AVR and voltage regulation
Generator AVR Faults Diagnosed, Repaired or Replaced
We repair and replace generator AVRs (automatic voltage regulators) on site across Gauteng. Before condemning the AVR, we check engine speed, residual voltage, the exciter, the rotating diodes and the sensing wiring, because a fault in any of them can look exactly like a failed AVR. Then we fix the real cause, fit a regulator that matches your alternator if one is needed, and set it up under load.
- No voltage, low or high voltage, or voltage that hunts
- Speed, residual voltage, exciter field and diode checks
- Replacement AVRs matched to the alternator make and model
- Voltage and stability set, then proven under load
Established 2006
20 years of experience
24/7 operations
We Arrive Within 1–2 Hours
(usually within 1 hour)
Free callouts and quotations
One-year warranty
Qualified and certified technicians
We Service & Repair All Generator Brands
All generator makes and models
1 kVA–20,000 kVA capability
Signs of an AVR fault, and what else causes them
| Symptom | Likely causes | Repair route |
|---|---|---|
| No output voltage, engine running normally | Likely causesFailed AVR or its fuse, lost residual magnetism, open exciter field, failed rotating diodes, tripped output breaker | Repair routeRunning but no power |
| Low voltage at every load | Likely causesEngine running slow so the AVR rolls the voltage back, a drifted voltage setting, a weak AVR or exciter | Repair routeVoltage problems |
| High voltage, lights unusually bright | Likely causesBroken or disconnected sensing lead, a failed AVR driving full excitation, a wrong setting. Switch off sensitive electronics first | Repair routeVoltage problems |
| Voltage hunts, lights pulse or flicker | Likely causesGovernor hunting, AVR stability set wrongly, a loose sensing or supply connection, a failing AVR | Repair routeFrequency and speed problems |
| Voltage collapses when load is switched on | Likely causesShorted rotating diode, overload, a self-excited AVR losing its supply, engine bogging down | Repair routeVoltage problems |
| Voltage builds for a few seconds, then drops away | Likely causesThe AVR’s over-excitation protection cutting out because of a diode or winding fault | Repair routeWinding tests and rewinding |
| Over- or under-voltage alarm on the controller | Likely causesA real voltage fault, or a sensing or setting fault on the controller itself | Repair routeControl panel repairs |
Check this first
Speed first: why we check the engine before the AVR
An alternator’s output depends on how fast it turns. A four-pole alternator has to run at 1,500 rpm to give 50 Hz, and many AVRs include under-frequency roll-off: when the speed falls below a set point, the AVR deliberately lowers the voltage to protect the exciter and help the engine recover. A worn governor, a blocked fuel filter or an overloaded engine therefore shows up as low voltage, and a new AVR will not fix it.
It works the other way too. If the governor hunts, the voltage hunts with it, and the AVR stability trimmer gets turned in an attempt to cure what is really an engine fault. So the first measurement on every AVR callout is frequency, at no load and under load. If it is wrong, the speed problem is repaired first.
- 50 Hz needs 1,500 rpm on a four-pole set
- Low speed lowers the voltage by design
- A hunting governor looks like a hunting AVR
Diagnosis
How we prove an AVR fault
- 01
Speed and frequency
Measured at no load and with load applied. The AVR cannot hold the voltage on an engine that will not hold its speed.
- 02
Residual voltage
With the AVR disconnected, a healthy alternator still gives a small voltage from the magnetism left in its exciter and rotor iron. On a self-excited set, if that has gone the AVR has nothing to build from, and the exciter field is re-magnetised with a controlled DC flash rather than a new AVR being fitted.
- 03
AVR output to the exciter
If the AVR is sending field voltage but the alternator gives little or nothing, the fault lies downstream in the exciter, diodes or windings. If it sends nothing, the AVR, its supply or its sensing is suspect.
- 04
Sensing and supply wiring
A loose or broken sensing lead makes the AVR see low voltage and drive the output dangerously high. The supply terminals, fuse and the auxiliary winding or PMG feed are checked as well.
- 05
Rotating rectifier
The diodes and the surge suppressor on the rotor are tested one by one. A single shorted diode makes the AVR work far harder and is a common reason a new AVR fails soon after fitting.
- 06
Windings
Insulation and resistance tests on the exciter and main windings. A burnt or earthed winding means rewinding, not a new regulator.
Repair it, reset it or replace it?
| What the tests find | What we do |
|---|---|
| Voltage or stability setting drifted or changed | What we doReset voltage, stability and roll-off with the set at rated speed, then check under load |
| Loose, corroded or broken sensing or supply connection | What we doRepair the connection and re-test regulation; the AVR itself is kept |
| Residual magnetism lost after a long standstill or a fault | What we doRestore it with a controlled field flash, then confirm the alternator builds up voltage on its own |
| AVR failed, with diodes and windings healthy | What we doFit a replacement matched to the alternator and set it up |
| AVR failed because a rotating diode shorted | What we doReplace the diode set and the AVR together, otherwise the new AVR fails too |
| AVR damaged by lightning or a surge | What we doReplace it and check the rest of the excitation circuit for damage |
| Exciter or main winding burnt or earthed | What we doArrange an alternator rewind, quoted first, and replace the AVR if it was damaged too |
Lightning damage is most common in the Highveld summer storm season. A set that stands idle for months between outages is also more likely to have lost its residual magnetism.
Matching a replacement AVR to the alternator
AVRs are not interchangeable. The replacement has to suit the alternator’s excitation system (powered from the main output, from an auxiliary winding, or from a permanent magnet generator), its sensing (one phase or all three), the exciter field’s voltage and current, and the set’s voltage and frequency. A regulator that is wrong on any of these can leave the voltage hunting, refuse to build up, or overheat the exciter.
We identify the alternator from its rating plate: the make, the model or frame size and the serial number. Stamford, Leroy-Somer and Mecc Alte alternators are common on standby sets in South Africa, and each maker lists which regulators suit which machines. Smaller and imported sets often carry other alternators, and their AVRs are matched on the same electrical ratings.
Generic “universal” AVRs exist and can work, but only when their ratings genuinely fit the machine. Some installations need more: sets that run in parallel need a droop input, fed by a current transformer, so they share reactive load, and unbalanced or sensitive loads such as a server room are better served by three-phase sensing. The replacement has to support whatever the original did.
Setting up and testing under load
A new or repaired AVR is set with the engine at rated speed: voltage first, at no load, then stability, so the voltage settles quickly after a change without overshooting or hunting. Where the AVR has under-frequency roll-off, its setting is checked so the AVR protects the alternator without dropping the voltage on every normal load step.
Then the set is loaded. We apply load in steps, using the building’s own loads where the site allows, and check that the voltage holds on every phase, dips only briefly when a large motor such as a borehole pump or compressor starts, and recovers cleanly. Where full-load proof is needed, a load bank test can be arranged and quoted.
During spells of load shedding, sets run every day, often lightly loaded late at night or while a house or office is empty. A regulator that is slightly unstable at light load then shows up as lights that pulse on every outage, so light-load stability is checked as carefully as full load.
FAQ
Frequently asked questions
How do I know if my generator’s AVR has failed?
Typical signs are no output with the engine running normally, voltage that is clearly too high or too low, or lights that pulse with the load. Speed, diode and wiring faults cause the same symptoms, so the AVR is tested before it is replaced. Our guide to what an AVR does explains why.
Can an AVR be repaired, or does it have to be replaced?
Settings and connections can be corrected, and lost residual magnetism restored, without a new AVR. When the electronics inside a sealed modern AVR fail, it is replaced. What matters most is finding why it failed, so the new one survives.
Why did my new AVR fail so quickly?
Usually because the original cause was never found: a shorted rotating diode, a winding fault, a loose sensing lead or a regulator that does not match the alternator. That is why we test the whole excitation circuit before and after fitting one.
Can a cheaper universal AVR be used instead?
Sometimes, if its ratings genuinely match the alternator’s excitation system, sensing and field current. A wrong one can fail on start-up or damage the exciter, so we check the ratings before fitting one. Please do not buy and fit one yourself: the work is inside a live terminal box.
Can a faulty AVR damage my appliances?
Yes. High or unstable voltage can damage electronics, chargers and motors. If you notice flickering lights or equipment behaving oddly on generator power, switch off sensitive loads and call us.
Is AVR repair the same as rewinding?
No. AVR work deals with the regulator and its circuit. If the tests show a burnt or earthed winding, the alternator needs rewinding, which is a separate job and quoted on its own.
Do you work on all alternator brands?
Yes. We Service & Repair All Generator Brands, including the alternators fitted to them.
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