Generator problems
Low, High or Fluctuating Voltage from Your Generator
The short answer
Wrong voltage from a generator comes from one of four places: the excitation system (the AVR, its sensing and the rotating diodes), the engine’s speed, the load, or the wiring between the set and your equipment. When it happens (all the time, only under load, or each time a motor starts) and whether every phase is affected tells you which. Until it is fixed, switch off computers, televisions and chargers, and reduce the load.
- Note whether the voltage is low, high, swinging or collapsing
- Note whether it changes with load or when motors start
- On three-phase sets, compare all three phase readings
- Check the frequency too: a speed fault shows up as a voltage fault
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What the voltage reading points to
| Symptom | Likely causes | Repair route |
|---|---|---|
| Low at every load, frequency normal | Likely causesAVR voltage setting drifted, a weak AVR or exciter, a failed rotating diode, a poor connection in the AVR supply or exciter leads | Repair routeAVR tests and set-up |
| Low, and the frequency is low too | Likely causesThe engine is running slow and the AVR is lowering the voltage on purpose: a governor or fuel fault, or overload | Repair routeFix the engine speed first |
| Collapses when load is switched on | Likely causesOverload, a shorted rotating diode, a self-excited AVR losing its supply on a heavy load step, an engine bogging down | Repair routeLoad, diode and excitation tests |
| Deep dip when a pump, compressor or air conditioner starts | Likely causesMotor starting current too large for the set, or slow AVR or governor response | Repair routeSizing and motor starting |
| High voltage, lights unusually bright | Likely causesBroken or loose sensing lead, a failed AVR driving full excitation, a wrong setting, or power-factor-correction capacitors left in circuit at light load | Repair routeSwitch off, then AVR and wiring checks |
| Voltage swings, lights flicker or pulse | Likely causesEngine speed hunting, AVR stability set wrongly, a loose connection, or a large load cycling on and off | Repair routeSpeed and stability checks |
| One phase high, another low | Likely causesBadly unbalanced single-phase loads, single-phase AVR sensing, or a loose or broken neutral (dangerous) | Repair routeSwitch off, then call us |
| Correct at the generator, low at a far building | Likely causesVoltage drop in a long or undersized cable to an outbuilding, borehole pump or gate motor | Repair routeCable check by an electrician |
Causes explained
What lies behind each kind of voltage fault
When the voltage is low: AVR repairs
The AVR holds whatever value it is set to, so a drifted setting leaves every reading low while the regulator works perfectly. A poor connection in its supply or exciter leads, or a failed rotating diode, can also leave it unable to reach the set value. More often the engine is to blame: many AVRs deliberately reduce the voltage when speed falls below a set point, to protect the exciter. Fuel starvation or an overloaded engine pulls the speed down, the regulator follows it, and replacing the regulator changes nothing.
When the voltage is high
High voltage does the most damage. If the AVR’s sensing lead breaks or works loose, the regulator sees too little voltage and drives the exciter harder; some AVRs shut down when they lose sensing, but not all. A failed AVR can also stick at full output. On business premises, power-factor-correction capacitors on a lightly loaded set can push the voltage up as well.
When it swings or the lights flicker: Surging and hunting
Rhythmic swinging usually follows the engine: if the governor hunts, frequency and voltage hunt together. If the frequency holds steady while the voltage moves, the AVR stability setting, a loose connection or a load that switches on and off is more likely. The eye picks up even small, rhythmic changes in brightness, so flickering lights are often the first sign.
When it collapses under load
A healthy set dips briefly as load comes on and recovers within a second or two. If it collapses and stays down, the set may be overloaded, the engine may be bogging down, or a shorted rotating diode may be limiting excitation. A self-excited AVR draws its power from the output itself, so a very heavy load step can starve it; sets with a PMG hold up better.
What to do while the voltage is wrong
These steps protect your equipment and give us the readings that point to the cause.
- Switch off computers, televisions, chargers and other sensitive electronics
- Reduce the load and watch whether the voltage settles
- Note the voltage on each phase, and the frequency, from the controller
- Note what was running or starting when it went wrong
- Start large motors such as pumps, compressors and air conditioners one at a time
- Check whether the problem is at every socket or only in one building or circuit
If the frequency is also wrong, the engine’s speed is the likely cause: see generator surging and hunting.
Three-phase sets: unbalanced loads and the neutral
On a three-phase set, a home or office is usually fed as single-phase circuits shared across the three phases. If most of the load ends up on one phase (the kitchen, the geysers and a borehole pump, say), that phase sags while the lightly loaded ones rise. Many AVRs sense only one phase, so they hold that phase correct and let the other two drift. A three-phase-sensing AVR averages all three, but it still cannot correct a badly unbalanced load. Our guide to single- and three-phase power explains the basics.
The cure is to rebalance the circuits across the phases, which is electrician’s work in the distribution board. A loose or broken neutral is more serious: loads on different phases end up in series across 400 V, so the lightly loaded side can see far more than 230 V and appliances fail within seconds.
- Spread single-phase loads across all three phases
- Single-phase sensing hides the other two phases
- Bright and dim rooms at the same time: suspect the neutral
How the type of set and load shedding change the picture
Small single-phase units that use a capacitor instead of an AVR regulate less tightly, so their voltage moves more with load by design; a sudden change in that behaviour points to the capacitor or the windings. Inverter generators produce a stable electronic output, so a voltage problem on one usually means overload or an inverter module fault. Three-phase standby sets are the ones most affected by unbalanced loads and by AVR or sensing faults.
Load shedding adds two patterns of its own. When the changeover transfers, the generator takes everything that was left switched on at once, so fridges, pumps and air conditioners start together and the dip can trip equipment or stall a motor. Later, in the evening, the set may run for hours with little connected, and a governor or AVR that is only just stable at light load shows up as lights that pulse. Mention either pattern when you call, because each points the diagnosis in a different direction.
Diagnosis
How we track down a voltage fault
- 01
Speed before voltage
Frequency at no load and under load. If the engine cannot hold its speed, that is repaired first.
- 02
Measure at both ends
Voltage at the alternator terminals and at the affected equipment separates generator faults from cabling faults.
- 03
Step the load
Readings as load is applied in stages show whether the voltage dips and recovers properly, collapses or swings.
- 04
Check excitation and sensing
AVR settings, sensing and supply wiring, the rotating diodes and the neutral connection.
How each voltage fault is repaired
| Cause found | What the repair involves |
|---|---|
| AVR setting drifted | What the repair involvesReset voltage and stability at rated speed, then confirm under load |
| AVR failed or sensing lead broken | What the repair involvesRepair the wiring, or fit and set up a matching regulator: see AVR repairs |
| Shorted rotating diode | What the repair involvesReplace the diode set, then check the AVR it has been overworking |
| Engine speed low or hunting | What the repair involvesRepair the governor or the fuel system first, then re-check the voltage |
| Unbalanced load | What the repair involvesRebalance circuits across the phases (electrician) |
| Motor starting dips | What the repair involvesStagger starts, fit soft starters on large motors, or size a larger set |
| Cable voltage drop | What the repair involvesLarger cable on the affected circuit |
| Loose or broken neutral | What the repair involvesRepair by a qualified electrician, then check what was damaged |
| Capacitors pushing the voltage high | What the repair involvesArrange for power-factor correction to switch out while on generator power |
Where a repair needs proof at full load, a load bank test can be arranged and quoted.
FAQ
Frequently asked questions
What voltage should a generator give in South Africa?
Nominally 230 V between live and neutral, and 400 V between phases on a three-phase set, at 50 Hz. A few volts of movement as the load changes is normal; most equipment is designed for a supply within about 10% either side of 230 V.
Why do my lights flicker on generator power?
Usually because the engine speed or the AVR is not holding steady, a connection is loose, or a large load is switching on and off. It should be checked, because unstable voltage also stresses electronics.
Can low or high voltage damage my equipment?
Yes. Motors draw more current at low voltage and overheat, and some electronics behave erratically. High voltage is worse: it can destroy chargers, televisions and control boards quickly, sometimes before anyone notices the lights are too bright.
Is a voltage dip when my borehole pump starts normal?
A brief, shallow dip is normal. A deep dip that stalls the pump, trips a breaker or makes lights go out means the generator is struggling with the starting current.
Why is the voltage different on each phase?
Usually because the load is unbalanced across the phases. If the difference is large or appeared suddenly, switch off and have the neutral checked, because a broken neutral destroys appliances.
Can I adjust the AVR to raise the voltage?
No. The trimmers sit in a live terminal box, and low voltage is often a speed or wiring fault that the AVR is correctly responding to. Turning it up hides the cause and can overload the exciter. Our guide to what an AVR does explains why.
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