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Generate PWM signals for 3-phase inverter using space-vector modulation technique
Control / Modulators
The space vector modulator generates a reference voltage vector, , at the ac
terminals of a three phase voltage source converter shown below. The reference vector
is defined in the
coordinate system:
.
The construction of the reference voltage vector, , is graphically depicted
below.
Construction of the reference vector .
The sector detection calculation determines the sector in which the reference
voltage vector resides. The relative on-times,
, for the switching vectors
and
are then calculated. In each sector, two unique switching
vectors named
and
are available. Two zero vectors, named
are also available. The relationship between the relative on-times and the
reference vector is shown below for an arbitrary sector. The relative on-times
are calculated by projecting the reference vector onto the vectors
and
.
The vector generation and sequencing function creates a switching cycle by
time-averaging the switching vectors according to their on-time values. There are
many possible switching sequences that can be implemented since the order in which
the vectors are applied during a switching cycle is arbitrary. In addition,
one or both of the
vectors can be used. For further information, please read the
documentation that accompanies the demo model "Space Vector Control of a
Three Phase Rectifier using PLECS." This documentation can be found at
http://www.plexim.com/examples.
Relationship between relative on times, , switching vectors,
, and
reference vector,
.
With symmetrical modulation, the two switching vectors are used: one
at the beginning and one at the middle of a switching sequence. All three
converter legs are switched during a switching sequence.