Three-phase square wave VSI
Three-phase Square Wave – VSI:
Fig : A 3-phase Voltage Source Inverter (VSI) feeding a balanced load
Fig: Relevant voltage waveforms outputs by three phase VSI
- The power circuit of a 3-phase VSI can be a three single-phase half-bridge inverter circuits put across the same dc bus. The individual pole voltages 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 wave inverter are shifted in time by one third of the output time period.
- The chosen numbering the switches turn on in the sequence:- Sw1, Sw2, Sw3, Sw4, Sw5, Sw6, Sw1, Sw2, ….and so on. Identifying the switching cycle time as 360 degrees (2π radians), it can be seen that each switch conducts for 1800 and the turning on of the adjacent switch is staggered by 60 degrees.
- The upper and lower switches of each pole (leg) of the inverter conduct in a complementary manner. To reverse the output phase sequence, the switching sequence may simply be reversed.
- Considering the symmetry in the switch conduction pattern, it may be found that at any time three switches conduct. It could be two from the upper group of switches, which are connected to positive dc bus, and one from lower group or vice-versa (i.e., one from upper group and two from lower group).
- According to the conduction pattern indicated in the waveform there are six combinations of conducting switches during an output cycle:- (Sw5, Sw6, Sw1), (Sw6, Sw1, Sw2), (Sw1, Sw2, Sw3), (Sw2, Sw3, Sw4), (Sw3, Sw4, Sw5), (Sw4, Sw5, Sw6).
- Each of these combinations of switches conducts for 600 in the sequence mentioned above to produce output phase sequence of A, B, C. The fundamental component of the three output line-voltages will be balanced.