Scope of this technical record
Fault-level triage for the Series 15H Bus Undervoltage condition, keeping input-supply, DC-link and auxiliary-control symptoms distinct.
The fact that a keypad can display a fault does not remove hazardous voltage risk or prove the power conversion path is sound. Follow lockout, discharge and rated-test procedures.
Meaning of the indication
The official Series 15H manual lists Bus Undervoltage as a detected low DC-bus voltage condition. It is a useful protective statement, but the repair path depends on whether the low-bus event occurs at initial energization, while accelerating or under a sustained load. Those scenarios differ in likely external and internal causes.
A low bus may reflect inadequate input voltage, phase or protective-device issues, rectification/precharge trouble, excessive loading or an internal sensing problem. It must also be separated from an auxiliary-control-supply problem: a drive can show status while its high-energy path is unable to sustain operation, or it can behave erratically because low-voltage electronics are unstable.
Build an evidence sequence, not a parts list
Begin with the full drive identifier, incoming supply context and when the trip occurs. Note whether the symptom follows installation changes, after long storage, under mechanical load or following repair. Before opening the unit, external electrical evidence and machine behavior should already narrow the question.
If internal assessment is justified, document the condition of the rectification/bus region, storage components and related sensing connections under appropriate professional procedures. Do not replace a control assembly merely because undervoltage appears alongside erratic display or enable behavior; where auxiliary-power symptoms exist, use the HW Power Supply route as a separate but linked investigation.
Undervoltage decision route
| Observation | Evidence priority | Related route |
|---|---|---|
| Fails on energization | Supply / protection / conversion path | DC-bus workflow |
| Trips under load | Supply sag / load demand / DC storage | DC-bus workflow |
| Unstable controls also present | Separate auxiliary rails from bus | HW Power Supply workflow |
How the database adds value
A manual fault table can tell a technician that the bus is low. It generally cannot store the growing relationship among a stopped legacy drive, its auxiliary board evidence, public repair availability and the proof a customer should collect before requesting repair. This record connects those layers without presenting unsupported component diagnoses.
For a legacy industrial asset, that structure matters commercially. An obsolete drive may justify repair only when the evidence points to a repairable assembly and the machine downtime or configuration risk makes a verified replacement attractive. A low-bus page should therefore guide the user toward a disciplined technical request rather than an impulsive parts purchase.
Future verification target
The available supply drawing does not itself map the complete bus/precharge section across the Series 15H range. A stronger future record would link a known frame or catalogue identifier to photographs or service documentation of the input rectifier, bus storage and measurement path. Until then, the page correctly remains a fault-triage and evidence-gathering reference.
That conservative scope is deliberate: it creates a useful page for real search demand while protecting the reliability of the database from speculation.
Field record checklist
- Full drive and voltage identity
- Occurrence timing and load state
- External supply and protective-device evidence
- Any linked HW Power Supply behavior
- Repair-versus-replacement context
Technical basis and reference documents
This is an independent editorial technical reference. Original manufacturer documentation remains controlling for installation, repair and commissioning decisions.
Official source for Series 15H operating scope, fault terminology and first-line troubleshooting guidance.
Reviewed schematic identifying UCC3802/UCC3804, TL431/CNY17-4 feedback and derived low-voltage rails; original source drawing is not redistributed.
Public repair-service evidence supporting continuing maintenance demand for the obsolete/legacy family.