Generator running but not producing power
The engine starts, holds speed and sounds healthy — and the panel shows no volts. This is not an engine fault. Everything downstream of the flywheel has stopped doing its job, and the diagnosis runs along one chain in one direction.
Everything on this page happens near the alternator terminal box, the most dangerous place on a generator. The output terminals carry lethal voltage whenever the engine is running, and a set that appears dead can be back-fed from the mains through a wrongly wired changeover — treat the terminal box as live at all times. Voltage measurement on a running set is competent-person work with correctly rated instruments and leads. Anything that involves touching conductors — fuses, sensing leads, diode testing, insulation resistance testing — is done with the set stopped, the controller in Stop, the starting battery isolated and the output breaker locked off. Prove dead before you touch. If you are not equipped and trained for that, stop here and call.
The chain
Work it from the cheap end
Speed, then excitation, then AVR, then rotating diodes, then main windings, then breaker, then load. A blown sensing fuse and a burnt stator show the same reading on the panel, and one of them costs almost nothing.
- Engine speed — voltage follows speed. Nothing after this is meaningful until the frequency is right.
- Excitation supply — sensing fuses and sensing leads in the terminal box.
- AVR — the regulator itself, judged only after the sensing chain is proved good.
- Rotating diodes — the bridge on the exciter rotor.
- Main windings — insulation resistance. Last, not first.
- Breaker and load — the output device and whatever it is feeding.
Before you call anyone
Four checks you can make safely
Is the breaker closed?
Obvious, and it is the answer often enough to be worth ten seconds. Check the main output breaker, any downstream isolator, and the changeover device. A breaker that has tripped is reporting a real fault — read why it trips before you simply reset it.
Is the frequency right?
Read frequency at no load. Voltage follows speed: a set turning too slowly cannot build volts however healthy its AVR is. Low, unstable or absent frequency is an engine, governor or speed-sensing fault — see under-frequency faults.
Zero volts, or a little?
There is a real difference. A small residual voltage means the machine is generating something and the excitation system is not amplifying it — AVR, sensing, or the excitation circuit. Absolutely nothing means lost residual magnetism, an open excitation circuit, or a winding or diode failure.
AVR sensing fuses
The single most common cause of "runs but no output". The AVR takes its voltage reference through small fuses in the terminal box; with one blown it has no reference and does not excite. Minutes to check, with the set stopped and isolated.
A fuse that blows again is not a fuse problem
Something downstream is pulling it. Fitting a larger fuse is how a cheap fault becomes a fire. Find what is loading the sensing circuit before anything else goes back in the holder.
Past the fuses
What needs a technician, and why
From here the diagnosis needs instruments, and more importantly it needs interpreting.
Lost residual magnetism
- A brushless alternator uses a small amount of residual magnetism in the rotor to bootstrap its own excitation. A machine that has stood idle a long time, been run backwards, or had its field leads disturbed can lose it — then it turns at correct speed forever without building voltage.
- The remedy is flashing the field: a brief external DC supply to re-magnetise the rotor. Routine, but the AVR is easily destroyed if the field leads are not disconnected from it first, and polarity matters. Not a first-attempt job with a car battery and a guess.
- If flashing produces voltage that holds, the machine was simply demagnetised. If it collapses the moment the external supply is removed, the fault is elsewhere — usually the AVR or a diode — and flashing has proved the machine itself can generate.
The AVR
- AVRs fail in two directions. Failed dead gives no excitation and no output — this page. Failed hard on gives uncontrolled over-excitation and rising output — see over and under voltage faults.
- Both diagnoses are only sound once the sensing chain above them is proved good. Condemning an AVR with a blown sensing fuse still in the holder is the commonest unnecessary parts change on a generator.
- Identification and replacement: control panel and AVR repair.
Rotating diodes
- On a brushless machine the exciter output is rectified by a diode bridge mounted on the rotating shaft.
- An open diode gives low, unstable, rippling output. A shorted diode overloads the exciter and can take the AVR with it.
- Tested with the machine stopped and the bridge disconnected. A failed bridge is replaced as a set, not individually.
PMG machines behave differently
- If the alternator carries a permanent magnet generator on its non-drive end, the AVR is supplied independently of the main output.
- That changes both the symptoms and the sequence — a PMG machine does not lose residual magnetism the way a shunt-excited one does, so a failure of excitation points somewhere else.
- Identify which type you have before working the chain.
The windings — last, not first
- Insulation resistance testing on stator, rotor and exciter tells you whether the windings are sound, degraded, or failed to earth.
- The interpretation matters as much as the number: a machine that has stood in humid air reads low and recovers after drying; a machine that has been overloaded and cooked will not.
- A burnt varnish smell, visible discolouration, or one dead phase with two healthy points to a winding fault — that is alternator repair and rewinding. The winding bay, the varnish and the oven do not travel, so this part of the job comes to Industrial Area 15 while the set and its cabling stay where they are.
Read the symptom
What you see, and what it usually means
| What you see | What it usually means |
|---|---|
| No output at all, no residual voltage | Lost residual magnetism, open excitation circuit, failed AVR, or a winding fault |
| Small residual voltage that will not build | AVR, sensing fuses, sensing leads, or the excitation circuit |
| Voltage builds, then collapses | AVR failing under load, weak excitation, a failed diode, or a genuine overload |
| Low output even with the trim wound up | Weak excitation or an open diode. Winding the trim up hides a fault, it does not fix one |
| Voltage present but unstable or hunting | Check frequency first. If speed is hunting, the governor is the fault and the AVR is chasing it |
| One phase dead, two healthy | Open winding, broken connection, blown fuse, or a failed breaker pole |
| Output collapses only when a large motor starts | Starting inrush exceeding what the set can deliver, or weak excitation under load |
| Burnt smell or scorching in the terminal box | Stop. A winding fault — cranking it again spreads the damage |
Urgency
Stop the set now, or book a planned visit?
Stop and call
Do not run it again
- Any burnt smell, visible scorching or discoloured windings
- Smoke from the terminal box
- A breaker that trips again the moment it is reset
- A set that is the only supply to a site currently in the dark
Can wait
Book it properly
- A standby set found without output during a routine test
- No smell, no scorching, mains healthy
- Fix it in daylight rather than at 2am — but do not leave it
- The whole point of the set is the day the mains are not healthy
Do not keep restarting a set that produced smoke or a burnt smell to see whether it does it again. It will, and the second run is the one that turns a repairable winding into a rewind. Site in the dark: emergency generator repair.
The capital question
Repair or replace the alternator?
That is a measurement, not an opinion.
What decides it is the condition of the windings, the state of the insulation, whether the core has been damaged by a rotor rub, and whether the frame and bearings are sound. Those are things you find out by opening the machine, testing it and measuring it — not by reading its age off a nameplate.
The winding work is done in our own workshop rather than subcontracted, so the assessment and the repair are the same conversation and the same queue. A machine that is economically rewindable gets rewound; one that is not, we will say so plainly rather than strip it first. Either way you get the readings the decision was made on.
- Measured, then judged
Insulation resistance, winding condition, core and bearing state — before anyone quotes.
- The alternator travels, the set stays
Cabling, exhaust and foundations are left untouched.
- One record
You get the readings, whichever way the decision goes.
Before you phone
What to have ready when you call
Send three photographs to +971 50 767 9050 on WhatsApp. Between them, most of this fault is identified before an engineer leaves Sharjah — and the right part goes on the van first time.
Three photographs
- The controller screen, alarms and all
- The alternator nameplate
- The AVR and the terminal box wiring
Three readings
- Frequency at no load
- Output voltage — zero, or a small residual figure
- Whether the breaker is closed, tripped or open
Planned work and quotations: jacob@abradiesel.com. What is fixed on site and what comes to the workshop is set out on generator repair and service.
Common questions
No output, answered
Why is my generator running but not producing electricity?
Almost always excitation. In order of likelihood: a blown AVR sensing fuse, a failed AVR, lost residual magnetism after a long idle period, a failed rotating diode, and last of all a winding fault. Check the breaker is closed and the frequency is correct first — both make the rest of the diagnosis meaningless if they are wrong.
What is flashing the field, and can I do it myself?
It is applying a brief external DC supply to the alternator field to restore the residual magnetism a brushless machine needs to start exciting itself. A standard procedure — but the AVR must be disconnected first or it can be destroyed, the polarity matters, and it is done on a machine with lethal voltages at its terminals. If you are not equipped and trained for that, this is the point to call someone.
How do I know whether it is the AVR or the alternator windings?
By working the chain in order and proving each stage: sensing fuses, sensing leads, excitation supply, AVR, diodes, then windings. A winding fault usually announces itself — a burnt smell, discolouration, one phase dead while the others are healthy, or a low insulation resistance reading. If the machine will build voltage at all, the windings are generally not the problem.
My generator produces power but not enough. Is that the same fault?
The same chain, further along. Low output that will not reach setpoint points to weak excitation, an open rotating diode, or an engine that is not holding speed. Check the frequency first — if the speed is low the voltage will be too, and the AVR is not the fault.
Can this be fixed on site, or does the alternator have to come off?
Sensing fuses, AVRs, sensing wiring, excitation circuit faults and rotating diodes on many frames are site repairs, provided the part is available. Rewinding is not — that comes to the workshop. The useful part is that the alternator travels and the set stays, so the cabling, the exhaust and the foundations are untouched.
Do you work on alternators from any manufacturer?
We are an independent repairer with no brand dealership, and we work across the alternator frames common on UAE gensets as well as the OEM-badged machines fitted to them. Photograph the alternator nameplate, the AVR and the terminal box wiring and send them to +971 50 767 9050 — usually enough to identify what is fitted before anyone travels.
Engine running, site still dark?
Call or WhatsApp. Send the controller screen, the alternator nameplate and the AVR, and tell us the frequency reading. Open 7 days, 07:00–20:00.