Specifications

Space Vector PWM
6-60
6.7 Space Vector PWM
Space vector PWM refers to a special switching scheme of the six power tran-
sistors of a 3-phase power converter. It generates minimum harmonic distor-
tion to the currents in the windings of a 3-phase AC motor. It also provides more
efficient use of supply voltage in comparison with the sinusoidal modulation
method.
6.7.1 3-Phase Power Inverter
The structure of a typical 3-phase power inverter is shown in Figure 6–24,
where V
a
, V
b
, and V
c
are the voltages applied to the motor windings. The six
power transistors are controlled by DTPH
x
and DTPH
x_
(x = a, b, and c). When
an upper transistor is switched on (DTPH
x
= 1), the lower transistor is switched
off (DTPH
x_
= 0). Thus, the on and off states of the upper transistors (Q1, Q3,
and Q5) or, equivalently, the state of DTPHx (x = a, b, and c) are sufficient to
evaluate the applied motor voltage U
out
.
Figure 6–24. 3-Phase Power Inverter Schematic Diagram
U
dc
DTPH
a
DTPH
a_
GND
DTPH
b
DTPH
b_
DTPH
c
DTPH
c_
Q
1
Q2
Q
3
Q
4
Q
5
Q
6
V
a
V
b
V
c
Power Inverter Switching Patterns and the Basic Space Vectors
When an upper transistor of a leg is on, the voltage V
x
(x = a, b, or c) applied
by the leg to the corresponding motor winding is equal to the voltage sup-
ply U
dc
. When it is off, the voltage applied is zero. The on and off switching of
the upper transistors (DTPH
x
, x = a, b, or c) have eight possible combinations.
The eight combinations and the derived motor line-to-line and phase voltage
in terms of DC supply voltage U
dc
are shown in Table 6–14, on page 6-61,
where a, b, and c represent the values of DTPH
a
, DTPH
b
, and DTPH
c
, respec-
tively.