SV0432

Measure Incoming AC Voltage

With the machine powered down, use a multimeter to measure voltage between L1-L2, L2-L3, and L1-L3 at the main disconnect. Ensure readings are within ±10% of rated voltage (200-230VAC or 380-480VAC depending on system).

Check during peak facility load times to catch intermittent brownouts.

Ensure Lockout/Tagout (LOTO) is applied before accessing live terminals.

Verify Phase Balance

Confirm that the voltage readings between phases are balanced within 2%. Imbalanced phases can cause uneven DC bus voltage and trigger this alarm.

If imbalance exceeds 2%, contact facility electrician to investigate upstream power distribution.

Do not proceed with troubleshooting if power is unstable.

Monitor Voltage Under Load

Power up the machine and monitor incoming voltage during operation using a clamp meter or power analyzer. A significant drop under load indicates a facility power issue.

Log voltage readings over 15-30 minutes to identify patterns or intermittent drops.

Avoid direct contact with live terminals during measurement.

Power Down and Apply LOTO

Shut down the machine and apply Lockout/Tagout procedures to ensure no power is present before accessing the servo amplifier cabinet.

Double-check with a voltmeter to confirm zero voltage before proceeding.

High voltage risk—wait 5 minutes for capacitor discharge.

Inspect DC Link Fuses

Locate the DC link fuses (typically F1-F3) on the converter module or PSM unit. Visually check for signs of a blown fuse (discoloration or broken filament).

Keep spare fuses of the correct rating (20-50A fast-blow) on hand for quick replacement.

Do not bypass fuses or use incorrect ratings.

Test Fuse Continuity

Use a multimeter set to continuity mode to test each fuse. Replace any fuse that shows no continuity with an identical rated fuse and retest after powering up.

If fuses blow repeatedly, investigate for short circuits or overloads downstream.

Ensure replacement fuses match OEM specifications.

Check DC Bus Voltage

Power up the machine and navigate to DIAGNOSIS > SERVO > PARAMETER screen. Monitor parameter 2051 for DC link voltage and compare to expected values (280-310VDC for 200V class, 530-680VDC for 400V class).

Record voltage during different load conditions to identify drops.

Do not open cabinet while powered on.

Power Down and Inspect Capacitors

After applying LOTO and waiting 5 minutes for discharge, visually inspect DC bus capacitors on the converter module for bulging, leaking, or discoloration indicating failure.

Capacitors often show physical signs of failure before complete breakdown.

Residual voltage may remain—verify 0VDC with a meter.

Measure Capacitor ESR

Use an ESR meter to test the equivalent series resistance of the capacitors. An ESR value above 0.5Ω indicates degradation, and the converter module should be replaced.

If ESR testing equipment is unavailable, consider proactive replacement if capacitors are over 5-7 years old.

Do not attempt repair of capacitors—replace the entire module.

Apply LOTO and Access Cabinet

Power down the machine, apply Lockout/Tagout, and wait 5 minutes for capacitor discharge before opening the servo amplifier cabinet.

Use a torque wrench for consistent tightening to manufacturer specs.

High voltage risk—verify 0VDC before proceeding.

Inspect Power Terminals

Check converter input terminals (R, S, T) and DC bus terminals (P, N) for looseness, corrosion, or overheating signs (discoloration or melted insulation).

Look for carbon buildup or pitting on terminal surfaces indicating arcing.

Do not touch terminals without confirming zero voltage.

Tighten and Clean Connections

Tighten all connections to specified torque values and clean corroded terminals with electrical contact cleaner and a wire brush. Retest DC bus voltage after powering up.

Apply dielectric grease after cleaning to prevent future corrosion.

Avoid over-tightening, which can strip threads or damage terminals.

Rule Out Other Causes

Ensure all previous checks (input power, fuses, capacitors, connections) have been completed and show no issues before suspecting converter module failure.

Document all test results to provide to FANUC support if escalation is needed.

Do not disassemble the converter module without authorization.

Swap with Known-Good Module

If available, replace the suspected converter module with a known-good spare of the same model and test for alarm resolution after powering up.

Label modules clearly during swapping to avoid mix-ups.

Ensure LOTO is applied and capacitors are discharged before swapping.

Contact FANUC Support

If swapping resolves the issue or no spare is available, contact FANUC for diagnostic support or to arrange a replacement converter module.

Have machine serial number, alarm history, and test data ready for faster support.

Do not attempt internal repairs without FANUC guidance.