Scope of this technical record
Industrial VFDs with line power present but no display, weak boot, keypad flicker, relay clicking or repeated restart behaviour.
A dark keypad is not a discharge indicator. The input and DC-link circuit can remain lethal; internal inspection belongs to qualified personnel with verified isolation.
No-display control-power route
A dark keypad is routed through energy and control-power evidence before a keypad or control board decision.
No-display control-power map
What the symptom really means
A dark VFD display is a symptom of missing usable control power, not proof that the keypad is defective. The failure may be upstream of the electronics, inside the DC-link/precharge path, inside the auxiliary SMPS, or on a downstream load that drags the supply down.
This page answers the service question that short fault-code notes usually skip: where does the power path stop, and what evidence is needed before a keypad, control board or entire drive is replaced?
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 |
Repair-request evidence
A useful 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, stored or contaminated drives, capacitor aging, dust, moisture 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.
Functional circuit route for control-power supply decisions.
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.
Turn this record into a qualified service request
A repair decision is much more reliable when the request includes the exact identity of the drive, the first fault evidence and the machine condition when the symptom appeared.
- Complete drive type code / MLFB or nameplate model
- Fault code, fault value and first event before reset
- When the event appears: power-up, enable, ramp, run, decel or stop
- Motor/cable connected or isolated during the symptom
- Visible board, option-card, module and connector identifiers
- Previous repair history, replacement parts and repeat-failure pattern