Note that these illustrations do not show any difference in the number of primary and secondary turns. If it did, more would be in the primary than in the secondary since both are reducing the primary voltage. The turns ratio determines the increase or decrease in voltage and current between the primary and secondary coils. What stands out in Figure 1 is that only two connections are at any point on the schematics. Both primary and secondary coils have two. The secondary on the left is connected hot to ground, and the one on the right is connected hot to hot.
The two center tapped voltages are also hot to ground. With three incoming phases, the connection scheme is different, and that is the purpose of Wye and Delta connections. Three-phase transformers consist of three separate sets of coils, each of which is connected to an individual phase.
For voltage and current to flow through the coils, some common connection must be among them. Figure 2 shows the two possible connections. The Delta connection joins the coils as an equilateral triangle and applies the individual phases at each of the vertices. The Wye connection joins together one end of each of the coils and applies the individual phases to the open ends.
These two connections produce very different results when power is applied. An advantage of the Delta connection is higher reliability.
If one of the three primary windings fails, the secondary will still produce full voltage on all three phases. The only requirement is that the remaining two phases must be able to carry the load. If one of the windings in a Wye primary fails, two of the phases of a Delta secondary will see a reduced voltage.
If the secondary is also Wye connected, two phases will have reduced voltage and the other will have zero volts. An advantage of the Wye connection is that it can provide multiple voltages without the need for additional transformers. This can reduce cost in many applications. The primary is wound as Delta, and the secondary is wound as Wye. The incoming phase voltages are applied at P1, P2 and P3.
S1, S2 and S3 are the output voltages. I mentioned earlier that the output of the two connections is different. Either can be wound to produce a particular phase voltage, but the phase-to-phase voltages will be different for the Wye and Delta connections. Figure 4 shows the secondary output side of a Wye-connected, three-phase transformer.
The green line is a center tap that leads to ground. In Figure 4, the individual phases are volts, and each produces volts when connected to the center tap. When connected phase to phase, the voltage is only —not the volts we might expect. The answer is Wye. Wye connections produce a different phase angle among the phases, and the phase angle determines the phase-to-phase voltage. The benefit is that a constant allows you to compute the phase-to-phase voltage produced by a Wye connection.
The phase-to-phase voltage will always be 1. Figure 5 shows the secondary output side of a Delta-connected, three-phase transformer. As in the Wye example, the individual phases produce volts. Figure 5. The secondary output side of a Delta-connected, three-phase transformer. In this example, the phase-to-phase voltages are twice the individual phase voltages, or volts.
It may appear that the Delta is a more efficient design, but phase angle also has a role here. The phase-to-phase current in a Delta circuit is only 1. Single-phase loads are connected line to neutral for v operation, whereas 3-phase equipment, such as motors, are connected line to line for operation at v. Figure shows the phasor diagram for a wye-delta transformation. From this diagram it is evident that there is a large phase angle between the line-to-line voltages on the wye side and the corresponding line-to-line voltages on the delta side.
Capacitors, Magnetic Circuits, and Transformers is a free introductory textbook on the physics of capacitors, coils, and transformers. See the editorial for more information Wye-Delta Connection The wye-delta connection affords the advantage of the wye-wye connection without the resulting disadvantage of unbalanced voltages and third harmonics in the line-to-neutral voltages when operating without the neutral wire. Figure
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