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Homework Help: AC Power and Combining Volt/Current

  1. Jul 13, 2009 #1
    1. The problem statement, all variables and given/known data
    One of the reasons we use AC power instead of DC is that with AC, we can use a single wire with a maximum voltage of 220V to return 660V. This is done by combining 3 lines of 220V into a single one. How can this be done without exceeding the wire's max voltage of 220V?

    My teacher also added this to clarify, "Ha, I realized that I was thinking in European terms, where they use 220V. Anyway, suppose I have an appliance (like an AC) that needs 660V to run. I can run this by connecting 3 220V wires to it, just like you'd use 3 4V batteries to run something that needs 12V. Suppose each wire can hold a maximum of 10A, and that's what's going through the 3 incoming wires. Now, there is a trick I can use to only need a single return wire (as opposed to the 3 incoming wires), even though it seems it would have to carry 30A (which is more than it can hold). What is the trick?"

    2. Relevant equations
    My only thought was that one could use a transformer to take (incoming) 660V in 3 wires into 1 wire of 220V. But then would I have 3 turns on one end and one on the other?

    [tex]\Delta[/tex]V2 = N2/N1 * [tex]\Delta[/tex]V1

    3. The attempt at a solution
    220V = x/3 * 660V
    But then x=1 turn, and what current or voltage would it carry?
  2. jcsd
  3. Jul 13, 2009 #2


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    Not really - the reason for using AC is that it is easier to step up/down voltages.

    No, the largest voltage difference between two 230V phases is 415v - I don't know where you get 660V from

    The trick is that the return line is at 0 volts, power flows back and forth through the live phases - in an ideal case there is no current in the return line.
    Think of it has 2 batteries back-to-back both powering separate 1.5V lamps, you can connect the point between the two batteries to ground
  4. Jul 13, 2009 #3
    The first and second paragraph look like 2 different questions. Which question are you answering? For example, first paragraph the wire has max voltage of 220V, but in second paragraph this new wire only has max current of 10A.

    Edit, nevermind, hah.
  5. Jul 13, 2009 #4
    Yes, I know it's confusing. The second part was an email from my teacher where he was trying to clarify the question for a student.

    I got 660V because there is three wires each carrying 220V towards the appliance. So the return line would have no current, or no voltage? I guess I don't understand how a closed circuit could have part of the circuit at 0 volts.
  6. Jul 13, 2009 #5
    Can anyone else help me with this problem?
  7. Jul 14, 2009 #6
    Don't you hate Lex?
  8. Jul 14, 2009 #7
    in case of a 3 wire system, the difference between the wire and ground is 220V. But the three wires have a phase difference of 60 degrees each. Hence, when u calculate the rms voltage between the wires, the difference is about 440V only. Here, no wires are joined to make one wire. There are 2 systems of connecting 3-phase supplies -Star and Delta. These links might help you better than me. http://eece.ksu.edu/~starret/581/3phase.html
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