Environmental Engineering Reference
In-Depth Information
V
DC
/2
−V
DC
/2
V
DC
/2
−V
DC
/2
π
/2
π
3
π
/2
2
π
0
π
/2
π
3
π
/2
2
π
0
ωt/rad
ωt/rad
(a)
v
aN
V
DC
/2
−V
DC
/2
V
DC
/2
−V
DC
/2
π/2
π
3π/2
2π
0
0
π/2
π
3π/2
2π
ω
t/rad
ω
t/rad
(b)
v
bN
v
ab
v
o
v
ab
V
DC
V
DC
v
o
−V
DC
−V
DC
π/2
π
3π/2
2π
0
0
π/2
π
3π/2
2π
ω
t/rad
ω
t/rad
(c)
v
ab
and
v
o
Figure 1.20
Unipolar operation of a single-phase inverter: with only one leg operated at the switching
frequency (left column) and both legs operated at the switching frequency (right column)
1.2.3.3 Operation of Three-phase Inverters
For three-phase inverters shown in Figure 1.22(a), three phase voltages can be compared with
the carrier waveform to generate three sets of bipolar PWM signals, as shown in Figure 1.18 to
drive the three phase legs separately. The corresponding curves are shown in Figures 1.22(b)
and 1.22(c). It can be seen that the maximum amplitude of the phase voltages is half of the
DC-bus voltage. It is worth noting that the average voltage between the reference point
N
of
the three phase legs and the common point
N
of the capacitors over a switching period is 0
but the instantaneous voltage is not.
1.2.4 AC-AC Conversion
The AC-AC conversion can be performed indirectly via AC-DC-AC with the addition of
a DC bus or directly without a DC bus. The indirect AC-AC conversion is basically the
combination of AC-DC conversion and DC-AC conversion, as discussed in Sections 1.2.1
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