how to find harmonics from a data

[varsha88]

hi,

can anyone guide me on how to convert the data to fft. the reason i use fft is to find the harmonics in the data as the data represents leakage current from a surge arrestor. i need to find 1st,3rd,5th and 7th harmonic. please do suggest if there is any other method to find the harmonics in the data. the harmonics need to be plot out as a graph. attache is the vi and also the data file. 

thanks

[johnsold]

You can simply take the FFT of the data or use one of the spectral VIs such as Amplitude and Phase Spectrum.vi.  They are found in the Signal Processing palette and the Spectral or Transforms subpalettes.

Your data looks rather strange. Is it one small data set repeated twice?  The result is that there is a dominant component at about 57 Hz but also components at ~1/3 and 2/3 of that frequency as well as harmonics.

In the VI I attached I have padded the data by concatenating arrays of zeros before and after the data set to improve the spectral resolution.

Lynn

[varsha88]

hi,

is there a way to extract the 1st,3rd,5th and 7th harmonic from the data.

[johnsold]

Once you know the location of the first peak (fundamental frequency or what you appear to be calling the 1st harmonic), you then look at the signals at 3, 5, and 7 times that frequency.  Depending on the resolution and the relationship between the sampling rate and the signal frequency, you may need to combine the values in several adjacent bins to get all the energy in each harmonic.  Search for information on "spectral leakage" for the reasons.

Lynn