| Output Default Failure | Output Default Failure or 3-phase Imbalance Fault |
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ERR12 – Output Fault / 3-Phase Imbalance
What this means
ERR12 indicates that the VFD has detected a fault in its output stage, typically related to an abnormal or imbalanced three-phase output to the spindle motor.
In practical terms, the VFD believes it is no longer able to deliver a healthy, balanced three-phase signal to the motor.
Why This Happens
The VFD constantly monitors its output current and voltage on all three phases.
If it detects that one phase is missing, unstable, or behaving differently than the others, it will fault to prevent damage to:
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The spindle motor
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The VFD’s output transistors
ERR12 is a protective shutdown, not a software or configuration warning.
Most Common Causes We See
1. Loose or intermittent motor wiring (most common)
This is the number one cause.
Examples:
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Motor connector partially seated
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Loose terminal screw
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Wire pulled slightly out of a crimp
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Cable strain from drag chains or repeated motion
Even brief interruptions can trigger ERR12.
2. Damaged motor cable
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Internal conductor break
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Pinched or crushed cable
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Excessive bending near connectors
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Coolant or debris intrusion into connectors
Often the spindle will run unloaded but consider cutting begins.
3. Connector contamination
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Coolant ingress
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Fine dust buildup
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Corrosion or oxidation on contacts
This can cause uneven phase delivery under load.
4. Internal VFD output fault (less common)
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Failed output transistor
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Driver board issue
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Internal protection circuitry triggering
This is typically only considered after wiring and motor are ruled out.
5. Motor internal fault (rare)
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Winding imbalance
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Insulation breakdown
This is uncommon, especially if the spindle previously ran reliably.
What to Check First
Step 1: Power down completely
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Turn off and unplug the VFD
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Allow time for internal capacitors to discharge
Never inspect motor wiring while powered.
Step 2: Inspect all motor connections
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Verify the motor connector is fully seated
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Check for bent, recessed, or damaged pins
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Gently tug each conductor to confirm it is secure
Step 3: Inspect the motor cable
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Look for tight bends or pinch points
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Inspect drag chain entry and exit areas
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Check strain reliefs closely
Step 4: Verify terminal tightness (if applicable)
If your setup uses screw terminals internally:
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Confirm all three motor phase terminals are tight
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Ensure no stray wire strands are present
Step 5: Test under light load
After reassembly:
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Run the spindle unloaded
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Apply a light cut
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Monitor for recurrence of ERR12
What ERR12 Is Not
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Not a controller or software issue
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Not related to spindle RPM commands
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Not caused by feed rate or toolpath logic
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Not EMI related
ERR12 is almost always electrical or mechanical at the motor output level.
When to Seek Technical Support
If ERR12 persists after:
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Verifying all wiring and connectors
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Inspecting cables
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Eliminating obvious mechanical issues
Then technical support may be required to evaluate:
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VFD output stage health
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Motor winding condition
Clear photos of connectors and wiring are very helpful at this stage.
Summary
ERR12 indicates an output fault or three-phase imbalance detected by the VFD.
Most common causes:
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Loose motor wiring
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Damaged motor cable
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Poor electrical contact
Start with a thorough inspection of all motor connections.
In most cases, correcting a wiring issue resolves the fault immediately.
