SV3

Check PSM LED Display for Alarms

Power down the machine safely and inspect the LED display on the PSM for any alarm codes. Refer to the FANUC PSM manual to decode the alarm and follow specific troubleshooting for that code.

Take a photo of the LED display for reference in case the alarm clears unexpectedly.

Ensure machine power is off and locked out before accessing the PSM to avoid electrical shock.

Verify PSM Output Voltage

If no alarm is present, use a multimeter set to DC voltage mode to measure the output at the PSM DC bus terminals. Compare the reading to the expected range (280-325VDC for 200V class, 560-650VDC for 400V class).

Use insulated probes rated for high voltage to ensure accurate readings and safety.

High voltage present; only trained personnel should perform this measurement.

Reset or Replace PSM if Necessary

If an alarm is present or output voltage is out of spec, attempt a power cycle reset of the PSM. If the issue persists, document the alarm code and prepare to replace the PSM with a compatible unit.

Keep a spare PSM on hand for critical machines to minimize downtime during failures.

Do not bypass PSM alarms; this can lead to further damage.

Measure AC Input Voltage at PSM

Using a multimeter set to AC voltage mode, measure the voltage across all three phases at the PSM input terminals (L1, L2, L3). Ensure readings are within spec (200-240VAC or 380-480VAC depending on machine).

Check voltage under load if possible, as idle readings may not reveal intermittent drops.

High voltage present; ensure proper lockout/tagout procedures before measurement.

Inspect for Missing Phases

Confirm all three phases show similar voltage readings. A missing phase (near 0V on one line) indicates a power supply or wiring issue upstream of the PSM.

Use a phase rotation tester to confirm correct phase sequence if voltages are present but unbalanced.

Do not operate the machine with a missing phase; this can damage components.

Correct Power Supply Issues

If voltage is low or a phase is missing, trace the issue to the facility power supply, circuit breakers, or disconnects. Contact an electrician if the issue is beyond the machine’s input terminals.

Install a power quality monitor to log input voltage over time for recurring issues.

Avoid temporary fixes like jumpering phases; resolve the root power issue.

Check for Pre-Charge Alarm on PSM

Inspect the PSM LED display for specific pre-charge circuit alarms (often indicated in the manual). Note any related codes or blinking patterns.

Some PSM models log pre-charge faults in memory; access diagnostic menus if available.

Power off the machine before inspecting to avoid electrical hazards.

Test DC Bus Voltage During Startup

With a multimeter, monitor DC bus voltage at PSM output terminals during machine power-up. A slow rise or failure to reach nominal voltage (e.g., 280-325VDC) indicates pre-charge failure.

Use a data-logging multimeter to capture voltage trends during startup for analysis.

High voltage risk; only qualified personnel should perform live measurements.

Replace PSM or Repair Pre-Charge Circuit

If pre-charge failure is confirmed, the PSM likely needs replacement or internal repair of the pre-charge resistors or contactors. Consult FANUC service for repair options if replacement isn’t immediate.

Check warranty status of the PSM before ordering a replacement to save costs.

Do not attempt internal PSM repairs without manufacturer guidance.

Review Machine Operation at Alarm Time

Check operator logs or CNC program to identify if the alarm occurred during heavy load conditions like rapid axis moves or high-torque cutting. Note the specific operation.

Use CNC event logs if available to pinpoint exact timing and axis activity.

None for this step.

Monitor DC Bus Voltage Under Load

Run a test program with similar load conditions while monitoring DC bus voltage with a multimeter or oscilloscope. Look for significant voltage drops below nominal levels during peak demand.

Set up a scope to trigger on voltage dips for precise capture of transient events.

High voltage risk; ensure proper safety protocols during live testing.

Adjust Parameters or Reduce Load

If voltage drops under load, reduce acceleration rates or feedrates in the CNC program. Alternatively, consult FANUC to verify if the PSM is adequately sized for the machine’s servo demands.

Backup CNC parameters before adjustments to revert if changes cause issues.

Avoid overloading other components by excessive parameter tweaks.

Rule Out PSM and Input Power Issues

Confirm that the PSM has no alarms and AC input power is within spec as per earlier cause checks. If both are normal, suspect internal servo amplifier issues.

Document all prior test results to justify focusing on the servo amplifier.

None for this step.

Inspect Servo Amplifier for Physical Damage

Power down and visually inspect the servo amplifier for bulging capacitors, burnt smells, or discoloration near the DC bus section. These are signs of capacitor degradation.

Use a flashlight and magnifying glass to spot subtle capacitor leaks or cracks.

Ensure complete power isolation before opening amplifier covers.

Arrange Servo Amplifier Repair or Replacement

If capacitors appear degraded or prior steps confirm low bus voltage despite a healthy PSM, the servo amplifier needs professional repair or replacement. Contact FANUC or a certified repair center.

Request a detailed repair report to identify if environmental factors contributed to capacitor failure.

Do not attempt capacitor replacement without proper training or tools.