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While using the FFT in Simulink tool, I have observed that if more number of cycles are selected then frequency steps in which harmonics percentage is shown is also divided accordingly.

For example:

Number of cycles = 4 , frequency steps = (fundamental/4) = 50Hz/4= 12.5 Hz. (Please refer the image shared).

enter image description here

This spreadsthe harmonics order( h1, h2 ...) values to their near frequencies.

I am not sure why is it happening like this. Should I take only one cycle? I read that to analyse the data better more number of cycles are required. I am interested to know THD and individual harmonics of the waveforms.

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1 Answer 1

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I am not sure why is it happening like this

Information: -

enter image description here

Number of cycles = 4

So, with 3150 samples per cycle, the time for one cycle is 6.35e-06 x 3150 = 20 ms. That's an equivalent frequency of 50 Hz and, with a 4 cycle analysis, the lowest frequency is 12.5 Hz.

Should I take only one cycle?

The lowest resolution will then be 50 Hz - your choice.

I read that to analyse the data better more number of cycles are required

More is usually better but, if you are happy with a base frequency of 12.5 Hz, why take more?

Summary

If your 4 cycle window occupies a time frame of 80 ms then, apart from the DC component, the minimum FFT resolution is 1/0.08 = 12.5 Hz. The maximum FFET frequency component is determined by your sampling time.

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