Unstable voltage
Generator Voltage Fluctuating or Unstable
Voltage that flickers, hunts or surges usually comes from the AVR, unsteady engine speed or a loose connection. That is a different fault from voltage that is steady but too low. We record voltage and frequency together to separate a regulation problem from a speed problem, then check the connections, the load and the alternator’s excitation.
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- Voltage and Hz recorded together
- Engine speed checked with the AVR
- Sensitive equipment kept in mind
- Onsite across Gauteng
Voltage that will not settle
Why does generator voltage fluctuate?
The AVR, the engine speed and the load all affect how steady the voltage stays. If the flicker follows the engine note, the speed is moving. If it follows a vibration or one particular appliance, a connection or the load is the likelier cause.
Steady voltage matters because motors, controls, UPS units and lighting are built for a narrow band, not a supply that swings. We find what is moving and fix it, rather than hiding the swings with an adjustment.
Please do not adjust the AVR yourself. We test the sensing wires and the regulator with instruments, and the alternator terminals stay closed until we make them safe to measure.
Measured, not guessed
What we record while it flickers
Voltage and frequency together
Your equipment is designed for South Africa’s standard 230 V between phase and neutral (NRS 048-2) at a nominal 50 Hz (South African Grid Code). If the voltage swings with the frequency, engine speed is the cause. If it swings on its own, we look at the AVR and the connections.
Motor starts
A motor started direct-on-line typically draws 6 to 7 times its full-load current until it reaches speed (Schneider Electric). A short dip as a large motor starts can be normal, so we look for swings that carry on once the motor is running.
Fault guide
Generator voltage unstable or fluctuating: what it can mean and what to do
Voltage that hunts, surges or flickers usually traces back to the AVR, unsteady engine speed or a loose connection. We record voltage and frequency together to tell them apart before we change parts. 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 |
|---|---|---|
| Lights flicker, voltage swings at a steady engine note | A failing AVR or a loose sensing connection | AVR repairs |
| Voltage and engine note rise and fall together | The engine is hunting: fuel or governor | Surging diagnosis |
| Voltage dips each time a motor or pump starts | Starting loads close to or above what the set can carry | Load and sizing check |
| Unstable after electrical work on the panel | A sensing or control wiring fault | Control panel repairs |
01
What you may notice
- Lights flicker or pulse on generator power
- The voltage reading climbs and falls on the display
- Equipment resets or UPS units keep switching
- The engine sound rises and falls with the flicker
02
Possible causes
Any of these can produce the same symptom. Testing decides which one applies.
- Engine speed hunting from a governor or fuel-supply problem
- An AVR fault, incorrect AVR settings or a sensing fault
- A loose or intermittent connection
- A load that changes rapidly, such as motors cycling
- Alternator excitation faults
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.
- Does the engine sound change at the same time as the flicker?
- Does it happen with no load, or only with certain equipment running?
- What range of voltage and Hz does the display show?
- Is it constant or does it come and go?
04
Stop using the generator and call if you notice
- Sensitive equipment shutting down repeatedly or being damaged
- A burning smell or visible heat at connections
- Engine speed surging strongly
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.
- Voltage and frequency recorded together to separate speed from regulation
- Governor response and fuel supply under load
- AVR stability, settings and sensing
- Connection integrity on each phase, with the supply made safe
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
- Whether the engine sound changes with the flicker
- What equipment was running
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.
- Voltage fluctuation diagnosis and repair
- R1,500 – R5,000
- AVR replacement
- R2,200 – R9,000
Step 9: book a diagnosis visit.
Common Questions
Questions about fluctuating generator voltage
Why are my lights flickering on the generator?
Because the output voltage or the engine speed is not holding steady. We measure both to find which one is moving and why.
Is fluctuating voltage the same as low voltage?
No. Low voltage stays too low, while unstable voltage keeps changing.
Is it always the AVR?
No. Engine speed, a changing load, a loose connection and the alternator’s excitation can all make the voltage swing.
The engine note surges and the lights follow. What does that mean?
Speed and voltage are moving together, so we test the fuel supply and the governor as well as the regulator.
It is steady until a motor starts. Is that a fault?
A brief dip when a large motor starts is normal. A swing that carries on after the motor is running is the fault to book.
Can unstable voltage damage equipment?
It can stress power supplies and controls that expect a steady voltage. That is a good reason to have it diagnosed soon, and to switch off sensitive equipment if it keeps resetting.
Should I keep adjusting the AVR until the flicker stops?
No. Turning the regulator without a meter can leave the voltage too high or still unstable. We check the sensing and the engine speed first.
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
Describe the flicker
Tell us whether it hunts all the time or only when equipment starts, and whether the engine note changes with the lights.
- 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.