In short. Most generator voltage complaints are not caused by a failed AVR. The regulator sits in a loop with its power supply, its sensing input, the exciter field, the rotating rectifier and the main rotor, and a fault anywhere in that loop produces no voltage, low voltage, high voltage or hunting. Service means confirming speed and load first, measuring each stage in sequence, and replacing the regulator only when the loop around it has been proven healthy.
The excitation loop, stage by stage
On a brushless alternator, which covers almost every diesel generator set in India, the automatic voltage regulator does not generate the output voltage. It controls a small DC current in the stationary exciter field. That field induces AC in the rotating exciter armature, which is rectified by diodes on the shaft and fed to the main rotor field, and the main rotor’s field induces the 415 V output in the stator. The AVR senses the stator voltage, compares it with its set point and adjusts the exciter field current to close the loop.
The AVR draws its own power from the stator output on shunt machines, from an auxiliary winding, or from a permanent magnet generator on the shaft end. Which arrangement your set has decides its behaviour during motor starting and short circuits, and its fault symptoms. The guides on how AVRs work and PMG excitation explain the differences.
Rule out speed and load first
Output voltage follows frequency. A set running at 47 Hz with a hunting governor shows low and unstable voltage, and the AVR’s under-frequency roll-off pulls it down further by design. The first measurement on any voltage complaint is therefore frequency, followed by a look at the load: a large motor starting, a UPS rectifier or a VFD-heavy load produces dips and distortion that look like regulation faults but are load behaviour. Record frequency, three-phase voltage and current and power factor at no load and at the complaint load before deciding where the fault lies. The page on nonlinear loads on alternators describes what distorted loads do to sensing.
Symptom to probable cause
With speed confirmed correct, the symptom narrows the search. The table gives the order in which we test, from the cheapest check to the most intrusive.
| Symptom at correct speed | Check first | Then | AVR likely faulty only if |
|---|---|---|---|
| No voltage, no build-up | Residual magnetism and field flashing on shunt machines; PMG output on PMG machines | Exciter field resistance, rotating diodes, main rotor resistance | Supply present at AVR and field healthy, but no field current output |
| Low voltage that will not adjust up | Sensing wiring and fuse, under-frequency setting | Open rotating diodes, high-resistance field connection | Sensing correct and field healthy but output limited |
| High voltage, uncontrolled | Sensing circuit open, lost sensing phase or blown sensing fuse | Shorted output device in AVR | Sensing proven present at AVR terminals |
| Hunting or oscillation | Governor stability, load type | Stability setting, loose sensing or field connections | Speed is stable and connections are tight |
| Voltage collapses under load | Shorted rotating diode, overload | AVR supply dipping on shunt machines | PMG or auxiliary supply steady but output drops |
| Unbalanced phase voltages | Load unbalance, neutral connection | Stator winding fault | Rarely; the AVR senses an average or one phase |
Rotating diodes deserve particular attention: a shorted diode appears as low voltage that worsens with load, an open diode as slightly low voltage with ripple.
The measurements that settle the question
A regulator can be condemned with confidence only when five readings have been taken at its terminals. The AVR supply voltage during a start, which must reach the regulator’s minimum operating value; on a shunt machine this depends on residual magnetism, so a set that has stood idle for months may need field flashing before any other test. The sensing voltage, which must correspond to the stator voltage on every sensing phase; a blown sensing fuse is the most common cause of a sudden overvoltage. The exciter field voltage and current at rated voltage and no load, compared with the alternator data sheet. The exciter field resistance cold, measured with the AVR disconnected. The main rotor resistance measured at the diode plate after lifting the diode leads. These readings take under an hour and separate a diagnosis from a guess; where a recorder is available, logging voltage and frequency through a load step is stronger still, because many complaints are transient.
Choosing a replacement AVR
When the regulator is proven faulty, the replacement must match the alternator and not merely the shape of the old board. Compatibility depends on the excitation type (shunt, auxiliary winding or PMG), the sensing arrangement, the continuous and forcing field current the regulator can supply against the exciter field demand, the field resistance range, the under-frequency characteristic, and any droop or paralleling inputs the installation uses. A regulator rated for a small exciter field overheats on a large alternator; a shunt regulator fitted to a PMG machine is damaged by the PMG voltage. Where a different make is proposed, the supplier should state in writing why it is compatible and which settings must be transferred. The AVR, PMG and exciter parts page lists what we need to confirm a match.
Setting up and verifying after repair
A new regulator arrives with factory defaults and has to be commissioned. At no load and rated speed, set the output to 415 V line to line, then adjust stability so a load step produces only a brief overshoot and no sustained oscillation. Set the under-frequency knee at about 47 to 48 Hz on a 50 Hz set, so voltage rolls off if the engine is dragged down and the exciter is protected from overexcitation. On sets that run in parallel, set droop or quadrature compensation to match the other machines and confirm reactive load sharing with a kVAr reading on each set. Where the regulator has an overexcitation limit, set it from the alternator’s field data rather than at maximum.
Verification must reproduce the original complaint: if the problem was a dip during motor starting, start that motor and record the result. Record the final settings on the service report and leave a copy at the panel, because the troubleshooting guide starts from exactly those records.
Common mistakes
- Replacing the AVR for a high-voltage fault without checking the sensing fuse; the new regulator sees the same open sensing circuit and drives the voltage high again.
- Adjusting the voltage potentiometer to compensate for a low reading that was really a speed problem, so the set runs overvoltage once the governor is corrected.
- Fitting a regulator chosen for physical similarity, with the wrong field current rating or excitation type, which overheats or fails on the first heavy load.
- Leaving the stability setting at default, so the set hunts whenever a compressor starts and the complaint returns within a week.
- Diagnosing a shorted rotating diode as an AVR fault and replacing the regulator twice before anyone lifts the diode leads.
What to send us
Send a photograph of the alternator nameplate and of the existing AVR showing its part number and terminal markings, and tell us whether the voltage is absent, low, high or unstable, at what load and after how long. Readings of frequency and voltage at the time of the fault shorten the diagnosis. Use the service and AMC enquiry form, or the spare parts enquiry if you have already confirmed which component has failed.
Questions engineers ask
The generator produces no voltage at all. Is the AVR dead?
Not necessarily. On a self-excited machine, loss of residual magnetism after long standing, a blown AVR fuse, an open exciter field or an open rotating diode bridge all produce the same result. Check for AVR supply during cranking, flash the field if the machine is shunt-excited, and measure the exciter field resistance before condemning the regulator.
Why does the voltage drop when a motor starts and recover slowly?
A motor draws roughly six times rated current at start at a very low power factor, and the voltage dips until the AVR forces the field up. The dip depth depends on alternator reactance and excitation forcing; PMG machines recover faster. If the dip trips equipment, the answer is usually a soft starter or a larger alternator, not an AVR adjustment.
What voltage should a 415 V generator be set to?
Rated voltage at no load and rated frequency, unless there is a documented reason to run slightly higher for cable drop. Running at 430 V or more to mask a cable problem shortens motor life and can trip UPS input protection.
Can I fit a universal AVR from the market?
Only if its excitation type, sensing arrangement, field current rating and field resistance range match the alternator. Universal regulators cover many small shunt-excited machines but are not suitable for PMG machines or large exciter fields. Confirm against the alternator data sheet before fitting.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.