights the number.

* Paragraph numbers correct as of 24 October 2026 and refer to L22, fourth edition (2014, amended 2018), HSR25, third edition (2015), and HSG85, third edition (2013). Check HSE's HSG85 page for the latest.