The cycloconverter has traditionally been used only in very high power units, generally above a megawatt, where no other type of unit can be used. Examples are cement tube mill drives of more than 5 MW, German-Dutch wind tunnel fan propeller of 13 MW, reversible drives of rolling mills and boat propulsion drives. The reasons for this are that the traditional cycloconverter requires a large number of thyristors, at least 36 and generally more for good motor performance, coupled with a very complex control circuit, and has some performance limitations, the worst of which is an output frequency limited to about one third of the input frequency.
The cycloconverter has four thyristors divided into a positive and negative bank of two thyristors each. When a positive current flows into the load, the output voltage is controlled by the phase control of the two positive bank thyristors, while the negative bank thyristors are kept off and vice versa when the negative current flows into the load. It is important to keep the non-conductive thyristor bank switched off at all times, otherwise the network may be short-circuited through the two thyristor banks, which would cause distortion of the waveform and possible failure of the device by the short circuit current. A major problem in controlling the cycle converter is how to switch between banks in the shortest possible time to avoid distortion and at the same time ensure that the two banks do not drive at the same time. A common addition to the power circuit which eliminates the requirement to keep a bank switched off is to place an inductor with central tap called the current inductor circulating between the outputs of the two banks. Both banks can now drive together without short-circuiting the power grid. In addition, the circulating current in the inductor keeps both banks running at all times, resulting in improved output waveforms.