Dc to Ac Converter

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  • Topic: Pulse-width modulation, Alternating current, Wave
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Chapter 4 DC to AC Conversion (INVERTER)
• General concept • Basic principles/concepts • Single-phase inverter – Square wave – Notching – PWM • Harmonics • Modulation • Three-phase inverter

DC to AC Converter (Inverter)
• DEFINITION: Converts DC to AC power by switching the DC input voltage (or current) in a pre-determined sequence so as to generate AC voltage (or current) output. • TYPICAL APPLICATIONS: – Un-interruptible power supply (UPS), Industrial (induction motor) drives, Traction, HVDC

• General block diagram
IDC +
VDC Iac

+ Vac −



Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002

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Types of inverter
• Voltage Source Inverter (VSI) • Current Source Inverter (CSI)

"DC LINK" + VDC − C

Iac + Load Voltage −

L + VDC − IDC

ILOAD Load Current

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Voltage source inverter (VSI) with variable DC link
DC LINK + Vs C + Vin + Vo -

CHOPPER (Variable DC output)

INVERTER (Switch are turned ON/OFF with square-wave patterns)



DC link voltage is varied by a DC-to DC converter or controlled rectifier. Generate “square wave” output voltage. Output voltage amplitude is varied as DC link is varied. Frequency of output voltage is varied by changing the frequency of the square wave pulses. Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002 4

• •



Variable DC link inverter (2)
• Advantages: – simple waveform generation – Reliable • Disadvantages: – Extra conversion stage – Poor harmonics Vdc2 Higher input voltage Higher frequency Lower input voltage Lower frequency T1 T2 t

Vdc1

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VSI with fixed DC link
INVERTER + Vin
(fixed)

C

+ Vo − Switch turned ON and OFF with PWM pattern



• DC voltage is held constant. • Output voltage amplitude and frequency are varied simultaneously using PWM technique. • Good harmonic control, but at the expense of complex waveform generation Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002 6

Operation of simple squarewave inverter (1)
• To illustrate the concept of AC waveform generation
SQUARE-WAVE INVERTERS T1 D1 + VO IO T4 D2 T2 D4 T3 D3

VDC

S1

S3 EQUAVALENT CIRCUIT

S4

S2

Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002

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Operation of simple squarewave inverter (2)
S1,S2 ON; S3,S4 OFF vO for t1 < t < t2

S1 VDC S4

S3

VDC

+ vO − S2

t1

t2

t

S3,S4 ON ; S1,S2 OFF vO

for t2 < t < t3

S1 VDC S4

S3 t2 S2 -VDC t3

+ vO −

t

Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002

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Waveforms and harmonics of square-wave inverter
INVERTER OUTPUT Vdc

-Vdc FUNDAMENTAL V1
4VDC π

V1 3

3RD HARMONIC

V1 5

5RD HARMONIC

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Filtering
• Output of the inverter is “chopped AC voltage with zero DC component”.In some applications such as UPS, “high purity” sine wave output is required. • An LC section low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics. • In some applications such as AC motor drive, filtering is not required. (LOW PASS) FILTER L + vO 1 − C + vO 2 − vO 2 LOAD

vO 1

Power Electronics and Drives (Version 2): Dr. Zainal Salam, 2002

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Notes on low-pass filters
• In square wave inverters, maximum output voltage is achievable. However there in NO control in harmonics and output voltage magnitude. The harmonics are always at three, five, seven etc times the fundamental frequency. Hence the cut-off frequency of the low pass filter is somewhat fixed. The filter size is dictated by the VA ratings of the inverter. To reduce filter size, the PWM switching scheme can be utilised. In this technique, the harmonics are “pushed” to higher frequencies. Thus the cut-off frequency of the filter is increased. Hence...
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