SV0438 Inverter Current Abnormal fault in Fanuc drive

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SV0438 Inverter Current Abnormal fault in Fanuc drive

Description

SV0438 is a critical hardware protection alarm. It indicates that the current sensors within the Servo Amplifier have detected an instantaneous current spike that exceeds the safe operating limits of the IPM (Intelligent Power Module) or IGBTs (Insulated-Gate Bipolar Transistors). This is effectively a "Short Circuit" alarm. The drive shuts down immediately to prevent the transistor modules from exploding. Unlike software thermal alarms, this is a hardware-level trip that usually occurs the moment the servo system tries to energize.

Cause

The high current path is almost always on the output side of the drive.

  • Motor Power Cable Short: The most frequent cause is the insulation breaking down on the U, V, or W cables, causing them to touch each other or the shielded ground braid. This is common in cable tracks where cables constantly flex.
  • Motor Winding Short: The internal windings of the servo motor may have burned out and shorted internally or to the motor casing due to coolant ingress (coolant short).
  • Drive Transistor Failure: The IGBT module inside the amplifier itself may have failed. If an IGBT shorts out, the drive will detect this immediately upon power-up.
  • Noise/Grounding: Rarely, extreme electrical noise on the current sensing circuit can trigger a false positive.

Solution

WARNING: High voltage remains in the drive capacitors even after power is off. Wait for the "Charge" light to extinguish before touching terminals.

  1. Isolate the Drive: Power off the machine. Disconnect the motor power cables (U, V, W) from the bottom of the amplifier.
  2. Power Up Empty: Turn the machine back on with the cables disconnected. If the SV0438 alarm remains, the short circuit is inside the drive (blown IGBT). The amplifier must be replaced or repaired.
  3. Test External Components: If the alarm clears when the cables are unplugged, the fault is in the cable or motor. Use a Megohmmeter (Megger) to test the motor and cable insulation resistance to ground. A reading below 10 Megohms usually indicates coolant ingress or melted insulation.
  4. Check Resistance: Measure the resistance between U-V, V-W, and W-U. They should be balanced (e.g., all 0.5 ohms). If one is significantly lower, the motor winding is shorted.

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