In short. Three-phase stabilisers come in balanced-load and independent-phase types. The choice depends on how unevenly the phases are loaded and how the supply voltages differ between phases. Sizing, neutral arrangement and protection must be chosen for the actual plant.
Design types
| Type | How it regulates | Best for |
|---|---|---|
| Balanced-load (three-phase common control) | One control moves all three phases together | Balanced loads such as three-phase motors. See balanced-load |
| Independent-phase | Each phase regulated separately | Unbalanced loads and uneven phase voltages. See independent phase |
Choosing
- Measure phase voltages and currents over time.
- If phase voltages differ much or single-phase loads are significant, choose independent-phase.
- If loads are mainly balanced three-phase, balanced-load may suffice.
Sizing
kVA = √3 × V × I ÷ 1000
Use the highest phase current for independent-phase designs, and check that each phase module can carry its load plus starting surge. See selection and sizing.
Neutral and earthing
Keep a solid neutral through the stabiliser. Provide proper earthing. For generators with switched neutral, confirm the stabiliser wiring suits the arrangement. See earthing architecture.
Protection features
- Overload and short circuit.
- Over and under voltage cut-offs.
- Phase failure and sequence.
- Thermal protection.
- Bypass operation if the stabiliser fails.
See phase protection.
Illustrative example
A building with mixed single-phase lighting and three-phase lifts has a phase R at 380 V, Y at 400 V and B at 350 V. A balanced-load unit would move all phases together and leave imbalance. An independent-phase stabiliser corrects each phase to 415 V line to line.
Understanding why phase voltages differ
Phase voltages differ because phase currents differ. Heavy single-phase loads pull their phase down through the supply impedance, and the other phases stay higher. In a building with uneven distribution, the gap can be tens of volts. A three-phase motor sees that gap as voltage unbalance, and responds with extra current in the rotor and extra heat.
Voltage unbalance (%) = Maximum deviation from average ÷ Average voltage × 100
For phase voltages of 380, 400 and 350 V, the average is 376.7 V and the maximum deviation is 23.3 V, so unbalance is about 6.2 percent. Motor makers normally recommend keeping unbalance to a few percent, so a figure like this is a warning.
Neutral current and the fourth wire
In a four-wire system the neutral carries the vector sum of the phase currents. Unbalanced single-phase loads and harmonic currents from electronic loads add to it. The neutral conductor, its terminals and the stabiliser neutral path must be sized accordingly. Check the maker’s neutral rating.
Selection flow
- Record voltage and current on all three phases for a representative period.
- Calculate unbalance at the worst time.
- If unbalance is small and loads are mostly three-phase, a balanced-load unit may be adequate.
- If single-phase loads are significant or unbalance is large, choose independent-phase regulation.
- Consider moving circuits between phases to reduce the problem at source.
Protection settings to review
| Setting | Why it matters |
|---|---|
| Over and under voltage cut-offs | Prevent passing bad voltage to loads |
| Phase sequence | Prevents reverse rotation after maintenance |
| Overload per phase | Protects the weakest phase module |
| Time delay on restart | Lets the supply settle after an outage |
Generator supply
When the building runs on a generator, unbalanced loading also stresses the alternator. Voltage regulation helps the loads, but balancing the circuits helps the generator. Treat both.
Documentation
Keep a diagram showing which circuits sit on which phase. When loads are added, update it and recheck balance.
The master engineering question
Are the phase voltages and loads close enough for common control, or does each phase need its own regulation?
What to send us
Send phase readings and load split. Send your application details.
Questions engineers ask
Can I use single-phase units on three phases?
Three single-phase units can be used in some arrangements; check wiring and neutral.
What if the load is unbalanced?
Use independent-phase regulation.
Does it correct phase sequence?
No, but protection can block on wrong sequence.
What about harmonics?
A stabiliser does not remove harmonics. See harmonic mitigation.
Values are illustrative. Use manufacturer ratings.