Using autotransformer for delta-delta voltage bucking.

[Cncsparky]

Hey everyone, I ordered a transformer that said it was a 240d-208d transformer, and without seeing it said it was an autotransformer, I assumed it was isolation because of it being delta-delta. I’m wondering if I can use it to buck the voltage for my delta load from the high leg delta line feed. Would I just connect H 1, 2, 3 and x1, 2, 3 and ignore the common X0/H0 neutral point? Ground line and load to the ground bar and call it a day? I’m guessing this could be problematic if there is any significant control circuit current causing imbalance. I’m going to call the manufacturer tomorrow, but wanted to see if anyone has dealt with this before. The nameplate clearly says 240wye to 208 wye with a star configuration diagram.

The load is a CNC lathe (Doosan Lynx if that matters)

 

[jim dungar]

You have a wye-wye transformer.
As long as there is no H0-X0 bond at your transformer you will have an isolated system.
It is possible your transformer is an auto transformer design, but that woukd have a factory installed H0-X0 bond. Can you post a picture of the nameplate?

You can install it on your 240/120 center grounded system by simply ignoring the H0 connection. In general, unless you are a utility, you do not need to bring neutrals to isolation transformer primaries.

An isolated transformer gets treated like any typical 208Y/120 system.
An autotransformer needs to have the H0-X0 connection floating, because this point is not a neutral point for all of the input phases.