WEG CFW500 fault F0021: Undervoltage on the DC link
The DC link (intermediate circuit) voltage, shown in P0004, fell below the minimum for the supply voltage set in P0296, and the inverter tripped. WEG’s trip levels are 200 Vdc for 200–240 Vac supplies, 360 Vdc for 380–480 Vac and 500 Vdc for 500–600 Vac.
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Try Free Drive Diagnostic Upload a keypad photo, the fault history or a parameter export from your WEG CFW500. InduQT compares the evidence and suggests the next useful checks.
When it happens
- During supply dips or interruptions
- After an input phase is lost
- At power-up, if the pre-charge circuit is faulty
- After a supply loss longer than about 2 seconds with Ride-Through enabled
Likely causes and checks
1Supply voltage too low
- Points to it
- Trip coincides with a dip on the plant supply; other equipment reacts at the same time.
- Check
- Measure the supply voltage and compare it with the inverter label; read P0052 (DC link at the last fault).
2Wrong supply voltage or P0296 setting
- Points to it
- Trips after installation or after the inverter was moved to another supply.
- Check
- Check that the inverter label and P0296 match the actual supply.
3Phase fault in the input
- Points to it
- Trips under load more than at light load; a fuse or one supply phase is missing.
- Check
- Check input fuses, the supply cabling and all three supply phases.
4Fault in the pre-charge circuit
- Points to it
- Supply and fuses are healthy but F0021 repeats, often at power-up.
- Check
- The pre-charge circuit is inside the inverter; refer to WEG service.
Telling it apart
- If the trip lines up with a plant power event, the supply is the first suspect; if the supply is steady and F0021 repeats, look at the input phases and then the inverter’s pre-charge circuit.
- With Ride-Through (P0320 = 2 or 3) the inverter blocks its output during a dip instead of tripping, and only shows F0021 if the supply takes too long to return (over 2 seconds).
- WEG notes that too short a ramp time can also cause F0021, so compare the trip with the ramp settings when the supply looks healthy.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| P0004 | DC link voltage (Ud) | Live DC link voltage; compare with the F0021 level for the supply. |
| P0296 | Line rated voltage | Must match the actual supply; WEG lists a mismatch as a cause. |
| P0320 | Flying Start / Ride-Through | Ride-Through (2 or 3) avoids F0021 for momentary supply drops of up to about 2 seconds. |
| P0331 | Voltage ramp for FS and RT | Voltage ramp time when the output restarts after a Ride-Through. |
| P0100 / P0101 | Acceleration and deceleration time | WEG notes too short a ramp time can cause F0021. |
Evidence to collect
- Fault history P0050–P0055, especially P0052 (DC link at the trip)
- Measured supply voltage on all three phases
- Input fuse condition
- Setting of P0296 and the inverter label
- Plant power-quality records at the time of the trip
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