In short. The term AVR is used for two different things. On a generator it is the excitation controller that holds terminal voltage by adjusting field current. On the mains side it is a voltage regulator or stabiliser. Knowing which is meant avoids wrong diagnosis.
Generator AVR
Sense terminal voltage → compare with set point → adjust exciter field current → alternator output voltage corrects. It also shapes response to load steps, with under-frequency roll-off protecting against dips. See PMG excitation and transient response.
| Setting or feature | Role |
|---|---|
| Voltage set point and trim | Sets output voltage |
| Stability and gain | Controls hunting versus speed |
| Under-frequency roll-off | Reduces voltage as speed falls to protect the engine and recover |
| Droop and cross-current | Share reactive load in parallel. See kW and kVAr sharing |
Mains-side AVR
Regulates utility or site supply by buck-boost, tap-changing or electronic means. See servo, static and buck-boost regulation.
Typical faults and clues
- Voltage hunting: gain, wiring or governor interaction.
- No build-up: residual magnetism or excitation circuit fault.
- Over voltage: sensing loss or regulator failure; protection needed.
- Low voltage under load: excitation limits, speed drop or overload.
Good practice
- Record settings before adjusting anything.
- Check sensing leads and fuses first.
- Adjust one parameter at a time and observe.
- Verify under load steps and with nonlinear loads. See UPS and nonlinear loads.
Illustrative example
A generator hunts around 410 to 425 V at light load. Reducing AVR stability gain sensitivity and checking the governor settle the swings. Without recording the original values, the engineer would not have been able to restore them if the change had not helped.
Generator AVR: what the settings do
The AVR compares terminal voltage with a reference and adjusts the exciter field to correct the error. Several adjustments shape the response.
| Adjustment | Effect if too high | Effect if too low |
|---|---|---|
| Stability or gain | Voltage hunting | Slow response, larger dips |
| Under-frequency knee | Voltage drops too early | Engine may stall on load steps |
| Voltage trim | Over-voltage risk | Low voltage at the loads |
| Droop | Voltage falls with reactive load | Poor reactive sharing in parallel |
Interaction with the governor
When the engine speed falls on a load step, the AVR’s under-frequency function reduces voltage so the load power falls and the engine recovers. If the governor and AVR are both tuned aggressively, they can interact and cause oscillation. Tune the governor first, then the AVR, changing one parameter at a time.
Fault-finding guide
- No voltage: check residual magnetism, excitation fuses, sensing leads and the AVR output.
- Hunting: reduce AVR gain slightly; check the governor and any fuel system fault.
- High voltage: check sensing; if sensing is lost, the AVR may drive full excitation.
- Voltage collapse on load: check overload, speed drop, the AVR current limit and the rectifier.
Protection
An over-voltage trip on the generator protects connected loads if the AVR fails high. Check that the setting is sensible and tested. See generator protection.
When a mains-side AVR is also fitted
If a stabiliser follows a generator, the two regulators can chase each other. Tune the generator for steady voltage first, then decide whether the stabiliser is still needed. Document every setting after commissioning.
The master engineering question
Is the voltage problem in the regulation loop, the engine speed, or the load, and what evidence separates them?
What to send us
Send alternator model, AVR model and observed behaviour. Send your application details.
Questions engineers ask
Is AVR the same as stabiliser?
No. A generator AVR controls excitation; a stabiliser corrects supply voltage.
What causes hunting?
Gain settings, governor interaction, loose sensing or load behaviour.
Can I replace the AVR myself?
Use a compatible model and a qualified technician.
Why does voltage dip on load steps?
Alternator and engine response; see transient response.
Values are illustrative. Follow the alternator maker’s manual.