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How to know the VA rating of a transformer

  1. Sep 11, 2011 #1
    Hello to all

    I am an electrical engineer and in an interview I was asked to tell the MVA rating of a transformer of few KV , and nothing was given means neither the power factor and nor the current , so I want to ask you people is there any way to determine the MVA rating of the transformer from the KV or vice-versa.

    Please tell me with an example if possible.

    Thank you
  2. jcsd
  3. Sep 11, 2011 #2
    I am no transformer expert, but I think the KV in your case is only the voltage input or output of the transformer, You can have two transformer of the same KV in and same KV out but of totally different power rating. eg, if you take two transformers both 110V in and 12V out. One can be rated only 1A output, the second one can be 10A. Both has the same voltage but the second one can handle 10 times the power.

    For MVA, is this Mega Volt Amp? That is a mighty big transformer!!! My understanding VA rating is power rating. eg. 100V at 1A is 100VA, so is 50V at 2A or 25V at 4A. They all 100W rating in my understanding. I have no idea why they rate transformer like this rather than just simply by Watts.

    Don't trust everything I said, this is just my understanding.
  4. Sep 11, 2011 #3
    P = I x E

    to determine power you need to know volts and amps and that should have be on the nameplate of the machine. The interview process might have been testing you on this basic concept.
  5. Sep 12, 2011 #4
    What is the reason people don't use watts as specification instead using VA if they mean the same thing?
  6. Sep 12, 2011 #5


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    http://en.wikipedia.org/wiki/Kva" [Broken]
    Last edited by a moderator: May 5, 2017
  7. Sep 12, 2011 #6
    What did you do?
    The best answer would be to explain to them that the question isn't answerable.
  8. Sep 12, 2011 #7
    So real power in watts is the real part of VA?
    Last edited by a moderator: May 5, 2017
  9. Sep 12, 2011 #8
    This image is worth a thousand words

    Last edited by a moderator: Apr 14, 2017
  10. Sep 12, 2011 #9

    jim hardy

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    "What is the reason people don't use watts as specification instead using VA if they mean the same thing?"

    In AC, Watts = Volts X Amps X (cosine of angle between them)
    that cosine term being called "Power Factor"
    see 2mile's triangle in post immediately above

    If angle is not 0 , there's more amps than are necessary to deliver the watts.
    The copper in the transformer has to carry those extra amps.
    While those amps don't deliver power to the load, they do heat the copper windings in the transformer.

    so they rate transformers in volt-amps not watts.
  11. Sep 12, 2011 #10
    I learn something today!!
  12. Sep 13, 2011 #11
    Hmmm, some places toss out questions whose answers depend on experience. For example, I remember being asked about common part numbers during an interview. Without having worked in the business, the question was unanswerable....
  13. Sep 13, 2011 #12


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    They could have been checking on your experience, as transformers are fairly standardized, and the voltages they interact with are very standardized (433V, 1kV, 11kV, 33kV, 66kV, 132kV etc). For example, 433V (which is slated for machines using ~415V and allowing for a voltage drop across the connecting cables) is predominant in mines with 1500kVA to 2500kVA transformers.

    They could have been testing on the research you did regarding the company whom you were trying to get a job with (online and/or talking to people). Most company's have a rough operating range for transformer (power) size, i.e. 250kVA to 50MVA. This can be limited by the lifting capacity of the cranes they have, the expertise of the designers, the market they are aiming for or the voltages their equipment can handle.

    If they don't give you any values then it can be safe to assume they don't want you to do any math.
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