PFC & Harmonics

apfc harmonicsWhat are harmonics?

Non linear loads, such as: rectifiers, inverters, speed variators, furnaces, etc. that absorb periodic non-sine  wave currents from the network. Said currents are composed of a fundamental frequency component  rated at 50 or 60 Hz, plus a series of overlapping currents, with frequencies that are multiples of the  fundamental frequency. This is how we define HARMONICS.The result is a deformation of the current  (and, as a consequence, voltage) that has a series of associated secondary effects.

Basic concepts

Some fundamental terms related to harmonics must be defined in order to interpret any measurement  and study:

  • Fundamental frequency (f1): Frequency of the original wave (50/60 Hz)
  • Order of harmonics (n): Whole number given by the frequency relationship between a harmonic and  the fundamental frequency. The order is used to determine the frequency of the harmonic (Example: 5th  order harmonic ? 5•50 Hz = 250 Hz)
  • Fundamental component (U1 or I1): 1st order sine-wave component for the development of the  Fourier series, with a frequency identical to the original periodic wave.
  • Harmonic component (Un or In): Sine-wave component of the 2nd or higher order for the  development of the Fourier series, with a frequency that is a multiple of the original periodic wave in whole numbers.
  • Individual distortion rate (Un% or In%): Relationship in % between the root mean square of  harmonic voltage or current (Un or In) and the root mean square of the fundamental component (U1  or I1).
  • True root mean square (TRMS): Square root of the sum of the squares of all components of the wave.
  • Harmonic residue: Difference between the total voltage and current and the corresponding  fundamental value.
  • Total Harmonic Distortion (THD): Relationship between the root mean square of the harmonic  residue of voltage and/or current and the value of the fundamental component.


Most common harmonics


The following table shows the most common loads that generate harmonics, as well as the wave shape  of the current consumed and their harmonic spectrum.

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