Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AC Theory Question: Transformers

  1. Apr 25, 2006 #1
    Question: A short circuit test is performed on the low voltage side of a 40-kVA 4800/240V 60HZ transformer and the following data are obtained: W = 240W, V= 66V, and I = rated current. Determine the percent voltage regulation with a 0.88 lagging PowerFactor load connected.

    On the Short cct test I found
    Impendence (Ze) to be = 7.92<64 ohms

    Basically now we've got a simple series circuit with
    Source = 4800V
    Ze = 7.92<64 ohms
    ZL = x< -28.4 ohms

    I'm totally stuck on how to find either the current (I) or the magnitude of ZL (x). If I could find either of these I could solve the voltage drop acrossed ZL and find the VR.
  2. jcsd
  3. Apr 25, 2006 #2


    User Avatar

    Staff: Mentor

    Sorry for my dumb question, but how do you perform this short circuit test? Is the 66V the input voltage when the output is shorted? What is the assumed source impedance for the 240VAC mains feeding the transformer during this short circuit test?
  4. Apr 25, 2006 #3
    The short cct test is just shorting out the Load Impedence (ZL) this allows us to isolate Ze and calculate it by doing:

    Ze=V/I = 66/8.33 = 7.92
    To find the angle we break Ze down into Re (resistance Equivilant) and Xe (inductive). cosX = re/xe = 64 degrees.
    We found these values using the power triangle.

    The 66v tells us the amount of voltage the circuit uses I guess.

    I'm not sure what you mean by the last part, but we assume the sources are perfect for these questions (ie no internal resistance).

    The final circuit looks like this

    Source 4800VAC (no internal resistance)
    Load Ze = 7.92<64 ohms
    Load ZL = x< -28.4 ohms

    This is where im stuck how can I find the current when all I have is a source voltage and 1 of the 2 loads?

    Also, the teacher mentioned that he may give us the current of the circuit WITHOUT the angle, how would I go about finding the angle of the current?
    Last edited: Apr 25, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook