Danfoss VFD Warning 8 or W8 DC Undervoltage
Getting Warning 8 (W8) on your Danfoss VLT drive? Learn the common causes of DC undervoltage, from missing phases to blown fuses, and how to fix it fast.
If you are working with a Danfoss VLT drive and the display starts flashing Warning 8 (W8), the drive is telling you that its internal "fuel tank"—the DC bus—is running low on voltage. Specifically, the intermediate circuit voltage has dropped below the low-voltage limit set by the factory.
Unlike an Alarm 8, which trips the motor immediately, Warning 8 is often an early alert. However, if the voltage drops further, it will escalate into a full trip. Here is a human-made, step-by-step guide to finding the root cause and restoring your power stability.
What Exactly is Warning 8?
Inside every Danfoss VFD, the incoming AC power is converted into DC voltage. This DC voltage is stored in a bank of capacitors. The drive monitors this level constantly. If the voltage dips too low, the drive can no longer provide enough torque to the motor, and its internal electronics may become unstable. Warning 8 is the drive's way of protecting itself from these "brownout" conditions.
Common Causes of DC Undervoltage
- Mains Supply Failure: A temporary dip or sag in the utility power.
- Missing Input Phase: One of the three AC legs has been lost (blown fuse or loose wire).
- Blown Fuses: An upstream fuse has failed, leaving the drive running on "single-phase" power.
- Voltage Sags: Another large motor starting up in the plant is pulling the grid voltage down.
- Internal Rectifier Failure: The drive's internal bridge that converts AC to DC is damaged.
Step-by-Step Troubleshooting Guide
1. Check the Incoming Mains Voltage
The most frequent cause is a problem with the building's power. Use a multimeter to measure the AC voltage across the input terminals (L1 to L2, L2 to L3, and L1 to L3).
- Are all three phases present and balanced?
- Does the measured voltage match the drive's nameplate rating? If you have a 400V drive but are only receiving 340V, W8 is expected.
2. Inspect Fuses and Circuit Breakers
Check the high-speed semiconductor fuses or the branch circuit breakers feeding the drive. If one fuse is blown, the drive might still power up its display using the remaining two phases, but it will trigger a Warning 8 because the DC bus cannot reach full charge.
3. Monitor the DC Link Voltage (Parameter 16-30)
You don't need a multimeter to see what is happening inside the drive. You can use the LCP (Local Control Panel):
- Navigate to Parameter 16-30 (DC Link Voltage).
- For a 400V drive, the DC Link should ideally be around 540V to 560V DC (Mains AC x 1.35).
- If this value is significantly lower than the calculation, your input power is definitely the issue.
4. Verify Parameter 14-11 (Mains Voltage)
Sometimes the drive is "confused" because its settings don't match the environment. Check Parameter 14-11. Ensure it is set to the correct nominal voltage of your facility (e.g., 380V, 400V, or 480V). If the setting is higher than the actual supply, the drive will think the supply is too low.
5. Check for Transient Dips
Does Warning 8 only appear when the motor starts or when a nearby machine cycles on? This indicates a "soft" power supply. You may need to:
- Install an AC Line Reactor to smooth out the power.
- Increase the capacity of the supply transformer.
- Check for loose terminal screws—vibration can cause high-resistance connections that drop voltage under load.
Summary Checklist
| Checkpoint | Desired Result |
|---|---|
| Input Terminals | Balanced 3-phase AC voltage. |
| Upstream Fuses | All phases showing continuity. |
| Parameter 16-30 | DC Voltage = AC Voltage x 1.35. |
| Connections | Tight and free of charring or heat marks. |
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