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A cycloconverter is a circuit designed to convert the frequency of ac voltage directly to another frequency of ac voltage without first converting the voltage to dc voltage. The history of this circuit dates back to the 1930s when mercury arc rectifiers were used to control the frequency of railroad engines in Germany. The supply voltage for these original circuits was a fixed 50 Hz ac sine wave common in Europe. The train engines used low frequency (16.6 Hz) so their electric motors would turn slowly, creating a tremendous amount of torque. These earliest rectifiers were rather large tube thyristors. The input circuit for the cycloconverter used a large transformer, and the output section used the thyristors to adjust the timing of the output stage, which allowed frequency to be changed.

Since modern electronic technology provides many ways to control voltage and frequency, the cycloconverter circuit is no longer useful. In fact many advantages are provided by converting the ac voltage to dc before the frequency is converted back to ac and adjusted for the output section. For instance, when the ac input voltage is rectified to dc and filtered, all transient signals and voltage spikes are, removed so that when the dc is converted back to ac, the output circuit is effectively isolated from the input.

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