As a preface, this is purely hypothetical. I know better than to use a transformer without knowing anything about it. I'm just trying to get an idea of whether this is possible, not whether it's a good idea.

Let's say I have a three-phase transformer that I know nothing about. I have three primary leads and three secondary leads, and that's it. No markings or anything, no datasheet, just a box with wires sticking out of it.

Is there any way to determine with external measurements how the windings are wired? I know that I can look at phase shift between primary and secondary to determine whether it's Y-Δ, but if there is no phase shift, is it possible to distinguish Y-Y from Δ-Δ? If there is phase shift, is it possible to determine which winding is Y and which is Δ?

Ideally I'd like answers that only involve measurements and physical properties. I'm aware there are conventions like using Δ on the high side of a Y-Δ transformer, but assume this transformer was wired by someone completely ignorant of any and all conventions and common sense. That means that either side could be Y or Δ without regard to relative voltages, and the neutral wire on a Y connection may not be present. You can assume the transformer is balanced, however, with three identical primary windings and three identical secondary windings.

  • \$\begingroup\$ Where's the motivation given that even if you knew the star delta stuff you wouldn't be able to use it without knowledge of supply voltage and VA rating? \$\endgroup\$
    – Andy aka
    Commented Mar 16, 2020 at 13:36
  • 1
    \$\begingroup\$ The side that has the neutral terminal on it is the 'Y' side. \$\endgroup\$
    – Neil_UK
    Commented Mar 16, 2020 at 13:52
  • 3
    \$\begingroup\$ If you only have three primary and three secondary wires I would guess \$ \Delta - \Delta \$ connection as I would expect to see neutral out if either was a 'Y' connection. But without details you can't use it. \$\endgroup\$ Commented Mar 16, 2020 at 14:02
  • \$\begingroup\$ @Andyaka This is a hypothetical scenario, I'm wondering if it's possible, not if it's useful. \$\endgroup\$
    – Hearth
    Commented Mar 16, 2020 at 16:20
  • \$\begingroup\$ @Neil_UK A Y connection without a neutral is possible too, though. Sure, you don't normally see that, but it's possible. \$\endgroup\$
    – Hearth
    Commented Mar 16, 2020 at 16:21

1 Answer 1


There is no way to determine, by external measurements, whether a 3 phase transformer's primary and secondary windings are connected in star or delta configuration.

That's because there is no parameter intrinsic to either star or delta connection.

  • \$\begingroup\$ If there's no way to distinguish them, does it matter which one you have? \$\endgroup\$ Commented Mar 17, 2020 at 12:54
  • \$\begingroup\$ @user253751: see quora.com/…. \$\endgroup\$
    – Transistor
    Commented Mar 17, 2020 at 14:42
  • \$\begingroup\$ Not being able to differentiate between delta and star connection of transformer windings in a black box does not mean that the two have no significance. They most certainly do but that's another subject altogether. \$\endgroup\$
    – vu2nan
    Commented Mar 17, 2020 at 17:44
  • \$\begingroup\$ This is what I thought would be the case, at least without doing weird things like harmonic analysis, but I wanted confirmation from others. Thank you! \$\endgroup\$
    – Hearth
    Commented Mar 18, 2020 at 14:25
  • \$\begingroup\$ Hi Hearth, Could you please accept my answer? \$\endgroup\$
    – vu2nan
    Commented Jun 26, 2020 at 10:15

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