The article deals with a method by means of which the purposefulness of 729 variously summed possible switching variants of two input voltage levels of all the three phases are evaluated in each time interval. Choosing integral equality of the difference of input voltages and the converter output reference voltage on both sides of the reference voltage as the basis of evaluation for each switching variant, we obtain, as a result, a limited number of switching variants of input voltages that suit best of all the formation of the output voltage within the given interval. The results obtained may be used further to compile control programmes of direct frequency converters, as well as for students` methodological training in the related discipline at the Riga Technical University.