Re: Oregon Wye Calc. requirements ?????????
Originally posted by ronaldrc:In the three phase configuration if the phases did not overlap and interfere with each other each other would you have 240 or 208?
I think I know what you are trying to say, and I hope you continue to ask questions, give opinions, and offer views. But I think a little electrical theory would help here. The following discussion is about the ?ideal transformer,? one in which we can disregard any internal losses, and just focus on the transformation process.
First, a three-phase transformer is nothing more than a set of three single-phase transformers wired together. The three primary windings can be connected to each other in either a WYE or a Delta configuration. The three secondary windings can be connected to each other in either a WYE or a Delta configuration. That gives you four choices: (1) WYE-WYE, (2) WYE-Delta, (3) Delta-WYE, and (4) Delta-Delta.
Secondly, the voltage on any of the secondary windings depends on two things, and on
NOTHING ELSE: (1) The voltage on the primary, and (2) The ?turns ratio,? defined as ?(the number of times the primary wire is wrapped around the primary core) DIVIDED BY (the number of times the secondary wire is wrapped around the secondary core).? Let us say that the primary voltage is 480 volts. Suppose the primary winding has 100 turns of wire, and the secondary has 50, a ratio of 2:1. Then the secondary voltage will be half the primary, or 240 volts. Now suppose the primary had 500 turns of wire and the secondary had 250. This is still a ratio of 2:1, so the secondary voltage will still be 240. Next let us suppose the primary had 400 turns and the secondary had 100. This is a ratio of 4:1, so the secondary voltage will be 120 volts (i.e., one quarter of 480). The bottom line here is that once the transformer is built, once the wires are wrapped a specified number of times around the core, then the turns ratio is permanently set, and forever thereafter the secondary voltage will be the same. If the transformer was built for 240 volts, then it will never give you 208. If it was built for 208, then it will never give you 240. No change in phase angles, no overlapping, no interference can cause the wires inside the transformer to unwind themselves, and therefore the secondary voltage is permanently fixed.
Finally, how do we build the commonly used 120/240V transformer (like the one that feeds my house) and the commonly used 120/208V transformer (like the one you might find in a large apartment building)?
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- <font size="2" face="Verdana, Helvetica, sans-serif">The 120/240 across the street from my house is not a set of three transformers connected together. It is a single transformer. It has a turns ratio of about 50:1, so that the primary voltage of about 12,000 volts is transformed down to 240 volts. The secondary winding has a tap connected in the center. That point is grounded (connected to planet Earth) and is used as the neutral point. You can read 120 volts from either ?hot? to ?neutral, and 240 volts between the two ?hots.?
The most common 120/208 is a set of three single-phase transformers, each of which has a turns ratio of 4:1. The primary voltage of 480 volts is transformed down to 120 volts in each of the three transformers. The three secondaries are connected WYE. The common point is grounded (connected to planet Earth) and is used as the neutral point. Here, for the first time, is where phase angles come into play. When you read voltage across any secondary winding, you are reading from ?hot? to ?neutral,? and you will see 120 volts. But the three secondary voltages are actually (1) 120 volts at an angle of 0 degrees, (2) 120 volts at an angle of 120 degrees, and (3) 120 volts at an angle of 240 degrees. So when you read voltage from line to line (or ?hot? to ?hot?), you don?t get 120 + 120 = 240. In fact, you are not really adding voltages, you are subtracting (i.e., reading ?voltage difference?). Rather, you get 120 (angle of 0) minus 120 (angle of 120) = 208 (angle of -30). Regardless of whether you do the mathematics by drawing vectors on a page or by using trigonometry, you are going to get that ?square root of three? factor. That is, if you multiply 120 by the square root of three, the answer is 208.</font>
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One final note. In an earlier post, I mentioned that there is a phase shift in the Delta-WYE transformer. You can see that when I subtracted the two 120 volt signals, the answer had a final angle of minus 30 degrees. That angle is the reason for the phase shift in the Delta-WYE. The phase shift has nothing to do with the 208, or the 120, or the 240. It comes from one side of the transformer being a Delta and the other side being a WYE.
