Cycloconverter
Converts AC to lower-frequency AC without a DC link.
A cycloconverter (CCV), also called a cycloinverter, converts a constant-amplitude, constant-frequency AC waveform to another AC waveform of a lower frequency. It does this by piecing together segments of the supply waveform, without using a DC link in between. The two main types are the circulating-current type and the blocking-mode type, with most high-power commercial units being the latter. In terms of characteristics, phase-controlled silicon controlled rectifiers (SCRs) can be used across all CCV types, while cheaper TRIAC-based designs are limited to resistive loads. Both the output voltage’s amplitude and frequency can be adjusted. For three-phase units, the output frequency must be less than about one-third of the input frequency for circulating-current types, or less than half for blocking-mode types. The quality of the output waveform improves when the pulse number of the switching bridges is increased in a phase-shifted arrangement. CCVs can be configured as single-phase to single-phase, three-phase to single-phase, or three-phase to three-phase, with the last being the most common. As for applications, standard CCVs typically handle power ratings from a few megawatts up to tens of megawatts. They are used to drive mine hoists, rolling mill motors, ball mills, cement kilns, ship propulsion, Scherbius drives (slip power recovery for wound-rotor induction motors), and aircraft 400 Hz power generation. Because the output frequency can be reduced nearly to zero, very large motors can start under full load at very slow speeds and then ramp up gradually. This is especially useful for ball mills, which can start fully loaded instead of needing to be emptied first. In processes like hot-rolling steel mills, variable speed and reversing are essential. Older SCR-controlled DC motors required frequent brush and commutator maintenance and had lower efficiency, whereas cycloconverter-driven synchronous motors need less upkeep and offer better reliability and efficiency. Single-phase bridge CCVs have also been widely used in electric traction, for example producing 25 Hz power in the US and 16⅔ Hz in Europe. Although phase-controlled converters like CCVs are gradually being replaced by faster PWM self-controlled types using IGBTs, GTOs, IGCTs, and similar devices, the older designs are still used at the high end of the power range.
- Output to input frequency ratio limit ci
- less than about one-third
- Output to input frequency ratio limit bl
- less than about one-half
- Power rating span
- a few megawatts up to many tens of megawatts
- Pulse number start
- 6
- Typical input output configuration
- 3-phase/3-phase
Lore & Background
Cycloconverters use phase-controlled semiconductor controlled rectifier devices (SCR) throughout their range, while low-cost, low-power TRIAC-based CCVs are inherently reserved for resistive-load applications. The amplitude and frequency of the output voltage are both variable. Output waveform quality improves as the pulse number of switching-device bridges in phase-shifted configuration increases in the CCV's input. CCVs can be configured as 1-phase/1-phase, 3-phase/1-phase, and 3-phase/3-phase input/output, with most applications being 3-phase/3-phase. CCVs are used for driving mine hoists, rolling mill main motors, ball mills for ore processing, cement kilns, ship propulsion systems, slip power recovery wound-rotor induction motors (Scherbius drives), and aircraft 400 Hz power generation. The variable-frequency output can be reduced essentially to zero, allowing very large motors to start on full load at very slow revolutions and gradually brought up to full speed. This is important for ball mills, allowing starting with a full load rather than having to start with an empty barrel. Single-phase bridge CCVs have been used extensively in electric traction to produce 25 Hz power in the US and 16 2/3 Hz power in Europe. While phase-controlled converters including CCVs are gradually being replaced by faster PWM self-controlled converters based on IGBT, GTO, IGCT and other switching devices, these older classical converters are still used at the higher end of the power-rating range.
Reader's Guide
Cycloconverters are notable for enabling variable-speed, reversing control of very large AC motors without an intermediate DC link, which simplifies the power conversion stage. Their ability to reduce output frequency essentially to zero allows massive motors—such as those in ball mills—to start under full load, avoiding the need for empty-barrel startup and progressive loading. In hot-rolling steel mills, variable speed and reversing are essential; previously, SCR-controlled DC motors were used, requiring regular brush/commutator servicing and delivering lower efficiency, whereas cycloconverter-driven synchronous motors need less maintenance and give greater reliability and efficiency. Although newer PWM self-controlled converters are gradually replacing phase-controlled converters including CCVs, the older classical converters remain in use at the higher end of the power-rating range, from a few megawatts up to many tens of megawatts. Their harmonic impact on input and output is characterized by a specific equation involving pulse number and output frequency, producing sideband interharmonics and subharmonics.
Did You Know?
- A cycloconverter synthesizes an output waveform from segments of the AC supply without an intermediate DC link.
- Most commercial high-power cycloconverters are of the blocking-mode type.
- The output-to-input frequency ratio for a three-phase blocking-mode CCV must be less than about one-half.
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