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

How to calculate power loss from a transformer

  1. Aug 4, 2011 #1
    I am currently trying to figure out how much power loss there is with a small transformer we have running VERY hot.

    XF info:
    120/208 to 277V

    I can't recall any power equations dealing with real transformers.
  2. jcsd
  3. Aug 4, 2011 #2


    User Avatar

    Staff: Mentor

    Why is it running hot? If it's rated for 5kVA at 60 Hz, what load is it seeing? What is the power factor of the load?
  4. Aug 4, 2011 #3
    load is approximately 3.84 kW. Its in an office building so the pf is most likely very near 1. I'm trying to figure an efficiency % or power loss to include in an energy audit type write up dealing with savings to be had if the transformer was removed from the system. its technically a step down 277 to 120/240 transformer but we have it hooked up the other way turning our 120/208v to 277v for office lighting.

    Here is a picture of the nameplate,


    im just trying to get a reasonable guesstimate on a effiency % to get a rough idea on yearly savings from removing the transformer.
  5. Aug 4, 2011 #4
    with a 5% impedance,



    this is my methodology behind it, using the Iload of the circuit.
  6. Aug 4, 2011 #5
    Maybe the transformer has aged and changed its property. If you are in dubious of Transformer, then you need to do a rigorous Transformer Testing.
    But, before that, you may like to use power analyzer, to check the input Power and Output power from transformer to check its efficiency.
    But, even before that, you may be assuming the transformer to be too hot, while its a normal temperature for it. Transformers do run hot.
    According to name plate, its internal temperature may rise to as high as 115 C
    Best of luck.
  7. Aug 4, 2011 #6
    The nameplate says that transformer temperature rise is 115 deg C and max ambient is 40 deg C.
    Transformer can have a maximum temperature of 155 deg C.
    Remember water boils at 100 deg C so if transformer is below boiling water temperature, it is very cold.
  8. Aug 5, 2011 #7
    I think the temperature is that of the hottest part of the transformer, which isn't its outer body. So, the outer body temperature must be much lower.
  9. Aug 5, 2011 #8
    "I think the temperature is that of the hottest part of the transformer, which isn't its outer body. So, the outer body temperature must be much lower."

    Depending on the transformer design, the outer body temperature may be much lower or it may be nearly the same as the hot spot temperature.
    The point is that the transformer's outer body can still be extremely hot and still be within the transformer's specifications.
    Many people will touch a transformer and when it burns their hand, will say the transformer is too hot. The transformer usually isn't too hot unless it is being overloaded.
  10. Aug 5, 2011 #9


    User Avatar
    Science Advisor

    Hi yopy, I notice that you're running it as 120:277 and that it's got two 120 volt windings. Do you have them correctly paralleled (X1 - X3 and X2 - X4)? If not then you are doubling the winding losses on the 120V side.
  11. Aug 5, 2011 #10
    Transformer losses can be categorized into 3 types:
    1) Coil (resistance losses)
    2) lamination losses (eddy currents)
    3) magnetization losses (iron saturation)

    Running the transformer at no-load can determine whether the problem is coil resistance or one of the other two.

    2) is cased by too high a frequency.

    3) is caused by too low a frequency or too high an input voltage.

    Because you are using two coils in parallel as the primary, be absolutely sure that both primary cols are connected and drawing current.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook