A 4-wire delta transformer-secondary interconnection using three 1ø transformers is
A 4-wire open-delta transformer-secondary interconnection using two transformers is
Illustration links at http://home.att.net/~benmiller/elecsys.htm Phase-to-phase voltages and phase-to-neutral voltages are effectively the same for the two transformer arrangements. Both are 3ø systems, but are generally intended to supply smaller 3ø loads and a combination of 1ø loads.
One way of explaining the difference is by the change in voltage from no-load to loaded conditions. Comparing the two systems, if a 1ø 2-wire 240V load is connected to Bø and Cø, there will be less voltage drop with the three-transformer arrangement than in the two-transformer arrangement.
Three-phase induction motors operate most efficiently {meaning with minimal excess heating} when voltage at their terminals is closest to that on their nameplates. In 3ø circuits, there two parts to line voltage—besides the gross quantity of, say, 230V, there is also balance of the three 1ø voltages that must be considered. At 3% unbalance, an induction-motor horsepower load should be derated about 10%. At 5% voltage unbalance, the motor output should be derated by about 25%.
[This message has been edited by Bjarney (edited 08-20-2003).]