SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.44 – ZS components of some popular PWM methods.
(Modified from [126].)
Clearly, since the ZS component adds in the neutral and since any triple-frequency waveform has
spectra that are only ZS, there is a freedom to choose the ZS signal, as the figure has shown. This
can be accomplished by “injecting” a ZS signal into the command of a ramp-comparison
modulator, or it can be accomplished by using different switching rules in an SVM
implementation. Since both methods use a 180° inverter, they are really only different ways of
controlling the inverter. Thus, one should perhaps expect that there is a relationship between the
control over the ZS signal in THI and SVM. Indeed there is: the freedom to choose the injected
signal in THI corresponds to the placement of the V0 and V7 in SVM [72, p.151], [128], [70,
ch.6]. The relationship of SPWM methods to those of SVM is described in [134], [135], [136].
The SVM inverter is often praised for its compatibility with digital implementation (since the
switching rules and calculation of times can be done in software) and for its compatibility with a
vector control structure (the commanded input is in the form of two SV components). However,
“SVM” can be implemented using the exact same hardware as THI—the only difference is that a
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