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EET421
Power Electronic Drives
- DC to AC converter / Inverter
Abdul Rahim Abdul Razak
ABD RAHIM 2008
ABD RAHIM 2008
Summary
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dc-to-ac converters are known as inverters
The function of an inverter is to change the dc input voltage to an ac output
voltage of desired magnitude and frequency
The output voltage waveforms of ideal inverters should be sinusoidal
However, the output of practical inverters contains harmonics
For high power applications, low distorted sinusoidal waveforms are required
Harmonic contents could be minimized by the use of high-speed
semiconductor switching techniques
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Inverters are widely used in industrial applications
- motor drives, UPS, induction heating, standby power supplies, etc.
- input may be a battery, fuel cell, solar cell, or there dc source
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dc-to-ac inverters can make smooth transition into the rectification mode,
where the flow of power reverses from the ac side to the dc side
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Two types of inverters: single-phase inverters and three-phase inverters
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Switch-Mode DC-AC Inverters
Applications:
• ac motor drives
• Uninterruptible ac power supplies
• Where a sinusoidal ac output is required whose
magnitude and frequency both have to be controlled
Terminal voltage is adjustable
in its magnitude and frequency
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Switch-Mode DC-AC Inverter: Bi-directional power flow
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Voltage control of 1-phase inverter
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The needs to have a controllable output voltage :
1) To cope with variations of DC input voltage
2) For inverter voltage regulation
3) For constant volts/frequency control requirement
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Most efficient techniques is by incorporating PWM control within
the inverter :
a) Single pulse width modulation - PWM
b) Multiple pulse width modulation - MPWM
c) Sinusoidal pulse width modulation - SPWM
d) Modified sinusiodal pulse width modulation - MSPWM
e) Phase displacement control
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a) Single pulse width modulation - PWM
Modulation index:
M=Ar/Ac
Varying Ar from 0 to
Ac, will increase the δ
thus the output will
vary from 0 to Vs
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a) Single pulse width modulation - PWM
Modulation index:
M=Ar/Ac
DF increased badly
significantly
at low
Varying Ar from
0 to
output
voltage
Ac, will
increase the δ
thus the output will
vary from 0 to Vs
3rd harmonic more
dominant on single
PWM
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b) Multiple pulse width modulation - MPWM
fo will set the output
frequency while fc will
determines the pulses
per half cycle,p.
fc / fo = mf, frequency
modulation ratio
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b) Multiple pulse width modulation - MPWM
-By using several pulses, (p=5)
The harmonic content are
reduced compared to single
fo will set the output
PWM
frequency while fc will
- the DF isdetermines
also reduced
the pulses
per half cycle,p.
significantly.
- but switching loss would
increased.
- if p increased, the amplitude of
lower order harmonics would be
lowered but it will increase the
high-orders harmonics
fc / fo = mf, frequency
modulation ratio
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c) Sinusoidal pulse width modulation - SPWM
-Pulse width varied proportional to
the amplitude of sinewave ref
signal.
- the gating signal generated by
comparing the triangular carrier
wave and sinusoidal ref signal.
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Sinusoidal pulse width modulation - SPWM
Modulation index:
M=Ar/Ac
Varying Ar from 0 to
Ac,compared
will increase
the δ
-DF is less
to MPWM
thus the output will
- eliminates
lower
varythe
from
0 toorder
Vs
harmonics of 2p-1, for this p=5,
lowest order harmonics is 9th.
- the harmonics were pushed to
the high frequency range of fc.
So it would be much easier for
low-pass filtering process.
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Overmodulation leads to
squarewave operation
and add harmonics to the
system
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d) Modified sinusiodal pulse width modulation MSPWM
- instead of 100%
pulses, carrier wave is
applied on 1st and last
60 degree of half cycle.
- advantages :
a) Fundamental
component increased
b) Harmonics
characteristics are
improved.
c) Reduce switching
loss.
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d) Modified sinusiodal pulse width modulation MSPWM
advantages :
a) Fundamental
component increased
b) Harmonics
characteristics are
improved.
c) Reduce switching
loss.
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e) Phase displacement control
- multiple inverter are used,
output is taken from
summation of output voltage
of the individual inverter.
2 half bridge inverter
output
Output with 180 degree
displacement.
Output with β degree
displacement
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Three-Phase Inverter
• Used to supply three-phase loads
• Three single-phase inverters could be used with 120 degree displacement
or shifting between phases, however, 12 switches are necessary
• Consists of three legs, one for each phase
• One of the two switches in a leg is always ON at any instant
• Output of each leg depends on Vd and the switching status
Current source inverters
- a large inductor component
inserted at the input
-input behaves like a current
source
-The output current will
maintained constant at any
loads but output voltage
forced to change
-Diodes in series are required
to block the reverse voltage
on the transistors.
- when two device in different
leg conduct – IL flows through
load
- when two device in same
leg conduct – IL is bypassed
from load (buck-bost
regulator circuit)
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Current source inverters
-when two device in different leg conduct – IL flows through load
- when two device in same leg conduct – IL is bypassed from load
(buck-bost regulator circuit)
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Current source inverters
- CSI are simpler than VSI 
only required capasitor for
comutation
- a large inductor component
inserted at the input
-input behaves like a current
source
-The output current will
maintained constant at any
loads but output voltage
forced to change
-Diodes in series are required
to block the reverse voltage
on the transistors.
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Current source inverters - operations
- assume T1 and T2
conducting, cap C1 and C2
are charged per picture
- once T3 and T4 are fired,
T1 and T2 will be reverse
biased and turned OFFF
(impulse commutation).
- the circuit now flows thru
T3C1D1-load-D2C2T4. the
caps will be discharged and
recharged at constant rate.
-once fully charged, caps will act as open circuit. Current falls to zero. Load
current will now be transferred thru D3 –load-D4.
- caps C1 and C2 is now ready to turn OFF T3 and T4 once T1 and T2 fired
in next half cycle.
- the commutation time will depend on magnitude of load current and
voltage.
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Current source inverters – 3-phase
- only 2 thyristor conducts at same time
- each device conducts for 120 degrees
- phases current can be stated as:
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Current source inverters – 3-phase
Other advantages of CSI:
1) the input DC current is
controlled and limited, so misfiring
of switching devices or short circuit
would not be serious problem.
2) The peak current of power
device is limited.
3) The commutation circuit for
thyristor are much simpler.
4) each device conducts for 120
degrees
Disadvantages :
1) Requires large reactor.
5) Requires no freewheeling
diodes.
2) Need extra front converter stage.
3) Dynamic respond is slow.
4) Due to current transfer between switches, output filter is required to
suppress the output voltage spikes.
ABD RAHIM 2008