The cycloconverter provides direct ac to ac transformation with control over both the output voltage magnitude and output frequency. It employs phase-angle-controlled bridges to synthesize voltages.
The most common form is the three-phase to three-phase converter illustrated below:
Each output needs two three-phase thyristor bridges. One bridge is used to synthesize positive voltages at the output; the other is connected in reverse and synthesizes the negative voltages. The phase-angles of the bridges are gradually altered to give an approximately sinusoidal variation of the voltage at the output. Both the output voltage and the input current are rich in harmonics and some form of filtering is normally required. The frequency of the output voltage is constrained to less than about 1/3 of the input frequency by the allowable distortion of the input current. To prevent shoot-through paths between the phases while thyristors are commutating, the input phases are isolated using transformers. Cycloconverters are used for very high power; low speed machine drives where the complexity of the circuit is less of a problem and where the line-frequency thyristors are the only viable device. Cycloconverters are able to transfer power in either direction and can form the basis of a four quadrant drive.
Also read: Basic Features of Power Semiconductor Controlled Drives
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