Scope of this technical record
No-display and weak-display VFD symptoms where line input, DC-link charging and auxiliary low-voltage rails must be separated before keypad or CPU replacement.
A dark display does not prove the DC bus is discharged. Internal inspection requires qualified isolation and measured discharge verification.
What the symptom really means
No display is one of the easiest VFD symptoms to misread. The drive can have a good keypad and still stay dark because the input path is open, the precharge stage never charges the DC link, the auxiliary SMPS cannot start or a downstream load collapses the low-voltage rail.
The correct route is therefore supply-first, not keypad-first. The page links the symptom to the rectifier/precharge path and the SMPS control-power path so a repair request can be evaluated with evidence rather than guesses.
Evidence order
A qualified technician should establish incoming supply at the drive terminals, DC-link charging, precharge state and auxiliary rail behavior. A pulsing relay or flickering keypad often indicates an SMPS trying to start into overload or insufficient bus/startup energy. A totally dead unit may still have hazardous voltage on the DC bus.
No-display evidence route
| Evidence | Likely boundary | Next decision |
|---|---|---|
| No line at drive input | External supply | Fix upstream supply before drive repair |
| Line present, no DC bus | Rectifier / precharge | Inspect input fuses, rectifier and charge path |
| Bus present, no logic rail | Auxiliary SMPS | Inspect startup, controller and secondary rails |
| Rail pulses or collapses | SMPS load or feedback | Separate regulator from shorted downstream branch |
| Rails stable, keypad dark | Keypad / cable / CPU interface | Proceed to control interface evidence |
Commercial repair implication
A no-display request should include photos of the nameplate, power board and control board, plus a description of whether fans or relays move at power-up. For old or contaminated drives, capacitor aging and board contamination can be as relevant as a visibly burned part.
Field record checklist
- Drive model and input voltage class
- Incoming supply condition at drive terminals
- DC-link voltage evidence under qualified procedure
- Keypad completely dark or pulsing/flashing
- Relay click, fan twitch or smell/visible damage
- Board markings and photos
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Linked workflow for no-display triage.
Diagnostic workflow
Drive line power is present, but the keypad is dark, flashing or repeatedly restarting.
Drive reports undervoltage/precharge fault, blows input fuses or shows no-display symptoms with uncertain DC-link charging.