Danfoss VLT AutomationDrive FC 302 fault A8: DC Undervoltage
Alarm 8 on a Danfoss VLT FC 302 is DC Undervoltage: the DC-link voltage dropped below the undervoltage limit. If no 24 V DC back-up supply is connected, the drive trips after a fixed delay that depends on unit size.
Diagnose this with AI
Try Free Drive Diagnostic Upload a keypad photo, the fault history or a parameter export from your Danfoss VLT AutomationDrive FC 302. InduQT compares the evidence and suggests the next useful checks.
When it happens
- During supply dips or interruptions
- After a fuse blows or a phase is lost
- At power-up if the soft-charge circuit does not charge the DC link
Likely causes and checks
1Supply voltage low or wrong for the drive
- Points to it
- Trips cluster with heavy plant loads starting, or the drive was installed on a lower supply than its rating.
- Check
- Check that the supply voltage matches the drive voltage rating and perform an input voltage test at the drive terminals.
2Supply dip longer than the drive can ride through
- Points to it
- Other equipment on the same supply also reacts; trip times match supply events.
- Check
- Compare alarm log times with power-quality records; review 14-10 Mains Failure and 14-11 Mains Fault Voltage Level.
3Lost phase, blown fuse or loose input connection
- Points to it
- Alarm 4 Mains Phase Loss may also be logged; problem worse under load.
- Check
- Lock out and check input fuses, contactor contacts and terminal tightness; measure all three phases under load.
4Soft-charge (precharge) circuit fault
- Points to it
- Supply is healthy but the alarm repeats, or appears at power-up.
- Check
- Danfoss lists a soft-charge circuit test; refer it to qualified service if the supply is confirmed good.
Telling it apart
- Warning 6 DC Link Voltage Low is the early warning with the drive still active; Alarm 8 is the trip.
- Warning/Alarm 36 Mains Failure is only active when 14-10 Mains Failure is not [0] No Function; Alarm 4 is a missing phase or high imbalance.
- Alarm 33 Inrush Fault means too many power-ups in a short time, not low DC voltage while running.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| 14-10 | Mains Failure | Function on mains loss: controlled ramp-down, coasting, kinetic back-up and trip variants. |
| 14-11 | Mains Fault Voltage Level | Threshold at which 14-10 acts; DC detection level is this value x 1.35. |
Evidence to collect
- Alarm log times for A8 and any A4/A36
- Supply voltage at the drive terminals, all three phases, under load
- Power-quality records
- 14-10 and 14-11 settings
- Whether a 24 V DC back-up supply is fitted
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