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Total Harmonic Distortion

  • 17-04-2015 07:24AM
    #1
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,692 Mod ✭✭✭✭


    I have a fundamental sine wave with high harmonic content. I have all of the information that I require to graph the fundamental and the 3 other sine waves. In fact I have graphed all 4 individually and the final combined wave in Excel.

    It has been a few years since college :)
    My question is, how do I calculate the THD and express it as a percentage?

    Many thanks,

    2011


Comments

  • #2
    rob w
    Registered Users, Registered Users 2 Posts: 1,208 ✭✭✭rob w


    2011 wrote: »
    I have a fundamental sine wave with high harmonic content. I have all of the information that I require to graph the fundamental and the 3 other sine waves. In fact I have graphed all 4 individually and the final combined wave in Excel.

    It has been a few years since college :)
    My question is, how do I calculate the THD and express it as a percentage?

    Many thanks,

    2011

    Sum the squares of all the harmonics amplitudes.....take the square root of that.....then divide by the fundamental....and multiply by 100 for your percentage! Make sure all values are in RMS though first!

    e.g.

    THD% = (sqrt(V2^2 + V3^2 + V4^2....)/V1)

    Where V1 is your fundamental and V2, V3 and V4 are your harmonics (In RMS)


  • #3
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,692 Mod ✭✭✭✭2011


    rob w wrote: »
    Sum the squares of all the harmonics amplitudes.....take the square root of that.....then divide by the fundamental....and multiply by 100 for your percentage! Make sure all values are in RMS though first!

    e.g.

    THD% = (sqrt(V2^2 + V3^2 + V4^2....)/V1)

    Where V1 is your fundamental and V2, V3 and V4 are your harmonics (In RMS)

    Thank you!


  • #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,692 Mod ✭✭✭✭2011


    rob w wrote: »
    Where V1 is your fundamental and V2, V3 and V4 are your harmonics (In RMS)



    Converting each to r.m.s. values doesn't change anything as its a common factor?


  • #5
    Michael Collins
    Moderators, Science, Health & Environment Moderators Posts: 1,855 Mod ✭✭✭✭Michael Collins


    2011 wrote: »
    Converting each to r.m.s. values doesn't change anything as its a common factor?

    Yes, it doesn't matter if it's RMS or not, since the harmonics are sine waves by definition.

    Sometimes the RMS value might be more readily available, so you might as well just use that, but peak amplitudes are also fine, once you're consistent.


  • #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,692 Mod ✭✭✭✭2011


    Michael Collins wrote: »
    Yes, it doesn't matter if it's RMS or not, since the harmonics are sine waves by definition.

    Sometimes the RMS value might be more readily available, so you might as well just use that, but peak amplitudes are also fine, once you're consistent.

    That makes sense, thanks.


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