Space Vector Modulator
Purpose
Generate PWM signals for 3-phase inverter using space-vector modulation technique
Library
Control / Modulators
Description
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:
.
Operation
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/support/application-examples.
Relationship between relative on times, , switching vectors,
, and
reference vector,
.
Parameters
- Modulation strategy
- The modulation strategy can be set to 'Alternating zero
vector' or 'Symmetrical' using a combo box. With alternating zero vector
modulation, only one of the two
switching vectors is used during a switching sequence. One switch leg is always clamped to the positive or negative dc bus voltage and only two of the three converter legs are switched.
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.
- Switching frequency
- The switching frequency in Hz.
- Switch output values
- The switch output values in the high and low state. The
values should be selected to match the converter's gate control logic so that a
high value turns on the upper switch in the leg and the low value turns on the
lower switch. The default values are
.
Inputs and Outputs
- DC voltage
- The input signal
is the voltage measured on the dc side of the converter.
- Theta
- The angular position of the
axis in radians. The value supplied must conform to:
. If
exceeds
,
is shifted into the range
using a modulus function.
- Reference voltage
- This input, labelled
, is a two dimensional vector signal comprising the elements
.
- Switch output
- The output labelled
is formed from three switch control signals,
, which control the converter legs A, B, and C. Each switch signal controls the upper and lower switches in the respective leg.
Probe Signals
- sector
- A value in the set of
that indicates the sector in which the refererence vector,
, is located.
- tau
- A vector signal comprising the three relative on time values,
.
- sw
- A vector signal consisting of the three gate signals for the converter legs,
.