# "Black box" transformer: Y-Y, Y-Δ, or Δ-Δ connected?

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.

• 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? Mar 16, 2020 at 13:36
• The side that has the neutral terminal on it is the 'Y' side. Mar 16, 2020 at 13:52
• 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. Mar 16, 2020 at 14:02
• @Andyaka This is a hypothetical scenario, I'm wondering if it's possible, not if it's useful. Mar 16, 2020 at 16:20
• @Neil_UK A Y connection without a neutral is possible too, though. Sure, you don't normally see that, but it's possible. Mar 16, 2020 at 16:21