Connection in Three-Phase System:
The three elements that means three phases of 3-phase source or three impedances of 3-phase load may be inter-connected in two different ways called
1. Star connection, and
2. Delta connection.
In star connection similar ends the 3-phases are joined together to compose the neutral as shown in Figure (a), star connection is mention by Y and the common or neutral point is denoted by n, it is also called star point.
(a) Star Connection (b) Delta Connection
In delta connection three-phases are connected in series, end-to-end to compose a closed mesh as illustrated in Figure (b). Two dissimilar ends are joined together. Delta connection is mentioned by Δ.
It may be shown that for 3-phase balanced system, the sum of instantaneous values of currents or voltages is always zero. Let instantaneous 3-phase voltages of a balance system are represented by :
Va = Vm sin ωt , Vb = Vm sin (ω - 120o ) , Vc = Vm sin (ω - 240o )
then, at any instant,
va + vb + vc = Vm sin ωt - Vm sin (ωt - 120o ) + Vm sin (ωt - 240o )
= Vm [sin ωt + sin (ωt - 120o) + sin (ωt - 240o)]
= Vm [sin ωt + 2 sin (ωt - 180o) cos 60o ]
= Vm [sin ωt - 2 sin ωt cos 60o]
Likewise, for 3-phase currents of a balance system, we can show that
ia + ib + ic = 0
It is the important property of 3-phase balanced system which has made possible the star and delta connection of three phases, without any circulating currents.