A new approach to real approximation of an ideal sinusoidal frequency conversion process is put forward in the present paper. This enables the output frequency range of line-commutated direct frequency converters to be expanded up to the value of input frequency. This is achieved by the application of a low frequency value sinusoidal modulating function and the substitution of input voltage and modulating function product with approaching to it input voltage level intervals. An example of this frequency conversion in terms of the simplest six thyristor circuit is considered for a fan drive.