Danfoss FC 302 trips A8 DC Undervoltage at power-up, what to check?
My Danfoss FC 302 drive is tripping with an A8 DC Undervoltage fault. It happens right at power-up, even though the supply seems fine. What should I check first?
Alarm 8 on a Danfoss VLT FC 302 indicates that the DC-link voltage has dropped below its undervoltage limit. If no 24 V DC back-up supply is connected, the drive will trip after a fixed delay that varies with unit size. This can occur during supply dips, after a fuse blows, or if the soft-charge circuit fails to charge the DC link at power-up.
Likely causes and checks
1Soft-charge (precharge) circuit fault
- Points to it
- The soft-charge circuit is responsible for gradually charging the DC link at power-up. If this circuit is faulty, the DC link may not charge correctly, leading to an undervoltage alarm even with a healthy supply.
- Check
- If the supply is confirmed good and the alarm repeats, especially at power-up, a soft-charge circuit fault is indicated. Danfoss lists a specific test for this circuit; if the supply is confirmed good, refer it to qualified service.
2Supply voltage low or wrong for the drive
- Points to it
- If the incoming supply voltage is consistently too low or does not match the drive's voltage rating, the DC link may not reach the required voltage level, triggering the undervoltage alarm.
- Check
- Verify that the supply voltage matches the drive's voltage rating. Perform an input voltage test at the drive terminals to confirm the actual incoming voltage.
3Lost phase, blown fuse or loose input connection
- Points to it
- A missing phase, a blown input fuse, or a loose connection on the input side can lead to an insufficient supply to the drive, causing the DC-link voltage to drop below the undervoltage limit.
- Check
- Lock out the power and check input fuses, inspect contactor contacts for wear or damage, and ensure all input terminal connections are tight. Measure all three phases under load to detect any imbalance or missing phases.
4Supply dip longer than the drive can ride through
- Points to it
- Transient dips or interruptions in the main supply voltage can cause the DC-link voltage to drop. If these dips are longer than the drive's ride-through capability, an undervoltage trip will occur.
- Check
- Compare the alarm log times with power-quality records to see if the trips correlate with supply events. Review parameters 14-10 Mains Failure and 14-11 Mains Fault Voltage Level for configuration related to mains failure response.
Next check: Given that the alarm appears at power-up with a seemingly healthy supply, the most likely cause is a fault in the soft-charge (precharge) circuit. Confirm the supply voltage is correct and stable, then investigate the soft-charge circuit as per Danfoss guidelines or contact qualified service.
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