ATV320 USF fault, supply undervoltage, what to check first?
I'm getting a USF fault on my Schneider ATV320 drive. It's a supply mains undervoltage error, and it seems to clear itself once the cause is gone. What should I check first to troubleshoot this?
The USF fault on a Schneider Altivar ATV320 drive indicates a supply mains undervoltage. This can be caused by the supply voltage being too low, transient voltage dips, or the loss of two or three input phases. The error clears once the underlying issue is resolved.
Likely causes and checks
1Supply voltage too low
- Points to it
- The most common cause for a persistent USF fault is that the incoming supply voltage is consistently below the drive's operating threshold.
- Check
- Measure the supply voltage at the drive input terminals while the drive is under load. Check the error record for the mains voltage (ULP1).
2Transient voltage dip
- Points to it
- Brief USF trips often coincide with large loads starting elsewhere on the system or utility power quality events, causing momentary voltage drops.
- Check
- Compare the exact times of the USF trips with power-quality records or logs of other large equipment starting. Review the [Undervoltage handling] USB- settings in the drive.
3Two or three phases lost
- Points to it
- If two or three input phases are lost, the drive will continue to run for a short period until the voltage drops sufficiently to trigger a USF fault.
- Check
- Inspect the input fuses and circuit breaker for the drive. Verify the integrity of the input connections. Note that a single missing phase typically results in a PHF fault, not USF.
4Loose or high-resistance input connection
- Points to it
- Poor connections can lead to intermittent undervoltage conditions, especially under load, and can also manifest as overheating at the terminals.
- Check
- After locking out power, check the tightness of the input terminal connections on the drive and inspect the contacts of any upstream contactors for signs of wear or poor contact.
Next check: Start by measuring the supply voltage at the drive input terminals under load to rule out a consistently low supply. If that's not the issue, review power quality records for transient dips.
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