What causes FANUC alarm SV0442?

Failed pre-charge relay (K1/K2) or stuck relay contacts in the converter module - relay coil open or contacts welded/corroded preventing proper switching. Open or damaged pre-charge resistor (typically 50-150 ohm, 50W ceramic) in converter inrush limiting circuit - resistor burned open from repeated power cycles or previous fault condition. Degraded or shorted DC bus capacitors in converter module preventing charge accumulation - ESR increased beyond acceptable limits or internal short reducing effective capacitance. Faulty converter rectifier bridge - one or more diodes shorted or open preventing proper AC-to-DC conversion and bus charging. Insufficient input voltage or missing phase on 3-phase power supply - check for voltage imbalance >3% or voltage drop below parameter 0 setting requirements (typically 200VAC minimum for 200V class). Failed DC bus voltage detection circuit or feedback sensor in converter preventing proper charge confirmation - Hall effect sensor or voltage divider circuit malfunction.

How do I fix FANUC alarm SV0442?

1. Step 1: Power down completely, lockout/tagout, and wait 10 minutes for DC bus capacitor discharge. Verify with DC voltmeter that bus voltage is 0VDC before proceeding (measure at converter P-N terminals). 2. Step 2: Check incoming 3-phase power at servo converter input terminals - verify all three phases present, balanced within 3%, and meeting voltage specification in parameter 0000 (200-230VAC for 200V class, 380-480VAC for 400V class). Check for loose connections or blown fuses. 3. Step 3: Navigate to SYSTEM > DIAGNOSIS > SERVO and check DC bus voltage reading. Visually inspect converter module for burned components, discolored resistors, or bulging capacitors. Look for thermal damage around pre-charge relay area. 4. Step 4: Perform resistance check: disconnect incoming AC power from converter, measure resistance between AC input lines (should be 50-150 ohms initially through pre-charge resistor, then drop to near-zero when relay closes). Infinite resistance indicates open pre-charge resistor or relay failure. 5. Step 5: Using PMC ladder monitor (PMC > LADDER > MONITOR), observe pre-charge relay output signal (typically G50.0 or similar depending on model) during power-up sequence. Signal should energize for 5-10 seconds then de-energize. No signal change indicates control board issue; proper signal change but no voltage rise indicates relay or capacitor failure. 6. Step 6: If pre-charge resistor and relay test good, measure DC bus capacitor bank capacitance with capacitance meter (after full discharge) - typical values 2000-4000µF for single axis modules. Less than 50% of rated value indicates capacitor replacement needed. 7. Step 7: If all components test good, replace entire converter module (PSM or power supply module) as integrated assembly. After replacement, verify parameters 1800-1899 series (servo motor parameters) match motor specifications and perform DC bus voltage verification test before enabling axes.

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FANUC Controller

Servo/Axis · Advanced · 1-4 hours

Alarm SV0442

Fanuc SV0442 - Servo alarm: n axis inverter cooling fan stop

The FANUC SV0442 alarm typically results from a failure in the servo amplifier converter module to pre-charge the DC bus capacitors, often due to a faulty pre-charge relay or resistor. The most likely fix involves inspecting and replacing the pre-charge circuit components or the entire converter module if necessary.

FANUCServo/AxisAdvanced1-4 hours

SV0442

Fanuc SV0442 - Servo alarm: n axis inverter cooling fan stop

SERVO ALARM: n AXIS INVERTER COOLING FAN STOP

Root Cause Summary

The FANUC SV0442 alarm typically results from a failure in the servo amplifier converter module to pre-charge the DC bus capacitors, often due to a faulty pre-charge relay or resistor. The most likely fix involves inspecting and replacing the pre-charge circuit components or the entire converter module if necessary.

Safety

DC bus capacitors retain lethal voltage (340VDC for 200V class, 680VDC for 400V class) for 5-10 minutes after power-down. Always wait minimum 10 minutes and verify 0VDC with meter before touching internal components.
Never bypass or jumper the pre-charge circuit - this safety feature prevents catastrophic inrush current that can destroy rectifier bridges, capacitors, and create arc flash hazard
Use insulated tools rated for CAT III 600V minimum when measuring or working inside converter module with power applied for diagnostic purposes

Causes and Fixes

1
Power Down and Safety Check
Power down the machine, apply lockout/tagout, and wait 10 minutes for DC bus discharge. Verify 0VDC across P-N terminals of the converter module using a CAT III 600V rated multimeter.
Always double-check voltage even after waiting, as residual charge can persist in damaged capacitors.
Lethal voltage (340-680VDC) may be present. Do not proceed until voltage is confirmed at 0VDC.
2
Inspect Relay Visually and Test Signal
Locate the pre-charge relay on the converter module (refer to module schematic). Visually check for burned or damaged contacts. Using PMC ladder monitor (PMC > LADDER > MONITOR), observe relay output signal (e.g., G50.0) during power-up. Signal should energize for 5-10 seconds then de-energize.
If the relay clicks but no voltage rise occurs on the DC bus, the contacts may be stuck or corroded.
Do not manually force relay contacts; this can cause arcing and damage.
3
Test Relay Continuity and Replace if Needed
With power off and discharged, use a multimeter to test continuity across relay contacts in both energized and de-energized states (if possible, manually actuate if safe). Replace the relay if continuity fails or contacts show high resistance (>1 ohm). After replacement, test power-up sequence for DC bus voltage rise.
Keep a spare pre-charge relay specific to your converter model on hand for quick replacement.
Ensure replacement relay matches OEM specifications to avoid mismatched current ratings.

DIY Feasiblemedium confidence

Call a technician if:

·If unable to safely access or test converter module components due to lack of tools or training
·If all diagnostic steps fail to resolve the issue and converter module replacement is required

Prevention

Perform annual inspection of converter module for signs of thermal stress or component wear.
Monitor incoming power quality regularly to detect voltage imbalances or drops before they cause failures.
Avoid frequent power cycling of the machine to reduce stress on pre-charge components.

Common Mistakes

Failing to wait for full DC bus discharge before working on the converter, risking electric shock.
Bypassing the pre-charge circuit to 'force' operation, which can destroy the rectifier and capacitors.

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