Generator over voltage and under voltage faults
A set that trips on high or low volts is telling you about its excitation, its engine speed or its load. Which of the three it is can usually be settled at the panel in a few minutes, before anyone opens a terminal box.
Read the panel. Do not open the terminal box.
Voltage faults put hands near the parts of a generator that will kill. An alternator terminal box on a running set is live at machine voltage, and it can be live with the engine stopped — back-fed from the mains through a changeover wired or interlocked badly. Almost every check on this page can be made from the controller display without opening anything.
Work inside the terminal box, the AVR enclosure or the outgoing breaker chamber is isolate, lock off, prove dead work for a competent person — not a facilities technician with a multimeter. Never work on a set left in Auto; it will start itself the moment its controller thinks the mains have gone. Never adjust an AVR on a set that is on load — a trim pot turned the wrong way puts full excitation onto a live building. Never strap out, bypass or widen a voltage protection setting to keep a set running; that protection exists because over voltage destroys everything downstream faster than it damages the generator. And windings that have been over-excited will be hot, with insulation damage that is often not visible. If the fault has already happened twice, stop resetting it — repeated over voltage events age insulation cumulatively, and the damage is not undone by the set behaving on the third attempt.
Do this first — it is free
Look at the frequency, not the voltage
Frequency is engine speed. Voltage is excitation. Reading both together is what separates a governor adjustment from a wasted AVR, and it is the first question we ask on the phone.
Under voltage
Output low, or collapsing on load
Causes, most likely first.
- Engine speed lowCheck frequency. A governor drifting low, a restricted fuel filter, a dust-choked air filter or an engine that has quietly lost power pulls speed down and voltage with it.
- Overload, or an inrush it cannot excite intoA genuinely overloaded alternator cannot hold voltage. Nor can a set correctly sized on running load but undersized for a large motor's inrush — the classic "voltage collapses when the chiller starts". Sizing is a calculation: load calculation and sizing.
- AVR trim set low, or driftedCommon after somebody else's site visit. If the set ran correctly for years and is now consistently a little low, ask what was done to it last.
- Weak excitationA partly failed exciter circuit, a high-resistance connection in the field path, or a failing rotating diode gives a set that builds voltage but cannot hold it — worst under load, where demand on excitation is highest.
- Failed rotating diodeA single open diode in the rotating rectifier gives low output with a characteristic ripple, and the set often still runs, badly. This is the fault most often misdiagnosed as an AVR.
- Sensing faultIf the AVR's sensing reads higher than reality — a wrongly connected sensing lead, a sensing transformer fault — the AVR reduces excitation and the real output sits low.
- Winding faultAn interturn short or an open connection in one phase gives low or unbalanced output that no AVR adjustment corrects. That is a rewinding job, not a panel job.
In this order
What to check, from the display
Frequency
If it is low, the fault is the engine. Stop here and go to the engine.
All six voltages
Three phase-to-phase and three phase-to-neutral. One phase low is a different fault from three phases low.
Current per phase
Against the nameplate. An overloaded set is not a faulty set.
When it happens
No load, a particular load, only when one motor starts, only after twenty minutes. Each points somewhere different.
What changed
A new load, a service, someone else's visit, a fuel delivery.
Everything past that — sensing fuses, excitation circuit resistance, diode testing, insulation resistance — is live or isolated work inside the machine, and belongs to control panel and AVR repair rather than to a troubleshooting session on your own.
Over voltage
The more urgent of the two
An alternator will survive a brief over voltage. The building's electronics, drives, lighting ballasts and control gear frequently will not.
Excitation causes
- AVR failed fully excited. The most common cause by a distance. When an AVR's output stage fails short it stops regulating and passes maximum available excitation to the field. Voltage runs away and the protection trips — which is the protection doing its job.
- Lost sensing. If the AVR cannot see the output it is regulating, most designs respond by increasing excitation, because zero sensed volts looks like a machine that needs more field. A blown sensing fuse, a broken sensing lead or a loose terminal produces a set that goes to maximum excitation and trips. Check the sensing fuses — small, cheap, frequently the whole fault. One that blows again immediately is a symptom, not a fix.
- Sensing on the wrong phase. Usually after a repair, a reconfiguration or a set rewired for a different voltage. The AVR is regulating something other than what you are measuring.
Speed and load causes
- Engine overspeed. Voltage rises with speed. A governor or actuator fault, a sticking fuel rack, or a sudden load rejection can all put speed above rating. An overspeed event is a stop-and-investigate fault in its own right — do not restart a set that has overspeed-tripped until the cause is known.
- Load rejection. A large block of load disconnecting suddenly gives a momentary voltage and speed overshoot while governor and AVR catch up. If the set trips every time a big load drops off, the subject is transient response, not a failed component.
- Protection set wrongly. A controller reconfigured with a narrow window trips a perfectly healthy set. Rare, free to check, and it happens after panel replacements.
Take the load off. Do not keep resetting it.
A recurring over voltage trip must not be reset repeatedly onto a live building — each event is another few seconds of stress on every connected device. If the site needs power urgently, that is a conversation about temporary arrangements, not about resetting the alarm again. Emergency generator repair, or call +971 50 767 9050.
The third case
Unstable voltage: hunting, flickering, surging
A set whose voltage will not sit still is diagnosed differently again.
Frequency hunting too
Engine speed is hunting and the AVR is faithfully following it. Governor, actuator, linkage or fuel delivery. Fixing the AVR will not help.
Frequency steady, voltage swinging
AVR stability, a loose or high-resistance sensing connection, a failing rotating diode, or interference on the sensing circuit.
Flicker tied to one load
A cycling motor, a welder, a lift or a drive is producing the disturbance and the set is reacting to it. A load characteristic question, not a fault.
Deep dip on every motor start
Excitation capability under transient load — a sizing and machine-capability question, best settled with a load bank test that records what happens at each load step rather than what someone remembers happening.
One phase adrift
Voltage imbalance
Worth separating out, because it is the fault most likely to be caused by the installation rather than by the generator. Measure all six voltages and the current on each phase, then read the pattern.
Not the generator
Currents badly unbalanced, voltages slightly
The load is unbalanced. Single-phase loads have been added to one phase over the years.
- A distribution board problem
- The cure is redistribution, not repair
- No amount of AVR work will correct it
The generator
Voltages unbalanced with the set off load
The fault is in the alternator or its connections.
- Open or high-resistance connection in the terminal box
- A failed breaker pole
- An open winding — one phase dead entirely
Unbalance matters beyond the numbers on the display. It heats the alternator's rotor, it heats every three-phase motor on the site, and it is a common hidden cause of nuisance tripping — see generator tripping the breaker.
Component change, or rewind?
When it is the AVR and when it is the alternator
This is the question the whole page is really about, because one is a part and the other is a machine.
| Evidence | Points to |
|---|---|
| Voltage high and uncontrollable, sensing fuses intact | AVR failed hard-on |
| Voltage collapses to residual, sensing fuse blown | Sensing circuit — but find out why the fuse went |
| No voltage at all, small residual at the terminals | Excitation. See runs but no power |
| Low output with pronounced ripple, set still runs | Rotating diode |
| Output low, one phase distinctly different | Winding or terminal connection |
| Low insulation resistance, discoloured windings, burnt smell | Winding failure — rewinding |
| Voltage tracks frequency exactly | Engine and governor, not the alternator at all |
The tests that separate the last few rows — insulation resistance, winding resistance balance, diode testing, controlled excitation — need instruments and an isolated machine. That is the point at which the sensible thing is to stop and call.
Local conditions
Why these faults are more common on Gulf sets
- Heat
Alternator output capability falls as ambient temperature rises, and a set in a poorly ventilated plant room fights its own exhaust heat as well as the outside air. A machine that holds voltage in February and sags in August is often not faulty — it is being asked for more than it can give at that air temperature. That is a ventilation and sizing question.
- Dust and moisture in the terminal box
Fine airborne dust, and humidity near the coast, get into enclosures and onto insulation. The result is exactly this population of faults: tracking, low insulation resistance, high-resistance connections and sensing circuits that behave differently in the morning and the afternoon.
- Load growth nobody recorded
Buildings add load over years without anyone recalculating the standby capacity. The set holds voltage until the day the mains drop and the full load arrives at once. A load bank test finds that before the power cut does.
- Light-load running
Standby sets that idle through short monthly tests never prove their excitation under real load. The first genuine demand is also the first real test.
What we do about it
Both halves of the set, measured
A workshop that only does panels will find an AVR fault; a workshop that only does windings will find a winding fault. We would rather measure and tell you which you have.
Before you phone
What to have ready when you call
Send it to +971 50 767 9050 on WhatsApp. A photograph of the panel and the alternator plate is usually enough to tell you whether this is a sensing fuse or a rewind before anyone travels.
From the panel
- The exact alarm text, and a photograph of the screen
- The event log — which alarm appeared first
- Voltage and frequency at the moment of the fault
- All three phase voltages and phase currents
From the machine and the site
- Alternator and AVR data plates
- Whether it happens at no load, on load, on a particular load, or only when hot
- What changed before the fault started
- How many times it has been reset
Planned work and quotations: jacob@abradiesel.com. What we repair and where is set out on generator repair and service.
Common questions
Voltage faults, answered
Why is my generator's voltage too high?
Most often a failed AVR passing full excitation, or a lost voltage sensing signal that makes the AVR behave as though the machine needs more field. Check the sensing fuses and leads. If the frequency is also high, the fault is engine speed rather than excitation and the voltage is simply following it. Do not reset an over voltage trip repeatedly onto a live building.
Why does my generator's voltage drop when load is applied?
Either the set is being asked for more than it can supply — most commonly the inrush of a large motor starting direct on line — or its excitation is weak, from a failing diode, a high-resistance field circuit or a tired AVR. Check the frequency at the same moment: if speed drops as well, the engine is the limitation, not the alternator.
Can I adjust the AVR myself to correct the voltage?
Not on a set that is on load, and not without knowing why the voltage is wrong. An AVR trim corrects a setting; it does not correct a fault. If the voltage was right last year and is wrong now, something has changed, and turning a potentiometer hides it. AVR work also means working live or on an isolated machine with the terminal box open, which is competent-person work.
What is the difference between an under voltage alarm and an under frequency alarm?
Under voltage is about excitation and output; under frequency is about engine speed. They frequently occur together, because a set losing speed also loses voltage. The one that appeared first in the event log is usually the cause and the other the consequence — which is why the log matters more than the alarm on the screen.
My generator shows no output voltage at all. Is that this page?
That is the neighbouring fault, and it has its own page: generator running but not producing power. Total loss of output is usually excitation, a blown sensing fuse or lost residual magnetism, rather than a regulation problem.
Does a voltage fault mean the alternator needs rewinding?
Usually not. The great majority are AVR, sensing, excitation circuit or diode faults — repairs rather than rebuilds. A rewind is indicated by low insulation resistance, unbalanced winding resistance, visible discolouration or a burnt smell, and those are measurements, not impressions. We test before we quote.
Set tripped on voltage and the site is off supply?
Call or WhatsApp with the alarm text, the voltage and the frequency. Do not keep resetting it onto a live building. Open 7 days, 07:00–20:00.