6 - Three-phase circuits

Abstract

This chapter discusses three-phase circuit. It also explains single-phase alternating circuits, that is, the source of supply has been a single ac generator. It is possible to generate such a single-phase supply by rotating a coil between the poles of a magnet. To obtain three completely separate circuits from a three-phase generator or alternator, each of the three outputs may be independently connected to its own particular load. If the loads are completely identical, the system is said to be balanced and the emf's currents and phase angles will be equal for each output. The two methods of interconnection are known as (1) star or Y or (2) delta or mesh. A star-connected circuit is obtained by connecting together the starts (or the finishes) of the windings on the alternator as a common point. The common point of the three windings is called the neutral or star point, and the line joining the common point of the star-connected loads to this point is the neutral line. Whereas in delta or mesh connection, the load is assumed to be balanced and there is no neutral point or neutral line. The star-connected impedances carry the line currents and the voltage across each is the phase voltage.