# A Simulation Study of the PWM Strategy for Inverters

```Power Electronic Systems &amp; Chips Lab., NCTU, Taiwan
A Simulation Study of the PWM
Strategy for Inverters

2014年11月21日

Power Electronic Systems &amp; Chips Lab.

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A Simulation Study of the PWM Strategy for Inverters
Gate Driver
S1
ma*
*
b
m
Switching
logic
P
S1
S3
S4
vc*
a
S2
S5
S3
o
Vdc
S2
N
S4
ia
S5
S6
b
ib
c
ic
van
vbn n
vcn
S6
vtri
 Two-level three-phase inverters are widely used in industrial applications, home appliances, and
distributed power generation systems, such as motor drives, PV inverters, UPS, etc. We can
make a study of characteristics of PWM strategy for a grid-connected three-phase inverter by
using computer simulation.
 The load is a three-phase R-L load in series connected with a three-phase ac source.
 Natural sampling sinusoidal PWM is adopted.
 The modulation signals has a phase shift of 120&ordm; between each other.
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van
a
S1
1
Vdc
2
Vdc
S3
S5
S2
1
Vdc
2
S4
ra
La
ea
ea
vbn
b
o
ia
S6
c
ib
rb
Lb
eb
ec
eb
n
ic
rc
Lc
ec
vcn








Vdc = 400V
Phase voltage of the three-phase utility (ea, eb, and ec): 110VRMS, 60 Hz
La = Lb = Lc = 10 mH
ra = rb = rc = 0.2 W
Inverter switching frequency fs = 12 kHz
Rated output power: 4 kW
All power switches are assumed ideal.
Make a study of the three-phase PWM inverter with natural sampling sinusoidal PWM.
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Study Issues: PWM Principle &amp; Power Factor Control
1. Illustration of PWM Principle: Set the modulation index with 0.75 and modulation frequency
of 60 Hz, apply SPWM to the inverter Complete the simulation of (1) by using SPWM and show
some selected variables shown in the following figure to illustrate the operation principle of
PWM strategy. You may show its effect with a much lower switching frequency so that we can
make observations of the corresponding relationships between the selected variables, such as the
switching of the power switches and the corresponding current waveforms.
2. Load Power Factor Control: Control the phase voltages in open loop so that the inverter output
phase currents ia, ib, and ic will be in phase with the corresponding phase voltages ea, eb, and ec of
the three-phase utility voltages. To study the loading effect, you may set the output phase current
to be 1, 5, and 10 amperes, respectively. Make a description of the determination of the
generation of the modulating signal and make a computer simulation verification.
1. Illustration of PWM Principle
Vdc
2
1.0
a
Vdc
c
b
vao
90
180
270
o
360

-1.0
Vdc
2
1.0
vab
vca
vao
vbc
vbo
-1.0
Vdc
2.0
ias
ibs
ics
vab
van
vbn
-2.0
n
1.0
vcn
2
Vdc
3
van
1
Vdc
3
-1.0
The van is the voltage applied to the phase voltage with a
common neutral of a Y-connected ac motor.
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 Keep phase current ia in phase with ea: To get the maximum output power with the
minimum output voltage, the phase current must be in phase with the utility. Generate the
phase output voltages va, vb, and vc so that the phase currents ia, ib, and ic will be in phase
with the corresponding utility phase voltages ea, eb, and ec.
a
ian
Ra
La
vab
van
ea
eb
ibn
ec
Lb
b
Rb
icn
n
Lc
c
Rc
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 Calculate the magnitude of phase current at rated output
power.
v an1

i a1
Po  3
ji a1e La
ea
e ab 220 2

3
3
Po  3
1
ea
2
van1 
e
1
i a1 cos   4000W
2
 i a1    i a1 e La 
2
a
1
i a cos   4000W
2
where  is the angle between ea and ia1, for get the
maximum output power with minimum output voltage, the
phase current must be in phase with the voltage, therefore,
 =0. ea represents the magnitude of the utility phase
voltage.
i a1Ra
ea 
1
ea
2
2
i a1 
  tan 1
2 Po
3 ea
i a1 e La
ea  i a1
We need to generate a three-phase voltage with its phase-a voltage van1 has a
leading phase angle of  relative to ea.
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