Dc to Ac Converter

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

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

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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

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

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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