Servo stabilizer or UPS?
These two get compared constantly and they are not alternatives. One fixes voltage, the other fixes continuity. Knowing which problem you have takes a week and a logger.
Roughly half the enquiries we take open with a description of a symptom rather than a requirement: the machine keeps tripping, the readings drift, the controller resets. The right equipment follows directly from which of two very different faults is causing it.
What each one actually does
A servo voltage stabilizer uses a motorised variable transformer to hold its output within a tight band while the input wanders. It is electromechanical, efficient, repairable, and it will run for decades. What it does not do is store any energy. If the incoming supply disappears, the output disappears with it.
An online UPS rectifies the incoming supply to DC, floats a battery bank on that DC bus, and synthesises a fresh sine wave for the load. The load never touches the mains, so the supply can sag, distort or fail entirely and the output does not change.
| Symptom | Likely cause | What fixes it |
|---|---|---|
| Equipment trips on undervoltage during shift hours | Feeder sag under estate load | Servo stabilizer |
| Everything goes down when the power cuts | Outage | UPS |
| Controller resets on a sub-second dip | Brief interruption | Online UPS |
| Motors run hot, windings fail early | Sustained low voltage | Servo stabilizer |
| Instrument readings drift or are inconsistent | Common-mode noise, earth loop | Isolation transformer |
| Both — dirty supply and frequent outages | Both | Stabilizer upstream of a UPS |
Cost is why this matters
At small ratings the two are broadly comparable and the decision is technical. At large ratings they are not remotely comparable. A 200 kVA stabilizer costs a fraction of a 200 kVA UPS, needs no battery room, no ventilation, no cell replacement every four years, and rejects far less heat. If your problem is voltage rather than continuity, buying a UPS is spending several times more than you needed to and taking on a maintenance liability you did not need.
The reverse error is worse in a different way. A stabilizer bought to fix outages will not fix them, and the money is simply wasted.
Log the supply first
A week of voltage logging at the incoming panel answers this question definitively and costs almost nothing. Sag through the day points at a stabilizer. Actual dropouts point at a UPS. Do this before anyone quotes you.
The balanced versus unbalanced trap
If a stabilizer is the answer, there is one more decision, and it is the most common specification error we correct. A balanced stabilizer regulates all three phases together from a common reference. An unbalanced — independent phase — unit regulates each phase separately.
Measure all three phase voltages across a working day. If they track each other within a few volts, a balanced unit is fine and cheaper. If one phase sits noticeably lower, which is normal anywhere single-phase load is spread unevenly, a balanced unit will correct on the average and leave the low phase low. You will have bought a stabilizer and kept the problem.
When you genuinely need both
Plenty of sites do, and the order matters: stabilizer upstream, UPS downstream. The stabilizer hands the UPS a supply inside its input window, which means the UPS spends far less time on battery and the bank lasts considerably longer. Batteries wear by cycling, so anything that reduces the cycle count pays for itself.
That said, an online UPS already accepts 330 V to 480 V on a three-phase input. If your supply stays inside that, adding a stabilizer in front of it buys you very little, and we will tell you so rather than sell you one.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
Will a servo stabilizer protect against power cuts?
No. It corrects voltage, it does not store energy. No supply in means no supply out. If your equipment goes down when the power fails, a stabilizer will not change that no matter how it is sized.
Does a servo stabilizer save electricity?
Sometimes, indirectly. If your incoming voltage regularly runs above 415 V then resistive and lighting loads draw more power than they need to, and holding the output at 415 V trims that. Anyone quoting a fixed percentage saving is guessing — the honest answer depends entirely on your voltage profile and load type.
Can I put a UPS in front of a stabilizer instead?
You can, but it is the wrong way round. The point of the stabilizer is to hand a clean supply to the UPS so the UPS stays off battery. Put it downstream and it is correcting a supply the UPS has already regenerated, which achieves nothing.