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Transformer Confusion (Simple, Help!)

  1. Dec 17, 2011 #1
    1. The problem statement, all variables and given/known data

    See first figure attached.

    2. Relevant equations

    3. The attempt at a solution

    For this question I understand parts 1) and 2) entirely and have obtained the correct answers from the solutions.

    I am confused about part 3) and I think my confusion stems out from confusion related to the rated voltages of a transformer.

    Please take a look at the 2nd figure attached.

    In this figure, where are the rated voltages of a transformer?

    Would they be at E1 and E2? Or are they V1 and V2?

    Once that is sorted out I have one more confusion to figure out.

    In part 3) they tell you it is at rated load. Rated load means rated current, but rated current doesn't always imply rated voltage! (Only if you have the rated power, correct?)

    How is it that in the calculation for the voltage regulation they assume rated voltage of 1000V?

    Thanks again!

    Attached Files:

    Last edited: Dec 17, 2011
  2. jcsd
  3. Dec 17, 2011 #2


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    Staff: Mentor

    It would have to be the supply and load voltages while loaded, V1 and V2.

    The transformer operates on currents. It is current that pushes the magnetic core towards saturation, that's ultimately its limiting factor. Sure, voltages stress the various insulations, but otherwise don't affect the transformer action. So testing under full load would mean full rated current.

    What was your answer to Q1?
  4. Dec 18, 2011 #3

    I determined that the OCT was conducted on the LV side and the SCT was conducted on the HV side.

    The only part left that I'm confused about is how they assume rated voltage across the terminals of the load.

    I was told in class that rated load (which implies rated current) does not always mean rated voltage!

    Is this the case?

    The biggest difficulty I have is reading the questions and pulling out the information they are giving me based on rated "this", and rated "that".
  5. Dec 18, 2011 #4


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    Staff: Mentor

    That seems logical. Is that how you arrived at that--based on common sense? Because I suppose it would not be beyond the bounds of possibility for someone to come up with a disasterous design where this was not the case; where you really did have to apply 50% of the supply voltage in order to get the full rated current on s/c test! It would be a very inefficient transformer.
    You're saying "does not always mean exactly, precisely 1000.00 volts", are you?

    Short answer: I forget. :(
    Longer answer: It most likely is true that rated load (which implies rated current) does not always mean while delivering the exact rated voltage. For the simple reason that the answer you get would be very little different from what you'd get if, during tests, you really did go to all the trouble of adjusting the primary voltage until the secondary terminal voltage was precisely 1000.00 volts under load. And after all your extra effort, the figure would probably be less helpful to users, when in practice they are concerned with the actual terminal voltage when supplied from actual mains.

    Think about it. Choose any transformer model and calculate regulation (at rated current) when E is 1000.0 volts. Now calculate it again, but for E of 1013.0 volts, or whatever you'd like it to be so that V equals 1000.0 volts. With the higher E, the load voltage will now drop by just a tiny tiny bit more under full load, leading to your calculation of regulation being almost identical as when you didn't involve all the expense of providing a heavy-current variable 100-115vac supply for practical testing of the transformer.
    I recall, now with amusement, how my class had a lecturer who clearly didn't understand the per unit system for analysing transformers that he was trying to teach us. You may not have heard of it, but per unit is a way to simplify the analysis of large power systems. Well this particular lecturer folded his arms and refused to mark our lab reports until we re-worked them in the way he insisted we should, even though we pointed out that this was plainly wrong because it attributed to the transformer an efficiency of greater than 100%. But he was adamant. So we re-worked the calculations, and got our reports marked! Everyone passed the subject.

    It was a lesson in life, that no one is perfect! :smile: Though I didn't accept it with such equanimity at the time. :yuck:
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