Environmental Engineering Reference
In-Depth Information
U
s
*
U
s
*
U
Δ
U
a
Figure 6.11 Synchronized sampling of modulating wave
Now, the sine wave that can be synthesized from a fixed dc bus of magnitude
U
dc
is constrained to a peak value of
U
dc
, so it will have a modulation index equal
to the reciprocal of the modulation index listed in (6.13) or
1
m
0
¼
0
:
787
m
max
¼
ð
6
:
14
Þ
Referring again to Figure 6.8 one can say that with sinusoidal, or regular,
modulation the limit on depth of modulation within the zone marked full PWM is
the value given by (6.14). In the transition region of pulse dropping, the modulation
depth exceeds 0.787 and reaches a value of 1 at the boundary of six step square
wave. The value
m
max
is a real and significant limitation of sinusoidal modulation
and it results in less than optimum bus voltage utilization. Later in this section it is
shown that with SVPWM, this limitation is increased significantly so that max-
imum bus utilization is realized.
To further illustrate sinusoidal, regular sampling, refer to Figure 6.12, where it is
shown what the switch patterns will appear as for three different voltage vectors in
sector I and one vector in sector II of the inverter state space. The points illustrated
are angle of 20
with the
a
-axis and modulation index of 0.35 (A), angle of 40
with
the
a
-axis and modulation index of 0.8 (B), angle of 55
from the
a
-axis and
