SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 4.34 – SVM overmodulation region: locus of fundamental SV.
Comparing Figure 4.33 and Figure 4.34 should finalize the understanding that the hexagon is the
“maximum” volt-second output and the circle (F) is the maximum of the fundamental of the
output (for both pole and line-neutral voltages).
SVM Switching Schemes
The previous subsection explained how SVM synthesized a time-averaged SV by switching
between at least two active SVs and at least one null SV. Since those are the only basic
requirements, there are many different ways to realize a SVM inverter. This section will develop
one of the common methods in order to illustrate some of the details involved in SVM
implementation. Perhaps the most intuitive switching scheme would be to divide the modulation
period into three parts corresponding to two active and one null SV, as shown in Figure 4.35.
During the time TX, X S
would be asserted, and so on.
Figure 4.35 – One period in a simple switching scheme.
Similar to a single channel in a PWM inverter, the period consists of an ‘on’ segment (TX+TY)
and an ‘off’ segment (TN). But instead of simply modulating a pulse width we are modulating the
duration of a base SV, and there are two of them. There is a disadvantage to this simple scheme,
193