SV0433 DC Link Undervoltage (PSM) fault in Fanuc drive

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SV0433 DC Link Undervoltage (PSM) fault in Fanuc drive

Description

SV0433 is a specific DC Link voltage alarm that originates from the Power Supply Module (PSM) but is reported by the servo amplifier. Unlike SV0435 (which is the inverter measuring the voltage), SV0433 means the PSM itself has detected that the main DC bus voltage has dropped below the operational threshold (typically around 190-200V DC for a 200V system) while the machine is running. This is a critical power delivery failure. The DC Link is the "fuel line" for all servo motors; if pressure drops here, no motor can hold position.

Cause

This is frequently caused by incoming power issues or internal charging circuit failures.

  • Pre-Charge Circuit Failure: The PSM has a "soft start" resistor and a relay that bypasses it. If the relay fails to close, the current must squeeze through the resistor, causing a massive voltage drop when a motor tries to move.
  • Phase Loss: One of the three phases of incoming AC power is missing. The drive might idle fine on single-phase, but the moment a load is applied, the voltage collapses.
  • Contactor Failure: The main Magnetic Contactor (MCC) has pitted contacts that provide high resistance under load.
  • Loose Bus Bars: The screws connecting the PSM to the Spindle/Servo modules are loose, creating a high-resistance junction.

Solution

DANGER: Measuring DC Link voltage involves working with 300V-600VDC.

  1. Check Input Phases: While the machine is under load (moving an axis), measure the AC voltage across L1-L2, L2-L3, and L1-L3. They must remain balanced. A drop in one leg indicates a bad breaker, fuse, or utility transformer.
  2. Listen to the MCC: When the machine starts, listen for a solid "clunk" from the main contactor. If it chatters or sounds weak, the coil or contacts are bad.
  3. Bus Bar Torque: Power down and tighten all screws on the DC link (the top terminals linking the drives).
  4. Resistor Check: If the alarm happens immediately upon startup, the pre-charge resistor inside the PSM may be open (blown). This usually requires replacing the PSM.

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