Transformer, 3-Phase

Transformer, 3-Phase. A three phase transformer is constructed by winding three single phase transformers on a single core.


Three phase transformers are used throughout industry to change values of three phase voltage and current. Since three phase power is the most common way in which power is produced, transmitted, an used, an understanding of how three phase transformer connections are made is essential. A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.

There are only 4 possible transformer combinations:


Some typical 3-phase transformer connections.

  • Delta to Delta - use: industrial applications
  • Delta to Wye - use : most common; commercial and industrial
  • Wye to Delta - use : high voltage transmissions
  • Wye to Wye - use : rare, don't use causes harmonic and balancing problems. [1]


One voltage cycle of a three-phase system, labeled 0 to 360° ( 2 π radians) along the time axis. The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle will repeat 50 or 60 times per second, depending on the power system frequency. The colors of the lines represent the American color code for three-phase. That is black = Voltage Load (VL1); red = Voltage Load (VL2); blue = Voltage Load (VL3)

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection)

phase-phase connections
Load (L1) Load (L2) Load (L3) Neutral Earth
North America Black Red Blue White Green
North America (newer 277/480 installations) Brown Orange Yellow White Green


  1. Electricians Toolbox Etc. Three Phase Transformer Info. 2007. [1]


3 Phase Wiring

See also[]