non-order harmonics

[saltukhan]

I searched about non-order harmonics but couldnt be able to find anything, if there is such a concept, why do these non-order harmonics occur?

 

[marks1080]

I thought by definition any harmonic had to be of some order. Multiple orders of harmonics together might look strange on a graph. FFT would confirm what orders are present.

 

[waross]

Isn't that a little oxymoronic? Harmonics are ordered multiples of the fundamental frequency.
If a frequency is not a multiple of the fundamental frequency it is not a harmonic.
However, a square wave contains components of all frequencies and may induce ringing at a non harmonic frequency.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

Harmonics are integer multiples of the fundamental frequency. The actual frequency is the first-order harmonic, i.e, 50Hz for example. The second order is twice, i.e., 100Hz.

I've heard the term "sub-fundamental" harmonics, but that doesn't sit well with the classic definition. Nor are fractional harmonics. If somebody is "finding" these, there is something somebody's missing in the analysis.

old field guy

 

[electricpete]

The only context where it makes sense is when you have a spectrum of a signal with two fundamental frequencies that are not multiples of each other.

Such is encountered in vibration analysis. I may have present in my spectrum:

1xTS, 2xTS, 3xTS... (where TS is turning speed)
1*BPFO, 2*BPFO, 3*BPFO...(where BPFO is Ball Pass Frequency Outer race... in generall not an exact multiple of turning speed).

I agree with the others, clarification of the original question is in order (pun intended).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Terminology alert:
"signal with two fundamental frequencies"
is not stricly correct. I should have said
"signal which includes the sum of two periodic signals that have different fundamental frequencies"

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[PHovnanian]

signal which includes the sum of two periodic signals that have different fundamental frequencies

What you are seeing here are referred to as intermodulation products, or 'birdies' in the radio biz.

saltukhan needs to characterize the spectrum of of his signal a bit better. 'Non-harmonic' could mean some signal with distinct peaks in the frequency domain or broadband noise. The former are usually generated by some mechanism with a distinct period or resonant frequency, such as your shaft speed and bearing example. The latter are generated by aperiodic mechanisms (corona discharge comes to mind as one example in the T&D biz).

 

[MAGTiger]

Maybe the term you are looking for is interharmonics? The IEEE apparently has a task force on this issue. I remember hearing a discussion several years ago and the Power Meeting. It was covered at that time under some research being done for IEEE 519. Haven't heard anythin lately though. There is a research paper you can Google under IEEE and interharmonics. It is related to cycloconverters. But the presentation I eard some years ago was more to do with ballasts, I think. Other than than don't know much about it.

Neil

 

[saltukhan]

yes i think this means interharmonics dear magtiger;
Interharmonic components are caused primarily by two
sources:
• Variations of the amplitude and/or phase angle of the
fundamental component and/or of the harmonic
components,for example, inverter drives
• Power electronics circuits with switching frequencies
not synchronized to the power frequency, for
example, a.c./d.c. supplies and power factor
correctors

 

[unclebob]

When you have an unbalanced system, you have to play with symetrical components. This is still true when adding the harmonics to the mix. Direct, inverse and homopolar current sequences exist for the fondamental and each harmonic present in the current.

Direct, inverse and homopolar sequences are not sine anymore, they are a sum of distorded harmonic currents.

Maybe this could explain these interhamonics. Perfectly balanced systems do not exist.