SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.38 – Leg control signals; commanded SV in sextant s12.
Using the neutral voltages from Table 4.2, the line-neutral voltage of phase-A corresponding to
Figure 4.38 can be plotted, as shown in Figure 4.39. For the SVM period in these figures, the SV
would be at approximately 30° (equal time spent in S1 and S2). Examining the SPWM case shown
in Figure 4.24, the 30° position is approximately above the letter “r” in the word average (cosine
reference). Comparing the 30° position in Figure 4.24 with Figure 4.39 it is seen that around 30°
both methods are switching between similar inverter states and produce similar instantaneous
waveforms.
Figure 4.39 – Phase-A line-neutral voltage; commanded SV in sextant s12.
It is apparent from the above discussion that selecting a switching scheme is not trivial. In
addition to the issue of undesirable simultaneous leg transitions, the current measurement must be
synchronized to the switching scheme, which adds additional requirements. The algorithm shown
here is common in the popular literature but in the academic literature there are many others. In
several variants, a phase is clamped to a rail during certain periods in order to minimize switching
loss. These may be called two phase SVM, bus-clamped SVM, or discontinuous SVM, although
the terms can carry different meanings. In addition to the issues discussed here, an important
practical issue is the compatibility of the switching scheme with the gate drive circuitry [83,
p.258]. These variants and practical issues are not discussed here.
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