On a Delta-connected system, how many phases are required to energize a transformer?

Prepare for the Southeast Lineman Training Center Exam with quizzes and detailed explanations. Enhance your skills and knowledge through interactive tests. Get exam-ready!

In a delta-connected primary, the three windings form a closed loop. Because of that closed path, applying a line-to-line voltage between any two phases drives current around the triangle through the windings. That circulating current magnetizes the core and energizes the transformer even though only two of the three phases are connected to the source. So two phases are enough to energize a delta-connected transformer. Of course, for normal balanced three-phase operation you would apply all three phases, but starting the transformer can be done with two.