What is an Open Delta transformer?

An open delta transformer is a three phase transformer that only has two primary and secondary windings, with one side of the delta phase diagram “open”. Open delta transformers are rare and are typically only used for small loads where cost is important. More common is critical loads being wired with three single phase transformers in a banked configuration. Should one of these transformers fail, the three phase circuit can remain active although the two remaining transformers are limited to about 57% of the total load. This allows a circuit to remain powered during a failure of a transformer, albeit at a lower overall load factor.