Scope of this technical record
Input energy path from line terminals through fuses, contactor, rectifier, precharge and the DC-link capacitor bank.
DC-link capacitors can hold lethal energy. Do not bypass precharge or probe the DC bus without rated instruments and model-specific procedure.
Input / DC-link / precharge route
The route explains why precharge can appear as no-display, undervoltage, fuse or control-power trouble.
Input / DC-link / precharge image
Why precharge sits at the center of many symptoms
A failed or overloaded precharge path can present as no display, undervoltage, blown input fuse, precharge fault, weak boot or contactor chatter. It is also affected by downstream shorts in the inverter bridge, brake chopper or capacitor bank.
This circuit page makes the energy route visible so the technician checks how the DC bus charges before blaming the keypad or main control board.
Path decisions
Use charge timing, fuse behavior and bus behavior to determine where the energy path stops.
Input and precharge diagnosis
| Bus behavior | Likely area | Service note |
|---|---|---|
| Never rises | Input, rectifier, open precharge | Check fuses/rectifier/charge resistor path |
| Rises slowly then collapses | Precharge overloaded / downstream short | Inspect capacitor, chopper and inverter bridge |
| Rises but contactor never closes | Precharge control / relay | Review auxiliary power and relay command |
| Fuse opens during charging | Shorted rectifier/bus/chopper | Do not upsize fuse; locate overload |
| Panel bus value not credible | DC-bus feedback | Inspect sensing divider and feedback path |
Field record checklist
- Input voltage and phase condition
- Fuse/contactor state
- DC-link charging behavior
- Precharge resistor or relay evidence
- Downstream bridge/chopper condition
- Capacitor visual condition
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Workflow linked from this circuit path.
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