| Larger Speed Deviation | Refer to the value of speed deviation larger than P6.1.23 and duration of P6.1.24 |
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ERR28 – Large Speed Deviation
What this means
ERR28 indicates that the VFD detected a significant difference between the commanded spindle speed and the actual spindle speed for longer than the allowed time.
In simple terms:
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The VFD asked the spindle to run at a certain RPM
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The spindle could not reach or maintain that speed
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The deviation exceeded the allowed tolerance and duration
This is a closed-loop speed accuracy fault.
Why the VFD Cares About This
The VFD continuously compares:
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Target speed (what it is told to run)
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Actual speed (what the motor is actually doing)
If the motor lags too far behind, runs erratically, or cannot stabilize, the VFD assumes something is wrong and triggers ERR28 to protect the system.
Most Common Causes We See
1. Excessive cutting load (most common)
The spindle is being asked to do more work than it can handle at the requested RPM.
Examples:
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Heavy depth of cut
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Large tooling
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Dense materials
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Low RPM with high torque demand
Result:
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Motor slows under load
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Speed deviation exceeds limits
2. RPM set too low for the load
Spindles are happiest at higher RPM.
Running:
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Large tools
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Heavy cuts
At very low RPM can cause speed instability, especially under load.
3. Encoder configuration issues (if applicable)
If your system uses an encoder for speed feedback:
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Encoder type may be incorrect
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Encoder wiring may be loose
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Encoder parameters may not match the motor
Incorrect feedback causes the VFD to misinterpret actual speed.
4. Motor parameters not identified
If motor parameter identification was never performed or was interrupted:
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Torque response can be poor
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Speed regulation becomes inaccurate
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Deviation faults are more likely
5. Overly strict deviation limits
Parameters controlling how much deviation is allowed may be set too tight for real-world cutting conditions.
Relevant parameters:
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P6.1.23 – Allowed speed deviation
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P6.1.24 – Duration before fault triggers
What to Check First
Step 1: Reduce the cutting load
This resolves the majority of ERR28 cases.
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Reduce depth of cut
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Reduce feed rate
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Increase RPM if tooling allows
If ERR28 disappears after load reduction, the cause was mechanical, not electrical.
Step 2: Verify RPM selection
Avoid:
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Low RPM with heavy cuts
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Lugging the spindle
Spindles regulate speed best when operating closer to their designed RPM range.
Step 3: Confirm motor parameter identification
If available in your VFD:
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Re-run motor parameter identification
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Ensure the spindle is not connected to a load during identification
This allows the VFD to properly model motor behavior.
Step 4: Check encoder setup (if used)
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Verify encoder type matches VFD settings
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Inspect wiring for loose or damaged connections
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Confirm encoder feedback is stable
If no encoder is present, ensure encoder-related features are disabled.
Step 5: Review deviation parameters
Advanced users only:
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Confirm P6.1.23 and P6.1.24 are set to reasonable values
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Overly aggressive limits can cause nuisance faults during normal cutting
What ERR28 Is Not
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Not an under-voltage or power supply issue
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Not EMI-related
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Not a wiring short
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Not a random software glitch
ERR28 reflects a real mismatch between commanded and achievable spindle speed.
When to Contact Support
If:
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ERR28 occurs during very light cutting
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Speed deviation happens immediately at startup
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Encoder parameters appear correct but fault persists
Provide:
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Tool size
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RPM
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Feed rate
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Material
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Whether an encoder is used
Summary
ERR28 means the spindle cannot maintain the commanded speed within allowed limits.
Most common causes:
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Excessive cutting load
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Low RPM under heavy torque
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Incorrect encoder configuration
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Missing motor identification
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Overly tight deviation settings
In most cases, reducing load or increasing RPM resolves the issue.
