Frequency
Generator Low Frequency & Frequency Faults
A low or wandering Hz reading usually means the engine is not holding its correct speed, because on most generators frequency follows engine speed. The governor, the fuel supply and the load are the usual causes. We also rule out a controller or sensing fault that shows the wrong number, and we set the speed against a meter, never by turning the governor until the display looks right.
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- Speed and Hz measured together
- Fuel and load checked
- Governor set against a meter
- Onsite across Gauteng
The hertz reading
What does low generator frequency mean?
It means the frequency on the display is below what your set should hold, which is 50 Hz on a South African machine. Because frequency is tied to engine speed, a low reading usually means a slow engine. We still confirm it, because a sensing or controller fault can show a low number while the engine runs at the right speed.
A reading that is fine with no load and falls when equipment connects is a different job from one that is low all the time. Under load, we look at fuel delivery and at whether the set is simply too small for the starting current, as well as at the governor.
A zero on the frequency display while the engine is clearly running points to the sensing circuit. We note what the screen shows, then check it against the engine’s real speed.
Measured, not guessed
The figures we check your set against
Frequency: 50 Hz
South Africa’s grid runs at a nominal 50 Hz (South African Grid Code, Network Code), so the equipment in your building expects 50 Hz from the generator too. We measure the output frequency off load and on load.
Engine speed: 1,500 or 3,000 rpm
Frequency is locked to speed by the number of alternator poles: speed in rpm equals 120 × frequency ÷ poles (Oriental Motor). For 50 Hz, a four-pole alternator needs 1,500 rpm and a two-pole alternator needs 3,000 rpm, so we check the engine against the right figure for your set.
Fault guide
Generator low or unstable frequency: what it can mean and what to do
On most generators the Hz reading follows engine speed, so a low or wandering reading usually points to the governor, the fuel supply or the load. We check those first. This guide separates what you can safely notice from what a qualified technician tests. One symptom never proves that a particular part has failed.
Symptom, likely cause and repair
Find the line closest to what you see. The cause is the most likely one, not a diagnosis: testing confirms it before any part is replaced.
| What you see | Likely cause | Usual repair |
|---|---|---|
| Below 50 Hz and the engine sounds slow | Speed set too low, or a governor or actuator fault | Generator fault finding |
| Frequency falls when the load comes on | Fuel restriction or overload slowing the engine | Fuel system repairs |
| Display reads low but the engine sounds right | A speed or frequency sensing fault | Controller repairs |
| An inverter or UPS rejects the generator supply | Frequency or voltage outside the device’s limits | Unstable voltage diagnosis |
01
What you may notice
- The display shows less than the normal frequency (50 Hz in South Africa)
- Frequency wanders up and down
- Frequency drops when load comes on and recovers slowly
- Clocks, motors or UPS units behave strangely on generator power
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- The engine not reaching rated speed
- A governor or actuator fault
- Fuel restriction that stops the engine holding speed under load
- Overload for the size of the set
- A controller or sensing fault misreporting frequency
03
What you can safely check
Look and note only, from outside the canopy. Do not open panels, covers, fuel lines or the cooling system.
- What Hz reading does the display show, off load and on load?
- Does the engine sound slower than normal?
- Is the voltage low at the same time?
- Did it start after a fuel delivery or a filter change?
04
Stop using the generator and call if you notice
- Motors overheating or failing to start
- Engine speed surging or hunting strongly
- Black smoke with the engine labouring
If it is safe to reach, stop the generator with its normal stop control or emergency stop, keep people away, and call a technician.
05
What a qualified technician typically tests
These are technician checks, not steps to try yourself. Several involve live electrical measurement, fuel or pressurised systems.
- Frequency against measured engine speed
- Governor and actuator response to load changes
- Fuel delivery under load
- Controller frequency sensing
06
Common repair categories
The repair follows the test result. These are the services this symptom most often leads to.
07
What to send when you book
- Your suburb and how we reach the generator
- Make, model and kVA from the data plate (a photo is ideal)
- Fuel type: diesel, petrol or gas
- The Hz reading off load and on load
- What was connected at the time
08
Related faults and services
Not sure which symptom fits? Try the generator symptom finder.
Safety before you check anything
- Exhaust contains carbon monoxide, which you cannot see or smell. Run a portable generator outdoors only, away from doors, windows and vents, and never run any generator in a house, garage or other enclosed space.
- A standby generator set to automatic can start at any moment, when the mains fails or on its exercise timer. Keep the canopy closed and stay clear of fans, belts and the exhaust.
- Generator output, the changeover and the control panel can be live, and some parts hold stored electrical energy after shutdown. Leave panels, covers and wiring closed for us to open.
- The engine, exhaust and coolant stay hot after running. Never open a radiator or coolant cap on a hot engine.
- Do not refuel a running or hot generator. If you smell gas at a gas-fuelled set, do not operate switches; leave the area and contact your gas supplier.
Carbon monoxide guidance: US Consumer Product Safety Commission: Generators and engine-driven tools (carbon monoxide).
Send the details on WhatsApp
This opens WhatsApp with a short message already typed: brand, model, capacity, symptoms and suburb. Fill in what you know and send it. Blank lines are fine.
- The data-plate photo is added inside WhatsApp after the chat opens: tap the attachment or camera icon and choose the photo. A link cannot attach the photo for you.
- Take the photo only when the generator is switched off and has cooled down. Do not reach over or into a running or hot generator, and keep away from the exhaust.
- A standby generator set to automatic can start by itself when the mains supply fails. If you are not sure how it is set, photograph the plate from a distance or send a photo of the whole set instead.
Typical repair costs for this fault
Published ranges, last reviewed October 2026. The callout is R400 – R700 in business hours and R700 – R1,400 after hours, and it is waived if you accept the quote. Your written quote after diagnosis is the price that applies.
- Fuel filter replacement
- R300 – R800
- Fuel system cleaning and polishing
- R1,500 – R3,500
- Voltage fluctuation diagnosis and repair
- R1,500 – R5,000
Step 9: book a diagnosis visit.
Related Pages
Load, voltage, fuel and the controller
Common Questions
Questions about low generator frequency
What does low frequency on a generator mean?
It means the Hz reading is below the 50 Hz your set should hold. Usually the engine is turning too slowly, and we confirm that with a speed reading rather than assuming it.
Is frequency the same as voltage?
No. Frequency follows engine speed, while voltage is the strength of the electrical output. They can fail together or separately, so we measure both.
The Hz falls only when I connect equipment. Why?
The engine cannot hold its speed under that load. Fuel restriction, overload and the governor are the usual causes.
Can I adjust the governor myself?
No. Changing the speed without proper readings can leave the set running fast or still unstable, and some linkages are dangerous to touch while the engine runs. We make any adjustment as part of the measured repair.
The display shows zero Hz but the engine is running. Is it stopped?
No. If the engine is clearly running, a zero reading points to the sensing circuit or the controller input. We test that separately from the engine speed.
Low frequency and low voltage arrived together. Which do you fix first?
Engine speed. Raising the voltage on the AVR while the engine is slow does nothing for the frequency, and it can leave the voltage too high once the speed is fixed.
The set shuts down when the Hz drops. Is that a frequency fault or a trip?
Often both. The protection stops the engine because the speed collapsed, so we look for the reason the speed fell.
What should I send with a low frequency reading?
Send the Hz figure if you can see it, say whether the engine sounds slow, and tell us whether it only happens under load.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Send us the Hz reading if you have it
Tell us whether the engine sounds slow, and whether the number only drops once equipment is connected.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.