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S c h o o l o f E l e c t r i c a l S y s t e m s E n g i n e e r i n g ABD RAHIM ABD RAHIM 2008 2008 EET421 Power Electronic Drives - DC to AC converter / Inverter Abdul Rahim Abdul Razak
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School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
EET421Power Electronic Drives
- DC to AC converter / Inverter
Abdul Rahim Abdul Razak
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
Summary
• 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
• 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
• 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
• Two types of inverters: single-phase inverters and three-phase inverters
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
Switch-Mode DC-AC InvertersApplications:
• 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 adjustablein its magnitude and frequency
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
Switch-Mode DC-AC Inverter: Bi-directional power flow
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
Voltage control of 1-phase inverter
- 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
- 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
School of E
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ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
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
School of E
lect rical S
ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
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
DF increased badly significantly at low output voltage
3rd harmonic more dominant on single PWM
School of E
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ABD RAHIMABD RAHIM 2008 2008
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
School of E
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ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
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
-By using several pulses, (p=5) The harmonic content are reduced compared to single PWM
- the DF is also reduced 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
School of E
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ABD RAHIMABD RAHIM 2008 2008
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.
School of E
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ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
Sinusoidal pulse width modulation - SPWM
Modulation index:
M=Ar/Ac
Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs
-DF is less compared to MPWM
- eliminates the lower order 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.
School of E
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ABD RAHIMABD RAHIM 2008 2008
School of E
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ABD RAHIMABD RAHIM 2008 2008
Overmodulation leads to squarewave operation and add harmonics to the system
School of E
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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.
School of E
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ABD RAHIMABD RAHIM 2008 2008
d) Modified sinusiodal pulse width modulation - MSPWM
advantages :
a) Fundamental component increased
b) Harmonics characteristics are improved.
c) Reduce switching loss.
School of E
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ystems E
ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
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
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
School of E
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ABD RAHIMABD RAHIM 2008 2008
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)
School of E
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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)
School of E
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ngi neer ing
ABD RAHIMABD RAHIM 2008 2008
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.
School of E
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ABD RAHIMABD RAHIM 2008 2008
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.
School of E
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ABD RAHIMABD RAHIM 2008 2008
Current source inverters – 3-phase
- only 2 thyristor conducts at same time
- each device conducts for 120 degrees
- phases current can be stated as:
School of E
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ABD RAHIMABD RAHIM 2008 2008
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
5) Requires no freewheeling diodes.
Disadvantages :
1) Requires large reactor.
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.
