Older three-phase UPS carried a heavy output transformer; most modern ones do not. Here is what that transformer did, why it disappeared, and when you still want it, including the isolation question.
For decades a three-phase UPS had a large iron transformer on the output of the inverter, and a lot of installed units still do. It was not there by accident, it did real work, and understanding that work is the only way to judge whether dropping it costs you anything on your site.
The transformer provided galvanic isolation between the inverter and the load: no direct electrical connection, just a magnetic coupling. That isolation gave you a clean separation of grounds, blocked DC and common-mode noise from passing through to the load, and let you derive a fresh neutral on the output. It also made the unit robust and fault-tolerant. A big transformer is hard to damage; it rides through short circuits and large inrush currents, soaks up transients, and gives the output stage a high tolerance to abuse on the load side. On a three-wire input it could create the output neutral the load needed, and it allowed the input and output voltages to differ, a built-in voltage match. Those are genuine engineering advantages, and they are exactly why transformer-based designs dominated for so long and why so many are still in service. Large and industrial units, and most legacy frames, a legacy Eaton 9395, an older MGE Galaxy, are transformer-based.
If the transformer was so useful, why did the industry abandon it? Because the cost of carrying it grew while the need for it shrank. A power transformer is big, heavy and expensive, and it burns energy, magnetising losses and copper losses that are there every hour the unit runs, loaded or not.
Power-electronics improved to the point where the inverter could do the regulating and conditioning the transformer used to help with, without the iron. Dropping the transformer makes a UPS smaller, lighter, more efficient and cheaper: less floor space and weight, a real bump in operating efficiency because you have removed a standing loss, and a lower purchase price. For a modern IT load, which is what most three-phase UPS now feed, those gains matter more than the isolation the transformer provided, the load does not need a derived neutral from the UPS, and the input and output voltages already match. So the industry moved. Most current platforms are transformerless: the Eaton 93PM, the APC Galaxy VS and VL, and their peers. Transformerless is now the default, and for the typical data-hall load it is the right default.
The transformer did not disappear because it stopped being useful, it disappeared because the typical load stopped needing it. Where the load or the environment is not typical, a transformer-based unit is still the better engineering choice. It still makes sense when:
The environment is harsh or industrial. Dust, heat, wide voltage swings, demanding fault current, a transformer-based unit brings ruggedness and a high tolerance to abuse that a compact transformerless cabinet does not. This is why industrial installations still specify them.
You need galvanic isolation or specific grounding. Some loads, codes or grounding schemes require true isolation between source and load, or a separately derived neutral. A transformer delivers that inherently.
You need voltage matching. When the input voltage and the load voltage differ, the transformer matches them in one device, instead of adding a separate transformer downstream.
You want maximum fault tolerance. For the most demanding loads, the transformer’s ability to absorb short circuits, inrush and transients is a margin of safety some sites still want by design.
Transformerless is the right default, but it is a set of trade-offs, not a free win. The advantages are concrete: smaller footprint, lower weight, higher efficiency (no standing transformer loss), lower capital cost, and the higher power density that makes modular designs possible, you cannot build a compact hot-swap module around a large iron transformer.
The cost is what you give up. There is no galvanic isolation by default, so source and load share a ground reference and the unit cannot derive a fresh neutral on its own. The output stage relies on the electronics, not a heavy transformer, for its fault tolerance, well-engineered and more than adequate for a clean IT load, but without the brute resilience of iron. And for an industrial environment, a transformerless cabinet is generally less rugged than a transformer-based unit built for the same conditions. None of this matters for the load transformerless was designed for. It matters when the load or the site is the exception.
Isolation is the point that causes the most confusion, so it is worth stating plainly: a transformerless UPS gives no galvanic isolation, but isolation can always be added externally when you need it. If a particular load, code requirement or grounding scheme calls for true isolation or a separately derived neutral, you fit an external isolation transformer on the output of the transformerless unit. You get the efficiency, size and cost of a transformerless UPS for the bulk of the installation, and add isolation only on the circuits that actually require it, rather than paying the weight, loss and cost of a built-in transformer across the whole system whether you need it everywhere or not.
That is usually the better answer when only part of the load needs isolation. Where the whole load needs it, or the environment is harsh enough that you want the transformer’s ruggedness as well as its isolation, a transformer-based unit in one package is cleaner than a transformerless unit plus an external transformer. The decision turns on how much of the load needs isolation and how demanding the environment is, not on a blanket preference for one topology.
Two practical points decide a clean install. Grounding: because a transformerless unit does not derive its own neutral, the neutral and grounding arrangement has to be worked out with the building’s electrical system, the source neutral carries through, and the install has to respect the site’s grounding scheme and code. A transformer-based unit can sidestep this by deriving a neutral itself, which is sometimes the simpler path on an older or unusual electrical system. Derating: a transformer is heat and standing loss, but it also buys margin; a transformerless unit relies on its electronics, so on difficult load mixes, high harmonic content, high crest factor, you confirm the unit is rated for it or apply a derate, rather than assume the iron will absorb it. We work both through against the measured load when we size the system, because neither shows up on a nameplate.
There is no single right answer, there is the answer that fits your load and your environment. For a clean, modern IT load in a controlled room, transformerless is almost always right: smaller, more efficient, cheaper, and the foundation for a modular build. For a harsh or industrial site, a load that needs true isolation or a derived neutral, a voltage mismatch to resolve, or the maximum fault tolerance, a transformer-based unit earns its weight, and where only some circuits need isolation, a transformerless unit with an external isolation transformer on those circuits is often the most efficient whole. Because we supply every major brand in both topologies, we are not steering you toward a product line. Tell us the load, the upstream source, the environment and any isolation or grounding requirement, and we will spec the topology that actually fits and size it properly. Compare current platforms like the Eaton 93PM, or browse systems for demanding sites on the industrial page.
A transformer-based UPS has an iron transformer on the inverter output that provides galvanic isolation, a derived neutral, voltage matching and high fault tolerance, at the cost of size, weight, standing energy loss and price. A transformerless UPS removes the transformer and relies on its power electronics, which makes it smaller, lighter, more efficient and cheaper. Most modern three-phase units are transformerless; many large, industrial and legacy units are transformer-based.
Because power electronics improved enough to handle the regulating and conditioning the transformer used to assist with, while the transformer’s downsides, bulk, weight, cost and a standing energy loss every hour the unit runs, stayed the same. For a typical IT load, which does not need a derived neutral or voltage matching from the UPS, dropping the transformer gives a smaller, more efficient, cheaper unit with no real loss. It also enables compact modular designs.
No, not by default, source and load share a ground reference and the unit does not derive its own neutral. If a load, code or grounding scheme requires true isolation or a separately derived neutral, you add an external isolation transformer on the output, ideally only on the circuits that need it. For loads where the whole system needs isolation, or in harsh environments, a transformer-based unit in one package is often the cleaner choice.
When the environment is harsh or industrial, when the load or grounding scheme needs true galvanic isolation or a separately derived neutral, when input and output voltages differ and need matching, or when you want the maximum fault tolerance of a heavy transformer. For a clean IT load in a controlled room, transformerless is almost always the better fit on size, efficiency and cost.
Tell us the application and we will come back within one business day, sizing, the right system, install and a price. Three-phase installs usually need a licensed electrician, so let us know if you have one.