SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

was fed to the SVM inverter as a SV). The SVM inverter is a 180° inverter with the same
allowable states as a PWM inverter. SVM is simply a method of controlling the inverter as a
whole rather than controlling each phase leg independently. Even though the method of control is
radically different, it is still a pulse width modulator and for this reason it is sometimes called
space vector PWM (SVPWM). Figure 4.28 is redrawn as Figure 4.29 without the circular
trajectories shown and the sextants have been labeled.
Figure 4.29 – Base SVs.
The figure shows all the possible SVs but of course, only one SV can be asserted at one time. In
SVM, one base SV is asserted for a period of time, another for a different period of time, and so
on. There are different implementations but there are two obvious minimum requirements. First,
the only way to synthesize a time-average SV is to switch between two active base SVs. Second,
the only way to control the magnitude of the synthesized time-average SV is to switch between
null and active base SVs. Thus the general SVM algorithm is to switch between two active and at
least one null base SVs during one switching period. Like regular PWM, the switching period
may be selected arbitrarily, although there are many practical limits. The SVM synthesis of a
time-average SV S is shown in Figure 4.30. Simple as it is, the figure is somewhat deceiving. It
shows the time-average SV S as being composed of two vectors aligned with the base SVs. But
the two shorter SVs do not exist—they are time-averages as well. In reality, one active base SV is
asserted, then the other active base SV, then the null base SV, and the cycle repeats (as will be
shown later). Thus, the magnitude of each shorter SV represents the duty cycle of that active SV.
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