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The efficiency of a transformer

  1. May 14, 2017 #1
    1. The problem statement, all variables and given/known data
    A step-down transformer has an efficiency of 90%, the voltage of it primary coil is 200V and its current is 0.5A, and its secondary coil has a voltage of 9V and a current of 10A and its number of turns is 90 turns.
    Calculate the number of turns of the primary coil.

    2. Relevant equations
    η =VsIs/VpIp * 100

    Vs/Vp = Ns/Np

    Is/Ip = Np/Ns

    3. The attempt at a solution

    I solved the problem in two slightly different methods, the first one:


    And the second one:


    The solution in the first method (Np = 1800) is the correct one, but I don't understand why the second one is wrong. I thought that both will lead me to the same answer, but apparently they didn't, could you explain it for me, please?
  2. jcsd
  3. May 14, 2017 #2


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    The transformer is not ideal.
    this is not true.
  4. May 14, 2017 #3
    OK, but why (Is/Ip = Np/Ns) is true?
  5. May 14, 2017 #4


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    It is because of the mmf balance in the core. (Magetizing current is neglected as it is very small compared to the load current).
  6. May 14, 2017 #5
    I didn't know about the MMF balance in my text book, but I googled it and understood it. Many thanks!:smile:
  7. May 14, 2017 #6


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    Thank google:wink:!
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