Harmonics with rectifiers

[frostie]

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 f_harmonics = f_{groundharmonic} x 6 x n +/- 1 x f_{groundharmonic}
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 f_modulation. Why is that, and why not harmonics with a frequency of 1 x f_modulation

My third and final question is related to the second one. Sometimes you do not get a frequency of exactly 2 x f_modulation 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