Page 243 - Power Electronics Handbook
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Envelope cycloconverters  233

                   transformer. From this figure it can be seen that the load voltage will have
                   a  very  high  harmonic content  and  this  can  be  reduced,  as  in  Figure
                   10.13(b),  by  using  a  six-pulse converter.  Once again  the  load  voltage
                   follows the envelope of the various phase voltages and the output voltage is
                   almost rectangular in shape.
                     Since most cycloconverters have an input transformer, it is possible to
                   vary  the  ratio  of  their  secondaries  with  respect  to  one  another.  For
                   instance, Figure 10.13(c) shows the output voltage obtained from a system
                   in which phase 1 has 33%, phases 2 and 6 have 73%, and phases 2 and 5
                   have 97% of the voltage of  that of phase 4. The operation is once again of
                   the  envelope type, where  the  load  voltage commutates naturally from
                   phase to phase, so that it is always equal to that of the most positive phase.
                   This system gives an output with a good approximation to a sine wave and
                   was widely used io the 1920s and 1930s for traction applications, to provide
                   a stable 16.66Hz (20-   for 6OHz supplies) voltage.
                     It  can be seen from  the above discussions that  the  half  cycle output
                   waveform from envelope cycloconverters is identical to that obtained if the
                   thyristors were all replaced by  diodes, so that conduction begins at the
                   commencement of  the supply cycle. The function of  the thyristor is now
                   purely to ensure that one of  the converter groups is switched off when the
                   other group is conducting, so that there is no short-circuit path across the
                   supply.
                     Figure 10.14 illustrates an alternative form of  envelope cycloconverter,
                   often referred to as asynchronous to distinguish it from the synchronous
                   type of  envelope  converter  described  with  reference  to  Figure  10.13.














                   Positive group                          Negative group
                   (a)
                                 Output load
                                 voltage









                   (b)
                        1e.M One form of asynchronous envelope cycloconvener: (a) circuit arrangement; (b)
                   waveforms
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