Table of Contents
- Balanced vs Unbalanced Servo Stabilizer: The Short Answer
- Start With Your Supply: Single Phase or Three Phase?
- What Is a Balanced Servo Stabilizer?
- What Is an Unbalanced Servo Stabilizer?
- Why Do Phase Voltages Become Unbalanced?
- How Does Phase Imbalance Damage Motors?
- What Is the Difference Between Balanced and Unbalanced Servo Stabilizers?
- How Do You Calculate Voltage Unbalance?
- How Do You Size a Servo Stabilizer for an Unbalanced Load?
- Which One Should You Choose?
- What Happens If You Choose the Wrong Type?
- What Should You Ask Your Supplier?
- How Should a Three-Phase Stabilizer Be Installed?
- Three-Phase Homes: Questions From Indian Forums
- Key Takeaways
- Not Sure Which One You Need? Ask SG Power
- Frequently Asked Questions
- Related Reading
- Sources
Balanced vs Unbalanced Servo Stabilizer: The Short Answer
Choose a balanced servo stabilizer when your three phase voltages stay close to each other and the load is mainly three-phase motors and machines. Choose an unbalanced servo stabilizer when the phase voltages differ, or when the building runs many single-phase loads such as ACs, lights and computers spread unevenly across the phases. If you're unsure, unbalanced is the safer choice.
| Balanced | Unbalanced | |
|---|---|---|
| How it corrects | All three phases together, by the same amount | Each phase separately, by the amount that phase needs |
| Best for | Three-phase motors, CNC machines, pumps, compressors | Homes, offices, hospitals, hotels, malls, mixed loads |
| Supply condition | Phase voltages close to each other | Phase voltages that differ |
| Price | Lower | Higher |
Start With Your Supply: Single Phase or Three Phase?
The balanced vs unbalanced choice only applies to three-phase stabilizers, so first confirm which supply you have. Your electricity bill, meter or main distribution board will show it, and an electrician can confirm in a few minutes.
Single-phase supply (most homes, small shops and small offices): you need a single-phase servo stabilizer. It corrects one live wire against neutral, so balanced vs unbalanced doesn't apply. See the 10 kVA and 15 kVA single-phase models.
Three-phase supply (factories, large commercial buildings, hospitals, large homes): you need a three-phase servo stabilizer, and you choose between balanced and unbalanced. See the 3 phase servo stabilizer range.
A single-phase stabilizer can protect one single-phase load in a three-phase building, but it can't protect three-phase machines. For those, use a three-phase unit.
What Is a Balanced Servo Stabilizer?
A balanced servo stabilizer is a three-phase servo voltage stabilizer that uses one servo motor and one control to correct all three phases by the same amount. It suits three-phase motor loads on a supply where the phase voltages stay close to each other.
How it works: a single servo motor drives the variacs (dimmers) of all three phases together. Each variac feeds a buck-boost transformer that adds or subtracts voltage. The three variacs sit on one common shaft. The controller senses the supply and moves all three variacs by the same amount, so every phase gets the same boost or buck.
A balanced unit has fewer moving parts, so it costs less and needs a little less maintenance. Some balanced designs sense one or two phases, others an average of all three. Either way, the correction is the same for every phase.
Where balanced fits best: sites that take power at high voltage (HT) through their own transformer, often with an on-load tap changer (OLTC), and run mainly three-phase motors. On these sites the three phases usually move up and down together. Sites fed at low voltage (LT) from a shared utility transformer are far more likely to see unequal phases, which is where unbalanced fits.
Limitation: because every phase gets the same correction, a balanced unit can't fix a difference between phases. If one phase is at 225V and another at 245V, both receive the same correction, and the gap between them remains.
What Is an Unbalanced Servo Stabilizer?
An unbalanced servo stabilizer is a three-phase servo voltage stabilizer with a separate servo motor, variac and control for each phase, so each phase is corrected to the set voltage on its own. It suits sites where phase voltages differ or single-phase loads are spread unevenly.
How it works: the unit has separate sensing, a separate servo motor and a separate variac for each phase. Each phase is corrected to the set phase-to-neutral voltage on its own, whatever the other two phases are doing. That's why a three-phase unbalanced servo stabilizer is also called an unbalanced type or independent phase control stabilizer.
Because each phase is corrected with respect to neutral, an unbalanced stabilizer is a four-wire (R, Y, B and N), star-connected unit. A solid neutral connection is essential for it to sense and correct properly.
SG Power's unbalanced models build the three per-phase sections into one cabinet, which usually costs less and takes less space than three separate single-phase stabilizers.
Variac (dimmer) and linear (roller) designs
Both balanced and unbalanced stabilizers can be built with different voltage regulators. A variac or dimmer type uses a carbon brush moving over a circular winding, and is common in air-cooled ratings. A linear or roller type uses rollers moving along a straight winding, and is often used in larger, oil-cooled ratings. The type of regulator doesn't change whether a stabilizer is balanced or unbalanced, so ask about both. SG Power builds dimmer-type and linear servo stabilizers, with transformers and variacs wound and tested in-house.
If you want to understand the parts involved, read how a servo stabilizer works.
Why Do Phase Voltages Become Unbalanced?
Phase voltages become unbalanced mainly because single-phase loads are spread unevenly across R, Y and B, and because long lines, shared transformers and loose connections drop voltage differently on each phase.
In a perfect three-phase supply, all three phases carry the same voltage. In practice, especially in commercial buildings and residential colonies, they often don't. Common causes include:
Uneven single-phase loads: ACs, lights, geysers and computers connected more heavily on one phase than the others
Long distribution lines: voltage drops differently on each phase over long cables
Shared transformers: neighbouring buildings loading the phases unequally
Loose or corroded connections: on one phase
Large single-phase machines: switching on and off
Uneven loads also send current through the neutral wire. The more unequal the phases, the higher the neutral current, which heats the neutral conductor and can shift the phase voltages further apart. Spreading single-phase loads evenly across R, Y and B is the cheapest first step, and it often lets you choose a smaller stabilizer.
How Does Phase Imbalance Damage Motors?
Phase imbalance makes a three-phase motor draw unequal currents, so one winding runs hotter, insulation ages faster and efficiency falls. Even 2–3% voltage imbalance is enough to shorten motor life.
Single-phase equipment on a high or low phase simply sees the wrong voltage. Three-phase motors suffer more, because even a small voltage difference between phases causes a much larger difference in current:
Current imbalance is several times larger: as a rule of thumb, current imbalance in a motor is about 6 to 10 times the voltage imbalance. A 2% voltage imbalance can mean 12–20% current unbalance in one winding.
The winding runs hotter: the extra temperature rise is roughly 2 × (voltage imbalance %)². At 2% imbalance that is about 8% extra heating; at 4%, about 32%.
Insulation life drops: as a rule of thumb, motor insulation life halves for every 10°C of extra temperature.
Efficiency falls and vibration rises: so the motor uses more energy for the same work and bearings wear faster.
NEMA guidance: motor standards (NEMA MG 1) recommend keeping voltage imbalance at the motor terminals below 1%, and derating the motor above that. An unbalanced servo stabilizer corrects each phase to the set voltage, so the motors downstream see a balanced supply.
What Is the Difference Between Balanced and Unbalanced Servo Stabilizers?
The difference between balanced and unbalanced servo stabilizers is how they correct the phases: a balanced unit applies one correction to all three phases, while an unbalanced unit corrects each phase independently. The table compares them point by point.
| Feature | Balanced Servo Stabilizer | Unbalanced Servo Stabilizer |
|---|---|---|
| Control | One common control for all phases | Independent control for each phase |
| Servo motors and variacs | One motor drives all three variacs together | One motor and variac per phase |
| Output when input phases differ | The difference between phases remains | Each phase is corrected to the set voltage |
| Suits | Balanced supply and three-phase motor loads | Unequal phase voltages and mixed single-phase loads |
| Typical sites | Factories, CNC workshops, pumping stations | Offices, hospitals, hotels, malls, homes with three-phase supply |
| Protection against phase imbalance | Limited | Good: each output phase held within ±1% |
| Supply connection | Can work without a neutral reference | Needs a solid neutral (4-wire, star connected) |
| Typical supply | HT supply with own transformer or OLTC | LT supply from a shared utility transformer |
| Single-phase loads on output | Not recommended | Suitable |
| Moving parts | Fewer | More (three motors and variacs) |
| Maintenance | Slightly lower | Slightly higher (more brushes to check) |
| Price | Lower | Higher |
How Do You Calculate Voltage Unbalance?
Voltage unbalance (%) = the largest deviation of any phase from the average of the three, divided by the average, × 100. A qualified electrician with a multimeter can measure it in four steps:
Measure the voltage of each phase to neutral (R–N, Y–N and B–N) at the main distribution board.
Repeat at different times: morning, peak evening hours and when heavy loads are running.
For each set of readings, find the average of the three phases.
Find the phase that is furthest from the average, and work out how far it is as a percentage of the average.
Example: R = 235V, Y = 222V, B = 241V. The average is about 232.7V. The largest deviation is Y, about 10.7V below average. Imbalance = 10.7 ÷ 232.7 × 100 ≈ 4.6%.
That is more than four times the 1% limit recommended for motors.
For three-phase motors, also measure the three line voltages (R–Y, Y–B and B–R) and apply the same formula, since motor standards use line voltages.
Also measure the current in each phase with a clamp meter. If one phase carries much more current than the others, you have an unbalanced load, and that phase decides the stabilizer size (see the next section).
Rule of thumb: if the imbalance regularly goes above about 1–2%, or the phases differ by more than a few volts during peak hours, choose an unbalanced servo stabilizer. Share your readings with your supplier so they can confirm.
How Do You Size a Servo Stabilizer for an Unbalanced Load?
Size a servo stabilizer for an unbalanced load on the most heavily loaded phase: per-phase kVA = phase voltage × highest phase current ÷ 1000, then multiply by 3, add a 25% margin and round up to the next standard size.
Here's why this differs from sizing a balanced three-phase load. A three-phase stabilizer can carry one third of its rating on each phase. When one phase is loaded more than the others, that phase reaches its limit first. So size the stabilizer for the most heavily loaded phase, not the total or the average.
Measure the current on each phase with a clamp meter while your normal load is running.
Per-phase kVA = phase voltage (about 230V) × current on the highest phase ÷ 1000.
Required rating = per-phase kVA × 3 × 1.25 (25% safety margin).
Choose the next standard size up.
Example: clamp meter readings of R = 45 A, Y = 30 A and B = 25 A.
| Method | Calculation | Result |
|---|---|---|
| Wrong: total current | (45 + 30 + 25) × 230 ÷ 1000 = 23 kVA, × 1.25 = 28.8 kVA | 30 kVA: each phase can carry only 10 kVA, but R needs 10.35 kVA, so it is overloaded |
| Right: highest phase | 45 × 230 ÷ 1000 = 10.35 kVA per phase, × 3 = 31.1 kVA, × 1.25 = 38.8 kVA | 40 kVA: every phase has spare capacity |
In this example the right choice is the 40 kVA servo stabilizer, not the 30 kVA. If your electrician can rebalance the loads across the phases, the total stays the same but the highest phase drops, and a smaller stabilizer may be enough. For full sizing steps, read how to calculate servo stabilizer kVA.
Not keen on the maths? Send us the voltage and current on each phase, and we'll work out the imbalance and the right size for you, free. Ask an engineer.
Which One Should You Choose?
Choose an unbalanced servo stabilizer for offices, hospitals, homes, malls and mixed factories, and a balanced servo stabilizer only for three-phase motor loads on a steady, dedicated supply. If unsure, choose unbalanced.
| Site or Load | Recommended Design | Why |
|---|---|---|
| CNC machines, motors, compressors on a dedicated feeder | Balanced | Phases usually stay close; motors load all phases equally |
| Factory with mixed machines and office loads | Unbalanced | Office and lighting loads make the phases uneven |
| Offices, IT floors, banks, showrooms | Unbalanced | Many single-phase loads on different phases |
| Hospitals, clinics and labs | Unbalanced | Sensitive single-phase equipment needs each phase corrected |
| Hotels, malls, institutions | Unbalanced | ACs, lifts, lighting and kitchens spread across phases |
| Large homes and villas on three-phase supply | Unbalanced | Each phase feeds different rooms or floors |
| Data centres, server rooms, telecom | Unbalanced | Single-phase IT loads rarely split evenly across phases |
| Cold storage, printing presses, RO plants | Unbalanced | Motors mixed with controls and lighting |
| Rural or end-of-line supply | Unbalanced | Phase voltages often drift apart on long lines |
| HT consumer with own transformer, motor loads only | Balanced | Phases move together; no single-phase loads on the output |
| Lifts | Depends on the building | Check the supply; mixed buildings usually need unbalanced |
| Not sure | Unbalanced | It handles both balanced and unbalanced conditions |
For more on three-phase models and sizes, see our 3 phase servo stabilizer page.
What Happens If You Choose the Wrong Type?
Balanced unit on an unbalanced supply: the stabilizer corrects all phases by the same amount, so one phase can stay too low or too high. Single-phase equipment on that phase isn’t properly protected, and three-phase motors still see unequal voltages.
Unbalanced unit on a balanced supply: it works correctly, but you pay more than you needed to for features you don’t use.
What Should You Ask Your Supplier?
A lower quote is sometimes lower simply because it is for a balanced unit. Ask every supplier these questions in writing:
Is each phase sensed and corrected independently? (This is what makes it unbalanced.)
How many servo motors and variacs does the unit have? One means balanced; three means unbalanced.
What output accuracy is held on each phase when the input phases differ?
Is the rating per phase one third of the total, and how much load can the heaviest phase carry?
Which regulator is used: variac (dimmer) or linear (roller)?
Are phase reversal, single phasing and neutral failure protections included?
More points to check are in our guide on how to compare servo stabilizer quotes.
How Should a Three-Phase Stabilizer Be Installed?
Even the right stabilizer can perform badly if it is connected wrongly. Check these points at installation, especially for an unbalanced unit:
Neutral is compulsory: connect a full-size, tight neutral at the input. A loose or missing neutral causes wrong correction and high or low voltage on single-phase loads.
Keep the phase sequence R–Y–B: check it before commissioning. A wrong sequence trips the phase reversal protection and can run motors in reverse.
Never use neutral as earth: run separate earth conductors to the earth pit, with two earth connections on the stabilizer body.
Check neutral-to-earth voltage: a high reading points to an unbalanced load or a poor earth, and should be fixed before you blame the stabilizer.
Install after the DG changeover: so loads stay protected on both grid and generator supply.
Don’t swap input and output terminals: and label both cables for future service.
For full steps, follow our servo stabilizer installation guide. For a reliable earth, see SG Power's earthing electrodes and earth pit covers.
Three-Phase Homes: Questions From Indian Forums
On Indian forums such as Team-BHP, TechEnclave and HiFiVision, homeowners with a three-phase supply keep asking the same questions about whole-house servo stabilizers. Here are the answers:
One three-phase unit or three single-phase stabilizers?
Forum members often advise against a balanced three-phase unit for homes, because the voltage on each phase of a domestic supply differs. They're right about balanced units. An unbalanced three-phase stabilizer solves the same problem: it is effectively three single-phase stabilizers in one cabinet, each correcting its own phase. One cabinet usually costs less, takes less space and needs one installation, while three separate units let each phase run on its own. Either works; a balanced unit is the wrong choice for a home.
What happens if one phase fails?
Many three-phase stabilizers switch off the whole output when one phase fails. This single phasing protection saves three-phase motors from burning out. In a home with only single-phase loads (lights, ACs, fridge), that means the whole house goes dark even though two phases are healthy. Before you buy, ask the supplier how the unit behaves on phase failure, and whether a bypass or changeover switch is included so you can keep power on until service arrives. If your home has no three-phase motors, ask whether per-phase cut-off is available.
Why does my voltage jump on one phase but not the others?
A forum member in Bangalore traced wild swings in a new house to a loose neutral from the local transformer. A broken or loose neutral makes phase voltages drift apart as loads switch on and off, and no stabilizer can fully fix it. If one phase reads very high while another reads very low at the same moment, ask your electricity board (DISCOM) to check the neutral at the transformer and meter before you buy a bigger stabilizer.
Will a stabilizer survive lightning or an HT line fault?
No stabilizer can survive a high-voltage line touching the low-voltage line, or a direct lightning surge. Forum members report stabilizers failing in exactly these events. Fit a surge protection device at the main board, ahead of the stabilizer, add a lightning protection system on exposed buildings, and keep the earthing and earthing rods in good condition.
Do I need permission from the electricity board?
A whole-house stabilizer is installed on your side of the meter, at the main distribution board, so it usually needs no change to your connection. It doesn't change your sanctioned load either. Have a licensed electrician install it, and check with your DISCOM if you're unsure of local rules. A house with two meters needs a separate stabilizer for each meter.
Key Takeaways
Balanced vs unbalanced only matters on a three-phase supply.
A balanced stabilizer corrects all phases by the same amount; an unbalanced stabilizer corrects each phase on its own.
Most Indian sites with single-phase loads (offices, hospitals, homes, mixed factories) need unbalanced.
Measure the voltage and current on each phase; voltage unbalance above about 1–2% points to unbalanced.
Size an unbalanced load on the most heavily loaded phase, not the total.
If in doubt, choose unbalanced: it handles both conditions.
Not Sure Which One You Need? Ask SG Power
We make both balanced and unbalanced servo voltage stabilizers, so we have no reason to push one over the other. We recommend whichever your readings call for.
Our three-phase range runs from 10 kVA to 500 kVA (air cooled 10–50 kVA, oil cooled 60–500 kVA), and popular sizes include 20 kVA, 30 kVA, 50 kVA and 100 kVA. Every unit holds the output within ±1%, includes phase reversal and single phasing protection, and is tested through 173 checkpoints.
Send us your phase readings. Share the voltage and current on each phase, and our engineers will tell you which type and size you need, with a written quote.
Frequently Asked Questions
Sources
SG Power Products Pvt. Ltd., servo stabilizer catalogue (February 2026): models, input ranges and protections
Indian forum discussions on whole-house and three-phase servo stabilizers: Team-BHP, TechEnclave and HiFiVision (read November 2026)