Delta-wye transformer

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A delta-wye (Δ-Y) transformer is an electrical device that converts three-phase electric power without a neutral wire into 3-phase power with a neutral wire. It is generally built from 3 independent transformers. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.

In the United States, Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three different types of power from one power source:

Three-phase power, for 480 volt motors
Single-phase leg-to-leg, for 208 volt motors and high-current loads
Split-phase leg-to-neutral, for 120 volt lights and appliances

An older method of providing these three different forms of power from three-phase was with the high-leg delta transformer, but these have fallen out of use due to complex application rules and hazards.

In Europe, an example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing three 230V phases and an earthed neutral point.

Delta-wye transformers should not be confused with the Y-Δ transform, which is a mathematical transformation used in circuit analysis.

[edit] Output voltages

The single-phase voltage is the voltage between any phase wire and neutral. The three-phase voltage (which is equal to single-phase voltage x $\sqrt{3}$ ) is the voltage between any two phase wires: