Three-Phase Circuits

Three-Phase Circuits =)

       Initially we explored the idea of three-phase power systems by connecting three voltage sources together in what is commonly known as the “Y” (or “star”) configuration. This configuration of voltage sources is characterized by a common connection point joining one side of each source. (Figure below)

Three-phase “Y” connection has three voltage sources connected to a common point.

If we draw a circuit showing each voltage source to be a coil of wire (alternator or transformer winding) and do some slight rearranging, the “Y” configuration becomes more obvious in Figure below.

Three-phase, four-wire “Y” connection uses a "common" fourth wire.

The three conductors leading away from the voltage sources (winding's) toward a load are typically called lines, while the windings themselves are typically called phases. In a Y-connected system, there may or may not (Figure below) be a neutral wire attached at the junction point in the middle, although it certainly helps alleviate potential problems should one element of a three-phase load fail open, as discussed earlier.

Three-phase, three-wire “Y” connection does not use the neutral wire.

Overview and Insights

-The conductors connected to the three points of a three-phase source or load are called lines.

-The three components comprising a three-phase source or load are called phases.

-Line voltage is the voltage measured between any two lines in a three-phase circuit.

-Phase voltage is the voltage measured across a single component in a three-phase source or load.

-Line current is the current through any one line between a three-phase source and load.

-Phase current is the current through any one component comprising a three-phase source or load.

-In balanced “Y” circuits, line voltage is equal to phase voltage times the square root of 3, while line current is equal to phase current.

 

-In balanced Δ circuits, line voltage is equal to phase voltage, while line current is equal to phase current times the square root of 3.

-Δ-connected three-phase voltage sources give greater reliability in the event of winding failure than Y-connected sources. However, Y-connected sources can deliver the same amount of power with less line current than Δ-connected sources.