Would the step-up factor be 20:1, in order to achieve 4400 VAC Wye from 220 VAC Delta?
I.e. A-B leg in Delta = 220 VAC and A-B leg in Wye = 4400 VAC
To understand transformers, the key is to realize that the primary and secondary windings are wrapped on top of each other. It is more difficult to see because of the way the diagram is drawn on the bottom half (but is shown in the top half). In the bottom half, it would be better if X1 was southwest, X2 was northwest and X3 was east (in other words the wye diagram rotated 30? counter-clockwise). With that you could see that H1-H2 is wrapped with X1-X0, H2-H3 is wrapped with X2-X0, and H3-H1 is wrapped with X3-X0 (the 180? direction difference is due to the way the windings are wrapped around the core relative to each other).
Once you realize that, you can see the transformation ratio is a function of the ratios of the number of primary and secondary windings, then the rest is just vector math (or just drawing accurate lines and angles on paper).
So: we want 4400 line-to-line from 220 line-to-line.
1) On the delta side, the winding goes from line-to-line so the winding voltage is 220V
2) On the wye side, the line-to-line voltage is made up of the sum (or difference depending on reference) of two winding voltages. Also, the (line-to-line voltage) is sqrt(3)*(line-to-neutral voltage).
3) On the wye side, the winding voltage is the line-to-neutral voltage so the winding voltage is 4400/sqrt(3)=2540
4) The "step up factor" is the winding ratio. This is the wye side line-to-neutral voltage divided by the delta side line-to-line voltage which is 2540/220=11.55
Unless I fat-fingered something, there it is.
