Dc to AC Inverters

Chapter 8 Switch-Mode DC-AC Inverters

• conver converters ters for for ac motor motor drives drives and uninterruptible power supplies Copyri Copyright ght © 2003 by John Wiley & Sons, Inc.

Chapter 8 Switch-Mode DCSinusoidal AC Inverters

Switch-Mode DC-AC Inverter

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• Block diagram of a motor motor drive where the power flow is unidirectional Copyri Copyright ght © 2003 by John Wiley & Sons, Inc.



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• Block diagram of a motor motor drive where the power flow is unidirectional Copyri Copyright ght © 2003 by John Wiley & Sons, Inc.



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• Block diagram of a motor motor drive where the the power flow can be bi-directional Copyri Copyright ght © 2003 by John Wiley & Sons, Inc.



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• Four Four quadran quadrants ts of oper operati ation on Copyri Copyright ght © 2003 by John Wiley & Sons, Inc.


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One Leg of a Switch-Mode DC-AC Inverter

One Leg of a Switch-Mode DC-AC Inverter

• The mid-point shown is fictitious Copyright © 2003 by John Wiley & Sons, Inc.


Synthesis of a Sinusoidal Output by PWM

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Synthesis of a Sinusoidal Output by PWM

Copyright © 2003 by John Wiley & Sons, Inc.


Details of a Switching Time Period

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Details of a Switching Time Period

• Control voltage can be assumed constant during a switching time-period Copyright © 2003 by John Wiley & Sons, Inc.


Harmonics in the DC-AC Inverter Output Voltage

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Harmonics in the DC-AC Inverter Output Voltage

• Harmonics appear around the carrier frequency and its multiples Copyright © 2003 by John Wiley & Sons, Inc.


Harmonics due to Over-modulation

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Harmonics due to Over-modulation

• These are harmonics of the fundamental frequency Copyright © 2003 by John Wiley & Sons, Inc.


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Output voltage Fundamental as a Function of the Modulation Index

Output voltage Fundamental as a Function of the Modulation Index

• Shows the linear and the over-modulation regions; square-wave operation in the limit Copyright © 2003 by John Wiley & Sons, Inc.


Square-Wave Mode of Operation

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Square-Wave Mode of Operation

• Harmonics are of the fundamental frequency Copyright © 2003 by John Wiley & Sons, Inc.


Half-Bridge Inverter

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Half-Bridge Inverter

• Capacitors provide the mid-point Copyright © 2003 by John Wiley & Sons, Inc.


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Single-Phase Full-Bridge DC-AC Inverter

Single-Phase Full-Bridge DC-AC Inverter

• Consists of two inverter legs Copyright © 2003 by John Wiley & Sons, Inc.


PWM to Synthesize Sinusoidal Output

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PWM to Synthesize Sinusoidal Output

• The dotted curve is the desired output; also the fundamental frequency Copyright © 2003 by John Wiley & Sons, Inc.


Analysis assuming Fictitious Filters

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Analysis assuming Fictitious Filters

• Small fictitious filters eliminate the switchingfrequency related ripple Copyright © 2003 by John Wiley & Sons, Inc.


DC-Side Current

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DC-Side Current

• Bi-Polar Voltage switching Copyright © 2003 by John Wiley & Sons, Inc.


Output Waveforms: Uni-polar Voltage Switching

• Harmonic components around the switching

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Output Waveforms: Uni-polar Voltage Switching

• Harmonic components around the switching frequency are absent



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DC-Side Current in a Single-Phase Inverter

DC-Side Current in a Single-Phase Inverter

• Uni-polar voltage switching Copyright © 2003 by John Wiley & Sons, Inc.


Sinusoidal Synthesis by Voltage Shift

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Sinusoidal Synthesis by Voltage Shift

• Phase shift allows voltage cancellation to synthesize a 1-Phase sinusoidal output Copyright © 2003 by John Wiley & Sons, Inc.


Single-Phase Inverter

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Single-Phase Inverter

• Analysis at the fundamental frequency Copyright © 2003 by John Wiley & Sons, Inc.


Square-Wave and PWM Operation

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• PWM results in much smaller ripple current Copyright © 2003 by John Wiley & Sons, Inc.


Push-Pull Inverter

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Push-Pull Inverter

• Low Voltage to higher output using square-wave operation Copyright © 2003 by John Wiley & Sons, Inc.


Three-Phase Inverter

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Three-Phase Inverter

• Three inverter legs; capacitor mid-point is fictitious Copyright © 2003 by John Wiley & Sons, Inc.

ThreePhase PWM Waveforms


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ThreePhase PWM Waveforms



Three-Phase Inverter Harmonics

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Three-Phase Inverter Harmonics



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Three-Phase Inverter Output

Three-Phase Inverter Output

• Linear and over-modulation ranges Copyright © 2003 by John Wiley & Sons, Inc.


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Three-Phase Inverter: Square-Wave Mode

Three-Phase Inverter: Square-Wave Mode

• Harmonics are of the fundamental frequency Copyright © 2003 by John Wiley & Sons, Inc.


Three-Phase Inverter: Fundamental Frequency

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Three-Phase Inverter: Fundamental Frequency

• Analysis at the fundamental frequency can be done using phasors Copyright © 2003 by John Wiley & Sons, Inc.



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• PWM results in much smaller ripple current Copyright © 2003 by John Wiley & Sons, Inc.


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DC-Side Current in a Three-Phase Inverter

DC-Side Current in a Three-Phase Inverter

• The current consists of a dc component and the switching-frequency related harmonics Copyright © 2003 by John Wiley & Sons, Inc.


Square-Wave Operation

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Square-Wave Operation

• devices conducting are indicated Copyright © 2003 by John Wiley & Sons, Inc.


PWM Operation

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PWM Operation

• devices conducting are indicated Copyright © 2003 by John Wiley & Sons, Inc.


Short-Circuit States in PWM Operation

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Short-Circuit States in PWM Operation

• top group or the bottom group results in short circuiting three terminals Copyright © 2003 by John Wiley & Sons, Inc.

Effect of Blanking Time

• Results in


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• Results in nonlinearity




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• Voltage jump when the current reverses direction Copyright © 2003 by John Wiley & Sons, Inc.



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• Effect on the output voltage Copyright © 2003 by John Wiley & Sons, Inc.


Programmed Harmonic Elimination

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Programmed Harmonic Elimination

• Angles based on the desired output Copyright © 2003 by John Wiley & Sons, Inc.


Tolerance-Band Current Control

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Tolerance-Band Current Control

• Results in a variable frequency operation Copyright © 2003 by John Wiley & Sons, Inc.


Fixed-Frequency Operation

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Fixed-Frequency Operation

• Better control is possible using dq analysis Copyright © 2003 by John Wiley & Sons, Inc.


8-39

Transition from Inverter to Rectifier Mode

Transition from Inverter to Rectifier Mode

• Can analyze based on the fundamentalfrequency components Copyright © 2003 by John Wiley & Sons, Inc.


Summary of DC-AC Inverters

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Dc to AC Inverters

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