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Transformer question (between substation and home)

  1. Mar 21, 2015 #1
    1. The problem statement, all variables and given/known data
    A power station generates 100kW of power at a voltage of 10kV. Given that the connecting cables in the primary and secondary circuit each have a resistance of 10Ω, find the turn ratio required for an ideal step-down transformer to bring electrical energy to the home at 240V.

    2. Relevant equations

    Turn ratio:
    Ns/Np = Vs/Vp = Ip/Is
    (s- secondary coil, p- primary coil)

    V = IR
    P = V^2/R = IV = I^2R

    3. The attempt at a solution
    For the primary circuit,
    Current = 100000/10000 = 10A
    Potential difference across 10Ω resistance = 10*10 = 100V
    Potential difference across primary transformer = 10000-100 = 9900V

    I know the power across both transformer coils must be equal, so:
    Power output at secondary circuit = Power input at primary coil = 9900*10 = 99000W

    - What does this 99000W refer to? The power across the secondary coil or the power across the whole secondary circuit?
    - Or does the secondary coil act like cell in the secondary circuit? ie. Do I have to consider its resistance?
    - Does the home act like a lightbulb in a circuit? So it has a resistance?

    I'm stuck here because I can't find the potential difference across the secondary coil to use the turn ratio formula.

    Diagram of secondary circuit:
  2. jcsd
  3. Mar 21, 2015 #2


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    Science Advisor
    Homework Helper
    Gold Member

    The power is going somewhere, so yes, assume the home load (resistance) is R. What equations can you write down?
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