In short. A generator AVR measures the alternator’s terminal voltage, compares it with a set point and adjusts the exciter field current so the output stays within about one percent of nominal from no load to full load. Most “AVR failures” reported to us are engine speed, sensing wiring, rotating diode, exciter or load problems that no regulator can correct. A fixed test sequence separates them in under an hour and avoids replacing a regulator that was never faulty.
What the regulator does
In a brushless alternator the AVR does not touch the main field directly. It drives the stationary field winding of the exciter with a chopped or phase-controlled DC current. The exciter armature on the shaft produces AC, the rotating rectifier converts it to DC, and that current flows in the main rotor field. More exciter field current means more flux and a higher terminal voltage. The loop closes when the AVR senses the terminal voltage and adjusts the exciter current until the sensed value matches the reference. Gain and stability settings decide how fast and how smoothly that happens.
The regulator needs its own power supply. On a self-excited or shunt machine it takes power from the main stator output; on an auxiliary-winding or permanent magnet generator machine it takes power from a source independent of the terminal voltage. That difference matters when voltage collapses during a motor start or a fault, as explained under PMG and excitation, and it also means an AVR designed for one supply type cannot simply be fitted to the other.
Terminals and adjustments
| Function | What it does | Typical setting or behaviour | What happens if it is wrong |
|---|---|---|---|
| Voltage set point | Sets the regulated terminal voltage | 415 V line to line, usually adjustable about plus or minus 10 percent | Over- or under-voltage at all loads |
| Stability or gain | Damps the control loop | Set for fast recovery without hunting | Hunting voltage, lights pulsing, or slow recovery after load steps |
| Under-frequency roll-off (UFRO) | Reduces voltage in proportion when frequency falls below a knee point | Knee at about 47 to 48 Hz on a 50 Hz set | Set too high, voltage dips on every load step; too low, the alternator is over-fluxed when the engine lugs |
| Droop (quadrature droop) | Lowers voltage slightly with reactive load so paralleled sets share kVAr | Droop CT on one phase, 3 to 5 percent at full reactive load | Circulating reactive current between paralleled sets, one set tripping on over-excitation |
| Excitation or over-excitation limit | Caps field current to protect windings | Set per alternator data | Nuisance limiting on motor starts, or no protection during a sustained overload |
| Remote voltage trim or analogue input | Allows a synchroniser or power factor controller to adjust voltage | Potentiometer or 0 to 10 V, plus or minus a few percent | Unexpected voltage offset if a trim pot or input is left connected and floating |
Droop and UFRO are the settings most often mis-set in the field. A single set with droop enabled shows a voltage that falls with reactive load for no apparent reason. A set with UFRO too close to 50 Hz dips its voltage every time the engine hesitates on a load step, which looks like a regulator fault and is a governor interaction. Frequency behaviour is covered under frequency dip, and reactive sharing on paralleled sets under kW and kVAr load sharing.
A test sequence for voltage complaints
- Check frequency first. If the set is not at 50 Hz at the time of the complaint, stop; the problem is engine speed, and the AVR’s UFRO is doing what it should.
- Read the voltage at the alternator terminals with a meter you trust. Controller displays depend on their own sensing, and a loose wire or wrong scaling can misreport by many percent.
- Measure the AVR’s sensing input and power input. Sensing should equal the terminal voltage; power input should match the type expected, whether shunt, auxiliary winding or PMG. A blown sensing fuse produces full excitation and dangerous over-voltage.
- Check residual voltage. With the AVR disconnected and the set running, a shunt machine should show a few volts of residual at the terminals. None means the residual magnetism has been lost and the field needs flashing through a diode from a 12 V battery, per the manufacturer’s method.
- Test the exciter field and the rotating rectifier. Exciter field resistance against the data sheet, each rotating diode for forward conduction and reverse blocking, and the surge suppressor for a short. A single shorted diode produces unstable or low voltage and a hot exciter field.
- Apply a load step and watch recovery. Slow recovery with correct steady-state voltage points to stability setting or excitation limiting; a dip that does not recover points to excitation support or an overloaded machine.
- Only then condemn the AVR. A regulator that receives correct sensing and power, feeds a healthy exciter and still cannot hold voltage is faulty. Replace with the part specified for the alternator from AVR, PMG and exciter spares, and set it up from the data sheet, not by copying the pots on the old unit.
Replacing and setting up a regulator
AVRs are matched to alternators by excitation type, exciter field resistance and output current, not by physical size or terminal count. A regulator rated for a small shunt machine overheats on a larger exciter field; a shunt regulator wired to a PMG supply sees the wrong voltage and frequency. Set the voltage to nominal at no load, check phase balance, confirm stability with a load step, set UFRO per the manufacturer, and enable droop only if the set will be paralleled. Record the final settings. Our AVR and excitation service includes that commissioning check, because the number of regulators we see that were replaced twice for the same symptom is not small.
Generator AVR and mains voltage stabiliser are different things
The same three letters are used for a mains stabiliser that corrects the incoming DISCOM supply. A generator AVR controls excitation inside the set and does nothing for the utility supply; a servo stabiliser does the reverse. When a site reports “voltage problems”, establish which source and which load is involved before anyone quotes a device. The stabiliser side is covered in automatic voltage regulators and stabiliser systems.
Common mistakes
- Turning the voltage pot up to cure a dip under load; the no-load voltage becomes excessive and connected equipment is stressed every time the load drops.
- Replacing the AVR for a hunting voltage without checking the governor; the engine speed is hunting and the regulator is following it.
- Leaving the droop CT connected and droop enabled on a single set; voltage falls with reactive load and the complaint persists through two new regulators.
- Fitting a regulator with the right connector but the wrong excitation type; it either overheats or never builds voltage.
What to send us
Send photographs of the alternator nameplate, the AVR label and its wiring, and the controller screen showing voltage and frequency at the time of the complaint. Describe the symptom precisely, whether no voltage, low, high, hunting or a dip under a particular load, and say whether the set is paralleled. Send a spares or service enquiry and we will confirm the correct regulator part and the checks to make before fitting it.
Questions engineers ask
Why is there no voltage at all after the set has stood idle for months?
On a self-excited machine the regulator needs a small residual voltage to start building excitation. Long idle periods, especially in humid conditions, can let that residual fade. Flashing the field with a battery through a blocking diode, following the manufacturer’s method, restores it. If the fault returns, look for a failed rotating diode or a sensing fuse.
Can one AVR model be used across different alternator brands?
Only where the excitation type, exciter field resistance and output current rating are compatible, and the alternator manufacturer accepts it. Universal regulators exist and work on many shunt machines, but they must be set up from the alternator data, and they are not a substitute for the specified part on PMG or auxiliary-winding machines.
Why does the voltage dip on every motor start even though the set is large enough?
The regulator can only respond as fast as its gain allows and as far as its excitation supply permits. A shunt-excited machine loses regulator supply as the terminal voltage falls, so the dip deepens. Check stability setting and UFRO first; if they are correct, the excitation architecture is the limit and a PMG or auxiliary-winding machine would do better.
How accurate should the regulated voltage be?
Modern regulators hold steady-state voltage within about plus or minus 0.5 to 1 percent from no load to full load at constant speed. Phase-to-phase differences beyond a percent or two usually indicate unbalanced load or a winding problem rather than the regulator.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.