Space vector modulation
Encyclopedia
Space vector modulation (SVM) is an algorithm for the control of pulse width modulation (PWM). It is used for the creation of alternating current
(AC) waveform
s; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multiple class-D amplifiers
. There are various variations of SVM that result in different quality and computational requirements. One active area of development is in the reduction of total harmonic distortion
(THD) created by the rapid switching inherent to these algorithms.
To implement space vector modulation a reference signal Vref is sampled with a frequency fs (Ts = 1/fs). The reference signal may be generated from three separate phase references using the
transform. The reference vector is then synthesized using a combination of the two adjacent active switching vectors and one or both of the zero vectors. Various strategies of selecting the order of the vectors and which zero vector(s) to use exist. Strategy selection will affect the harmonic content and the switching losses.
More complicated SVM strategies for the unbalanced operation of four-leg three-phase inverters do exist. In these strategies the switching vectors define a 3D shape (a hexagonal prism
in
coordinates or a dodecahedron in abc Three-Dimensional Space Vector Modulation in abc coordinates) rather than a 2D hexagon.
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multiple class-D amplifiers
Switching amplifier
A class-D amplifier or switching amplifier is an electronic amplifier where all power devices are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states....
. There are various variations of SVM that result in different quality and computational requirements. One active area of development is in the reduction of total harmonic distortion
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
(THD) created by the rapid switching inherent to these algorithms.
Principle
A three phase inverter as shown to the right must be controlled so that at no time are both switches in the same leg turned on or else the DC supply would be shorted. This requirement may be met by the complementary operation of the switches within a leg. i.e. if A+ is on then A− is off and vice versa. This leads to eight possible switching vectors for the inverter, V0 through V7 with six active switching vectors and two zero vectors.| Vector | A+ | B+ | C+ | A− | B− | C− | VAB | VBC | VCA | |
|---|---|---|---|---|---|---|---|---|---|---|
| V0 = {000} | OFF | OFF | OFF | ON | ON | ON | 0 | 0 | 0 | zero vector |
| V1 = {100} | ON | OFF | OFF | OFF | ON | ON | +Vdc | 0 | −Vdc | active vector |
| V2 = {110} | ON | ON | OFF | OFF | OFF | ON | 0 | +Vdc | −Vdc | active vector |
| V3 = {010} | OFF | ON | OFF | ON | OFF | ON | −Vdc | +Vdc | 0 | active vector |
| V4 = {011} | OFF | ON | ON | ON | OFF | OFF | −Vdc | 0 | +Vdc | active vector |
| V5 = {001} | OFF | OFF | ON | ON | ON | OFF | 0 | −Vdc | +Vdc | active vector |
| V6 = {101} | ON | OFF | ON | OFF | ON | OFF | +Vdc | −Vdc | 0 | active vector |
| V7 = {111} | ON | ON | ON | OFF | OFF | OFF | 0 | 0 | 0 | zero vector |
To implement space vector modulation a reference signal Vref is sampled with a frequency fs (Ts = 1/fs). The reference signal may be generated from three separate phase references using the
transform. The reference vector is then synthesized using a combination of the two adjacent active switching vectors and one or both of the zero vectors. Various strategies of selecting the order of the vectors and which zero vector(s) to use exist. Strategy selection will affect the harmonic content and the switching losses.More complicated SVM strategies for the unbalanced operation of four-leg three-phase inverters do exist. In these strategies the switching vectors define a 3D shape (a hexagonal prism
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
in
coordinates or a dodecahedron in abc Three-Dimensional Space Vector Modulation in abc coordinates) rather than a 2D hexagon.See also
-
transform - Inverter (electrical)Inverter (electrical)An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
- pulse width modulation
External links
- Model based control of PMSM motor with space vector modulation Description and working VisSimVisSimVisSim is a visual block diagram language for simulation of dynamical systems and model based design of embedded systems. It is developed by Visual Solutions of Westford, Massachusetts....
source code diagram.