| Buffer Resistance | The bus voltage fluctuates strongly |
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ERR40 – Buffer Resistance / Bus Voltage Fluctuation
What this means
ERR40 indicates that the VFD is detecting strong or unstable fluctuations in the DC bus voltage. The DC bus is the internal high-voltage supply that feeds the inverter section of the VFD.
In simple terms:
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The internal voltage is rising and falling more than expected
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The VFD cannot stabilize the bus
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The VFD shuts down to protect internal components
This is primarily a power stability issue, not a cutting or spindle problem.
What the DC Bus Does
Inside the VFD:
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Incoming AC power is converted to DC
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That DC voltage is stored and smoothed by capacitors and buffer circuitry
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The inverter uses that stable DC to drive the motor
If the bus voltage fluctuates excessively, the VFD cannot operate safely.
Most Common Causes
1. Incoming power instability
The most common cause.
Examples:
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Voltage dips or surges
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Weak electrical circuits
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Long wire runs
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Shared circuits with large inductive loads
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Utility power fluctuations
Even short disturbances can trigger ERR40.
2. Contactor malfunction
If the VFD uses:
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An internal contactor
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An upstream external contactor
And that contactor:
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Chatter opens/closes
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Has worn contacts
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Is failing under load
The DC bus voltage can fluctuate rapidly, triggering ERR40.
3. Internal DC bus component degradation
Less common, but possible:
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Aging bus capacitors
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Failing buffer resistors
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Heat-related degradation
This is more likely on older or heavily used VFDs.
4. Rapid power cycling
Powering the VFD:
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On and off rapidly
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Through an unstable relay
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Via an undersized switch
Can cause erratic bus behavior.
What to Check First
Step 1: Verify incoming power quality
Check:
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Correct input voltage for the VFD
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Tight mains connections
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Dedicated circuit if possible
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No large motors or compressors starting on the same circuit
If available, monitor voltage while the fault occurs.
Step 2: Inspect contactors
If your setup includes:
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An external contactor
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A relay feeding the VFD
Check for:
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Audible chatter
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Loose wiring
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Heat discoloration
A failing contactor is a common trigger.
Step 3: Observe when the error occurs
Note whether ERR40 appears:
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At power-up
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During spindle start
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During cutting
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Randomly while idle
This timing helps identify whether the cause is external or internal.
What Will Not Fix ERR40
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Changing feeds or speeds
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Adjusting acceleration or deceleration
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Re-running motor identification
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Changing spindle parameters
ERR40 is not load-related.
When to Contact Support
If ERR40:
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Happens repeatedly on a known good power circuit
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Appears immediately on startup
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Persists after verifying power and contactors
Please contact support with:
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VFD model
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Input voltage
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Description of your power source
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Whether a contactor is used
In some cases, ERR40 indicates internal DC bus component failure.
Summary
ERR40 means the VFD cannot stabilize its internal DC bus voltage.
Most common causes:
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Unstable incoming power
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Failing or chattering contactor
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Electrical supply fluctuations
This is a power quality or internal power-stage issue, not a spindle or tuning problem.
