Environmental Engineering Reference
In-Depth Information
6.2 Essentials of pulse width modulation
Resonant pole inverters present an alternative to conventional hard switched
inverters in that switching losses are dramatically lower. In hard switched inverters,
the simultaneous high current and high voltage (HV) during commutation present
high switching loss stresses to the power electronic devices. The motivation behind
resonant pole, and resonant link, inverters was to reduce inverter losses when high
frequency modulation was required.
Figure 6.6 is an illustration of the fundamental dc to ac inversion process into
an
R-L
load. In this hard switched scheme, a stiff dc bus is connected across the
inverter poles and the load is connected from inverter pole-A mid-point to inverter
pole-B mid-point. The modulation afforded by PWM regulates the effective vol-
tage across the
R-L
load as shown. Load current through the inductive branch is
proportional to the net volt-seconds appearing across the branch inductance.
Because of this the branch current is synthesized to sinusoidal waveshape.
Pole-A
Pole-B
S1
D1
S3
D3
I
a
L R
a
b
U
dc
U
ab
S2
S4
D4
D2
U
dc
U
ab
S1 On
S3, 4 PWM
T
2
T
p
/
w
a
S4 On
S1, 2 PWM
2
p
/
w
a
U
dc
Figure 6.6 Hard switched inverter
The maximum voltage that can be synthesized from a fixed dc bus and applied
to the stator of a 3-phase electric machine is given by (6.1):
=
3
Þ
sin
ðp=
3
þ gÞ
sin
ð
p
2
3
U
dc
U
s
,
max
¼
ð
V
pk
Þ
ð
6
:
1
Þ
where
U
dc
is the dc link, or battery, voltage and
g
is the angle between the inverter
q
-axis (real) and the applied voltage vector,
U
s
. The modulation index,
m
i
, that
