- Cycloconverter Definition: A cycloconverter (also known as a cycloinverter or CCV) is defined as a device that converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency.
- Function of Cycloconverters: Cycloconverters change AC to AC while altering the frequency, usually lowering it without any DC stages.
- Types of Cycloconverters: The main types of cycloconverters are blocking mode cycloconverters and circulating current cycloconverters.
- Single Phase Cycloconverters: Single phase to single phase cycloconverters use two full wave converters connected back-to-back to convert voltage.
- Cycloconverter Applications: Cycloconverters are used in cement mill drives, rolling mills, ship propulsion, water pumps, washing machines, mine winders, and various industries.
What is Cycloconverter
A cycloconverter, also called a cycloinverter or CCV, directly converts AC at one frequency into AC at another frequency. Controlled semiconductor switches synthesize the output from segments of the supply waveform without an intermediate DC link. Most line-commutated cycloconverters produce an output frequency below the supply frequency.
Power converters are selected according to the required input and output. Rectifiers change AC to DC, inverters change DC to AC and DC choppers change one DC level to another. A cycloconverter performs direct AC-to-AC conversion while changing output frequency and controllable RMS voltage, as Figure 1 shows.
A cycloconverter controls both the fundamental output voltage and its frequency. In a conventional line-commutated design, many Silicon Controlled Rectifiers select portions of the input phases. The firing controller coordinates those devices, limits unwanted circulating current and shapes the low-frequency output.
Types of Cycloconverters
Cycloconverters are also classified by whether both converter groups may conduct at the same time.
- Blocking-mode cycloconverters
- Circulating-current cycloconverters
Blocking Mode Cycloconverters
In blocking mode, only the converter group that supports the required load-current direction receives firing pulses. The positive group supplies controlled voltage during positive load current, while the negative group is blocked. During negative current, the negative group operates and the positive group is blocked. This non-circulating-current form of cycloconverters does not need an intergroup reactor, but control must allow safe current transfer between groups.
Circuiting Current Cycloconverter
In circulating-current operation, the positive and negative converter groups may conduct together. An intergroup reactor limits the current driven by the instantaneous voltage difference between them. This permits smoother transfer between groups but adds reactor loss, size and control requirements.
A separate frequency-ratio classification gives the two categories below.
- Step-up cycloconverters
- Step-down cycloconverters
Step Up Cycloconverters
A step-up cycloconverter has an output frequency above the input frequency. This is a specialised category and generally needs switching that can commutate independently of the supply rather than a basic line-commutated thyristor bridge.
Step Down Cycloconverters
A step-down cycloconverter produces an output frequency below the input frequency. Conventional thyristor cycloconverters use line commutation and are normally step-down converters.
The following sections classify these converters by input and output phase count.
Single Phase to Single Phase Cycloconverters
A single-phase cycloconverter uses positive and negative full-wave converter groups connected in inverse parallel. The groups create positive and negative portions of the low-frequency load voltage.
For the waveform shown, the desired output frequency is one-quarter of the input frequency. The positive group supplies two input cycles for the positive output half-cycle. The negative group then supplies two cycles with opposite polarity. In blocking mode, one group is disabled before the other receives firing pulses, so no intentional circulating current flows between them.
Three-Phase to Single Phase Cycloconverters
A three-phase-to-single-phase cycloconverter uses the three supply phases to produce one controlled output phase. Its positive and negative converter groups can support positive or negative load current. Depending on firing angle and current direction, a group can transfer power from supply to load or regenerate power to the supply. In blocking mode, only one group conducts at a time.
Three Phase to Three Phase Cycloconverters
A three-phase-to-three-phase cycloconverter combines three three-phase-to-single-phase converter sections, one for each load phase. The output sections may be connected in wye or delta according to the motor and converter design. Their fundamental output voltages are displaced by 120o. The circuit below shows the arrangement.
Applications of Cycloconverter
Cycloconverters suit high-power, low-speed drives that need reversible torque and operation near zero speed. Examples include:
- Cement mill drives, especially large low-speed grinding mills
- Rolling mills with existing cycloconverter-fed drives
- Ship propulsion drives designed for low-speed motor control
- Large water pumps where a low-frequency direct converter is suitable
- Early or specialised washing-machine speed-control circuits
- Mine winders that need controlled low-speed torque and regeneration
- Other heavy industries with high-power, low-speed synchronous motors
