What is Total Harmonic Distortion?
Did you ever see specifications on harmonic distortion, and wonder how to check out the measurements? You can buy harmonic distortion analyzers to do this for you, but there is another way, that of using the equipment you have right in your laboratory — your random vibration controller/analyzer. Harmonic distortion, or total harmonic distortion (THD) is the summation of the amplitudes of various harmonics when a pure sine tone is put into a system. The system is traditionally accepted to be the amplifier, such as your home stereo system. However, there are many sources of harmonic distortion. The generator that generates the “pure” sine tone will generate harmonics. The system which you are exciting may be the source of the harmonic distortion. This might include the power amplifier and the shaker, which translates the electrical energy to mechanical energy. Lastly, the measuring equipment will have errors in it.
There is currently a lot of misleading information in the marketplace regarding “Total Harmonic Distortion” (THD) in relation to generators. Terms such as this, and also “Sine Wave”, are used along with “Clean Power” and “Perfect Wave Form” when referring to the output from generators. One Australian manufacturer is using these terminologies as a marketing tool, advertising their generating sets as “the cleanest power around”, due to their THD percentage. They claim that “most portable generating sets” have over 25% THD. Independent University testing has shown that the Total Harmonic Distortion of CROMMELINS Generators tested is only 6%, well below the “said” 25%!
