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Power factor for transformers

  1. Dec 20, 2014 #1
    I have read that transformers operate on high power factor but I cannot get what is the reason behind it..?? I mean..what if we are having a low power factor..how is that going to affect the working of transformer??
  2. jcsd
  3. Dec 20, 2014 #2
    PF depends on leakage inductance of xfm and the load itself.
  4. Dec 20, 2014 #3
    but what's the reason behind high pf??
  5. Dec 20, 2014 #4
    relatively small leakage inductance of xfm + operation near rated load point
  6. Dec 20, 2014 #5
    actually I have studied that pf is inversely proportional to the current...so I was seeing whether this thing point is the reason..??
  7. Dec 20, 2014 #6
    The no-load transformer p.f. is low [approx.0.1]. However, a transformer does not change much the load p.f.Since the active no-load power and reactive no-load power are low the load power factor [at full load] does not decrease more than 2% [usually].
    Of course, if the load will be low the p.f. could decrease more [up to 0.1 at no-load!].
  8. Dec 21, 2014 #7
    Another extreme case is a short circuit case where I>>Inom. PF also low.
  9. Dec 21, 2014 #8

    jim hardy

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    Gold Member

    That's a bit ambiguous question.
    I'd suggest the source from which you extracted that tidbit was suggesting that it is usual practice to use power transformers at reasonably highPF because:
    Z of transformer is largely inductive
    Draw your phasors..
    When PF of load is unity,what is phase angle between applied voltage and I X Z drop in transformer?
    When PF of load is zero, what is that angle?

    At high PF your transformer I X Z drop is at right angle to applied voltage so doesn't change it much, because the hypotenuse and long side of a right triangle are nearly the same when vertex angle is small. That's elementary geometry.
    At zero PF that I X Z drop Is in phase with applied voltage and subtracts or adds to it volt for volt.

    So you'll get better regulation with a load having reasonably high PF.
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