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Centre aligned PWM question

Question asked by Wood.Robert on Jun 14, 2012
Latest reply on Jun 22, 2012 by Wood.Robert
I've been using edge aligned PWM to turn a three phase induction motor and this works just fine. However, in an attempt to cut down on noise, with the possibility of two IGBTs switching at the same time, we thought it would be a good idea to try centre aligned PWMs.

Originally with edge aligned I was counting up to 4800 to give me a PWM time of 15k. For a 25Hz sine wave I was then plotting 600 points per cycle. When I went to centre aligned, as it counts up to ARR and then down, I changed to ARR being 2400 to give a total count of 4800, and redid the maths to account for the whole count up count down thing. However,  that doubled my frequency! If I went back to ARR of 4800 that gives me the correct frequency.

Problem is that when I take my cycle count to 300 it gives me a frequency of 37.5Hz and I think this all might be connected.

So, I must be missing something fundamental here! Anyone able to explain what is going on?