In short. Voltage regulation on a brushless generator is a chain: the AVR controls the exciter field, the exciter rotor feeds the rotating diodes, and the diodes feed the main rotor that produces the 415 V output. No voltage, low voltage, high voltage and hunting each have causes outside the AVR, so a replacement AVR is identified by alternator make, excitation type, sensing and field ratings, and is ordered only after the chain has been tested.
The excitation chain, stage by stage
An automatic voltage regulator does not make voltage; it decides how much field current the machine gets. On a conventional brushless alternator the AVR takes its power from the main stator output (a shunt or self-excited machine) or from a small permanent magnet generator on the shaft (a PMG or separately excited machine). It compares the sensed output voltage with its setpoint and feeds a controlled DC current into the exciter stator, a stationary field winding. The rotating exciter armature induces a three-phase voltage, the rotating diode bridge rectifies it, and the resulting DC flows into the main rotor field. The main rotor’s flux cuts the main stator and the terminal voltage appears.
Every link matters. A failed diode, an open exciter field, a blown sensing fuse, a wrong governor speed or a loose sensing wire all show up at the terminals as a voltage problem, and all of them have been “fixed” with a new AVR that then fails or does nothing. The mechanical parts of the chain, including the diodes, are covered on alternator parts; the machine as a whole is described on generator alternator architecture.
Self-excited, PMG and auxiliary-winding systems
The excitation power source changes both the behaviour of the set and the AVR that will fit it.
| Excitation power source | How the AVR is powered | Behaviour under disturbance | Replacement implication |
|---|---|---|---|
| Shunt (self-excited) | From the main output terminals | A heavy motor start or short circuit pulls down the output and with it the AVR supply; voltage can collapse | AVR must suit shunt input; residual magnetism must exist for build-up |
| PMG (separately excited) | From a permanent magnet generator on the shaft, typically a three-phase supply at a frequency several times line frequency | AVR power is independent of load; the machine sustains about three times rated current into a fault for protection to operate | AVR must accept the PMG input; a shunt AVR will not work without rewiring that must not be improvised |
| Auxiliary winding (AREP and similar) | From extra windings in the main stator that respond to both voltage and current | Close to PMG performance without a separate PMG | AVR is specific to the winding arrangement; alternator make and model identify it |
A PMG is therefore not a universal generator component. Many sets below about 100 kVA are shunt excited and have no PMG at all; asking for “the PMG” on such a machine wastes everyone’s time.
Diagnosing the four classic complaints
No voltage
Check engine speed first; many AVRs hold the field off below a frequency threshold. Then confirm the AVR has supply, check the AVR fuse, measure the exciter field resistance for an open circuit, test the rotating diodes and look for a lifted sensing wire. On a shunt machine that has stood for a long time, loss of residual magnetism is common and a brief, controlled flash of the exciter field from a battery through a diode will restore it. Only after these checks is a dead AVR the probable cause.
Low voltage
Read the frequency. At 48 Hz or below, most AVRs reduce voltage deliberately through under-frequency roll-off to protect the machine, so the governor or a fuel problem is the fault. If frequency is right, suspect a failed diode, a shorted main rotor turn, a mis-set voltage potentiometer or an overloaded machine.
High voltage
The AVR has lost its view of the output. A broken sensing wire, a failed sensing transformer or a blown sensing fuse makes the regulator believe voltage is low and drive the field to maximum. A shorted AVR output stage does the same. Over-voltage protection in the controller should trip the set; it is a backup for excitation faults and must never be disabled to keep the lights on.
Unstable or hunting voltage
Hunting can be electrical or mechanical. If frequency is also swinging, the governor is hunting and the AVR is following it. If frequency is steady, the AVR stability setting is wrong for the machine, a sensing connection is intermittent or a rotating diode is marginal. Adjust the stability potentiometer only with the manufacturer’s procedure and with the set on a steady load.
Frequency and speed must be right before any AVR work
f = N × P / 120
Here f is the electrical frequency in hertz, N is the shaft speed in revolutions per minute and P is the number of poles. (Illustrative) a four-pole alternator at 1,500 rpm gives 50 Hz; the same machine dragged down to 1,440 rpm by a tired governor gives 48 Hz, which is where under-frequency roll-off typically begins to lower the voltage.
Identifying a replacement AVR
Generator kVA does not identify an AVR. The information that does is listed below.
- Alternator manufacturer, model, frame and serial number from the alternator nameplate.
- Existing AVR manufacturer and model, with a photograph of its label and of the terminal strip with wires attached.
- Excitation type: shunt, PMG or auxiliary winding, which fixes the input the AVR must accept.
- Sensing arrangement: single-phase or three-phase sensing, and the sensing voltage, which may be taken through a transformer on 3.3 kV or 11 kV machines.
- Features in use: under-frequency roll-off knee, droop CT input for parallel operation, remote voltage adjustment, external excitation limiter, over-excitation protection, and any analogue input from a power management controller.
Where an AVR has configurable parameters or a digital interface, record the existing settings before removing it. A new unit at factory defaults may hold the right voltage at no load and behave badly under a motor start or in parallel. Sets in parallel need the droop current transformer connected and the droop setting matched between machines, as discussed under generator load sharing.
Repeated AVR failure is a root-cause event
A second AVR failure in a year is not bad luck. Look for a shorted rotating diode or varistor loading the AVR output, an intermittent sensing connection that drives the regulator hard, excessive heat in the terminal box, moisture or dust on the board, a loose earth that lets switching transients through, or a load that throws large reactive swings. Our AVR and excitation service replaces the part only after the cause is found, and the same philosophy applies to controllers on controllers and sensors.
Common mistakes
- Replacing the AVR for low voltage without reading frequency; the real fault is engine speed and the set runs over-excited once the governor is fixed.
- Fitting a shunt AVR to a PMG machine or the reverse because the terminal count looked similar; the set either fails to excite or over-excites and damages the exciter.
- Turning the stability potentiometer to cure hunting that is really a governor problem; the AVR becomes sluggish and voltage dips worsen on motor starts.
What to send us
Send photographs of the alternator nameplate, the AVR label and the AVR terminals with the wiring in place, plus the generator nameplate. Tell us the symptom, the voltage and frequency readings at the time, whether the set runs alone or in parallel, and whether the AVR, diodes or alternator have been replaced or rewound before. Send a spare parts enquiry.
Questions engineers ask
Can one AVR model suit several alternators?
Sometimes, if input, sensing and field ratings all match. The alternator maker’s approved list decides it, not the terminal count.
How do I check whether my alternator has a PMG?
Look at the non-drive end. A PMG sits as a small additional machine under its own cover beyond the exciter, with a separate cable to the AVR. The alternator model code and the AVR input terminals confirm it. Most sets under about 100 kVA do not have one.
Can I adjust the AVR to get 415 V when the set reads 400 V?
Only after confirming the frequency is 50 Hz and the reading is taken at the generator terminals with a calibrated meter. If the frequency is low, fix the governor. If the voltage drop is in the cable to the load, raising the generator setpoint over-volts the equipment nearest the set.
What does under-frequency roll-off do and should I disable it?
It lowers voltage in proportion to frequency below a knee, usually set just under 48 Hz on a 50 Hz machine, so that the alternator and connected transformers are not over-fluxed when the engine slows. It should stay enabled; the fix for low voltage caused by roll-off is engine speed, not the AVR.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.