In short. In an independent-phase stabiliser each phase has its own sensing, control and mechanism, so each output phase is held at its set value even when input phases differ or the load is unbalanced. It is the better choice for mixed single-phase and three-phase loads.
How it works
Three separate regulators correct line-to-neutral voltage per phase. They share a neutral and are protected as a group. Because each phase is separate, differences at the input are removed at the output.
Why it matters
- Voltage unbalance harms motors with extra negative-sequence current and heating. See unbalance studies.
- Single-phase loads on only one phase pull that phase down.
- Drives and sensitive loads may trip on unbalance.
Sizing
Size each phase for its own current plus starting surge. A phase heavily loaded may set the kVA rating of the unit, even if the others are lightly loaded. Compare cost with load redistribution, which can reduce the required size.
| Item | Consideration |
|---|---|
| Per-phase current | Highest phase current sets module rating |
| Neutral current | Check neutral conductor and terminal capacity for unbalance and harmonics |
| Input range per phase | Cover the lowest and highest measured on each phase |
| Protection | Per-phase overload, thermal and voltage cut-offs |
Neutral
Neutral current can be significant with unbalance and triplen harmonics. Size the neutral and terminals accordingly. See harmonics.
Operation on generators
Generators dislike unbalanced loading, which heats the rotor. Independent-phase regulation corrects voltage but does not remove the unbalanced current from the source; balance the load too. See negative-sequence limits and DG and stabiliser.
Illustrative example
An office building has phase currents of 220, 140 and 90 A and phase voltages of 370, 395 and 410 V. An independent-phase stabiliser brings all phases to 415 V line to line. The engineer also moves some circuits between phases to reduce neutral current and the size of the unit.
Why unbalance heats motors
An unbalanced supply can be described as a balanced positive-sequence part plus a smaller negative-sequence part. The negative-sequence voltage drives current in the opposite rotating direction relative to the rotor. This current is large compared with the voltage causing it, because the rotor sees the opposite field at nearly double slip frequency. The result is extra rotor and stator heating, reduced torque and shorter insulation life.
That is why a few percent of voltage unbalance can produce a much larger percentage of current unbalance, and why motor makers recommend derating or correction.
Sizing and loading per phase
| Item | Example approach |
|---|---|
| Phase R current | Peak recorded, with starting surge margin |
| Phase Y current | As above |
| Phase B current | As above |
| Module rating | At least the highest phase requirement, with low-input derating |
| Neutral | Recorded neutral current with harmonic allowance |
Reduce the problem at the source
- Draw which circuits are on which phase.
- Move single-phase loads to even out current.
- Recheck by recording again.
- Size the stabiliser for the improved, not the original, distribution where practical.
Balanced currents reduce neutral current, voltage drop and the size of the stabiliser needed.
Protection
- Per-phase overload protection.
- Over and under voltage cut-off per phase.
- Phase sequence and failure relays for motor feeders.
- Thermal protection on each regulator module.
Commissioning checks
- Measure each output phase against the set point at light and heavy load.
- Check neutral current.
- Test bypass operation.
- Test a single-phase failure response and confirm the protection acts.
Monitoring
Where practical, install a meter showing the three output voltages and currents. A drift on one phase is an early warning of a failing module or an overloaded circuit.
The master engineering question
Are the phase voltages and currents different enough that correcting each phase separately is the only way to protect the loads?
What to send us
Send phase readings over a representative period. Send your application details.
Questions engineers ask
Is it more expensive?
Usually, because each phase has its own mechanism.
Does it balance the load?
No. It corrects voltage; load currents remain unbalanced.
Do I still need phase protection?
Yes, for phase loss and sequence.
Can it serve a three-wire load?
Check the manufacturer’s wiring for delta-connected loads.
Values are illustrative. Use manufacturer specifications.