In short. A balanced-load stabiliser corrects all three phases together using one control. It suits plants where loads and phase voltages are close, such as groups of three-phase motors. It cannot fix differences between phases, so imbalance must be checked first.
How it works
One sensing and control circuit drives a common mechanism that adjusts all three phases together. The output follows the average of the three phases, so if one phase is higher than another at the input, that difference remains at the output.
When it suits
- Three-phase motors, pumps, compressors and drives with balanced current.
- Supplies where phase voltages track each other.
- Budget-sensitive installations where imbalance is small.
When to avoid
- Significant single-phase loads on the output.
- Large differences in phase voltages at the input.
- Equipment sensitive to voltage imbalance.
Checks before selecting
| Check | Method |
|---|---|
| Phase voltage difference | Record each phase over time; compare maximum difference |
| Load balance | Clamp or log phase currents |
| Voltage unbalance limit of equipment | Check motor and drive limits. See unbalance studies |
Sizing
kVA = √3 × V × I ÷ 1000
Add margin for starting surges and growth. See kVA guide.
Cost and reliability
A single mechanism means fewer parts, but a fault affects all phases. An independent-phase design has more mechanisms but can continue regulating the remaining phases in some failures, depending on design. Compare, with bypass arrangements in both cases.
Illustrative example
A pump house with four 55 kW motors sees phase voltages within a few volts of each other, ranging from 360 to 400 V. A balanced-load unit holds output near 415 V and the motors run cooler. A later change adds large single-phase lighting; the engineer rechecks balance and decides whether to upgrade to independent-phase.
A quick test of whether balanced-load control is enough
Take recordings of the three phase voltages at the input. For each moment, find the largest difference between any two phases. If that difference stays small compared with the output tolerance you need, common control may work. If it often exceeds that tolerance, the phases need separate correction.
| Measured difference between phases | Likely decision |
|---|---|
| Within a few volts | Balanced-load control is reasonable |
| Tens of volts at times | Review loads; consider independent-phase |
| Large and persistent | Independent-phase, plus load rebalancing |
These bands are illustrative. Use the tolerance of your own equipment.
What the output looks like under imbalance
With common control, the stabiliser chooses a correction that brings the average output close to the set value. A low phase and a high phase will both move together, so a 20 V gap at the input is still about 20 V at the output. Motors see that residual gap as unbalance.
Operating notes
- Check the output on all three phases at commissioning, not just one.
- Add phase failure and sequence protection.
- Record input and output after any change to site loads.
Maintenance
A common mechanism reduces parts but concentrates wear. Check brushes, rollers, the drive coupling and the limit switches. Test bypass operation periodically.
When to upgrade
If a plant adds single-phase loads such as lighting, office equipment or new machine controls, rerun the balance check. If the output imbalance has grown, plan the change to independent-phase during a scheduled shutdown, and keep the old unit as a spare or bypass if it remains serviceable.
Cost thinking
Compare not only the purchase price but also the cost of motor losses and failures caused by imbalance. A cheaper unit that leaves motors running with high unbalance may cost more over its life.
The master engineering question
Will the output of a common-control unit leave any phase outside the limits of the load?
What to send us
Send phase voltage and current records. Send your application details. See also three-phase stabilisers.
Questions engineers ask
Is balanced-load less accurate?
Accuracy applies to the average; individual phases may differ.
Can it be upgraded?
Usually replacement is needed for independent regulation.
Does it fix phase loss?
No. Use phase failure protection.
What if the loads change later?
Recheck balance and consider independent-phase if needed.
Values are illustrative. Use manufacturer specifications.