DC Link Overvoltage Alarm 7 (Drive Display) / SV0439 fault in Fanuc drive

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DC Link Overvoltage Alarm 7 (Drive Display) / SV0439 fault in Fanuc drive

Description

Often seen as "7" on the drive LED or SV0439/SV0440 on the screen. The DC Link is the high-voltage DC bus (approx 300V-600VDC) created by rectifying the incoming AC power. This bus powers the servo motors. When a motor accelerates, it consumes energy. When a motor decelerates, it acts as a generator (regeneration) and pumps energy back into the drive. The drive must dissipate this excess energy or return it to the grid. If the energy comes back too fast and has nowhere to go, the DC voltage rises. If it exceeds the capacitor limit (e.g., 800V), the drive trips with an Overvoltage alarm to prevent the capacitors from exploding.

Cause

This is almost always a regeneration (braking) issue.

  • Regenerative Discharge Unit Failure: The resistor or regen module responsible for burning off the excess energy is open-circuit or disconnected.
  • Input Voltage Too High: If your shop power is 240V or 250V AC instead of 200-220V, the base DC bus voltage starts too high, leaving little headroom for braking energy.
  • High Inertia Load: Attempting to stop a heavy table or spindle too quickly.
  • Impedance Mismatch: Using the wrong size Power Supply Module (PSM) for the number of motors connected.

Solution

Focus on the incoming power and the braking circuits:

  1. Check Input Voltage: Measure the incoming AC voltage. If it is above 230V, you may need a step-down transformer. Fanuc drives are sensitive to high input voltage.
  2. Inspect Regen Resistors: If the system uses external resistor banks, measure their resistance. If a resistor reads "Open Loop" (Infinity), it is burnt out and must be replaced.
  3. PSM Troubleshooting: If you have a regenerative PSM (one that puts power back to the building), check the fuses and IGBTs in the PSM.
  4. Reduce Deceleration: As a temporary fix, increase the time constant for deceleration in the parameters. This slows down the braking process, generating energy at a slower rate that the system can handle.

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