SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.24 – Instantaneous phase-A line-neutral voltage in SPWM inverter.
(Modified from [73, p.233].)
In the PWM inverter each phase leg was controlled separately by either reference-carrier
comparison or hysteresis. A SVM inverter treats the inverter as one unit. Rather than controlling
each leg in an attempt to command an average phase voltage, the SVM inverter is controlled
simply by selecting which of the eight states is desired. This concept is symbolized in Figure
4.25.
Figure 4.25 – Direct selection of inverter state.
Obviously some high-level control will be necessary to determine how to time-average the
correct states in order to synthesize the desired output waveforms; this will be developed in later
subsections.
Space Vectors, Magnitudes, and Trajectories
The output of the inverter is usually studied by plotting the phase voltage as a function of time.
Using SV theory allows a more compact representation of the inverter output. SV theory can also
be used to control using the concept of Figure 4.25; the result is SVM. Before examining SVM it
is necessary to study the inverter output voltages in terms of complex αβ0 components and to
study the trajectories of various SVs.
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