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Basic Power Distribution Questions

  1. May 16, 2005 #1
    Greetings :)
    Recently I have discovered how fascinating electrical engineering is, and so I have been doing some studies on my own time before entering college. I have several questions regarding electrical power distribution systems.

    1.) What exactly is the purpose of the neutral wire? From what I have read, I am assuming that it is a cable that carries the sum of all three phases so that in case there is an imbalance, the imbalanced voltage/current will flow from the neutral to the ground. Am I way off?

    2.) What would happen if there was no neutral wire?

    3.) What is the meaning of center-tapped? From what I have read, I am assuming that it refers to the neutral wire that is attached at a point equidistant from the two live wires exiting from the ends of the transformer secondary coil.

    4.) What do delta and wye formations represent?

    5.) What is the purpose of the neutral contact on the outlet? Is it similar to that of the one following the power lines?

    I may have more later, and I will add accordingly. Thanks in advance!
     
  2. jcsd
  3. May 17, 2005 #2

    Cliff_J

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    Ok, lets make sure we're all on the same page. Electricity needs a complete circuit to flow. So at the typical US household single phase outlet we have a hot wire that has 120 volts of alternating current and a path back to ground called the neutral wire (EDIT: the return wire that completes the circuit). The third plug is the ground wire and its is a path back to ground. When no current is flowing the neutral and ground are at the same potential, ground. But when current is flowing in the circuit, the neutral wire (EDIT: return wire) is carrying current and is therefore not a safe wire. So the ground wire that can be connected to the metal chassis of equipment to offer some protection to us from electrical shock (along with insulation). While you may very well be aware of this, I find it annoying how some people consider there to be no distinction between the concept of a neutral and ground conductor.

    You could think of the transformer secondary as being wound with two coils and the center tap is the point where the ends of the two coils are connected. The typical single-phase US household setup the 240V is the ends of the secondary coil and the 120V is from the end of one side of the coil to the center tap so there are two of these 120V 'legs' available for use. I say typical and US because this seems to vary a bunch around the world with different standards and practices. :smile:

    I don't have the practical experience to answer your question about the similarity and necessity of the neutral line in an overhead power line but a lot of the rest of your questions may be covered by another poster or:
    http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm
     
    Last edited: May 17, 2005
  4. May 17, 2005 #3
    Thanks cliff J:)

    So in other words, the neutral serves as a reference voltage for the live?
     
  5. May 17, 2005 #4
    the neutral wire is often refered to as the RETURN Wire..following the concept of completing a circuit..i am sure you did not mean to say that the neutral wire was connected to ground..
     
  6. May 17, 2005 #5

    dlgoff

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    What he said is correct. The RETURN wire is connected to ground.

    Regards
     
  7. May 17, 2005 #6

    Cliff_J

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    I edited my post. I didn't invent the labels nor the poor water analogies that make complete circuits mysterious to those who misunderstand it. I pass the buck to them. :smile:

    While on this topic of confusion, how did we end up with black being the typical color for negative for DC power and black for hot in our homes? Does this trace back to the fight between Edison and Westinghouse/Telsa?
     
  8. May 17, 2005 #7

    dlgoff

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    Heck. When I trouble-shoot some dc equipment circuit, color coded wire is just a way to trace where it goes. When in doubt, measure it.

    Regards
     
  9. May 17, 2005 #8

    dlgoff

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  10. May 17, 2005 #9
    The netural is as stated a reference point that acts as a return point for wye formation systems. A typical example would be that one side of a street may use a red phase loaded to the neutral wire, whilst the opposite of the street may yse a blue phase loaded to the neutral. All loading is brought down to the neutral point so hence it does carry the sum of all three phases.

    Contrary to what has been said the Neutral is not connected to ground at least not directly. Hence why you may found up to 5V between N and E. Some transformers particularly centre tapped have the centre tapping connected to ground for safety. Although much effort is made to balance the loads on the three phases in a wye system. There generally is a an imbalance. The neutral point resolves this.

    If there was no neutral there would be no return for the loads back to the other side of the winding, unless of course that winding is grounded, but then there is a risk of imbalance and hence losses in the transmission.

    Centre tapped is a why of having variable voltages. It is particularly used in the delta formation. For example, lets say you have a delta formation. with 240V per winding. If you tap the centre of one winding you can have two 120V lines, or take it to the other terminal to the oppisite corner, you can get 208V (remembering phase difference).

    In basic terms wye formations providing loading to a neutral point, especially good for single phase systems, in effect prodiving three supplies. With delta formation you can have variable voltages via centre tapping. It is used for high power systems (i.e. industry) because it can provide more current.
     
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