Topology
Transformer or transformerless: what current three-phase UPS models use
A transformerless (transformer-free) UPS converts power with IGBT electronics alone, with no isolation transformer in the power path; a transformer-based UPS adds one at the input, the output or both. Many current three-phase families are transformer-free, among them the Vertiv Liebert APM2 and EXL S1, the Mitsubishi 9900AEGIS and the ABB DPA 250 S4, and transformers now appear mainly as voltage options, such as the input isolation transformer on the Eaton 9355 and Tripp Lite S3M at 480 V and 600 V.
Transformer arrangement by model
| Model | Transformer | Phases and wiring | Status | Source |
|---|---|---|---|---|
| Vertiv Liebert APM2 Liebert APM2 UL, 208 V (data sheet SL-71300) | Transformer-free; dual input transformer cabinet available | Output 208/220 V, 3-phase 3-wire + N + PE | Current | Vertiv, Liebert APM2 data sheet SL-71300 |
| Vertiv Liebert EXL S1 Liebert EXL S1 (brochure for 625 to 800 kVA) | Transformer-free | Not stated | Current | Vertiv, Liebert EXL S1 625-800 kVA brochure |
| Vertiv Trinergy (newer range) Vertiv Trinergy, the newer range that follows the Trinergy Cube (brochure SL-71331) | Transformer-free | Not stated | Partly discontinued | Vertiv, Trinergy UPS brochure SL-71331 |
| Mitsubishi Electric 9900AEGIS 9900AEGIS, 80 to 225 kVA at 480 V | Transformerless | Not stated | Current | Mitsubishi Electric, 9900AEGIS UPS |
| ABB DPA 250 S4 DPA 250 S4 | Transformer-free IGBT converters, three-level | Not stated | Current | ABB, DPA 250 S4 |
| Eaton Power Xpert 9395P Power Xpert 9395P, 200 to 1,200 kVA | 600 V models need no external transformer (Eaton) | Not stated | Current | Eaton, Power Xpert 9395P page |
| Eaton 93PM 93PM 208V (North America) | Distribution cabinet with input and output transformer options | 208/120 V or 220/127 V, 4-wire input | Current | Eaton, 93PM 208V UPS page |
| Eaton 9355 9355, 10 to 30 kVA | 480 V and 600 V models use an input isolation transformer | Output 208/120 V or 220/127 V | Current | Eaton, 9355 UPS page |
| Tripp Lite (Eaton) SmartOnline S3M SmartOnline S3M, 10 to 100 kVA | 480 V and 600 V systems pair the UPS with an input isolation transformer; WR models add an input/output transformer | Not stated | Current | Eaton (Tripp Lite), 3-phase UPS solutions |
| XPC M90U-80 M90U-80, 15 to 80 kW at 208/120 V | 480 V input transformer as an option | 208/120 V three-phase in and out | Current | XPC, M90U-80 |
Sources, checked October 3, 2026: values as each manufacturer publishes them for the model shown; “Not stated” means the page we read does not give the value.
Where the transformer went
The transformer has moved out of the UPS and into the voltage options. Eaton lists the 9355 at 480 V and 600 V "with transformer", an input isolation transformer ahead of the UPS. Tripp Lite sells its 480 V and 600 V S3M systems as the UPS plus a matching isolation transformer cabinet: the S3M100K-100K6T, for example, pairs the S3M100K with the S3MT-100K600V, and its WR models add an input and output transformer. Vertiv offers a dual input transformer cabinet for the Liebert APM2, Eaton a distribution cabinet with input and output transformer options for the 93PM, and XPC a 480 V input transformer for the M90U-80.
What a transformer costs in heat
Eaton's 9355 page shows the difference in its heat tables. A 10 kVA 9355 dissipates 3,798 BTU/hr at 208 V or 220 V input and 6,294 BTU/hr at 480 V or 600 V with the input isolation transformer; at 30 kVA the figures are 9,220 and 13,831 BTU/hr. That heat is extra cooling load in the electrical room. Eaton quotes 91% typical efficiency for the 9355, against 97.5% in double conversion that Vertiv quotes for the transformer-free Liebert APM2.
When a native 600 V UPS changes the answer
Eaton offers the Power Xpert 9395P in 600 V models and says they remove the need for an external transformer. Its quoted double-conversion efficiency is up to 96% at 600 V and up to 97% at 480 V. The 600 V guide lists the other ways to reach 600 V.
Neutral and isolation
A transformer-free UPS passes its supply's conductor arrangement through to the load. Eaton lists the 9395XC as 480 V 3-wire in and out, so 277 V or 120 V loads behind it need a neutral created downstream, by a transformer in the PDU or a transformer option. Where a site needs galvanic isolation between supply and load, the transformer cabinets above provide it.
Transformer-based and transformerless frames both appear among the three-phase UPS families listed online, and isolation questions on an installed system are handled by three-phase UPS engineering for mission-critical sites.
Frequently asked questions
Is the Vertiv Liebert EXL S1 transformerless?
Yes. Vertiv describes the Liebert EXL S1 as a monolithic, transformer-free UPS.
Does the Eaton 9355 have a transformer?
Only in its 480 V and 600 V versions, which Eaton lists with an input isolation transformer. Eaton's heat figures for a 10 kVA unit are 3,798 BTU/hr at 208 V and 6,294 BTU/hr at 480 V or 600 V with the transformer.
Is the Tripp Lite S3M a native 600 V UPS?
No. Tripp Lite supplies the S3M for 600 V as the UPS with a 600 V input isolation transformer, for example the S3M100K-100K6T (S3M100K with S3MT-100K600V).
Is the Mitsubishi 9900AEGIS transformerless?
Yes. Mitsubishi Electric describes the 9900AEGIS as a transformerless UPS.
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