Regulators Selection Guide
A servo voltage regulator continuously measures the output voltage and corrects it steplessly; selection depends on single-phase or three-phase construction and kVA rating.
This guide covers what to look at when choosing products in the Regulators category.
How a servo voltage regulator works
In a servo regulator, an electronic circuit continuously measures the output voltage. When the voltage deviates from the set value, a servo motor moves the brush that travels over the winding of a variable autotransformer and brings the output back to the set value. The correction is stepless; no jump caused by step changes appears at the output. On the other hand, the correction speed is limited by the mechanical movement of the brush: a servo regulator corrects voltage that changes slowly during the day or stays low or high for a long time, and cannot cope with very short voltage sags.
Single-phase and three-phase models
The phase notation in the product name gives the input and output together: 1f/1f means single-phase input and single-phase output, and 3f/3f means three-phase input and three-phase output.
- Single-phase servo regulators: for single-phase lines and single-phase loads; ratings such as 5 kVA, 7.5 kVA, 20 kVA and 50 kVA are listed.
- Three-phase servo regulators: for three-phase enclosure and facility inputs and for three-phase motor-driven machines; ratings such as 10.5 kVA, 22.5 kVA, 45 kVA, 75 kVA and 150 kVA are listed.
The rated power of a three-phase regulator is divided equally among the three phases. If the load is not evenly distributed across the phases, the load of the most heavily loaded phase must not exceed the regulator's power per phase. Also check from the manufacturer data whether the regulator corrects each phase separately or the three phases together; a construction that corrects the phases separately brings each phase to its own set value on lines where the phase voltages differ from one another.
How to determine the regulator rating
- List the equipment to be fed from the regulator and add up their apparent power (VA or kVA). For a single-phase device whose label gives only current, the apparent power is the product of voltage and current.
- Mark motor loads separately. Compressor and pump motors that start direct-on-line draw more than their rated current at starting; the regulator must be able to carry this current too.
- Choose a rating above the total. A margin is left so as not to run the regulator at full load continuously and to leave room for loads added later; the size of the margin is determined by the load type and the manufacturer's recommendation.
- Check the input and output protection and the cable cross-section. For the same load, the input current rises as the input voltage falls; the input side is therefore selected according to the maximum input current given by the manufacturer. For a preliminary calculation you can use the current and fuse calculation from active power and cable current carrying capacity calculation tools; the final values belong to the project calculation.
Values to compare besides rating
The product names give only the rating and phase configuration; look up the following values in the manufacturer's technical data.
- Input voltage range: The regulator holds the output at the set value only within this range. Measure the line voltage at different times of the day before selecting and determine the lowest and highest values.
- Output tolerance: Shows how far the output voltage can deviate from the set value; it is compared with the voltage range the equipment to be fed allows.
- Correction speed: Shows how long it takes for a change in voltage to be eliminated; it matters on lines where the load changes suddenly.
- Protection and bypass: Check whether there is protection that cuts the output on overvoltage and undervoltage, and a bypass switch that transfers the load directly to the mains during a fault or maintenance.
Difference between a servo regulator and a static regulator
In a static regulator, the voltage is corrected with semiconductor switches without moving parts; its response is faster than a servo regulator and there is no brush maintenance. In a servo regulator, the motor and brush work mechanically, so the brush wears over time and periodic maintenance is required. All products in this category are of the servo type and are collected on the servo regulators page.
For loads sensitive to interruptions and very short voltage sags, UPS uninterruptible power supplies are considered instead of a regulator.
Ordering notes
The products are sold per piece and are of the Siel brand. The rating (kVA) and phase configuration are written in the product name. Because the 15 kVA and 30 kVA ratings are listed as both single-phase and three-phase, check the phase configuration before adding to the cart.
Comparison and selection table
| Selection Criterion | Single-Phase Servo Regulator (1 – 30 kVA) | Three-Phase Servo Regulator (10 – 200+ kVA) | Static (Thyristor / Solid-State) Voltage Regulator |
|---|---|---|---|
| Regulation Principle | Variable autotransformer (variac) + servo motor drive | Independent three-phase servo-motorized variac banks | Solid-state tap switching (zero moving mechanical parts) |
| Response Speed | ~80–100 V/s smooth stepless correction | ~80–100 V/s with independent phase-balance control | Ultra-fast (<20 ms / 500 V/s) response to sudden sags |
| Input Window | 130–250 V or 160–250 V AC → 220/230 V ±2% | 275–450 V AC → 380/400 V AC ±2% | Industrial grids with rapid welding or crane voltage dips |
| Typical Application | Homes, boilers, HVAC, offices & small workshops | Factories, CNC machines, elevators & commercial buildings | Laser cutters, medical imaging & sensitive automation |
Frequently asked questions
Why is regulator rating given in kVA rather than kW?
The windings and brush of the regulator are limited by the current they can carry; the quantity that determines current is apparent power, not active power. Apparent power is the product of voltage and current and is expressed in VA or kVA. A load with a low power factor draws more current for the same kW value; the regulator is therefore selected by comparing it with the load's kVA value.
Can a single-phase regulator be used in a three-phase installation?
A single-phase load is fed between one phase and the neutral of a three-phase installation; if only this load is to be protected, a single-phase (1f/1f) regulator can be connected to its supply line. Three-phase motors and machines require a three-phase (3f/3f) regulator. If the whole installation is to be protected, the three-phase regulator is connected to the main incoming supply and the single-phase loads are also fed from its output; in that case the distribution of the loads across the phases is compared with the regulator's power per phase.
Is a separate regulator needed where a UPS is used?
If the UPS input voltage range covers the variation on the line, a separate regulator is not needed in most applications; an online UPS generates its own output voltage. If the line voltage often goes outside the UPS input range, the UPS switches to the battery each time and the battery is used more often. In that case, whether a regulator should be placed upstream of the UPS is determined in the project according to the manufacturer data of the two devices.
What should be considered when choosing a regulator for a combi boiler, air conditioner or refrigerator?
These devices contain a compressor or pump motor; motors that are not inverter-driven draw more than their rated current at starting. The regulator rating is chosen not only by the running power on the label but also so that it can carry the starting current. Base your choice on the device's label values and, if there is one, the manufacturer's regulator recommendation; if more than one device is to be connected to the same regulator, the ratings are added up.