Applicable Systems
FC-30
FC-51
FC-101
FC-202
FC-302
Symptoms
The drive trips and the display shows Alarm 1. The drive has detected a low voltage (undervoltage) condition on the DC bus or the supply input.
Possible Causes
- Mains supply voltage dropped below the drive minimum
- Supply phase loss (one phase missing on 3-phase input)
- Supply wiring too thin causing voltage drop
- Drive DC bus capacitor aging
Troubleshooting Steps
- Step 1: Measure the mains supply voltage at the drive input terminals
Voltage is within the drive rated range (typically 380-480V +/-10% for 400V drives) - Step 2: Check all three input phases for presence and balance
All three phases are present and balanced within 2% - Step 3: Verify the supply cable gauge is adequate for the drive current rating
Cable cross-section meets local code and drive manual requirements - Step 4: If voltage drops are intermittent, check for loose connections upstream
All connections in the supply circuit are tight
How to Reset
To reset: 1) Restore the supply voltage to the rated range. 2) Press the RESET key on the drive keypad. 3) If the alarm persists, check the DC bus voltage with the drive powered. Caution: Undervoltage can cause the drive to lose control. Always fix supply issues before restarting.
Frequently Asked Questions
- Q: What is the minimum supply voltage for Danfoss FC-302?
The FC-302 for 400V mains typically operates down to 342V AC (15% below nominal). Below this threshold, the drive trips on undervoltage to protect the DC bus.
- Q: Can phase loss damage the drive?
Yes, running a 3-phase drive on two phases causes unbalanced currents, overheating of the input rectifier, and potential DC bus capacitor damage. The drive trips to protect itself.
Source: Danfoss VLT FC-302 Design Guide, 2023
Category: PLC & VFD