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How a Generator’s Automatic Voltage Regulator Works
An AVR (automatic voltage regulator) keeps a generator’s output voltage steady, normally at 230 V phase to neutral, whatever the load does. It measures the output and varies a small DC current to the exciter, which sets how strongly the main rotor is magnetised and so how much voltage the alternator produces. When the AVR, or anything else in that excitation chain, fails, the voltage disappears, runs high or hunts.
Technical guide · Last reviewed 16 October 2026
The control loop
How the AVR controls the voltage
- 01
Sense
Sensing wires carry the output voltage back to the AVR, from one phase or all three.
- 02
Compare
The AVR compares it with the voltage it is set to hold.
- 03
Adjust
It raises or lowers the DC current it sends to the exciter field.
- 04
Excite
The exciter’s output, rectified by the rotating diodes, sets the main rotor’s magnetism and so the output voltage.
Inside a brushless alternator
Most modern generator sets use brushless alternators. The AVR feeds DC to the stationary exciter field. The exciter rotor, on the main shaft, produces AC, which a rotating rectifier of diodes, guarded by a surge suppressor, turns into DC for the main rotor. The spinning, magnetised main rotor induces the output voltage in the main stator.
Each stage amplifies the one before, so an AVR handling a few amps can control hundreds of kVA. It also means a fault anywhere along the chain, from a shorted diode to a damaged exciter winding, looks like an AVR fault from outside.
Building voltage from residual magnetism
When a self-excited set starts, its AVR has no power yet. The alternator’s iron, chiefly the poles of the exciter field, keeps a little residual magnetism from its last run, enough for a few volts at the output. The AVR uses those few volts to start feeding the exciter, the voltage builds on itself, and within a second or two the set reaches its rated voltage.
If the residual magnetism is lost, after a long standstill, a fault or work on the alternator, the engine runs normally but the voltage never builds. A technician restores it by flashing the exciter field with a controlled DC supply; done wrongly, it can destroy the AVR. Running but no power covers the other causes of that symptom.
Three ways an AVR is powered
| Excitation system | AVR power comes from | How it behaves |
|---|---|---|
| Self-excited (shunt) | AVR power comes fromThe alternator’s own output | How it behavesSimple and common. Needs residual magnetism to build up, and loses supply when the voltage dips, so it recovers more slowly from motor starts. |
| Auxiliary winding | AVR power comes fromExtra windings in the main stator | How it behavesIts supply holds up better during load steps and faults, so the voltage recovers faster. |
| Permanent magnet generator (PMG) | AVR power comes fromA small permanent-magnet generator on the shaft | How it behavesIndependent of the output voltage, so it gives the best motor-starting response and sustains fault current for the breakers. Common on larger sets. |
Stamford, Leroy-Somer and Mecc Alte alternators each come with more than one of these systems across their ranges, so the excitation type is read from the alternator’s nameplate and model, not assumed from the brand. A replacement AVR must match it.
Sensing and over-excitation protection
Smaller AVRs sense one phase; larger ones sense all three and average them, which regulates better when loads are unbalanced. If a sensing lead breaks or comes loose, the AVR sees low or no voltage and drives the excitation to maximum, giving a dangerous overvoltage.
Many modern AVRs include over-excitation protection that cuts the field after a short delay when excitation stays too high, whether from lost sensing, a shorted diode or a winding fault. That is why some faults show as voltage that builds for a few seconds and then drops away.
Under-frequency roll-off: why voltage falls when the engine slows
An AVR trying to hold full voltage on a slowing engine would push ever more current into the exciter, overheating it and loading the engine harder still. So most AVRs have a knee point a little below rated frequency, below which they lower the voltage in proportion to speed. That protects the exciter and lets the engine recover after a big load step.
The consequence for diagnosis: low voltage under load may come from an engine that cannot hold speed, through overload, a blocked fuel filter or a governor fault, with the AVR doing exactly what it should. Voltage and frequency basics explains the speed side.
Settings
What the adjustments do
Voltage
Sets the voltage the AVR holds: 230 V phase to neutral, or 400 V between phases on a three-phase set.
Stability
Sets how quickly the AVR responds. Too fast and the voltage hunts; too slow and it recovers sluggishly after a load change.
Roll-off knee
Sets the frequency below which the AVR starts lowering the voltage, matched to the set’s rated frequency.
Droop
Used when sets run in parallel so they share reactive load evenly. A set that runs alone usually has little or none.
Portable generators
AVR, capacitor or inverter on small generators
Many portable petrol generators carry an AVR label, like the one on this unit’s tank, meaning a small electronic regulator controls the excitation. Cheaper units often use a capacitor instead: simple and robust, but the voltage wanders more with load and speed. Inverter generators have no AVR at all; electronics build the 230 V, 50 Hz output, which is why they suit laptops and chargers.
See portable and inverter generators for how these units differ.
Why AVRs fail
| Cause | What happens |
|---|---|
| Failed rotating diode | What happensThe AVR has to drive much harder to hold voltage, and overheats |
| Lightning and surges | What happensDamage to the AVR’s electronics, a particular risk in the Highveld summer storm season |
| Sustained overload or low engine speed | What happensThe AVR runs at its limit for long periods |
| Loose or broken sensing wire | What happensThe AVR sees low or no voltage and drives excitation to maximum |
| Moisture, dust and vibration | What happensCorrosion, tracking and cracked connections on the board |
| Wrong replacement or wiring | What happensA mismatched AVR, or one wired incorrectly, can fail as soon as the set starts |
Signs of an AVR or excitation problem
Each of these can come from the AVR, but also from engine speed, the diodes or the wiring, so testing comes before replacement.
- The engine runs normally but there is no output voltage
- Voltage well above normal, with lights unusually bright
- Voltage that pulses while the engine speed is steady
- Voltage that collapses when a load is applied
- An over- or under-voltage alarm on the controller
Switch off sensitive equipment, and leave the alternator terminal box to a technician: the AVR is live whenever the set runs. AVR repairs explains how we test the excitation chain and fit a matched regulator.
FAQ
Frequently asked questions
Can I adjust the AVR myself?
It is not recommended. The adjustments are on a live circuit, and wrong settings can cause hunting, overvoltage or damage to equipment. Ask a technician.
Can a faulty AVR damage my appliances?
Yes. An AVR that fails to full excitation, or loses its sensing, can push the voltage well above normal and damage electronics. Switch them off at the first sign of high voltage.
How do I know if my AVR has failed?
No voltage, very high voltage or voltage that hunts are typical signs, but the same symptoms come from diodes, wiring and the exciter. Testing tells which. See AVR repairs.
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