| Missing phase | Error of missing phase or unbalanced three phases |
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ERR11 – Missing Phase / Phase Imbalance
What this means
ERR11 indicates that the VFD has detected a missing phase or a significant imbalance in the motor output phases.
In plain terms, the VFD expects to see three healthy, balanced phases going to the spindle motor. If one phase is missing, weak, or inconsistent, the VFD shuts down to protect the motor.
Why This Happens
Spindle motors are three-phase devices.
If one phase is not delivered correctly, the motor can:
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Lose torque
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Overheat rapidly
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Suffer permanent winding damage
ERR11 is a protective fault, not a nuisance warning.
Most Common Causes We See
1. Loose motor wiring (most common)
This is by far the most frequent cause.
Common scenarios:
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Motor connector not fully seated
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Set screw or terminal loosened over time
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Cable strain from drag chains or repeated motion
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Recent maintenance where a connector was disturbed
Even a slightly loose connection can trigger ERR11 under load.
2. Damaged motor cable
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Internal conductor break
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Pin pushed back inside a connector
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Crushed or sharply bent cable
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Damage near strain relief points
The spindle may spin unloaded, then fault once cutting begins.
3. Connector contamination
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Coolant intrusion
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Fine dust or debris inside connectors
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Oxidation on contacts
This can cause intermittent phase loss that only appears during motion.
4. Internal motor fault (less common)
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Damaged winding
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Insulation breakdown
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Manufacturing defect
This is rare, especially if the system has been running reliably before.
5. VFD output fault (rare)
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Output driver issue
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Internal relay or board fault
This is typically a last-resort diagnosis after all wiring is confirmed good.
What to Check First
Step 1: Power down completely
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Turn off and unplug the VFD
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Allow bus voltage to fully discharge
Never check motor wiring with the VFD 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 wire to confirm it is secure
Pay special attention to connectors near moving axes.
Step 3: Inspect the motor cable end-to-end
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Look for tight bends or pinch points
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Inspect drag chain entry and exit points
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Check strain relief areas carefully
Step 4: Check terminal tightness (if applicable)
If your setup uses screw terminals internally:
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Verify all three motor phase terminals are tight
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Ensure no wire strands are loose or broken
Step 5: Test under light load
Once reassembled:
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Run the spindle unloaded first
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Then apply a light cut
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Observe whether ERR11 reappears
What ERR11 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 rates or toolpaths
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Not EMI related
ERR11 is almost always electrical or mechanical at the motor connection level.
When to Seek Support
If ERR11 persists after:
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Verifying all connections
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Inspecting cables
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Confirming no visible damage
Then technical support may be required to evaluate:
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Motor winding health
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VFD output stage
At that point, photos of connectors and wiring are very helpful.
Summary
ERR11 means the VFD is detecting missing or unbalanced motor phases.
Most common causes:
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Loose motor connectors
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Damaged motor cables
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Poor electrical contact
Start with a careful inspection of all motor wiring.
In the majority of cases, correcting a connection resolves the issue immediately.
