In short. A static stabiliser corrects voltage electronically using power-semiconductor switches and a transformer, with no motor or brushes. It responds in milliseconds and needs less mechanical maintenance, but typically has tighter overload limits and costs more than servo units of the same rating.
How it works
Sense voltage → control logic selects a transformer tap or switching pattern → power switches change the buck or boost → output corrected within a few cycles. Some designs use tap-changing; others use high-frequency inverter stages.
Comparison with servo
| Feature | Static | Servo |
|---|---|---|
| Response | Very fast | Slower; motor driven |
| Moving parts | None | Motor, brush, rollers |
| Overload | Limited by semiconductors | Generally more tolerant |
| Output accuracy | Tap-step or design dependent | Smooth, tight |
| Cost | Often higher | Often lower at higher kVA |
See the full servo vs static comparison.
Where it suits
- Loads sensitive to fast voltage changes.
- Sites where brush maintenance is hard to schedule.
- Rapidly fluctuating supplies.
Checks
- Tap step size and output tolerance.
- Switching transients on sensitive loads.
- Overload and inrush capability.
- Cooling and component derating.
- Behaviour on generator supply.
Illustrative example
An imaging lab has lifts causing quick dips in the supply. A static stabiliser corrects within a few cycles, protecting a scanner that a servo unit would follow too slowly. For a motor-heavy pump house, the same site chooses servo for overload tolerance.
Static designs in more detail
Static stabilisers fall into two broad families. Tap-switching designs use a transformer with several taps and thyristor or triac switches to select the tap. Inverter-based designs use a power converter to add a correcting voltage. Both avoid moving parts, but they differ in output smoothness, harmonic content and overload behaviour.
| Family | Output behaviour | Points to check |
|---|---|---|
| Tap-switching | Stepped output; step size sets accuracy | Step size, switching transients, overload |
| Inverter-based | Smooth correction | Harmonics, efficiency, thermal design |
Specification questions to ask the maker
- What is the response time to a step in input voltage?
- What is the output tolerance, and the size of each step?
- What is the overload rating and its duration?
- What is the harmonic distortion added at full load?
- What are the operating temperature limits?
- What happens on component failure: bypass, trip or pass?
Where static designs can disappoint
- Heavy motor starts that exceed the semiconductor current rating.
- High ambient temperature without derating.
- Poor supply with spikes that stress the switches.
- Fan failures that go unnoticed.
Maintenance
Without brushes and rollers, maintenance shifts to fans, capacitors, cooling paths and connections. Thermography and record keeping still apply. Capacitors in particular age with temperature, so track temperature.
Selection by load example
| Load | Static suitability |
|---|---|
| Imaging and lab equipment | Good, if the load is steady |
| Variable-speed drives | Check compatibility and harmonics |
| Large direct-on-line motors | Check inrush rating carefully |
| Lighting | Good |
The master engineering question
Is speed or overload tolerance the more important requirement for this load?
What to send us
Send load type and supply recordings. Send your application details.
Questions engineers ask
Is static the same as relay type?
No. See servo vs relay; relay types use electromechanical switching.
Does it distort the waveform?
Depends on design; check THD data.
Is it maintenance free?
No moving parts reduce wear, but fans and capacitors need checks.
Can it handle motor starts?
Check inrush rating.
Values are illustrative. Use manufacturer data.