Cause : DC Bus Under voltage during running comes when the DC bus voltage drops below the undervoltage detection level. Typically, this happens when the input voltage falls below ~15% of the drive's rated voltage and caused from a momentary power loss.

The power unit has detected an undervoltage condition in the DC link. When encountering a "Uu1" DC link voltage undervoltage fault in drive, it indicates that the voltage level in the DC link has dropped below the acceptable threshold.

1. Check the input power source.
2. Check input rectifier using multimeter.