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# Harmonics with rectifiers

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#### f2ostie

##### Newbie level 2
Hello, I'm new on this forum so I don't really know what to expect but, I have a question and I hope someone is able to help me

First question: If you use a 3 phase 6 pulse bridge rectifier (common diode rectifier), the harmonics you will get are fharmonics = fgroundharmonic x 6 x n +/- 1 x fgroundharmonic
I was wondering why this is.

My second question is: If you use a rectifier with Active Front End, the current you pull out of the net will have a harmonic of 2 x fmodulation. Why is that, and why not harmonics with a frequency of 1 x fmodulation

My third and final question is related to the second one. Sometimes you do not get a frequency of exactly 2 x fmodulation but then there are 2 peaks around that frequency (see on the figure)

If someone could answer or point me to a paper or website who deals with one of those problems, the help would also be greatly appreciated

For the first question I suggest a power electronics text book. You can show, that all 3*n harmonic currents are cancelled by the grid 3-phase symmetry, also all even order harmonics for a symmetrical bridge rectifier.

For the second, depends on what you consider as "modulation frequency"? In my view, the fundamental PWM frequency will show in the spectrum. The usual unipolar PWM scheme is however acting like a supressed carrier AM, thus you only get the side bands.

Thanks!

You really helped me out. Question one is solved, question 2 is because the afe uses the boost converter principle, and so the IGBT's have to short-circuit
for a moment and then have to make the circuit to the DC-link voltage, so they switch 2 times per cycle, hence the 2 x f modulation (I think =D).

The third question still isn't really clear to me. I found sites that told me that you get a DSBSC (Double sideband suppressed carrier) signal when you
modulate 2 bipolar signals. But I would like to know why this happens if you modulate 2 bipolar signals..

Once again, Thank you very much!

I must admit, that I used to think about AFE behaviour in terms of power electronics rather than of communication engineering. But for the unipolar modulation which is a standard in most AFE circuit, the phase of the PWM frequent component is switched according to the input voltage sign. That means, that the PWM carrier frequncy component is cancelled out in the average.

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