Finding the supply voltage in a single phase transformer (Urgent)

Discussion in 'Homework Help' started by Oscar Vazquez, Feb 13, 2015.

  1. Oscar Vazquez

    Thread Starter New Member

    Feb 13, 2015
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    I have worked this out and compared to other colleagues and they are getting different answers that do not match up to the turn ratios, and I am getting tired of this problem.

    A single-phase distribution transformer supplies four houses. The maximum load is 12 kW at a power factor of 0.78 (lagging). The load voltage is 235 V. The transformer rating is 20 kVA, with a primary voltage of 7.2 kV, a secondary voltage of 240 V, and a leakage reactance of 12%. Calculate the transformer supply voltage.

    I've worked it out but I'm not sure its correct because you are given a leakage reactance in the problem, but it does not need to be used.

    0213151339.jpg
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Of course the leakage reactance must be used.
     
  3. Oscar Vazquez

    Thread Starter New Member

    Feb 13, 2015
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    Ok, thats fine, and please bare with me as I am a newbie at this. According to the problem they want the supply voltage, does this mean that they want the induced voltage at the primary winding? And is the leakage reactance in the correct place, based on this problem, would the leakage reactance be on the secondary side instead of the primary? I will post the new schematic.
    new schem.jpg
     
  4. Oscar Vazquez

    Thread Starter New Member

    Feb 13, 2015
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    Here is my new solution, I read the problem wrong. It is asking for the voltage that the transformer is supplying, not the voltage that is being supplied to the transformer. Yes, 7kV would be what is supplying to it, but what I think is that it wants the induced voltage.
    new soltn.jpg
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    You've lost me. Your original question states that the load voltage is 235V. Post the question exactly as asked, then it might be possible to answer intelligently.
     
  6. Oscar Vazquez

    Thread Starter New Member

    Feb 13, 2015
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    I apologize, I will restate the specs for this problem. The transformer is rated at 20kVA, with a primary winding of 7200V and secondary winding of 240V, with leakage reactance of 12%. The load voltage is 235V with a max load of 12kW. Find the voltage supplied by the transformer.

    My approach is to find the load current which is the total current at the secondary winding. I then solve for the per unit reactance of the leakage reactance. This impedance is then used to find the voltage drop at the impedance by using the current at the secondary winding. Summing the load voltage and the voltage drop at the impedance is equivalent to the induced voltage at the secondary.

    After this, use the turn ratios to solve for the primary induced voltage and were done.

    Thank you in advance for helping me out.
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    That's the correct approach. Your solution in post #4 looks reasonable.
     
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