This is a simplified application—a case where a gas station has a combination of motor loads. There might be a 3ø 7½-hp 230V motor for an air compressor, and four 1ø ½-hp 115V motors in fuel dispensers. The smallest distribution transformers that utilities typically use are around 10kVA. If three transformers are used, the bank capacity is 30kVA, and is able to serve a load of about 30hp. For a single customer that may be a little wasteful, so instead two 1ø transformers could be used in the open-delta arrangement, and serve a load of 17kVA. Considering that the load for the gas station may be around 12kVA—the utility can adequately serve the gas station while not having to maintain a third transformer over its life of ~25 years.
There is a caveat in all this. In motors intended for dedicated applications like submersible water pumps and hermetic refrigeration compressors, manufacturers recommend not using the open-delta transformer configuration to serve their products. These two types of motors operate at higher current densities that most plain-vanilla NEMA T-frame motors. Consequently, at their rated mechanical load, {output} they are designed ‘closer to the edge’ in terms of heating and overall durability, so are generally more likely to run hotter at a particular degree of voltage unbalance compared to otherwise similar T-frame motors.
