When the engine is running, it is tempting to assume the alternator has failed. The real path may include breaker state, insufficient speed, lost excitation, sensing wiring, AVR supply, loose connections or a load-side fault.
Engineering decisions covered
Facility teams collecting evidence before qualified electrical diagnosis.
Do not reset or replace components before recording speed, voltage, frequency, alarms, breaker state and the sequence that produced the fault.
A fault package that separates engine-speed, alternator, control and downstream-load causes.
Do not reset or replace components before recording speed, voltage, frequency, alarms, breaker state and the sequence that produced the fault.
Decision 01
Is the generator truly connected to the measured point?
An open generator breaker, tripped protection, wrong selector position or failed ATS path can produce no voltage at the load even when the alternator terminals are healthy.
Record one-line position, breaker indication, controller mode and the exact measurement point.
- Photograph breaker and ATS indications
- Confirm controller operating mode
- Identify where voltage is absent
- Preserve the trip or alarm record
Decision 02
Why does speed matter to electrical output?
Frequency follows engine speed, and many excitation systems depend on reaching a valid speed range. Low or unstable speed may therefore appear as low, missing or unstable voltage.
Record rpm and frequency from start through stabilization before adding load.
- Compare rpm and frequency trend
- Note hunting before voltage appears
- Check whether the symptom changes with load
- Do not adjust governor or AVR blindly
Decision 03
What evidence points toward excitation or sensing?
Loss of residual magnetism, AVR supply, sensing fuse or wiring can prevent voltage build-up. Unstable sensing connections can make voltage oscillate even when engine speed is steady.
Preserve wiring history, recent work and phase-by-phase readings for the engineer.
- Record voltage on all phases
- Compare voltage and frequency stability
- Report recent alternator or panel work
- Check only externally visible loose or damaged parts
Decision 04
Can the downstream system cause unstable output?
Short circuits, severe imbalance, non-linear loads, motor starting and incorrect protection settings can collapse or distort output. The fault may disappear when the breaker is open.
Document the load sequence and whether stability returns when safely isolated.
- List loads connected at the event
- Record phase current if available
- Note motor or UPS starting events
- Retain protection and ATS event logs
Live generator terminals can be lethal. Site staff should observe displays and external conditions only; internal measurements and excitation work require qualified electrical personnel.
Decision record
Separate Site Evidence from Engineering Confirmation
Use traceable site data and a named engineering check to support each decision; specialist calculations remain with the qualified design or service team.
| Decision Point | Prepare on Site | Confirm in Design or Service |
|---|---|---|
| Is the generator truly connected to the measured point? | Photograph breaker and ATS indicationsConfirm controller operating mode | Identify where voltage is absentPreserve the trip or alarm record |
| Why does speed matter to electrical output? | Compare rpm and frequency trendNote hunting before voltage appears | Check whether the symptom changes with loadDo not adjust governor or AVR blindly |
| What evidence points toward excitation or sensing? | Record voltage on all phasesCompare voltage and frequency stability | Report recent alternator or panel workCheck only externally visible loose or damaged parts |
| Can the downstream system cause unstable output? | List loads connected at the eventRecord phase current if available | Note motor or UPS starting eventsRetain protection and ATS event logs |
Project record
The Review Should Leave a Usable Project Record
Record the verified condition, engineering conclusion, responsible party and retest or acceptance result for each topic.
Is the generator truly connected to the measured point?
Record one-line position, breaker indication, controller mode and the exact measurement point.
Why does speed matter to electrical output?
Record rpm and frequency from start through stabilization before adding load.
What evidence points toward excitation or sensing?
Preserve wiring history, recent work and phase-by-phase readings for the engineer.
Can the downstream system cause unstable output?
Document the load sequence and whether stability returns when safely isolated.