Cycloconverter operation modes: can a cycloconverter perform step-down and/or step-up frequency conversion?

Introduction / Context:
A cycloconverter directly converts AC at one frequency to AC at another frequency without an intermediate DC link, by controlling segments of the input waveforms. Understanding whether it can step frequency down or up is important in selecting drives for low-speed, high-power applications.

Given Data / Assumptions:

• Consider conventional line-commutated cycloconverters used in industrial drives.
• They synthesize output by phase-controlling input segments from a fixed-frequency AC source.
• We are not considering special resonant or forced-commutated laboratory variants here.

Concept / Approach:

Practical cycloconverters in industry are step-down devices: the output frequency is a submultiple of the input, typically less than one-third of the supply frequency, to maintain acceptable waveform quality and commutation margin. Step-up operation is not used in standard line-commutated cycloconverters due to severe commutation and waveform quality limitations.

Step-by-Step Reasoning:

Verification / Alternative check:

Applications like large synchronous motor drives at low speeds use step-down cycloconverters from 50/60 Hz to a few Hz to tens of Hz. Step-up needs are served by other topologies (DC link converters, matrix converters with advanced commutation).

Why Other Options Are Wrong:

• step up or step down: overstates capability for practical, line-commutated units.
• step up: contradicts common industrial practice and commutation constraints.
• none: incorrect because step-down is widely used.

Common Pitfalls:

• Assuming any AC–AC converter can both step up and step down frequency.
• Confusing cycloconverters with matrix converters or indirect AC–DC–AC converters.

Final Answer:

step down