Question 15 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is A ### Explanation for Option A (Correct) **A) The operating motor frequency is generally limited to less than one-third of mains line frequency.** A cycloconverter is a direct AC-to-AC power converter that synthesizes a lower-frequency output waveform from a higher-frequency AC input (mains line frequency) by directly switching segments of the input waveform. To produce an acceptable output waveform with low harmonic distortion, the output frequency must be significantly lower than the input frequency. * **Frequency Ratio:** Typical industrial or propulsion applications using cycloconverters (like those for large synchronous motors) often require the output frequency ($f_{out}$) to be constrained such that $f_{out} \leq 0.3 \times f_{in}$ (i.e., less than 30% or one-third of the input frequency). This limitation ensures adequate time to sample and switch the input waveform segments to accurately reconstruct the required low-frequency output sine wave, minimizing commutation failures and distortion. ### Explanation of Incorrect Options **B) The operating motor frequency is generally limited to three times the mains line frequency.** * **Incorrect:** A cycloconverter is fundamentally a step-down frequency converter. It cannot synthesize an output frequency higher than the input frequency; certainly not three times higher. Direct AC-to-AC conversion limits the output frequency to a fraction of the input frequency. **C) The operating motor frequency is generally not limited regardless of the mains line frequency.** * **Incorrect:** The output frequency is heavily dependent on the input frequency. Due to the need for commutation (switching) and waveform reconstruction, the output frequency must be strictly limited relative to the input frequency to maintain efficiency and acceptable waveform quality. **D) The operating motor frequency is generally limited to that equal to the mains line frequency.** * **Incorrect:** While it is theoretically possible to output a frequency equal to the input frequency, this would result in extremely poor waveform quality and high harmonics in a practical cycloconverter implementation, making the system inefficient and unreliable for motor propulsion. Therefore, the frequency is limited to a small fraction of the input frequency, not equality.