Cycloconverter

A cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link . They are most commonly used in three phase applications. In most power systems, the amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency . The quality of the output waveform improves if more switching devices are used ( a higher pulse number). Cycloverters are used in very large variable frequency drives, with ratings of several megawatts.

A typical application of a cycloconverter is for use in controlling the speed of an AC traction motor and starting of synchronous motor. Most of these cycloconverters have a high power output — in the order a few megawatts — and silicon-controlled rectifiers (SCRs) are used in these circuits. By contrast, low cost, low-power cycloconverters for low-power AC motors are also in use, and many such circuits tend to use TRIACs in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction, but it is also a three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used. However, it is common in very high power applications such as for ball mills in ore processing and for cement kilns.

The switching of the AC waveform creates noise, or harmonics, in the system that depend mostly on the frequency of the input waveform. These harmonics can damage sensitive electronic equipment. If the relative difference between the input and output waveforms is small, then the converter can produce subharmonics. Subharmonic noise occurs at a frequency below the output frequency, and cannot be filtered by load inductance. This limits the output frequency relative to the input. These limitations make cycloconverters often inferior to a DC link converter system for most applications.

Tap converter

A dramatically improved output waveform can be achieved by employing the circuit known as the Tap converter, invented and perfected between 1981 and 1984 by Sandler, Wrzesniewski, Wilner, and Fung in the United States. The tap converter utilizes a Scott transformer connection and somewhat more sophisticated control logic (adding several more SCRs) to switch among a variety of transformer taps and thereby fabricate a far smoother signal.