1. Power Electronic Drives - DC to AC converter / Inverter
EET421
Power Electronic Drives
- DC to AC converter / Inverter
Abdul Rahim Abdul Razak
Systems Engineering
School of Electrical
ABD RAHIM 2008

2. Systems Engineering
School of Electrical
ABD RAHIM 2008

3. Systems Engineering School of Electrical 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
Systems Engineering
School of Electrical
ABD RAHIM 2008

4. 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
Systems Engineering
School of Electrical
Terminal voltage is adjustable
in its magnitude and frequency
ABD RAHIM 2008

5. Switch-Mode DC-AC Inverter: Bi-directional power flow

6. Systems Engineering
School of Electrical
ABD RAHIM 2008

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School of Electrical
ABD RAHIM 2008

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School of Electrical
ABD RAHIM 2008

12. Systems Engineering
School of Electrical
ABD RAHIM 2008

13. Systems Engineering
School of Electrical
ABD RAHIM 2008

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ABD RAHIM 2008

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School of Electrical
ABD RAHIM 2008

16. Systems Engineering
School of Electrical
ABD RAHIM 2008

17. Systems Engineering
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20. Systems Engineering School of Electrical
Voltage control of 1-phase inverter
The needs to have a controllable output voltage :
To cope with variations of DC input voltage
For inverter voltage regulation
For constant volts/frequency control requirement
Most efficient techniques is by incorporating PWM control within the inverter :
Single pulse width modulation - PWM
Multiple pulse width modulation - MPWM
Sinusoidal pulse width modulation - SPWM
Modified sinusiodal pulse width modulation - MSPWM
Phase displacement control
Systems Engineering
School of Electrical
ABD RAHIM 2008

21. Systems Engineering School of Electrical
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
Systems Engineering
School of Electrical
ABD RAHIM 2008

22. Systems Engineering School of Electrical
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
Systems Engineering
School of Electrical
ABD RAHIM 2008

23. Systems Engineering School of Electrical
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
Systems Engineering
School of Electrical
ABD RAHIM 2008

24. Systems Engineering School of Electrical
b) Multiple pulse width modulation - MPWM
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
fo will set the output frequency while fc will determines the pulses per half cycle,p.
fc / fo = mf, frequency modulation ratio
Systems Engineering
School of Electrical
ABD RAHIM 2008

25. Systems Engineering School of Electrical
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.
Systems Engineering
School of Electrical
ABD RAHIM 2008

26. Systems Engineering School of Electrical
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.
Systems Engineering
School of Electrical
ABD RAHIM 2008

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Overmodulation leads to squarewave operation and add harmonics to the system
Systems Engineering
School of Electrical
ABD RAHIM 2008

29. Systems Engineering School of Electrical
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.
Systems Engineering
School of Electrical
ABD RAHIM 2008

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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.
Systems Engineering
School of Electrical
ABD RAHIM 2008

31. Systems Engineering School of Electrical 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
Systems Engineering
School of Electrical
ABD RAHIM 2008

32. 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

34. 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)
Systems Engineering
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ABD RAHIM 2008

35. Current source inverters
Systems Engineering
School of Electrical
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)
ABD RAHIM 2008

36. 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.
Systems Engineering
School of Electrical
ABD RAHIM 2008

37. 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.
Systems Engineering
School of Electrical
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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38. Current source inverters – 3-phase
Systems Engineering
School of Electrical
only 2 thyristor conducts at same time
each device conducts for 120 degrees
phases current can be stated as:
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39. 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.
Systems Engineering
School of Electrical
Disadvantages :
Requires large reactor.
Need extra front converter stage.
Dynamic respond is slow.
Due to current transfer between switches, output filter is required to suppress the output voltage spikes.
ABD RAHIM 2008