SV0401 V-Ready OFF fault in Fanuc drive

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SV0401 V-Ready OFF fault in Fanuc drive

Description

SV0401 is a status alarm rather than a component failure alarm. "V-Ready" stands for "Velocity Ready." This signal is the handshake between the CNC controller and the Servo Amplifier. When the CNC powers up and releases the Emergency Stop state, it sends a signal to the amplifier to energize the main contactor (MCC) and prepare the bus voltage. If the amplifier does not return the "Ready" signal to the CNC within a specific time window, the SV0401 alarm is generated. Essentially, the CNC is saying, "I told the drives to turn on, but they didn't answer."

Cause

Because this is a handshake failure, the cause is usually in the control chain *before* the drive fully initializes.

  • Emergency Stop (E-Stop) String: The most common cause is a break in the hard-wired 24V E-Stop chain. If the E-Stop button is pressed, or an over-travel limit switch is hit, the MCC cannot pull in.
  • Missing 3-Phase Power: If the Power Supply Module (PSM) does not have 200V/220V incoming power, it cannot become ready.
  • MCC Failure: The Magnetic Contactor (the large relay that clicks when you turn the machine on) may have a burnt coil or fused contacts.
  • Door Interlocks: Many modern machines route the ready signal through safety door switches. A misalignment here will prevent the V-Ready signal.

Solution

Troubleshooting SV0401 requires a multimeter and the electrical schematics of the machine:

  1. Check the PSM/Drive Display: Look at the LED display on the power supply in the electrical cabinet. Is it completely blank? If so, check the input fuses and breakers. If it shows a specific alarm number (like 02 or 03), troubleshoot that specific alarm first.
  2. Trace the E-Stop Chain: Verify 24V is passing through the E-Stop button, all Over-Travel limit switches, and safety relays.
  3. Listen for the "Click": When you release the E-Stop, do you hear the heavy click of the Magnetic Contactor? If not, check the coil voltage of the MCC. If you hear it click but still get the alarm, the auxiliary contacts on the MCC might be dirty or damaged, failing to send the confirmation signal back to the CNC.
  4. Check Communication Cables: Ensure the optical fiber cable or ribbon cable connecting the CNC to the drive is seated correctly.

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