Why Alarms Are Protection, Not Failure
Closed-loop steppers are one of the biggest upgrades you can make to a CNC machine. They add accuracy, safety, and confidence. They also introduce alarms, which can be confusing if you are coming from open-loop systems.
This article explains what closed-loop steppers are actually doing, why alarms happen, and how to tell the difference between a real hardware problem and a correct protective response.
What “Closed-Loop” Actually Means
Traditional stepper motors are open-loop. The controller assumes the motor moved where it was told, even if it stalled or skipped.
Closed-loop steppers are different.
They continuously monitor their actual position using an encoder and compare it to the commanded position. If the motor cannot keep up, it knows immediately.
This is a good thing.
It means:
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No silent lost steps
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No ruined parts without warning
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No guessing where the machine actually is
Why Closed-Loop Steppers Alarm
An alarm simply means:
“I was commanded to move, but I could not safely or accurately do so.”
The motor stops instead of blindly continuing.
That is protection, not failure.
Most alarms are caused by conditions outside the motor, not the motor itself.
Common Reasons for Closed-Loop Alarms
Mechanical Causes
These are the most common.
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Axis binding
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Misaligned couplers
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Overtightened motor mounts
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Debris on rails or screws
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Bent lead screws or ball screws
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Excessive acceleration or feedrate
If the motor physically cannot move, it will alarm.
Wiring and Electrical Causes
These often show up after a machine has been running fine.
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Poor grounding
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Floating shield drains
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Static discharge from a shop vac
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Loose motor connectors
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Encoder cable damage
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EMI noise coupling into signal lines
These issues can cause false alarms or multi-axis alarms at once.
Configuration Causes
Less common, but still possible.
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Acceleration set too aggressively
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Mismatch between controller settings and motor capability
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Improper alarm wiring or input assignment
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Incorrect DIP switch settings on the motor
What an Alarm Does NOT Mean
An alarm does not automatically mean:
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The motor is bad
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The driver is defective
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The kit is faulty
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Something needs to be replaced
In most cases, the motor is doing exactly what it is designed to do.
Wiring vs Mechanical Clues
Use these patterns to guide troubleshooting.
Likely Mechanical
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Alarm happens at the same physical location
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Alarm happens under load only
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One axis alarms consistently
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Motor gets hot before alarming
Likely Electrical or Grounding
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Multiple axes alarm together
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Alarm appears after static shock
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Alarm appears after vacuuming
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Alarm persists after reboot
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Alarm happens randomly
When Replacement Is Justified
Motor replacement is rare, but it does happen.
Replacement is usually justified when:
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An encoder has failed internally
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A motor has suffered sustained electrical damage
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Continuity tests fail on motor windings
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Alarms persist after correcting grounding, wiring, and mechanics
We always recommend ruling out external causes first. Replacing motors without fixing the underlying issue often leads to the same failure repeating.
How to Respond to an Alarm
Do this:
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Stop motion immediately
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Power down if alarms persist
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Inspect mechanics and wiring
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Check grounding and shield drains
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Correct the root cause before resetting
Do not:
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Repeatedly reset and continue running
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Ignore alarms
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Increase torque to “push through”
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Assume the motor is bad without diagnosis
Repeated resets can turn a protective event into permanent damage.
Final Thought
Closed-loop steppers are honest.
They tell you when something is wrong instead of hiding it.
Once you understand that alarms are information, not accusations, confidence goes way up and frustration drops fast.
If you are unsure what an alarm is telling you, reach out.
Photos and a short description of when the alarm occurs help us pinpoint the cause quickly.
