Fanuc SV0435 - Servo alarm: n axis psm regenerative failure
The FANUC SV0435 alarm, indicating a low DC link voltage, is most commonly caused by incoming AC power supply issues or degraded DC link capacitors in the servo amplifier. The most likely fix involves verifying and correcting the input power supply or replacing aging capacitor banks in units over 8 years old.
FANUCServo/AxisIntermediate30-90 minutes
SV0435
Fanuc SV0435 - Servo alarm: n axis psm regenerative failure
SERVO ALARM: n AXIS PSM REGENERATIVE FAILURE
The servo amplifier has detected that the DC link (bus) voltage has fallen below the minimum acceptable threshold, typically below 170VDC on 200V class systems or 280VDC on 400V class systems. This condition triggers an immediate axis disable to prevent erratic motor behavior and protect the amplifier power stage. The alarm indicates either inadequate input power supply, regenerative discharge issues, or internal amplifier component failure.
Root Cause Summary
The FANUC SV0435 alarm, indicating a low DC link voltage, is most commonly caused by incoming AC power supply issues or degraded DC link capacitors in the servo amplifier. The most likely fix involves verifying and correcting the input power supply or replacing aging capacitor banks in units over 8 years old.
Safety
DC link voltage exceeds 300VDC (200V systems) or 600VDC (400V systems) and remains present up to 5 minutes after power-down due to capacitor storage - use LOTO and verify 0VDC with multimeter before touching internal connections
Regenerative discharge resistors can reach 150°C during normal operation - allow 15+ minute cool-down before handling
Never bypass or disconnect regenerative resistor with power applied - can cause catastrophic DC bus overvoltage and amplifier explosion
Causes and Fixes
1
Measure AC Input Voltage
With the machine powered on, use a multimeter to measure AC voltage across L1-L2, L2-L3, and L1-L3 at the servo amplifier input terminals. Ensure readings are within ±10% of nominal (200-230VAC for 200V class, 380-480VAC for 400V class) and balanced within 2%.
Measure during peak facility load times to catch intermittent drops.
Ensure proper insulation on multimeter probes as live voltages exceed 400VAC.
2
Inspect Upstream Power Connections
Power down the machine, follow LOTO procedures, and inspect connections at the main transformer secondary, line reactor, and amplifier input terminals for looseness, corrosion, or heat damage. Re-torque to manufacturer spec (typically 12-15 N⋅m).
Use a thermal camera before shutdown to identify hot spots indicating poor connections.
Verify zero voltage on all terminals before touching.
3
Check Facility Power Stability
Consult with facility maintenance to confirm no recent power interruptions or phase imbalances. If issues are suspected, install a power quality analyzer on the incoming line for 24 hours to log voltage sags or spikes.
Request historical power data from the utility provider if available.
Do not bypass power monitoring equipment without authorization.
1
Monitor DC Link Voltage on CNC
From the CNC interface, navigate to DIAGNOSIS > SERVO screen and observe the DC link voltage for the affected axis. Normal values should be 280-320VDC (200V class) or 530-650VDC (400V class). Note any drops >10% during axis motion.
Record voltage trends during rapid axis moves for clearer evidence of capacitor issues.
Do not attempt internal measurements without proper training.
2
Check Amplifier Age and Service History
Review maintenance logs or serial number data to determine the servo amplifier’s age. Units over 8-10 years old are prime candidates for capacitor degradation, especially if operated in high-temperature environments.
Cross-check with FANUC’s recommended capacitor replacement interval (typically 7-10 years).
None for this step.
3
Measure DC Bus Ripple Voltage
If equipped with an oscilloscope, power down, LOTO, then access the DC bus test points (refer to amplifier manual). Power up and measure ripple voltage; excessive ripple (>5% of nominal DC voltage) confirms capacitor failure. Replace capacitor bank or entire amplifier module.
Use a high-voltage differential probe for safe and accurate ripple measurement.
DC bus voltages can exceed 600VDC; verify zero voltage before accessing test points.
1
Locate and Inspect Regenerative Resistor
Power down the machine, follow LOTO, and locate the external regenerative discharge resistor (often on the cabinet panel or heat sink). Check for physical damage, cracks, or burn marks indicating failure.
Allow 15+ minutes for resistor cool-down as it can reach 150°C during operation.
Do not touch the resistor immediately after operation due to high temperature.
2
Measure Resistor Continuity and Resistance
Using a multimeter, test for continuity across the resistor terminals. Then measure resistance and compare to the value in parameter 2007 (typically 40-80 ohms). Replace if open circuit or out of spec.
Double-check parameter 2007 in SETTING > SERVO PARAMETERS to ensure correct spec.
Ensure machine is fully de-energized before testing.
3
Verify Parameter Settings for Regeneration
Access CNC parameters and confirm parameter 2009 (regenerative load factor, default 100%) and 2020 (regeneration mode, 0 for resistor). Adjust 2009 to 80-90% if frequent multi-axis deceleration occurs and alarm persists.
Document original parameter values before making changes for easy rollback.
Incorrect parameter changes can cause overvoltage alarms; proceed cautiously.
1
Power Down and LOTO
Shut down the machine at the main disconnect and follow lockout-tagout procedures to ensure no power is present. Verify zero voltage at all terminals with a multimeter before proceeding.
Label all disconnected wires during LOTO for easier reassembly.
Failure to follow LOTO can result in severe injury or death due to high voltage.
2
Inspect Connection Points
Examine AC input connections at the transformer secondary, line reactor output, and amplifier L1/L2/L3 terminals for signs of corrosion, discoloration, or heat damage. Look for loose bolts or terminals.
Use a small mirror and flashlight to inspect hard-to-reach terminal areas.
Wear insulated gloves even after LOTO as a precaution.
3
Re-Torque and Clean Connections
Clean any corroded terminals with electrical contact cleaner and a wire brush. Re-torque all connections to manufacturer specifications (typically 12-15 N⋅m) using a calibrated torque wrench. Replace damaged terminals if necessary.
Apply dielectric grease after cleaning to prevent future corrosion.
Do not over-torque as it can strip threads or damage terminals.
1
Review Alarm History for Patterns
Check the CNC alarm log for recurring SV0435 alarms or related power supply errors (e.g., 401, 414). Consistent alarms under minimal load suggest internal amplifier issues like rectifier failure.
Export alarm history to a USB drive for detailed analysis if supported by the CNC.
None for this step.
2
Test DC Link Voltage Stability
Power up and monitor DC link voltage on the DIAGNOSIS > SERVO screen during idle and light load conditions. If voltage fails to reach nominal levels (280-320VDC for 200V class) even at rest, suspect rectifier or charging circuit failure.
Compare voltage readings across multiple axes to isolate the faulty amplifier.
Do not attempt internal repairs without proper training.
3
Consult FANUC for Amplifier Testing
If external checks (power supply, resistor, connections) are normal but the alarm persists, contact FANUC support for advanced diagnostics or arrange for amplifier bench testing. Replacement of the rectifier module or entire amplifier may be required.
Have the amplifier serial number and machine model ready when contacting support.
Do not disassemble the amplifier without manufacturer guidance due to high-voltage risks.
DIY Feasiblemedium confidence
Call a technician if:
·If DC link capacitor replacement or internal amplifier repair is required and you lack high-voltage training.
·If the alarm persists after verifying power supply, connections, and regenerative resistor.
Prevention
Schedule annual power quality checks to detect facility voltage issues early.
Replace DC link capacitors proactively every 7-10 years as part of preventive maintenance.
Keep servo amplifier cooling fans and vents clean to prevent overheating, which accelerates capacitor degradation.
Common Mistakes
Ignoring incoming power quality and focusing only on the amplifier, missing the root cause.
Failing to follow LOTO procedures when inspecting connections, risking severe injury.
