Electronic – Transformer to convert 220V split phase to 220V single phase

Writing my comment as an Answer (though it is not) as I do not have enough reputation (in electrical forum) to comment.

Last year I tried this by connecting R + Y + (reversed)B phases (R+Y-B) in series on the secondary of 3Ph Transformer.

As R + Y gives a vector exactly opposite to B, by inverting B we get -B in Phase with R + B. I simulated this at 210V using 3X3Ph Transformers and lamp bank.

As expected, I got single phase supply (with appropriate scaling) but got disappointed to see Unbalanced Phase currents on primary side. Later rechecking model on paper showed me that my idea was wrong. Primary being 3 Phase it will always load B Phase (which was reversed) unbalanced. Hence discarded the idea.

Imagine each single-phase transformer has an 'O'-shaped core. When you make a 3-phase, you combine one side of each 'O'. If the 3-phase system is balanced (equal magnitudes of currents in each leg), then the total flux through the combined core sums to zero -- basically the centre leg is not necessary. In practice, it is usually present -- it helps in case there is some imbalance in the phase currents, or even a shorted fault in one phase.

So, instead of needing three complete 'O' cores, you just need 3 'C' cores. This saves about half the iron.