Cycloconverters

A cycloconverter is a type of static power converter that directly converts AC electrical power from one frequency to another without an intermediate DC link. It uses thyristors or similar controllable switches arranged in one or more bridge configurations to produce an output AC voltage with a frequency different from the input, typically lower. Cycloconverters are often employed for high-power, low-speed motor drives where a variable output frequency is required.

Principle of operation: The device gates thyristors in a coordinated manner so that portions of input AC waveforms are selected and combined to form the desired output waveform. Because there is no DC intermediate link, the output frequency must be produced by sequentially conducting portions of successive input cycles. The resulting waveform is quasi-sinusoidal and rich in harmonics, requiring filtering and careful control. Commutation relies on the AC supply, so line conditions strongly influence performance.

Configurations and types: Line-commutated cycloconverters (LCCC) use thyristor bridges connected directly to the AC sources and are designed to produce a lower output frequency. Within this family, circulating-current cycloconverters introduce a current path between bridges to improve waveform control, at the cost of more complex commutation dynamics. Cycloconverters are typically implemented for large-magnitude power where a DC link or high-speed switching devices would be impractical.

Applications and limitations: They are used in very large motor drives for applications such as ship propulsion, rolling mills, cement plants, and hoisting, where very high power is required at low speeds. Limitations include high harmonic distortion, limited dynamic response, and complex control and filtering requirements. Modern variable-frequency drives often prefer PWM inverters for new designs, but cycloconverters remain relevant for specific very-high-power, low-speed applications.