| Current Detection Fault | Circuit fault after current detection |
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ERR31 – Current Detection Fault
What this means
ERR31 indicates that the VFD detected a fault in its internal current sensing circuitry. This is not a cutting load issue and not caused by feed rates, RPMs, or toolpaths.
In simple terms:
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The VFD cannot reliably measure motor current
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The fault occurs after current detection circuitry
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The VFD shuts down to protect itself and the motor
This is primarily an internal electronics fault, not a user configuration issue.
Why the VFD Triggers This Fault
The VFD continuously monitors motor current using:
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Hall-effect current sensors
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Signal conditioning circuits
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Driver board logic
If the signal from those components is invalid, unstable, or missing, the VFD cannot safely operate the motor and triggers ERR31.
Most Common Causes We See
1. Internal Hall sensor fault
The Hall device used to measure current may be:
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Failing
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Producing invalid readings
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Intermittent
This can occur due to:
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Component aging
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Electrical stress
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Prior over-current or short events
2. Driver board electronics fault
The fault may be in the circuitry after the current sensor, such as:
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Amplification stage
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Signal filtering
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Logic processing on the driver board
This is internal to the VFD and not field-adjustable.
3. Power quality issues causing internal stress
Less common, but possible:
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Voltage spikes
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Brownouts
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Poor grounding
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Repeated power cycling under load
These can damage current detection components over time.
4. Rare wiring-related false positives
In rare cases:
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Severe motor cable damage
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Short-to-ground events
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Internal arcing
May cause the VFD to falsely detect a current sensing failure.
What to Check First
Step 1: Power cycle the VFD completely
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Turn off the VFD
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Disconnect input power
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Wait at least 2 minutes
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Restore power and test again
If the error clears and does not return, it may have been transient.
Step 2: Inspect motor and spindle cable
Check for:
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Damaged insulation
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Pinched or crushed cable
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Evidence of short-to-ground
If a short occurred previously, internal damage may already exist.
Step 3: Verify grounding
Ensure:
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VFD ground is solid
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Motor ground is intact
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No floating or loose ground connections
Poor grounding can contribute to internal sensing faults.
What Will Not Fix ERR31
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Changing feeds or speeds
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Adjusting acceleration or deceleration
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Modifying V/F curves
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Re-running motor identification
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Replacing tooling
ERR31 is not a tuning or load-related error.
When to Contact Support
If ERR31:
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Appears consistently
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Persists after power cycling
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Occurs immediately on startup
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Reappears after clearing
Please contact support with:
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VFD model
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Approximate age
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Any recent faults or power events
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Photos of wiring and grounding
In most cases, ERR31 indicates an internal VFD hardware issue that may require repair or replacement.
Summary
ERR31 means the VFD can no longer reliably measure motor current.
Most common causes:
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Hall sensor failure
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Driver board electronics fault
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Prior electrical stress
This is an internal fault and is not caused by user operation or cutting conditions.
