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Harmonics,what are they?by RozzerETO
Tags: harmonics 
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#1
Feb2013, 02:15 PM

P: 2

so, I've been doing some research on harmonics recently and i can't actually find a good explanation of how they are created or what the effect. I have a feeling that they are something to do with 'clipping' the sinusoidal waveform to get the desired voltage. VSDs?



#2
Feb2013, 04:23 PM

P: 563

In their purest mathematical sense Harmonics are signals or part of the waveform at multiples of the fundamental frequency. Basically ANY distortion of the pure sinusoid  will result in harmonics. ( It is interesting / important to note  that ALL signals can be broken down into a sum of wave forms  all of which are multiples of some fundamental frequency  for noise this is true within a specific bandwith....)
For VFDs  one important measure of the quality of the drive is the THD  Total Harmoinic Distortion. Ideally this should not look like clipping  but I am sure some drives end up with a waveform that looks clipped. The typical effect of harmonics are overheating of magnetics ( Motors, Transformers etc) for "low order" harmonics, that is Harmonics between 2nd and 12th my 20th Harmonic. Another issue, more common with higher order ( Starting around the 12th upward), is increased voltage stress on the conductors and insulation systems ( also in the transformers and motors  etc). They can also cause interference with other devices ( EMI ) due to the power / frequency levels involved. 


#3
Feb2113, 02:10 AM

Sci Advisor
P: 1,724

I don't know your level of knowledge, but if you're asking from a signal analysis or mathematical point of view, any nonsinusoidal waveform will have harmonics because it's nonsinusoidal. As Windadct alludes to, in order to create that nonsinusoidal waveform, you need the contribution of higherpower sinusoids (determined via Fourier Analysis):
I just wanted to post to share the animation, which probably comes from this site: http://blog.matthen.com/post/4211270...escanbeused 


#4
Feb2113, 11:32 AM

P: 563

Harmonics,what are they?
Sorry but I have to disagree with lolone's whole post. Harmonics can be completly understood with out this info. Signal theory only, and photons what?



#5
Feb2113, 11:36 AM

Sci Advisor
PF Gold
P: 2,245

Yes, please ignore lolone's post.
Harmonics is a mathematical concept, you don't need any physics to understand it. Harmonics specifically refers to multiplies of a fundamental (for a 50 Hz system the harmonics would be 100, 150, 200 Hz etc), not just any frequency as Iolone's post would suggest. 


#6
Feb2113, 11:50 AM

P: 1,857

Here is a decent article on harmonics with regards to VFD's and THD.
http://literature.rockwellautomation...p011_enp.pdf http://support.fluke.com/findsales/...03_ENG_K_W.PDF the second site has some handy DMM tests for harmonics although those tests won't tell you what harmonic number is involved only that harmonics are present. 


#7
Feb2113, 12:46 PM

P: 2

thanks very much for all the replies, they have helped me grasp the subject! in reply to what my depth of knowledge is, I am currently doing a Marine electrical engineering degree! thanks very much for all your help!



#8
Feb2113, 01:15 PM

P: 8

This should be obvious since you are dealing with charge carriers and photons. This is what electricity is and you cannot measure something that is not a product of these quantum particles. I'm sure as engineers you have mathematical descriptions of what is happening, but in order to understand it fully everyone needs to understand the physics. 


#9
Feb2113, 01:43 PM

Sci Advisor
PF Gold
P: 2,245

Secondly, harmonics is a mathematical concept (from Fourier analysis) applicable to a very wide range of phenomena, some electrical some not. You can have harmonics in any driven nonlinear system, a mechanical system would be an obvious system. Hence, trying to explain it by refering to details of charge transport (which btw were incorrect) is just silly 


#10
Feb2113, 01:54 PM

P: 8

So, in the Fourier analysis, we are not looking at purely mathematical concept, we are looking at how the system really is, albeit with limited resolution. It is the "time domain" that is the mathematical concept. In terms of describing what harmonics is, its best to look at it in terms of collisions and particle exchanges. 


#11
Feb2113, 01:59 PM

P: 8

Actually, let me put that a little bit better:
Time Domain  Voltage  is a summation of group activity Frequency Domain  Factorization of the voltage In terms of harmonics, we are interested in the factorization of the voltage as it is these interactions that give rise to harmonics. 


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