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VFD DC bus undervoltage trip

Technical review pending. This guide is based on the cited manufacturer documentation; check it against the manual for your exact model and firmware before acting.

A drive trips on DC bus undervoltage when the voltage on its internal DC link falls below a limit. Usually the supply sagged or lost a phase, or an input connection is loose. Less often the cause is inside the drive (precharge circuit or worn DC bus capacitors), or the ride-through settings do not suit the plant supply.

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Where it shows up

Likely causes and how to tell them apart

1Supply sags and dips

Points to it
Trips coincide with large motors starting, utility events or storms; other equipment reacts too.
Check
Compare trip times with power-quality records; measure supply voltage at the drive under load.

2Lost phase or blown fuse

Points to it
Phase-loss code logged as well; one phase low or missing.
Check
Measure all three input phases at the drive and check fuses with the supply locked out.

3Loose or burnt input terminals or contactor contacts

Points to it
Intermittent trips under load; discoloured terminal or contactor pole.
Check
Lock out, inspect and re-torque input terminals; check contactor contacts.

4Precharge (soft-charge) circuit fault

Points to it
Supply is healthy but trips repeat, or the drive faults at power-up (Schneider CRF1, Danfoss soft-charge test).
Check
Follow the manual’s soft-charge or precharge check; refer to qualified service.

5Aging DC bus capacitors

Points to it
Old drive; trips on load steps that it used to ride through, with a healthy supply.
Check
Have the drive assessed by the manufacturer’s service; capacitor work is inside the power section.

6Ride-through settings not suited to the supply

Points to it
Short dips cause trips that the process could tolerate.
Check
Review Danfoss 14-10/14-11 or ATV320 USB, UST and STP against the plant supply and the process.

Diagnostic sequence

  1. Read the code and logNote the time and any phase-loss or mains-failure codes logged with it; on the ATV320 the error record stores mains voltage.
  2. Match to supply eventsIf other loads tripped or flickered at the same time, the supply is the first suspect.
  3. Measure under loadMeasure all three phases at the drive input while it runs loaded.
  4. Check connectionsWith the supply locked out, check fuses, input terminals and contactor contacts.
  5. Then the driveIf the supply and connections are good, follow the manual’s precharge checks and refer internal faults to service.
  6. Tune ride-through lastAdjust the mains-failure or undervoltage handling only once the supply is understood, and confirm the process can tolerate the chosen behaviour.

Evidence to collect

Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.

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