Open delta transformer connections

[Family man]

I am having a hard time with this one. I have someone who bought a Gardner Denver air compressor from someone at an auction. It consists of a 200 amp disconnect fused at 150 amps with a sticker labeling it as 480. Then apparently went into two single phase transformers and per adventure acheived 575 volts three phase. As that is what the compressor and air dryer need. It was then fused at 80 amps before going into a line reactor and then to the compressor. The two transformers are 10 kva and have a 480/240-240/120 rating. I am unfamiliar with this open delta but I opened one and found it wired H1 to a wire that was external, H2 and H3 wire nutted together, H4 to X3 to X1 to a wire that was external, X2 to X4 to a wire that was external. I am saying external because they were in a flex. Three wires in each flex.
First off I am questioning it because it has a primary coil rating of 480 and we need 575 to operate. Secondly not sure what voltage would actually be present. Could use some input to be at peace with this one.


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[GoldDigger]

The simplest case of an open delta is two separate single phase transformers with one primary (for example) wired between A and C of the input three phase and the other with its primary wired between B and C.
The two secondaries are wired together at one end to form the output C' terminal while the other two ends become A' and B' respectively.
The output delta is then A'B'C'.
As long as the primaries are wired in delta, the delta output voltage is just the turns ratio of each transformer times the input delta voltage.

Since your setup connects the primary (H) and secondary (X) windings in series it appears to be making an autotransformer from each isolation transformer.
Analyze it as a buck/boost design.

You cannot (easily) make a buck or boost three phase configuration using autotransformers which is closed delta, since the triangle will not close properly.
You can easily make a three phase buck or boost with three transformers if you wire them in wye instead of delta.

 

[Family man]

Open delta transformer connections

After analyzing my connections and looking at the wiring diagram for an open delta it looks like a open delta configuration. Looks like 480 A phase would go to H4, X3, X1. B phase to H1. C phase. X2, X4 would go to corner of delta.
Then H1 would connect to second transformer H1 and also to center of delta. So in the end it would be both H1's and B phase of 480 would also be B phase of 575v.
Then C phase would go to X3, X1, H4 of second transformer
X2, X4 of second transformer would become last corner of delta


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[Family man]

After analyzing my connections and looking at the wiring diagram for an open delta it looks like a open delta configuration. Looks like 480 A phase would go to H4, X3, X1. B phase to H1. C phase. X2, X4 would go to corner of delta.
Then H1 would connect to second transformer H1 and also to center of delta. So in the end it would be both H1's and B phase of 480 would also be B phase of 575v.
Then C phase would go to X3, X1, H4 of second transformer
X2, X4 of second transformer would become last corner of delta


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Sorry C phase of 480 does not enter first transformer.


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