pulse-width modulation (pwm) techniques
TRANSCRIPT
Pulse-Width Modulation (PWM) Techniques
Mohammed Salah Yousef
• To control inverter output frequency (fundamental)
• To control inverter output voltage (fundamental)• To minimize harmonic distortion
Why Use PWM Techniques?
Objective of PWM
Disadvantages of PWM
Increase of switching losses due to high PWM frequency
Reduction of available voltage
EMI problems due to high-order harmonics
Control of inverter output voltage
Reduction of harmonics
Basic principles of PWM
-Application of the equal-area theoremThis is sinusoidal
PWM (SPWM)-The equal-area
theorem can be appliedto realize any shape of
waveforms
Pulse-Width Modulation (PWM)
Fig. Pulse-width modulation.
Inverter output voltage
When vcontrol > vtri, VA0 = Vdc/2
When vcontrol < vtri, VA0 = -Vdc/2
A01A0
10
Vofcomponentfrequecnylfundamenta:)(Vwhere,
,2/
)(
dc
A
tri
control
V
Vofpeak
v
vm
Modulation Index (m)
Control of inverter output voltage
Amplitude is controlled by the peak value of vcontrol
Fundamental frequency is controlled by the frequency of vcontrol
PWM frequency is the same as the frequency of vtri
II. PWM METHODSA. Sine PWM (1)
Fig. Three-phase Sine PWM inverter.
Three-phase inverterTwo Level Voltage Source Inverter
• Inverter Configuration
Assumption:
dc capacitor very large dc voltage ripple free
dC
1S
2S
3S 5S
4S 6S
A
B
C
P
N
LOAD
Ai
OBi
CidV
• Modulating and Carrier Waves
• vcr – Carrier wave (triangle) • Amplitude modulation index
cr
ma V
Vm
ˆ
ˆ
• Frequency modulation index
m
crf f
fm
0
v mAv Bmv Cmvcrv
crV̂ mV̂t
• vm – Modulating wave (sine)
• Gate Signal Generation
1gv
4gv
dV
0
ANv
2
mAv crv
0
crmA vv 01 gv )0( 4 gv 1S on )off( 4S dAN Vv Phase A
crmA vv 04 gv )0( 1 gv 4S on )off( 1S 0ANv
Vg1 and Vg4 are complementary
• Line-to-Line Voltage vAB
ABv
BNv
ANv
0
0
0
v mAv Bmv Cmvcrv
crV̂ mV̂
dV
dV
dV
2t
t
t
t
1ABv
1S
2S
3S 5S
4S 6S
B
C
P
N
dV
A
VA
0V
B0
VC
0V
AB
VB
CV
CA
t
Fig. Waveforms of three-phase sine PWM inverter.
Three-phase sine PWM waveforms
where, VAB = VA0 – VB0
VBC = VB0 – VC0
VCA = VC0 – VA0
When vcontrol > vtri, VA0 = Vdc/2
When vcontrol < vtri, VA0 = -Vdc/2
Frequency of vtri = fs
Frequency of vcontrol = f1
Frequency of vtri and vcontrol
where, fs = PWM frequency
f1 = Fundamental frequency
Inverter output voltage
Amplitude modulation ratio (ma)
A01A0
10
Vofcomponentfrequecnylfundamenta:)(Vwhere,
,2/
)(
dc
A
tri
controla V
Vofvaluepeak
vofamplitude
vofamplitudepeakm
Frequency modulation ratio (mf)
frequencylfundamentafandfrequencyPWMfwhere,, 1s1
f
fm sf
mf should be an odd integer
if mf is not an integer, there may exist sunhamonics at output voltage
if mf is not odd, DC component may exist and even harmonics are present at output voltage
mf should be a multiple of 3 for three-phase PWM inverter
An odd multiple of 3 and even harmonics are suppressed