In short. A servo stabiliser on generator supply sees voltage and frequency that vary more than on the grid. Check its input window and frequency tolerance, sequence the load to avoid hunting, and decide whether the stabiliser sits before or after the changeover.
What changes on a generator
- Voltage dips during load steps.
- Frequency droops and recovers.
- Higher source impedance, so more distortion with nonlinear loads.
- Variable loading at start-up.
Placement options
| Option | Effect |
|---|---|
| Stabiliser before ATS (on utility side) | Corrects utility only; generator supply bypasses it |
| Stabiliser after ATS (common load side) | Corrects both sources; sees generator behaviour |
| Separate stabiliser per source | Tailored but more equipment |
See architecture and ATS service.
Sequencing
- Let the generator reach stable voltage and frequency.
- Transfer, with stabiliser start delay.
- Start loads in steps. See motor starting.
- Watch for hunting during step changes.
Common problems
- Stabiliser cuts out on generator frequency below its window.
- Generator voltage and stabiliser correction chase each other.
- Stabiliser inrush overloads a lightly loaded generator.
See frequency dip and wide-range units.
Illustrative example
A plant puts a servo stabiliser after the ATS. On generator, the AVR is tuned so voltage is steady within a narrow band and the stabiliser’s input window includes the generator’s loaded range. A start delay avoids transferring while voltage is still settling.
Interaction between regulators
Three control loops can interact: the engine governor holding frequency, the alternator AVR holding voltage, and the stabiliser holding its output. If the stabiliser sees the generator voltage swing after a load step, it corrects, which changes the load on the generator slightly, which changes the voltage again. If the loop speeds are too similar, hunting appears. Well-set systems have a clear hierarchy: the generator is stable on its own, and the stabiliser trims only slow, small errors.
Commissioning sequence
- Run the generator without the stabiliser on a load bank; record voltage and frequency at steps.
- Adjust governor and AVR for stable response.
- Connect the stabiliser and repeat the steps.
- Check for hunting and adjust stabiliser sensitivity if allowed.
- Decide whether the stabiliser is needed on generator supply, or can be bypassed by control logic.
See load bank testing.
Sizing note
The stabiliser draws inrush if it has an input transformer. Combined with other loads at transfer, the generator needs capacity for the starting sequence. See generator sizing and motor starting.
Decision guide
| Situation | Suggested approach |
|---|---|
| Generator voltage stable within load tolerance | Bypass the stabiliser on generator supply |
| Generator voltage drifts beyond load tolerance | Keep the stabiliser, with checks |
| Frequency outside the stabiliser window | Bypass or select a wider-range unit |
| Mixed loads | Feed sensitive loads through the stabiliser and motors directly |
Documentation
Record AVR, governor and stabiliser settings together, so that after maintenance, they can be restored. Note the date and the person.
The master engineering question
When the generator voltage is itself correct, does the stabiliser add value or just another thing to hunt?
What to send us
Send generator and stabiliser data. Send your application details.
Questions engineers ask
Do I need a stabiliser on a generator?
Often the AVR is enough; add one only if data shows need.
Why does it hunt?
Interaction between AVR, governor and stabiliser.
Can I bypass it on generator?
Possible with design; control logic needed.
What frequency window?
Check the model; generators drift under load.
Values are illustrative. Test on site.