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I have inherited a control software for ACIM scalar control and I need to do some changes in this software. Before I can do any change I need to understand current implementation. So I have started to read the source code and analyze it.

I have identified the main parts and one of them is a module which is responsible for PWM generation. But here I have encountered several lines of code which I am not able to understand what they do. For sake of readibility I have attempted to describe them via below given Scilab scheme, where \$U^*_1\$, \$U^*_2\$ and \$U^*_3\$ are modulation sinusoidal signals and \$U_{dc}\$ is voltage in dc link of the three phase inverter.

Scilab scheme

Here is the input into the model

Model input

and here is its output

Model output

Based on these figures I guess that the aforementioned code somehow implements sinewave modulation with injected third harmonics but I still don't understand how. Does anybody have any idea what is the principal of this algorithm? Thank you for any ideas.

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