Scope of this technical record
Workflow for drives that appear dead or unstable at boot despite line power being expected at the equipment.
The display state is not a safety indicator. Isolate input power and verify DC-link discharge according to model-specific procedure.
No-display workflow route
The workflow separates missing bus, weak SMPS and downstream short before panel replacement.
No-display workflow image
Workflow goal
The workflow prevents three common mistakes: replacing the keypad before proving control power, ignoring precharge/DC-link evidence, and energizing a contaminated or shorted board after a weak-rail symptom.
Decision sequence
Use the steps as an evidence order for repair intake.
No-display workflow sequence
| Step | Evidence | Decision |
|---|---|---|
| 1 | Input at drive terminals | No input means upstream problem |
| 2 | DC-link charging | No bus means rectifier/precharge/bus path |
| 3 | Auxiliary rail state | Missing/pulsing rails define SMPS route |
| 4 | Load isolation evidence | Collapsed rail may be downstream load |
| 5 | Keypad/control interface | Only after rails are stable |
Evidence package
For a repair shop, the minimum useful evidence is model, voltage class, board photos, whether any relay/fan/display movement occurs, and whether the unit was stored, wet, dusty or previously repaired.
Field record checklist
- Input evidence
- DC-link charge evidence
- Rail behavior
- Fan/relay/keypad symptoms
- Board photos
- Contamination/storage history
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Functional path behind this workflow.
Linked records
The symptom belongs to the input, DC-link, precharge and auxiliary control-power boundary until proven otherwise; a dark display is not proof that the keypad itself failed.
Maps the auxiliary supply route that powers the CPU, keypad, I/O, relay logic, fan control and often isolated driver supplies.
Routes incoming three-phase supply through protection, rectification, precharge and DC-link storage before the inverter stage is allowed to run.
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