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FANUC Controller
Power Supply · Intermediate · 30-90 minutes
Fanuc Alarm 434 - DC Bus Undervoltage
Alarm 434 indicates the servo amplifier (SVM) has detected a low voltage condition on the 24VDC control power supply circuit. This can be caused by insufficient voltage from the power supply unit, excessive voltage drop in wiring/connectors, or high current draw from faulty components on the 24VDC bus.
434
Fanuc Alarm 434 - DC Bus Undervoltage
CNV DC Link Undervoltage
Communication error: cnv dc link undervoltage. Data transfer between devices has failed or timed out.
Root Cause Summary
Alarm 434 indicates the servo amplifier (SVM) has detected a low voltage condition on the 24VDC control power supply circuit. This can be caused by insufficient voltage from the power supply unit, excessive voltage drop in wiring/connectors, or high current draw from faulty components on the 24VDC bus.
Safety
- Check power off before inspecting
Causes and Fixes
- 1
At the power supply unit, measure 24VDC output voltage under load. Verify voltage is between 24VDC ±10% (21.6-26.4VDC). If below 22VDC, power supply is failing or overloaded.
Always use proper lockout/tagout procedures before working on power supply circuits.
- 2
Check DIAGNOSIS screen > SYSTEM > I/O for 24VDC voltage reading if available on your control series (30i/31i/32i typically show this). Compare with multimeter readings at amplifier terminals.
- 3
Inspect power supply cooling fans and ventilation. Overheating causes voltage regulation failure. Clean dust buildup and verify fan operation.
- 4
If voltage is low, disconnect non-critical 24VDC loads (indicators, auxiliary relays) and retest. If voltage recovers, power supply is undersized or failing. Replace PSU with adequate capacity rated for servo amplifier requirements.
Call a technician if:
- ·DC bus capacitor replacement required
- ·Servo amplifier module replacement needed
- ·Complex power quality issues requiring specialized test equipment
Prevention
- Regular 24VDC power supply voltage monitoring
- Scheduled replacement of DC bus capacitors every 5-7 years
- Installation of power quality monitoring equipment
- Proper ventilation to reduce capacitor stress from heat
Common Mistakes
- Assuming alarm is always power supply related without checking DC link voltage
- Replacing power supply without checking actual current draw
- Ignoring power quality issues that cause intermittent alarms
- Not verifying proper wire gauge for power supply connections
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Always verify with your machine manual or a certified technician before performing service procedures.