Generator problems
Why a Generator Surges, Hunts or Drifts Off 50 Hz
The short answer
A generator surges or hunts when its governor cannot hold the engine at a steady speed, and because the output frequency comes straight from engine speed, the 50 Hz supply swings with it. The usual causes are fuel starvation or air in the fuel, a worn or badly set governor, a sticking actuator or linkage, or a load that is too large or very light. Left alone it upsets clocks, motors and UPS units, and it usually gets worse.
- Read the frequency (Hz) on the controller or meter
- Note whether it hunts with no load, or only under load
- A recent run-dry or filter change often means air in the fuel
- Inverter generators change engine speed by design
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Surging, hunting or sagging: reading the pattern
| Symptom | Likely causes | Repair route |
|---|---|---|
| Rhythmic rise and fall with little or no load (hunting) | Likely causesGovernor gain or droop set wrongly, worn or sticking linkage, a sticking actuator, or very light load on a mechanically governed engine | Repair routeGovernor set-up and linkage repair |
| Surges after running dry, refuelling or a filter change | Likely causesAir in the fuel from a system that was not bled properly or a loose suction-side fitting, or a weak lift pump | Repair routeFuel system repairs |
| Random surges that get worse as the tank runs low | Likely causesPickup drawing air or sludge, a blocked tank vent, water or diesel bug in the fuel | Repair routeFuel contamination checks |
| Speed drops sharply when load comes on | Likely causesOverload, a partly blocked fuel filter or water separator, slow governor response, worn injection components, a blocked air filter | Repair routeLoad, fuel and air checks |
| Steady, but slightly slower under load | Likely causesNormal droop on a mechanically governed engine, if it stays within limits | Repair routeOften normal |
| Small petrol unit surges at no load | Likely causesCarburettor jet partly blocked by stale fuel, an air leak at the intake, a sticking governor spring or linkage | Repair routePetrol generator repair |
| Engine races, then an overspeed shutdown | Likely causesSticking fuel rack or actuator, a governor fault, or a false trip from a faulty speed signal | Repair routeShutdowns and trips |
| UPS or battery inverter rejects generator power | Likely causesFrequency or voltage moving outside the range the equipment accepts, or a distorted waveform from a small set | Repair routeSpeed and voltage checks |
How it works
Speed is frequency: why 50 Hz depends on the engine
A conventional generator’s alternator is turned directly by the engine, so the output frequency is set by engine speed and the number of poles. For 50 Hz, a four-pole alternator must turn at 1,500 rpm, which suits most standby diesels; many small petrol and air-cooled units use a two-pole alternator at 3,000 rpm. Any change in speed is a change in frequency, which is why surging shows up on the hertz reading before anything else.
Mechanical governors are set with a little droop: the speed sits a few percent high with no load and falls towards 50 Hz as load rises. Electronic governors and engine control units can hold the speed constant at any load (isochronous control). Inverter generators work differently again: the engine speeds up and slows down with the load, and the electronics produce a steady 50 Hz regardless. Our guide to voltage and frequency basics explains the relationship in more detail.
- Hz = rpm × poles ÷ 120
- 1,500 rpm for 50 Hz on a four-pole set
- Droop is normal; hunting is not
Mechanical, electronic and inverter speed control
| Speed control | How it holds speed | Where it goes wrong |
|---|---|---|
| Mechanical governor | How it holds speedFlyweights in the engine or injection pump move the fuel rack (or the throttle on a petrol engine) through a spring and linkage | Where it goes wrongWorn flyweights and pivots, a tired spring, sticking or loose linkage, wrong speed or droop setting |
| Electronic governor | How it holds speedA magnetic speed pickup reads the flywheel teeth, and a governor controller drives an actuator on the fuel pump | Where it goes wrongDirty or badly gapped pickup, sticking actuator, loose wiring, gain or stability set wrongly |
| Engine control unit | How it holds speedThe engine’s own electronics read crank and cam sensors and meter the fuel electronically | Where it goes wrongSensor and wiring faults, low fuel pressure, configuration; the unit usually logs a fault |
| Inverter generator | How it holds speedEngine speed follows the load; the inverter module makes 50 Hz electronically | Where it goes wrongAn unsteady engine at a constant load points to fuel or throttle control; wrong output frequency points to the inverter module |
Causes explained
Where surging and hunting come from
Fuel starvation: Fuel system repairs
A governor can only ask for more fuel; it cannot deliver fuel the system will not supply. Partly blocked filters, a clogged water separator, a weak lift pump or a restricted tank pickup starve the engine as demand rises, so it sags and recovers in surges.
Air in the fuel
Running the tank dry, a filter change that was not bled properly, or a loose fitting on the suction side lets air in. Bubbles reaching the injection pump give random surges and can stall the engine.
Governor and linkage wear
Free play in the linkage, worn flyweight pivots or a weak spring make the governor late to respond, so it overcorrects in both directions. That is the classic rhythmic hunt.
Actuator and speed pickup
On electronically governed sets, a sticking actuator or a dirty, loose or badly gapped speed pickup gives erratic speed, overspeed trips or a shutdown for loss of speed signal.
Very light load
With almost nothing connected, the governor works near its minimum fuel position, where linkage friction and small settings matter most. Long evening runs at light load during load shedding are a common trigger, and they also cause wet stacking.
Overload and big load steps: kVA sizing guide
A load beyond the set’s rating, or several motors starting at once, pulls the speed down faster than the governor can recover. Black smoke with a falling frequency usually means overload.
Why the frequency matters in your building
Mains power in South Africa is 50 Hz, and a lot of equipment assumes it. Induction motors in pumps, compressors and fans run faster or slower with the frequency, and low frequency at full voltage makes motors and transformers run hot. Clocks on ovens, microwaves and some timers count the supply’s cycles, so they gain or lose time on a generator that runs off speed.
A frequent complaint during load shedding is a UPS or battery inverter that refuses the generator. Most UPS units, and the backup and hybrid inverters many homes now have, only accept input within a set window of frequency and voltage. A hunting generator drifts in and out of that window, so the UPS keeps switching to battery, runs it down and may drop the load. Some units have a sensitivity or generator setting, described in their manual or set by their installer, that widens the window. That can help with a healthy set’s normal droop, but it will not cure a generator that is genuinely hunting.
Checks you can make without opening anything up
- The frequency at no load and again with load applied, from the controller or meter
- Whether the surging is rhythmic (usually the governor) or random (usually fuel or electrical)
- Whether it eases when you switch off large loads
- The fuel level, and whether the set recently ran dry, was refuelled from containers or had a filter change
- The air filter restriction indicator, if one is fitted, with the engine stopped
- Any alarms on the controller, and the hour-meter reading
Tell us whether the set has been running long evening hours on light load. It changes where we look first.
Repair options for a surging engine
| Cause found | What the repair involves |
|---|---|
| Fuel restriction or air leak | What the repair involvesReplace filters, drain the water separator, find and fix the leak, bleed the system and test lift pump delivery |
| Contaminated fuel | What the repair involvesDrain water and sludge, arrange cleaning or replacement of the fuel, and change the filters |
| Governor out of adjustment | What the repair involvesSet speed, droop and gain so the engine holds steady from no load to full load |
| Worn linkage or mechanical governor | What the repair involvesRepair or replace the worn parts, then set the governor up again |
| Sticking actuator or faulty speed pickup | What the repair involvesClean, re-gap or replace, and check the wiring and governor controller |
| Carburettor fault (petrol units) | What the repair involvesClean or replace the carburettor and refill with fresh fuel |
| Overload or large load steps | What the repair involvesStagger motor starts, move loads, or size a larger set |
| Long runs at very light load | What the repair involvesPlan load during long runs and test under load; a load bank test can be arranged |
FAQ
Frequently asked questions
Why is my generator surging?
Most often because the engine is not getting a steady supply of clean fuel: a blocked filter, air in the lines or a weak lift pump. A worn or badly set governor, a sticking actuator and very light or very heavy loads are the other common causes. Rhythmic surging points to the governor; random surging points to fuel.
Why does my UPS switch to battery when the generator is running?
Most UPS units only accept input within a set range of voltage and frequency. A generator whose speed hunts, or whose voltage swings, falls outside that range and the UPS rejects it. Stabilising the generator usually solves it, although some small sets produce a waveform that sensitive units reject even at a steady speed.
Is it normal for the engine to slow slightly under load?
On mechanically governed engines, a small drop is normal and designed in. A large drop, or one that keeps getting worse, is not.
What frequency should a generator run at?
50 Hz in South Africa. A four-pole set needs 1,500 rpm for that and a two-pole set 3,000 rpm. Mechanically governed sets run slightly above 50 Hz with no load and settle close to 50 Hz at full load.
Why does my inverter generator speed up and slow down?
Most inverter generators have an economy mode that runs the engine only as fast as the load needs. The output frequency stays at 50 Hz because the inverter produces it electronically.
Can surging damage the generator?
It can. A hunting engine wears the governor and linkage, the AVR works harder to follow the changing speed, and running short of fuel or on air can damage an injection pump that relies on diesel for lubrication. Connected motors and electronics suffer too.
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