Schematic diagram of a three-pulse phase,three-phase cycloconverter is shown in figure 1.a.It is clear from the schematic diagram that there are two groups of SCR per phase.One group is responsible for producing the positive alternations and it is known as the positive group shown by group P,the other which producees the negative alternations is called as positive the negative group shown by group N in figure 1.a. Each group consist of three SCRs,thus in total six SCRs are required per phase.This means,in all 18 SCRs are required for the whole circuit.The basic circuit diagram for this scheme of cycloconverter has been shown in figure 1.b

The firing schedules of thyristors of phase group B and C are same as that of group A but lag $120^0$ and $240^0$ respectively.The average value of the output voltage can be varied by varying the firing angles of the SCR in conduction,whereas the frequency of the output voltage can be varied by changing the sequence of firing SCRs. With a balanced load the neutral connection is no longer necessary and may be omitted,thereby suppressing all triplen harmonics.

It has been assumed that the output frequency of the cycloconverter is less than the supply frequency.In practice,with the basic cycloconverter circuit of figure 1.b,reasonable power output,efficiency and harmonic content are obtained in the step-down region with an output frequency from zero to about one-third of the input frequency.As the cycloconverter output frequency approaches the supply frequency,harmonic distortion in the output voltage increases because the output voltage waveform is composed of fewer segment of the supply voltage.

As a result,losses in the cycloconverter and in the a.c motor become excessive,and there is a drop in overall efficiency.By using more complex converter circuits with higher pulse numbers,the output voltage waveform is improved and the maximum useful ratio of output to input frequency is increased to about one-half.The a.c. motor normally presents a high impedance at the ripple frequency and hence, the output current is nearly sinusoidal and no additional filtering is necessary.