Delta and WYE

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kencoel

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I"m sure this question has been asked several times before. So perhaps just a link to the answer would be great! Please explain what a Delta is? I know it has a stinger leg (150V-180V) and had something to do with the early days in industrial applications. Perhaps a diagram of how this looks. And while were at it perhaps show a WYE system. THANKS!
 
There are hundreds of diagrams around, so I'll paint you pictures with words instead.

They're both supplied by a three-phase primary system that feeds three single-phase transformers. How the secondaries are connected together is what we're concerned with here. Let's assign the ends of each secondary with an A and an A'.


In a Delta system, the three secondaries are connected together in a way that resembles a triangle, or a Delta. Secondary 1's A' connects to secondary 2's A, secondary 2's A' connects to secondary 3's A, and secondary 3's A' connects back to secondary 1's A.

The corners where each A' meets A are the points that power is taken. The voltage between any two wires is the voltage of each secondary. At this point, the voltage to earth is floating, since we have not earthed any wires yet.

If all of the loads are 3-phase, we need do nothing else, nor ground any wire. However, unless it's imperative to keep the system energized even during a fault, it's safer to ground one of the wires, which is called 'corner grounding'.

There is 240 between any two wires, and 240 bewteen either of the ungrounded wires and the grounded wire, and of course, between either of those two wires and earth. The grounded wire will of course measure 0 volts to earth.

If we need 120v 1-phase as well as 240v 3-phase, we can instead add, and ground, a center tap to one (and only one) of the three secondaries, creating a neutral. This is exactly, and I mean exactly, the same as the 240/120v system in your house.

There is 120v between either end of this secondary and the neutral (and earth), and 240v between the two wires of the secondary with the tap. However, there will be approximately 208v between the third hot wire (the 'stinger', or 'high leg') and earth.

Interestingly, if you were to omit one of the transformers, the resulting 2/3 of the triangle, aka the 'open Delta', will still finction the same, except for a bit higher impedance (lower stability) and lower power availability. This is used where the 3-phase load is a small part of the total.

Note that the Delta system, especially the high-leg and open-Delta high-leg varieties, is considered passe these days. More common is a 480v Delta for industrial and other high-power systems for more efficient power distribution.


More common is the Y system. Here, we take three transformer secondaries again, but connect them with all three A terminals connected together, and we take the power from each of the A' terminals. This common point is grounded, and is the system neutral.

The Y system comes in two popular flavors: 208/120v and 480/277v. The higher voltage is line-to-line, and the lower voltage is line-to-neutral, and is the voltage of a secondary. The Y is used where the majority of the load is 120v, and is the most popular in new installations.


I hope this gives you a start. Ask away if you have more questions.
 
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