Three-Phase Transformers are one of the work horses of a power transmission and distribution system. They are actually among the most often used on the transmission part of the system and are also an integral part of the industrial distribution system.
The more efficient generation systems are in three phases. APhase is essentially one portion of a period in which the armature of a generator completes one revolution. As it rotates, it passes by either fixed and permanent magnets or electromagnets attached to its casing on the inside, just a breath away from the armature and its windings or magnets.
Since there are commonly three circuits independent of each other, either on the armature or on the fields, it is considered that electricity produced this way is in three phases. Consequently, there will be three output lines and any transformer used to modify power coming from such a generator will also have to have three lines; one to handle each phase. Thus you have what is called a Three-Phase Transformer.
There may also be a fourth neutral line that allows simultaneous service for both high-voltage and single-phase applications.
Three-Phase Transformers are characterized by the presence of three separate coupled windings; each designed to transform one of the three incoming lines from the main generator, transmission or distribution source. In effect, a Three- Phase Transformer is actually a group of Single-Phase Transformers built into one. Therefore a Three-Phase Transformer can also be considered to be one of many others, as in Step-Up Transformer, Step-Down Transformer and Auto Transformer.
Using a Three-Phase Transformer assures proper power service for industrial applications with 240V supply needs. It is also common to split off a single-phase service from a Three-Phase Transformer. Once a service has been supplied from a single-phase source, as is found in most residential applications, it is necessary to use a special application, like a Rotary Phase Converter to run Three-Phase equipment.
Three-Phase is often cheaper and more efficient as it carries higher voltages that travel with less resistance over smaller lines. Cost of lines and losses to resistance are reduced and saves money.