FANUC 0i-F SV0401 IMPROPER V_READY OFF on multiple axes, what to check?
I'm getting a FANUC 0i-F SV0401 IMPROPER V_READY OFF alarm on multiple axes at once. It happens during operation, and I've seen other servo alarms pop up with it. What should I check first?
The FANUC 0i-F SV0401 IMPROPER V_READY OFF alarm indicates that the velocity-control ready signal (VRDY) from the servo amplifier was OFF while the position-control ready signal (PRDY) was ON. This means the amplifier stopped being ready to drive the motors. When multiple axes show this alarm, especially with other servo or power-supply alarms, it often points to a root cause that made the amplifier drop out.
Likely causes and checks
1Another amplifier or power-supply alarm dropped the MCC
- Points to it
- If other SV or SP alarms appear simultaneously or in the alarm history, or if several axes show SV0401 together, it suggests a primary alarm caused the Motor Current Contactor (MCC) to drop, leading to SV0401 as a consequence.
- Check
- Read the full alarm list and check the amplifier displays for any other active alarms. Clear the first alarm that appeared, as it is likely the root cause.
2Emergency stop chain or converter not ready
- Points to it
- If the alarm appears when releasing the E-stop or at power-up, or if DGN 358 bit *ESP or CRDY is 0, it indicates a failure in the initial readiness sequence of the converter or the E-stop circuit.
- Check
- Read DGN 358 as a binary value and identify the first '0' between bits 5 and 14. This bit corresponds to the specific step (*ESP, CRDY, INTL, DRDY, SRDY) that failed to become ready.
3Dynamic-brake relay or amplifier hardware not reaching ready
- Points to it
- If DGN 358 shows *ESP and CRDY are set (1) but INTL or DRDY are missing (0), and the issue repeats on the same amplifier, it points to a problem with the dynamic-brake relay or the amplifier's internal hardware preventing it from becoming ready.
- Check
- Note the amplifier LED status and the exact value of DGN 358. Compare these with the FANUC alpha i servo amplifier maintenance manual before considering amplifier replacement.
4Low supply to the power supply module
- Points to it
- If the alarm coincides with plant supply dips, other machines starting, or appears with SV0433 or SV0435 (low DC link), the power supply module may not be receiving adequate input voltage.
- Check
- Have a qualified electrician measure the three-phase input voltage to the power supply module while the machine is under load and compare it against the module's rated specifications.
Next check: Given that multiple axes are affected and other servo alarms are present, the first step is to thoroughly review the alarm list and amplifier displays for any other active or historical alarms. The SV0401 is likely a secondary alarm, so identifying and addressing the primary fault is crucial.
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