Cycloconverter

For the rotating electrical machine, see Rotary converter

Connection of the thyristors in a 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 (Dorf 1993, pp. 2241–2243 and Lander 1993, p. 181). They are most commonly used in three-phase applications.

Usually silicon-controlled rectifiers (SCRs) are used in these circuits. By contrast, low cost, low-power cycloconverters for low-power AC motors 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. Cycloconverters are not common in low power range of variable frequency drives, which is dominated by transistor-based inverters.

In most power systems, the amplitude and the frequency of input voltage to a cycloconverter are fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter are variable. The output frequency of a three-phase cycloconverter must be less than about one-third to one-half the input frequency (Lander 1993, p. 188). The quality of the output waveform improves if more switching devices are used (a higher pulse number of the thyristor bridges).

Cycloconverters are used in very large variable frequency drives with ratings from few megawatts up to many tens of megawatts. Typical applications of cycloconverters are rolling mill drives^[1], ball mills for ore processing, cement kilns and ship propulsion^[2].

Cycloconverters have also been used to produce single phase 16 2/3 Hz or 25 Hz power for electric railroads from the 50 Hz or 60 Hz AC grid.

The switching of the AC waveform creates current harmonics in the power system that depend mostly on the frequency of the input waveform. These harmonics can disturb or overload sensitive electronic equipment in the AC grid. Cycloconverters produce subharmonics and interharmonics in addition to harmonics. Subharmonic noise occurs at a frequency below the output frequency. The interharmonics are at frequencies that are between harmonic frequencies. Subharmonics and interharmonics are often problematic for the AC grid that is feeding the cycloconverter and their mitigation may require filters with considerable size. One of the subharmonic components will pass through zero frequency when the output frequency of the cycloconverter reaches half of the input frequency. Because DC component in the input current will saturate the transformer feeding the cycloconverter badly this limits the output frequency relative to the input. Another reason to limit the maximum output frequency below half of the input frequency is the deterioration of the output voltage waveform with increasing output frequency. 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.

References

In-line references

1. ^ http://www05.abb.com/global/scot/scot271.nsf/veritydisplay/c2be0f4cc18285dac1256ddd00346eed/$file/28-33m214.pdf retrieved August 5, 2011
2. ^ http://www05.abb.com/global/scot/scot271.nsf/veritydisplay/2314773146eb2d77c1256ec300341511/$file/12-18%20ENG%209902.pdf retrieved August 5, 2011

General references

• Dorf, Richard C., ed. (1993), The Electrical Engineering Handbook, Boca Raton: CRC Press, ISBN 0-8493-0185-8 
• Lander, Cyril W (1993), Power Electronics (3rd ed.), London: McGraw-Hill, ISBN 0-07-707714-8