Voltage Regulation & Power Quality
Servo and static voltage stabilizers: where each fits
How servo and static voltage stabilizers correct voltage, how they differ in speed, maintenance and rating, and which duties suit each.
Servo and static voltage stabilizers solve the same problem: they hold the output voltage close to a set value while the incoming supply varies. They do it in different ways, and the difference shows up in correction speed, maintenance, rating range and cost. Neither is better in general. The supply condition and the load decide which fits.
How a servo voltage stabilizer works
A servo voltage stabilizer uses a motor-driven variable transformer. A control circuit compares the output voltage with its setting and drives a small servo motor that moves a carbon brush or roller along the transformer winding. The changed tapping feeds a buck-boost transformer in series with the load, which adds to or subtracts from the incoming voltage.
Because correction is continuous, the output can be held within a narrow band, typically ±1% of the set value within the stated input window. Correction is mechanical, so it takes time. The correction rate is typically 6–25 V per second, lower on larger ratings, so a large step is corrected over a few seconds.
Where servo designs fit
- Supplies that drift slowly across a wide range, such as voltage that falls through the day at the end of a long feeder.
- Larger ratings, where servo designs are well established; Invitrona builds them up to 500 kVA.
- Mixed commercial and industrial loads that tolerate a short correction time.
How a static voltage stabilizer works
A static voltage stabilizer has no motor-driven parts in the correction path. The common form is the tap-changing stabilizer: the control electronics measure the output and switch between taps on a transformer, using thyristors or, in simpler designs, relays. Some designs use IGBTs to control a buck-boost stage continuously.
Correction is fast. Thyristor switching can act within a few cycles of the supply, which is tens of milliseconds at 50 Hz, and relay switching typically completes a tap step in under a second. Tap-changing designs correct in steps, so output accuracy depends on the number of taps and is typically ±2–5%; designs with continuous electronic control can hold a narrower band.
Where static designs fit
- Loads that react to fast dips and swings, such as some electronic equipment, controls and medical or laboratory instruments.
- Locations where maintenance access is limited and brush or motor wear is a concern.
- Dedicated loads and smaller supplies that tolerate a few per cent of variation. Tap-changing stabilizers are typically built from about 1 kVA to 100 kVA.
Comparing the two
| Parameter | Value |
|---|---|
| Correction method | Servo: motor-driven variable transformer; Static: electronic switching |
| Correction speed | Servo: typically 6–25 V per second; Static: thyristor within a few cycles; relay under 1 s per step |
| Output accuracy | Servo: typically ±1%; Static tap-changing: typically ±2–5%, by number of taps |
| Wearing parts | Servo: brush or roller, motor, gearing; Static: relay contacts in relay designs; none with thyristors |
| Typical rating range | Servo: up to 500 kVA; Static tap-changing: typically 1–100 kVA |
Maintenance and service life
A servo stabilizer has wearing parts: the brush or roller contact on the variable transformer, the motor and its gearing. These need periodic inspection and, over time, replacement. The work is routine and parts are simple, but it must be planned.
A thyristor-switched static stabilizer avoids mechanical wear, but its power electronics depend on cooling and on protection against surges; relay-switched designs have contacts that wear with switching. Heat sinks, fans and filters need to be kept clean, and a failed semiconductor generally needs a trained technician to replace.
Questions that decide the choice
- How fast does the incoming voltage change, and how far? Slow drift over a wide range favours servo correction; fast dips favour thyristor switching.
- How sensitive is the load to a short deviation while correction takes place?
- What rating is needed, and is it within the economical range of each type?
- What maintenance can the site support, and who will carry it out?
- What does the site's harmonic and transient environment look like?
The supply condition comes first
Both types correct the voltage magnitude. Neither removes harmonics, filters fast transients on its own or provides back-up during an outage. If the site problem is interruption rather than variation, the answer is a different product, and a stabilizer is at most one part of it.
Recording the incoming voltage over a representative period answers the first two questions above. With that record and the load list, the choice between servo and static correction is usually clear.
On this site, servo designs are listed under Servo Voltage Stabilizers, up to 500 kVA. Tap-changing stabilizers, switched by relays or thyristors, are listed under Voltage Stabilizers & Voltage Controllers. The correction method for a given rating is stated in the proposal.
Where to go next
- Servo Voltage Stabilizers
- Voltage Stabilizers & Voltage Controllers
- Servo and static stabilizers compared
- Discuss your requirement: share the load list and recorded voltage.
Let’s find the right equipment.
Tell us what you’re working on. We’ll start with your application, load and installation needs.
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