Page 242 - Power Electronics Handbook

232 Direct a.c. frequency converters

symmetrical cycloconverters although, as expected, the transformer and
thyristor utilisation have now also been considerably reduced. Once again,
however, this circuit provides a low-cost medium-performance converter,
which is economical in many applications.

10.4 Envelope cycloconverters

In the discussions so far the operation of a cycloconverter has been
illustrated with reference to its phase-controlled mode. As mentioned
above, this is the most common technique and gives a system in which the
load voltage and frequency can be readily controlled. However, in certain
applications, especially where the ratio of input to output frequency is
fixed, envelope converters can prove simpler and more economical.
Figure 10.13(a) shows the output from a single-phase supply in which the
ratio of input to output frequency is three. The converter thyristors are
operated at the beginning of every half cycle, so that they follow the
envelope of the a.c. supply waveform. Voltage control is now obtained by
a correct choice between primary and secondary ratios on the input

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. . Mean voltage
,

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. - . Mean voltage

Flporc 10.13 Load-voltage waveforms for synchronous envelope converters: (a) single phase;
(b) three phase; (c) three-phase stepped output