dc to ac inverters

7/28/2019 Dc to AC Inverters

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Copyright 2003by John Wiley & Sons, Inc.

Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters

8-1

Chapter 8

Switch-Mode DC-AC Inverters

converters for ac motor drives and

uninterruptible power supplies


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8-3

Switch-Mode DC-AC Inverter

Block diagram of a motor drive where the power

flow can be bi-directional


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8-4

Switch-Mode DC-AC Inverter

Four quadrants of operation


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8-5

One Leg of a Switch-Mode DC-AC Inverter

The mid-point shown is fictitious


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8-6

Synthesis of a Sinusoidal Output by PWM


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8-7

Details of a Switching Time Period

Control voltage can be assumed constant during

a switching time-period


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8-8

Harmonics in the DC-AC Inverter Output

Voltage

Harmonics appear around the carrier frequency

and its multiples


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8-9

Harmonics due to Over-modulation

These are harmonics of the fundamental

frequency


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8-10

Output voltage Fundamental as a Function

of the Modulation Index

Shows the linear and the over-modulation

regions; square-wave operation in the limit


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8-11

Square-Wave Mode of Operation

Harmonics are of the fundamental frequency


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8-12

Half-Bridge Inverter

Capacitors provide the mid-point


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8-13

Single-Phase Full-Bridge DC-AC Inverter

Consists of two inverter legs


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8-14

PWM to Synthesize Sinusoidal Output

The dotted curve is the desired output; also the

fundamental frequency


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8-15

Analysis assuming Fictitious Filters

Small fictitious filters eliminate the switching-

frequency related ripple


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8-16

DC-Side Current

Bi-Polar Voltage switching


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8-17

Output Waveforms:

Uni-polar Voltage

Switching

Harmonic

components around

the switchingfrequency are absent


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8-18

DC-Side Current in a Single-Phase Inverter

Uni-polar voltage switching


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8-19

Sinusoidal Synthesis by Voltage Shift

Phase shift allows voltage cancellation to

synthesize a 1-Phase sinusoidal output


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8-20

Single-Phase Inverter

Analysis at the fundamental frequency


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Copyright 2003by John Wiley & Sons, Inc. Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters8-22

Push-Pull Inverter

Low Voltage to higher output using square-wave

operation


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Three-

Phase

PWMWaveforms


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Three-Phase Inverter Harmonics


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Three-Phase Inverter Output

Linear and over-modulation ranges


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Three-Phase Inverter: Square-Wave Mode

Harmonics are of the fundamental frequency


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Three-Phase Inverter: Fundamental

Frequency

Analysis at the fundamental frequency can be

done using phasors


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Square-Wave and PWM Operation

PWM results in much smaller ripple current


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DC-Side Current in a Three-Phase Inverter

The current consists of a dc component and the

switching-frequency related harmonics


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Square-Wave Operation

devices conducting are indicated


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Copyright 2003by John Wiley & Sons, Inc. Chapter 8 Switch-Mode DC-Sinusoidal AC Inverters 8-32

PWM Operation

devices conducting are indicated


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Short-Circuit States in PWM Operation

top group or the bottom group results in short

circuiting three terminals


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Effect of Blanking

Time

Results innonlinearity


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Effect of Blanking Time

Voltage jump when the current reverses direction


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Effect of Blanking Time

Effect on the output voltage

Programmed Harmonic Elimination


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Programmed Harmonic Elimination

Angles based on the desired output


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Tolerance-Band Current Control

Results in a variable frequency operation


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Fixed-Frequency Operation

Better control is possible using dq analysis


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C i ht 2003 Ch t 8 S it h M d DC 8 41

Summary of DC-AC Inverters

Functional representation in a block-diagram form

dc to ac inverters

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