The basic configuration of a Voltage Source Inverter (VSI) has been described in Lesson 33.Single-phase half-bridge and full-bridge configurations of VSI with square wave pole voltageshave been analyzed in Lesson 34. In this lesson a 3-phase bridge type VSI with square wave polevoltages has been considered. The output from this inverter is to be fed to a 3-phase balancedload. Fig. 35.1 shows the power circuit of the three-phase inverter. This circuit may be identifiedas three single-phase half-bridge inverter circuits put across the same dc bus. The individual polevoltages of the 3-phase bridge circuit are identical to the square pole voltages output by single phase half bridge or full bridge circuits. The three pole voltages of the 3-phase square waveinverter are shifted in time by one third of the output time period. These pole voltages along withsome other relevant waveforms have been plotted in Fig. 35.2. The horizontal axis of thewaveforms in Fig. 35.2 has been represented in terms of ‘ωt’, where ‘ω’ is the angular frequency(in radians per second) of the fundamental component of square pole voltage and ‘t’ stands fortime in second. In Fig. 35.2 the phase sequence of the pole voltages is taken as VAO, VBO andVCO. The numbering of the switches in Fig. 35.1 has some special significance vis-à-vis theoutput phase sequence.
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