By applying the PWM command to the three arms of a tri phased inverter, we impose the potentials 
to follow, as an average value, the reference wave forms
(figure 1).
Figure 1
· If the load is tri phased, balanced, with insulated null, we have the following instantaneous values:
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· These relations are also valid as average values:
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· As we wish the average values 
to follow the reference values
, and 
, we have to impose the same relation to the reference values
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It is sufficient to consider (see principle)
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Finally, we thus have:
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But we also obtain the desired result if we consider:
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The term 
only modifies the widths of the intervals during which we simultaneously have KA, KB and KC ON, or OFF, respectively, that means modifying the manner in which we obtain 
.
