What is a cycloconverter?

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A cycloconverter is a device that converts alternating current, or AC, power at one frequency into AC power of an adjustable, but lower frequency, without any direct current, or intermediate DC stage. It can also be considered a static frequency changer and typically contains silicon-controlled rectifiers. The device consists of an array containing consecutive, parallel connected switches that are used to manufacture the desired output AC waveforms. You can control the frequency of these output AC waveforms by opening and closing the switches in a controlled manner.

This converter converts single-phase or three-phase AC power into single-phase or three-phase power with varying frequency and magnitude. Normally, the AC power output frequency is lower than the input frequency. A cycloconverter is capable of operating with varying power factor loads and also allows for bidirectional power flow. They can be broadly classified into two types – phase-controlled cycloconverters and envelope cycloconverters. In the former, the firing angle control is carried out by means of adjustable gate impulses, while in the latter, the switches remain in the on state and conduct in consecutive half-cycles.

They are primarily used to control the speed of inverters and to convert variable input frequency power to constant frequency output, as in high power applications including synchronous motors and induction motors. Some of the places where cycloconverters are employed include cement mill units, winders and ore mills. They are also used in ship propulsion units, scherbius units and rolling mill units.

Offering many advantages, a cycloconverter can be used in some low speed applications and is also a compact system. Its ability to directly affect power frequency conversion without any intermediate stages involving DC power is another big advantage. If the cycloconverter has a commutation failure, the results will be minimal, such as individual fuses blowing.

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It also has regeneration ability, covering the entire speed range. Another great advantage of the cycloconverter is its ability to provide a sine waveform at a lower output frequency. This advantage comes from its ability to synthesize the output waveform using a large number of input waveform segments.

This technology has some drawbacks, however. First, the output power frequency is about a third or less of the input frequency. It is possible to improve the quality of the output waveform if a greater number of switching devices are employed. A cycloconverter requires a very complex control mechanism and also uses a large number of thyristors. Its use is also limited by severe harmonics and the low-output frequency range.

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