SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

called the overmodulation region (also called the pulse-dropping region because transitions that
were previously present are “dropped” from the pulse train).
Figure 4.17 – Overmodulation in carrier-based SPWM; not to scale.
When operating in the overmodulation region the relationship between the reference amplitude
and the fundamental of the output is no longer linear. Further, in examining Figure 4.17 it is clear
that when the reference amplitude is made large enough the pole voltage will become a
squarewave. In that case the inverter is operating exactly the same as the 180° six-step
squarewave inverter presented earlier. Therefore the operation of the inverter can be divided into
three regions or modes. These modes and a plot of the amplitude of the fundamental output
voltage as a function of the modulation index are shown in Figure 4.18. The shape of the curve in
the overmodulation region and the value of the modulation index (marked x) at which the inverter
operates in six-step squarewave mode depend on the frequency modulation ratio m f . The
literature describes a host of different schemes to handle the nonlinearity of m a in the
overmodulation region and the transition between regions. In this report only the linear region of
PWM inverters is of concern.
174