SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

similar convention is to use V BUS as shown but replace V DC with V BUS /2.
This practice has a
definite advantage in that it does not require the creation of an “artificial” voltage definition
( V DC ) but it has the disadvantage that the factor of ½ will combine with other numerical factors
in formulae. This leaves the voltages specified in terms of V BUS which again loses the clear
association with the midpoint shown in the figure and is not preferred in this report.
Figure 4.2 – Definitions of DC bus midpoint and voltage magnitudes.
Figure 4.3 shows a scale showing the two measures of voltage. On the left it is clear that the
voltage scale is centered at the midpoint and bipolar voltages result (as we expect in an AC
waveform). On the right the scale is referenced to the low side of the bus and is therefore
unipolar. However, there is nothing to prevent us describing a bipolar voltage (relative to M) in
terms of V BUS or describing a unipolar voltage (relative to the low side of the bus) in terms of
V DC . It is difficult to use only one convention and references in the literature are testament to
this. As much as possible, voltages in this report are referenced to the midpoint M and therefore,
the measures V DC and V BUS refer to magnitudes of a bipolar voltage, as shown in the center of
Figure 4.3.
Figure 4.3 – Voltage measurements.
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