Star Connection

On Picture 1 is shown classic Star (Y) connection of the three-phase load. The loads on the picture are marked as R1, R2 and R3, assuming that they are pure resistance loads. In general, the loads are more complex than pure resistance, so, in that case the loads should be represented as impedance Z1, Z2 and Z3.

Picture 1: Star (Y) Connection of the three-phase load


The line voltage between phases R and S is Urs = Ul. The phase current is actually the load current, Ir = Ir1 (the current that flow through phase R is same as the current that flow through load R1, the current that flow through phase S is same as the current that flow through the load R2, and the current that flow through the phase T is same as the current that flow through the load R3). The voltage of the load R1 is Ur1 = IrR1 = Uf. Only in case when the three-phase load is symmetric, which means that all three resistances are equal R1 = R2 = R3 or all three impedances are equal Z1 = Z2 = Z3, then the line voltage Ul (between each two phases Urs = Ust = Utr = Ul) is equal to the voltage of the load Uf times square root of 3, as shown on Picture 1.

Picture 2: Star (Y) Connection of the three-phase source


On Picture 2 is shown Star (Y) connection of the three-phase source. The coils of the source are connected in star configuration. The coil 3-0 provides L1 phase voltage, the coil 2-0 provides L2 phase voltage and the coil 1-0 provides L3 phase voltage. In this case, also is present the Null conductor (null line). The null line is connected to the center of the star, the common node 0 of the three coils. On the Picture 2 are also shown all the phase and line voltages. The voltages from the each phase to the null line are phase voltages (Ul1,n ; Ul2,n ; Ul3,n) and the voltages between each two phases are line voltages (Ul1,l2 ; Ul2,l3 ; Ul3,l1).

Just to mention here, that the labeling of the phases on the Picture 1 is R-S-T, and the labeling of the phases on the Picture 2 is L1-L2-L3. In practice and in the literature we can find both of them, depending of the convention, however, they refer to the same phases. In other words, Phase R corresponds to the Phase L1 and both of them refer to the Phase 1 of the three-phase system. Phase S corresponds to the Phase L2 and both of them refer to the Phase 2. Phase T corresponds to the Phase L3 and both of them refer to the Phase 3. Also, the star connection on the Picture 2 has null line (which is actually 4-wired connection), and the star connection on the Picture 1 has not (which is actually 3-wired connection). So, if we want to connect the three-phase load from the Picture 1 to the three-phase source from the Picture 2 we should add null line wire to the load, which will be connected to the center of the star, the common node of all three loads R1, R2 and R3.