Scope of this technical record
Auxiliary SMPS and logic-rail triage for VFDs with dark keypad, flickering keypad, relay clicking, fan twitching or no ready state.
A dark keypad does not prove the DC bus is discharged. Treat the input and DC link as hazardous until verified by qualified procedure.
Why no-display cases need an energy path
A dead keypad can start upstream at line power, precharge or DC-link charging; it can also start inside the auxiliary SMPS or a shorted load on the 24 V, 15 V or 5 V branches. Replacing the keypad first often wastes time because the display is usually only the visible consumer of a deeper supply path.
The strongest page angle is the distinction between no bus, bus but no auxiliary rail, rail pulsing into overload and stable rails with a bad panel/interface. That distinction is missing from many short troubleshooting pages.
Control-power boundary table
The table turns a no-display symptom into a field evidence package for a repair shop. It also prevents unsafe assumptions about discharge state.
No-display boundary decision
| Observation | Boundary | Evidence to collect |
|---|---|---|
| No input at drive terminals | External supply | Disconnect, fuses, contactor, upstream protection |
| Input present, no bus | Rectifier / precharge | Charge resistor, relay/contactor, fuse and rectifier evidence |
| Bus present, rails missing | Auxiliary SMPS | Startup feed, controller, secondary rectifiers, opto feedback |
| Rails pulse or collapse | Shorted load / feedback | Fan, I/O, driver supply and control-board load isolation |
| Rails stable, display dark | Keypad / cable / CPU interface | Panel cable, connector and control-board interface |
Field record checklist
- Line voltage at drive input
- DC-link charge/no-charge behavior
- Any relay/fan/display pulsing
- Visible SMPS damage
- Board and keypad cable photos
- Storage or moisture history
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Workflow used when line power exists but the drive does not boot.
Related technical records
The drive has line power present but the keypad stays dark, flashes briefly, clicks repeatedly or never reaches a ready state.
Routes incoming three-phase supply through protection, rectification, precharge and DC-link storage before the inverter stage is allowed to run.
Maps the auxiliary supply route that powers the CPU, keypad, I/O, relay logic, fan control and often isolated driver supplies.
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