TL;DR
Bus undervoltage is a supply problem or aging DC bus capacitors. Measure the AC input voltage under load first. A sagging supply needs upstream correction before the drive can run reliably.
What you might see
- F006 or UV fault on drive display
- drive trips under load
- bus voltage reading low on HIM
- fault appears during other heavy loads starting nearby
Likely causes
AC supply voltage sagging below the drive input range, often when large nearby loads start
Loose or corroded input power connections increasing impedance under load
Degraded DC bus capacitors with reduced capacitance, allowing larger voltage dips
Undersized feeder or input fusing increasing voltage drop under load
Required tools
- True-RMS multimeter rated for 600V
- LOTO kit
- Arc-flash PPE
- Torque wrench for terminal lugs
Safety first
- VFDs retain lethal DC bus voltage after input disconnection. Wait at least 5 minutes and verify bus voltage below 50V DC before working inside the enclosure.
- All live-voltage measurements inside the drive require arc-flash PPE and an Energized Electrical Work Permit.
Procedure
- 1
Read the fault code. F006 is the PowerFlex 525 undervoltage fault.[1]
- 2
Measure the AC input voltage at the drive input terminals under normal load conditions with a true-RMS multimeter. It must be within the drive nameplate input rating (typically 480V, -10%).
Warning: Measuring drive input terminals requires opening an energized enclosure. Use arc-flash PPE. - 3
Note whether the fault correlates with a nearby large motor starting. Simultaneous starts on a shared feeder will cause voltage sags.
- 4
Inspect the input terminal block for loose or corroded connections. Lock out the drive, then torque each input terminal lug to the drive manual specification.[1]
- 5
Check the input fuse ratings. Undersized fuses increase voltage drop. Verify the fuse rating matches the drive input current rating.
- 6
If supply voltage is in range and connections are tight, the DC bus capacitors may be degraded. Capacitor replacement requires a Rockwell Automation service technician.
- 7
Clear the fault and run a start. Monitor bus voltage on the HIM display during the full load cycle to confirm the fault does not return.
Sources
Allen-Bradley PowerFlex 520-Series AC Drives User Manual (Pub 520-UM001), Rockwell Automation (Allen-Bradley)
Allen-Bradley PowerFlex 520-Series User Manual, general undervoltage fault and input power connection procedures (general)
View source
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Stop fixing the same fault twice.
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