Open-delta transformer banks always consist of
two 1ø transformers, where
delta banks {sometimes called ‘closed-delta’} use
three 1ø transformers. See the two compared in Scott35’s Technical Reference Area under
https://www.electrical-contractor.net/ubb/Forum15/HTML/000054.html It’s important to understand that [in elecbob’s sketch] that there should almost always be very close to 240V across the two shown transformer-secondary windings. But also, there should be 240V across the open-delta ‘ends’ at 12 o’clock and 4 o’clock in elecbob’s sketch.
For most utilities, open-delta configurations are reserved for smaller motor loads, on the order of 7½-20hp. Voltage imbalance is the biggest problem—and 3ø induction motors are sensitive to stator heating—with reducing the motor’s mechanical load to prevent overheating.
Note that one way to illustrate open-delta imbalance is using two 10kVA transformers and a 10kW 240V 1ø 2-wire resistive load. If the 10kW load is connected across the lower or left winding in elecbob’s sketch, voltage drop will be about this same. But if the same load is connected across the open ends of the two windings [still at 240V] somewhat greater voltage drop will be produced.
