In short. A servo stabiliser corrects slow voltage variation by moving a carbon-brush contact on an autotransformer, driven by a servo motor, to add or subtract voltage. It gives accurate output and handles overload well, but it is not fast enough for sudden sags.
How it works
Sense output voltage → compare with set value → servo motor moves the variable transformer → buck-boost transformer adds or removes voltage → output returns to the set point.
Characteristics
| Property | Typical behaviour |
|---|---|
| Output accuracy | Tight band, such as plus or minus 1 percent in many models |
| Correction speed | Tens to hundreds of volts per second at best, not instant |
| Waveform | Preserved; no added distortion |
| Overload | Good short-time capability |
| Efficiency | High |
| Maintenance | Brushes, rollers and motor wear over time |
Where they suit
- Sustained low or high supply voltage.
- Loads with inrush such as motors, compressors and air conditioners.
- Industrial and commercial machines needing steady voltage.
- Sites on long feeders.
Where they do not
- Sudden dips faster than the servo can respond.
- Loads needing no-break supply: add a UPS.
- Noise and earth problems: add isolation. See stabiliser vs isolation.
Selection summary
- Phase configuration: single or three phase. See selection and sizing.
- kVA with margin for inrush and growth. See kVA guide.
- Input range from measured data.
- Cooling: air or oil. See air-cooled and oil-cooled.
- Bypass and protection features.
With generators
On generator supply voltage and frequency vary; check the stabiliser input range and frequency tolerance, and sequence the load so the stabiliser does not hunt. See DG and stabiliser integration.
Illustrative example
A plant at the end of a long feeder sees 340 to 400 V on a 415 V supply. A three-phase servo stabiliser with a wide input range holds 415 V plus or minus a small tolerance. Motors stop tripping on undervoltage, and the starting current falls because voltage is higher.
The physics behind buck-boost correction
A servo stabiliser uses a variable autotransformer to set a voltage, and a series (buck-boost) transformer adds or subtracts that voltage from the incoming supply. The correction needed is the difference between the target and the actual input.
Correction = Target voltage − Input voltage
For a 415 V target and a 360 V input, the series element must add about 55 V. Its rating is only a fraction of the load kVA, which is why the design is economical. At the extremes of the range the series winding carries more of its capacity and the unit is working hardest.
Why response speed is limited
The motor must move the brush along the winding. The control loop deliberately avoids overshoot and hunting, so correction takes time. A step change of 40 V can take a second or more to remove. Loads that cannot tolerate even that need a UPS or fast conditioner behind the stabiliser.
| Event | Servo stabiliser result |
|---|---|
| Slow rise or fall over minutes | Followed accurately |
| Step change of tens of volts | Corrected after a delay |
| Dip of a few cycles | Not corrected |
| Supply loss | Output lost |
What to inspect during service
- Carbon brush length and surface; uneven wear marks a mechanical problem.
- Roller track cleanliness and smooth travel.
- Servo motor current and noise.
- Control card voltage settings against a calibrated meter.
- Terminal temperature, using thermography under load.
- Over and under voltage cut-off operation.
Diagnosis guide
| Symptom | Likely cause | First check |
|---|---|---|
| Output swings back and forth | Hunting from control settings or worn brush contact | Sensitivity setting and brush condition |
| Output stuck high or low | Control card or motor drive fault | Control voltages and limit switches |
| Tripping on overload | Undersized unit or low input raising current | Measured current against rating |
| Excess heat | Poor ventilation or loose terminals | Room temperature and thermography |
Commissioning checklist
- Verify input and output wiring and neutral.
- Check protection set points.
- Test with varied input voltage if a variac or supply change is available.
- Test bypass transfer under load.
- Record voltages and currents as the baseline.
Local conditions
Dust and heat in industrial areas of Hyderabad and Secunderabad accelerate brush and roller wear and clog ventilation. Plan a cleaning interval that reflects the site, not the brochure default.
The master engineering question
Does the voltage problem change slowly enough for a servo to follow, and does the load need protection from faster events?
What to send us
Send voltage records and load details. Send your application details.
Questions engineers ask
How fast is a servo stabiliser?
Speed depends on the model. Check the rated correction rate.
Does it need maintenance?
Yes, periodic inspection of brushes, rollers and motor.
Can it run on a generator?
Yes, if input range and frequency tolerance suit.
Does it fix power cuts?
No. It needs input voltage to work.
Values are illustrative. Use manufacturer specifications.