Cycloconverter

A cycloconverter or a cycloinverter converts an alternating current (AC) waveform, such as the mains supply, to another AC waveform of a different frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct current (DC) link. (DC) link is used to first convert AC signal to DC and then again from DC to AC signal.

The input voltage, v_s is an ac voltage at a frequency, f_{i .} Assume that all the thyristors are fired at α=0° firing angle, i.e. thyristors act like diodes.The firing angles are named as  αP for the positive converter and  αN for the negative converter.

Consider the operation of the cycloconverter to get one-fourth of the input frequency at the output. For the first two cycles of v_s, the positive  converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen . In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. The current waveforms are not shown  because the resistive load current will have the same waveform as the voltage. It should be kept in mind that when one of the converters operates the other one is disabled, so that there is no current circulating between the two rectifiers.

The frequency of the output voltage, v_o  is 4 times less than that of v_s, the input voltage, i.e. f_o/f_i=1/4. Thus, this is a step-down cycloconverter. On the other hand, cycloconverters that have f_o/f_i>1 frequency relation are called step-up cycloconverters. The  step-down cycloconverters are more widely used than the step-up ones.  The frequency of v_o can be changed by varying the number of cycles the positive and the negative converters work.