Page 360 - Power Electronics Handbook
P. 360

350  D.C. link  frequency changers
                       (v)  The RFI generated in a current-fed inverter is much lower than that
                            in  voltage-fed  inverters,  since  RFI  is  essentially  generated  by
                            changing  currents  and  in  a  current-fed  inverter  the  current  is
                            substantially constant.
                       (vi)  The current drawn from the supply in a current-fed inverter is much
                            smoother  than  that  in  a  voltage-fed inverter,  so  that  the  filtering
                            requirements are also much less.
                         In spite of  some of  the advantages of  current-fed inverters, voltage-fed
                       inverters continue to be used, since they are more versatile in a variety of
                       applications, and they are easier to control.



                       13.6 Inverter control circuits
                       A variety of  control circuits exist for inverters, dependent on the system
                       used to vary the output voltage and the inverter configuration. All these
                       will  consist  of  the  power  semiconductor  drive  circuit  and  a  form  of
                       sequencing to turn the devices on at the appropriate instances in the cycle,
                       usually incorporated into a single integrated circuit.
                         Figure 12.23 illustrated a method of  mark-to-space control of the output
                       voltage, using a sawtooth waveform and a pedestal reference, and this was
                       incorporated into a control circuit for a chopper in Figure 12.24. The same
                       technique can be used to provide mark-to-space control within an inverter,
                       although  Figure  13.65  shows an  alternate  technique  using  two  shifted
                       square waves to achieve the same effect, as illustrated in Figure 13.41. The


                                 Variable frequency   Comparator   Sequencer   Drive   Inverter
                       Fr~enc)L, square wave   -+      --+       -D   circuit  -+  switches
                       control   generator






                       Figure 13.65 System for mark to space control of the output voltage from an inverter


                       square waveform from the generator is passed through a phase shifter and
                       the two direct and shifted waveforms are compared to provide the input to
                      the sequencer  and drive circuits, which turn the inverter semiconductor
                      switches on at the correct instances. The load voltage may be fed back and
                      used  to regulate  the  output,  and the current can also be monitored  to
                      provide a current limit function, if  required. The frequency of  the inverter
                      is controlled by varying the frequency of the square wave generator and the
                      output voltage by regulating the phase shift between the two square waves
                      prior to the comparison stage.
                        Selected harmonic reduction, of  the type illustrated in Figure 13.36, can
                      be obtained by the control circuit shown in Figure 13.66. The output from
                      the pulse generator,  which determines the frequency of  the inverter, is
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