Generator under and over frequency faults
Frequency is engine speed. That is the whole page in three words, and it is what makes this fault far easier to diagnose than it looks — an under-frequency alarm is not an electrical fault at all.
One reading splits every generator fault in two
Watch the frequency, not the alarm
Output frequency is fixed by how fast the engine turns and how many poles the alternator has. A four-pole machine at 1,500 rpm gives 50 Hz. Nothing else in the generator influences it. So the controller is reporting that the engine lost speed, and the only question is why.
Frequency steady, voltage wrong
Not this page
The fault is on the alternator, AVR or excitation side. The engine has done its part.
- AVR and sensing circuit
- Excitation and rotating diodes
- Windings
- Go to generator voltage problems
Frequency wrong or unstable
This page
The fault is the engine, the fuel or the governor. Any voltage misbehaviour is a consequence, not a cause.
- Load, and load steps
- Fuel delivery under load
- Governor, actuator and linkage
- Engine power, and speed sensing
Under-frequency
Six causes, ranked by how often they are the answer
- More load than the engine can carryThe commonest cause and the cheapest to check. A genuine overload against the nameplate, or a load step the set cannot absorb in one go — a large motor started direct on line, a chiller cutting in, a fire pump. The engine slows before the governor can add fuel, and if the dip goes below the trip point the controller shuts the set down. That is not a fault: it is a set being asked to do something outside its capability. And high ambient temperature reduces what an engine can actually produce, so a set that carried its load comfortably in February may be marginal in August against exactly the same load.
- Fuel starvationThe engine is trying to make the power and cannot get the fuel to do it. Blocked primary or secondary filters, a restricted supply line, a failing lift pump, a partly blocked tank outlet, air drawn in on the suction side, or contaminated fuel. The symptom is characteristic: fine at no load and on light load, drops speed the moment real load arrives. If your set passes its no-load test start every month and trips on under-frequency whenever it is actually needed, look here first.
- Governor or actuator faultsThe governor is what adds fuel when the engine slows. Worn linkage, a sticking fuel rack, a slow or seized electronic actuator, a failed speed reference, or a governor controller out of adjustment will all fail to respond quickly enough. On an electronically governed engine the same job is done inside the ECU, and a fault there gives the same symptom — see ECU warning lights.
- The engine cannot make its rated powerWorn injectors, a turbocharger not making boost, a blocked air filter, poor compression, or an exhaust restriction. The set runs, holds speed at light load, and simply runs out of engine when asked for more. Black smoke alongside the under-frequency alarm points firmly here.
- Speed sensingThe controller reads engine speed from a magnetic pickup on the flywheel ring gear, or from the alternator output frequency. A pickup with a wrong air gap, a chafed screened cable, or a bad earth will report a speed the engine is not running at — and the set is shut down on a fault it does not have.
- The trip setting itselfTrip points and time delays are configurable. Set too close to normal running speed, or with too short a delay, and the controller trips on a load step the set actually rode out perfectly well. Sets that trip on under-frequency with no mechanical cause are sometimes configured to.
Over-frequency
The same subject from the other direction
Why speed goes high
- Load rejection. A large load disconnects suddenly, the engine is still being fuelled for it, and speed overshoots before the governor pulls the fuel back. Momentary and usually harmless, unless the trip setting is tight.
- Governor or actuator failure in the fuelling direction. A sticking rack, a failed actuator or a failed speed reference leaving the engine over-fuelled. This is the serious one, because its extreme is an overspeed.
- Speed setting wrong. Someone adjusted the no-load speed and never checked it on load, or a replacement governor controller was fitted without setting it correctly.
- Speed sensing again. A pickup reading extra pulses, or a controller configured for the wrong number of flywheel teeth, reports a speed higher than the engine is running.
An overspeed shutdown means the engine ran away. Do not reset and restart it. A governor or fuel system fault that has run away once will do it again, an overspeeding diesel can destroy itself in seconds, and it can throw parts. That is a stop-and-call, without exception — +971 50 767 9050.
The third case
Hunting: the frequency that will not settle
A set whose frequency swings continuously without settling is hunting, and its causes are different again.
- Governor gain or stability out of adjustment — the governor is over-correcting and chasing itself
- Worn or sloppy linkage, with lost motion between actuator and fuel rack
- Air in the fuel system, giving inconsistent fuelling cycle to cycle
- A sticking rack or injector
- Interaction with the load — some electronic loads and some AVR settings can excite an oscillation
If the frequency is moving, the governor is the fault
Hunting is frequently misdiagnosed as an AVR fault, because the voltage swings visibly along with the frequency. It does not cause it — the voltage is following the speed. Adjusting the AVR on a hunting set makes things worse.
Before you call
What you can safely check yourself
None of this requires opening a live enclosure.
From the panel, in a minute
- Read frequency and load together, at no load and again with the site load on. Two numbers, thirty seconds, and they decide most of the diagnosis
- Compare the load in kW against the nameplate rating — free, and often the answer
- Check the trip settings, if you have access, against the engine's rated speed. A trip point sitting very close to normal running speed will trip on normal transients
From watching it work
- Identify what starts when it trips. If the shutdown always coincides with a particular motor, compressor or pump, you have found it
- Check the fuel filter service history; drain the water separator and look at what comes out
- Note the pattern — only under load, only when hot, only in the afternoon, only when a specific machine runs
- Look for smoke on load acceptance. Heavy black smoke with a speed drop points at air, injectors or turbocharger rather than the governor
You can check the governor linkage for free movement and the actuator for obvious mechanical seizure — but do not adjust anything. A governor set by guesswork on a running machine is how an overspeed happens.
Past the panel
What needs a technician, and why
The argument this page settles
Is it the set, or is it the load?
It cannot be settled by argument. It is settled by measurement.
What we do is establish what the engine can actually deliver under controlled load, and what the site is actually asking of it, and then compare the two. One of three answers comes out — and we would rather tell you the second one, and lose the repair, than sell you a governor overhaul for a problem that is a chiller starting across the line.
Being wrong about which of the three you have is expensive in both directions. Tell us what it is doing and we will say which we think it is before anyone travels.
- The engine is down on power
Fuel, air, injection or mechanical condition. That is a repair, and it is worth doing.
- The set is healthy, the load is beyond it
In total, or in a single step. A staging, soft starter or sizing conversation. No repair will change it.
- The controller is tripping wrongly
Settings or speed sensing. The set and the load are both fine.
Urgency
When it is urgent, and when it can wait
Stop and call now
Any overspeed shutdown. Any under-frequency trip with a knock, heavy smoke, a new noise, or a temperature or pressure alarm alongside it. Any set that is the only supply to a site now dark — emergency generator repair.
Deal with it this week
Repeated under-frequency trips on load with no other symptom. The set is telling you it cannot do the job it exists to do, and it will demonstrate that during the next power cut.
Plan it
A single trip on a known large motor start, with the set otherwise healthy. A load staging or sizing conversation rather than a repair — worth having properly rather than resetting it every time.
Never
Raise a trip setting to stop the alarm appearing. The trip protects the alternator and the connected load from running at the wrong frequency, and moving the goalposts leaves the fault in place with the protection removed.
Before you phone
Four things that narrow it a long way
Call or WhatsApp +971 50 767 9050 with these and most of the diagnosis is done before anyone travels. Planned work and quotations: jacob@abradiesel.com.
- The load in kW at the moment it trips
- The nameplate rating of the set
- The exact alarm text
- A photograph of the event log — which alarm came first
Common questions
Frequency faults, answered
What does under frequency shutdown mean on a generator?
That the engine speed dropped below the controller's trip point for longer than its time delay, so the set was shut down to protect the alternator and the connected load. Frequency is engine speed, so the cause is always on the engine, fuel or governor side — never the AVR.
Why does my generator trip on under frequency when the load comes on?
Because the engine cannot deliver, or cannot deliver quickly enough. Either the load or the load step is beyond what the set can take, or the engine is being starved of fuel by blocked filters or a restricted supply, or the governor is not responding fast enough. Check the load against the nameplate first, then the fuel filters, then the governor.
How do I fix a generator under frequency problem?
Establish which of three things it is: too much load, not enough fuel reaching the engine, or a governor that is not responding. Reduce or stage the load and see if the trip goes away — that is free. Change the fuel filters and drain the water separator. If it still trips with a reasonable load and clean fuel, the governor, the injection equipment or the engine's mechanical condition needs measuring properly.
What causes over frequency on a generator?
Most often a sudden load rejection, where the engine overshoots before the governor can pull the fuel back. Otherwise a governor or actuator fault leaving the engine over-fuelled, a speed setting that was never checked on load, or a speed sensing error. An over-frequency alarm is a nuisance; an overspeed shutdown is not, and must never be reset and rerun.
My generator's frequency keeps swinging up and down. Is that the AVR?
No. If the frequency is moving, the governor is the fault. The voltage swinging along with it is following the speed, not causing it. Adjusting the AVR on a hunting set makes it worse. Look at governor stability settings, linkage wear, air in the fuel, and a sticking fuel rack.
Can I just raise the under frequency trip setting to stop the shutdowns?
No. The trip is protecting the alternator and everything connected to it from running at the wrong frequency. Widening it leaves the fault in place and removes the protection. If the trip setting is genuinely configured too tightly for the engine and load, that is a correction made deliberately with the engine's specification in front of you — not a way of silencing an alarm.
Tripping on under frequency with a site to carry?
Call or WhatsApp with the load in kW, the nameplate rating and the alarm text. If it was an overspeed, do not restart it. Open 7 days, 07:00–20:00.