SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

has been made of using THI in the SVM inverter. Indeed, one of the advantages of the SVM
inverter is that it can produce an average SV anywhere inside the THI limit without having to
“manually” inject any harmonics—it achieves this inherently, and not by injection but by
“rearranging.” Returning to the discussion that compared Table 4.2 with Table 4.3, we saw the
instantaneous output of a 180° inverter always has a ZS component. This is true of SPWM as
well, but it was distributed throughout the modulation period as to have a zero average (hence the
output was limited to the SPWM limit, R). In SVM it is distributed throughout the modulation
period so as to have a nonzero average (a very particular nonzero average that allows the output
to reach the THI limit, T). We have not seen any evidence of this, however, because (per the
previous discussion) the SV cannot contain this component. When we synthesize the SV using
the base SVs and draw it on the two-dimensional diagram, we will never see it. This important
understanding seems to be entirely absent in the popular literature. Plots of the phase voltage can
indeed be found that show the phase voltage with the ZS component present (and the 2/ 3 gain
in fundamental voltage is always touted) but there is no mention of how or why SVM generates
such a waveform. Figure 4.43 shows the shape of the average pole voltage in a THI inverter and
also shows the shape of the same for an SVM inverter. There is clearly a difference in shape; the
former has rounded corners and a smooth dip, while the latter has steeper sides, sharp corners,
and a pointed trough. This is because the ZS component in a THI inverter is a sinewave but in an
SVM inverter it is a triangle wave.
Figure 4.43 – Average pole voltages for THI and SVM inverter.
The various SVM switching strategies mentioned earlier will produce a ZS component (and
hence, pole voltages) that are very different from that shown above. Although they are not
discussed here, a sample of these waveforms is shown in Figure 4.44, where the darker line is the
ZS voltage generated. In each case there is a lighter line that is the line-neutral voltage and
another line that is the pole voltage. In each case the line-neutral voltage is sinusoidal. This shows
that the different modulation schemes produce radically different pole waveforms but in each
case the ZS component generated is such that the phase voltage (pole minus ZS) is sinusoidal.
The annotated graphs are those of the techniques discussed earlier in this report.
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