18-09-2012, 11:42 AM
A Simple Space-Vector PWM Algorithm for VSI-fed AC Motor Drives
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Abstract
Space-Vector Pulse Width Modulation (SV-PWM) is
a well-known modulation strategy in electric drives (also used in
power electronics). However, a simple explanation of its analysis
and its hardware implementation is lacking, which is the focus
of this paper.
INTRODUCTION
In electric drives and other power electronic applications,
such as in switch-mode rectifiers, the intent is to supply
three-phase sinusoidal currents. In the system of Fig.1a, the
neutral “n” may be hypothetical in case of delta-connected
analysis. At switching frequencies relatively high compared
to the fundamental frequency of synthesis, it is possible to
represent the switching circuit of Fig.1a by means of ideal
transformers in terms of switching average quantities, as
shown in Fig.1b [1]. Average variables with a bar (“—“) on
top do not contain switching frequency ripple.
The intent to supply balanced sinusoidal currents into a
balanced ac motor implies that the line-to-line voltages across
the motor terminals be sinusoidal in steady-state. In a
balanced ac motor, it can be shown (Appendix 1) that the
motor phase voltages (van, vbn and vvn) are also sinusoidal and,
similarly to the motor currents, the three phase voltages sum
to zero on an instantaneous basis (see [2] and [3]).
SINUSOIDAL PULSE-WIDTH MODULATION
A traditional approach that is discussed in most textbooks
is sinusoidal pulse width modulation, where sinusoidal
control voltages are compared with a triangular carrier signal
(see Fig.2a) to generate the switching functions qa,b,c in
Fig.1a. The resulting duty factors (transformer turn-ratios in
Fig.1b) are shown in Fig.2b,
CONCLUSION
As discussed in this paper, Space-Vector PWM control of a
voltage source inverter can be easily explained and
implemented using the classical “carrier-based” approach.
Since the method can be applied both to electric drives and
switch-mode power converters, its derivation can be
straightforward, without any use of additional definitions
such as “sector calculations”, “hexagon of states”, “vector
decomposition” etc.