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Homework Help: Transformer copper loss calculation

  1. Aug 2, 2009 #1
    Hello guys have a little problem

    see if anyone can help

    1. The problem statement, all variables and given/known data

    Q the primary winding resistance is 125Ohms, Secondary is 0.7ohms calulate the transformers copper loss at full load.

    now the transformers name plate is 240V 50Hz 28VA the earlier question explains it is a step down transformer delivering 12V

    2. Relevant equations

    so primary Current = Vp/Rp 240/125 = 1.92A

    now im not sure what figures to use to calc the current in secondary winding at full load

    i get values from 40A 38.4A

    and using formula Full load current = VA rating/primary voltage i get .116A

    i need Is value for copper loss formula

    Copper loss W= Ip(sq)xRp+Is(sq)+Rs
    Copper loss W= 1.92(sq)x125+Is(sq)+0.7 dont know the equation for secondary current Is

    any help appreciated
  2. jcsd
  3. Aug 2, 2009 #2


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    First of all, you're not calculating the primary current correctly. Hint: what does 28 VA mean?

    Once you have that, remember that power in = power out to work out the secondary full-load current.
  4. Aug 2, 2009 #3
    thanks for the reply but im lost now.

    not entirely sure what to do with the 28VA but thought it was used somehow to calc secondary current

    thought i did the priomary current ok though
    I=V/R 240/125

    am i on the wrong track?
  5. Aug 2, 2009 #4


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    Very much so. Let's start at a more basic level, then:

    What is VA? What does it mean and how is it calculated?
  6. Aug 2, 2009 #5
    "The VA rating is the math formula used in determining the amperage at a given voltage."

    So my secondary voltage of 12 volts from a 28VA transformer will produce 2.33 amps...?

    e.g., 28 VA ÷ 12 Volts = 2.33 Amps. am i correct.
  7. Aug 2, 2009 #6


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    Right. Now, what do you know about the relationship of primary current to secondary current?
  8. Aug 2, 2009 #7
    does this apply to the primary current also

    primary voltage of 240volts from a 28VA transformer will produce .116 amps...

    e.g., 28 VA ÷ 240 Volts = .116 Amps.

    thanks for you help in this
  9. Aug 2, 2009 #8


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    You've got it! Now you can plug those values into I2R to find the primary and secondary copper watt losses.
  10. Aug 2, 2009 #9
    Brilliant thanks for the advice

  11. Feb 11, 2011 #10
    hey guys
    i have a doubt
    will turns ratio affect the amount of copper loss or core loss
    i am a little bit confused in ds matr
  12. Feb 11, 2011 #11
    Insufficient data.

    Is 12 VRMS the unloaded secondary voltage?
    Is this an imaginary transformer with the unloaded primary power dissipation equal to zero?
    I don't know if this one counts, but are we to pretend that 'full load' is a purely resistive load?
    Last edited: Feb 11, 2011
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