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Why am i getting worng answer from the Fourier Block in Simulink when generating interharmonics in my input sine signal

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Aubai on 17 Jan 2017
Commented: Aubai on 25 Jan 2017
Hi All,
I am trying to understant why am i not getting the correct Mag and Phase Output of the Fourier Block.
for testing you can use the Example in Simulink for the Fourier Block, there you can add an interharmonics of lets say 3.8 for the Frequency and an Amplitude of 40 V and Phase of lets say 50 degree. and then dont Forget to add the 3.8 to the Fourier Block also.
The steps for doing that are:
- Open the Fourier Example by writting 'power_Fourier' in your Comandwindow
- edit the Src1 Block by adding an interharmonics as said before for example 3.8 Order
- edit the Fourier1 Block by adding the new Order Harmonic:[0:3 3.8]
I have added a figure for the selected configuration.
so my question why am i getting wrong answers?
Best Regards

Accepted Answer

Maitreyee Mordekar
Maitreyee Mordekar on 24 Jan 2017
Hi Aubai,
I tried to execute the code by changing parameters as you suggested and I could see the concerns that you face. This happens because the fundamental frequency of the signal is not 60 anymore. Since the 3.8 term is added to the frequency field, we get the following as the frequencies present in the signal-
[ 1 2 3 3.8 ]*2*pi*60= [ 60 120 180 228 ]*2*pi =[5 10 15 19]*2*pi*12
Thus, the fundamental frequency is 12 and the harmonics are 5, 10,15 and 19 respectively.
Double click on the 'Fourier 1' Block and make the following changes-
  1. Enter '12' as the 'Fundamental Frequency (Hz)'
  2. Enter [0 5 10 15 19] as the 'Harmonic n'.
I have attached a snapshot of the changes below. This should yield you the desired amplitude and phase of the signal.
  1 Comment
Aubai on 25 Jan 2017
thx very much for the answer it did work :)
I am still skeptic on the principle itself. do you suggest any references?
anyway this acutally means that every time i call the Fourier block with Inter-Harmonics Input the suggested Transformation should be done, right?
again that did help thx alot :)
Best Regards

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