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Efficiency in a transformer

  1. Mar 25, 2012 #1
    When I calculate efficiency of a transformer, I use output power/imput power x100%. Then what are these power values? Is the power the power in a load or the total power coming out of the transformer (meaning it includes the joule heating)? Then also what about the input power, which values do I use? Thanks for the help!
  2. jcsd
  3. Mar 25, 2012 #2
    You need to consider any source of joule heating in the transformer, plus anything that creates a power factor <1. There are several classes of things you need to consider:

    1) Self inductance and power factor (primary), and inductive (excitation current) losses.
    2) mutual inductance (coupling efficiency to secondary circuit)
    3) Core losses (eddy currents in laminations, hysteresis losses, magnetostriction*)
    4) copper winding losses(I2R losses, primary and secondary, also eddy currents in copper)
    5) secondary circuit losses (in load).

    The overall efficiency is 100 x Pload/Pin, where Pin is measured at the wall plug. Are you talking about joule power (real) or reactive power (volt-amps)?

    * If you can hear the transformer or feel vibrations, there are magneotstrictive losses.
  4. Mar 25, 2012 #3
    Oh so the cables getting heard is not included in the Pin or Pout? So its the power at the load, and the power from the generator?
  5. Mar 25, 2012 #4
    If we know the input and output current and voltages ( for example by measurement) then we can calculate the efficiency easily. All the losses are included in Pin-Pout. However , If we would like to calculate the efficiency analytically ( as in design process), we need to consider and calculate the losses as a function of the load ( and the input voltage). Core losses are not easy to calculate and just some approximations may be made.
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