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What Is a Cycloconverter?

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  • Written By: Lakshmi Sandhana
  • Edited By: PJP Schroeder
  • Last Modified Date: 19 October 2014
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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 DC, stage in between. It can also be considered as a static frequency changer and typically contains silicon-controlled rectifiers. The device consists of an array containing back-to-back, parallel, connected switches, which are used to fabricate the desired output AC waveforms. It's possible to control the frequency of these output AC waveforms by opening and closing the switches in a controlled fashion.

This converter converts single-phase or three-phase AC power to single-phase or three-phase power having a variable frequency and magnitude. Typically, the output frequency of the AC power is lower than the input frequency. A cycloconverter has the capacity to operate with loads of variable power factors and also allows bidirectional power flow. They can be broadly classified into two types — phase-controlled cycloconverters and envelope cycloconverters. In the former, control of the firing angle is accomplished through adjustable gate impulses, while in the latter, the switches remain in an on state and conduct in consecutive half cycles.

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They are mostly used to control the speed of drives and for converting variable input frequency power into constant frequency output, such as in very high-power applications, including driving synchronous motors and induction motors. Some of the places where cycloconverters are employed include cement mill drives, mine winders, and ore grinding mills. They are also utilized in ship propulsion drives, scherbius drives, and rolling mill drives.

Offering many advantages, a cycloconverter can be used in quite a few low-speed applications and is also a compact system. Its ability to directly affect the frequency conversion of power without any intermediate stage involving DC power is another huge advantage. If the cycloconverter experiences a commutation failure, the results are minimal, such as the blowing off of individual fuses.

It also has the capacity of regeneration, covering the total range of speeds. Another huge advantage of the cycloconverter is its ability to deliver a sinusoidal waveform at a lower output frequency. This advantage comes from its ability synthesize the output waveform using a large number of segments of the input waveform.

This technology does have some disadvantages, though. Firstly, the frequency of the output power is around one third or less of the input frequency. It's possible to improve the quality of the output waveform if a larger number of switching devices are employed. A cycloconverter requires quite a complex control mechanism and also uses a large quantity of thyristors. Its use is also limited by severe harmonics and the low-output frequency range.

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