8-1

Chapter 8

Switch-Mode DC-AC Inverters

• converters for ac motor drives and uninterruptible power supplies

8-2

Switch-Mode DC-AC Inverter

• Block diagram of a motor drive where the power flow is unidirectional

8-3

Switch-Mode DC-AC Inverter

• Block diagram of a motor drive where the power flow can be bi-directional

8-4

Switch-Mode DC-AC Inverter

• Four quadrants of operation

8-5

One Leg of a Switch-Mode DC-AC Inverter

• The mid-point shown is fictitious

8-6

Synthesis of a Sinusoidal Output by PWM

8-7

Details of a Switching Time Period

• Control voltage can be assumed constant during a switching time-period

8-8

Harmonics in the DC-AC Inverter Output Voltage

• Harmonics appear around the carrier frequency and its multiples

8-9

Harmonics due to Over-modulation

• These are harmonics of the fundamental frequency

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

8-11

Square-Wave Mode of Operation

• Harmonics are of the fundamental frequency

8-12

Half-Bridge Inverter

• Capacitors provide the mid-point

8-13

Single-Phase Full-Bridge DC-AC Inverter

• Consists of two inverter legs

8-14

PWM to Synthesize Sinusoidal Output

• The dotted curve is the desired output; also the fundamental frequency

8-15

Analysis assuming Fictitious Filters

• Small fictitious filters eliminate the switching-frequency related ripple

8-16

DC-Side Current

• Bi-Polar Voltage switching

8-17

Output Waveforms: Uni-polar Voltage

Switching

• Harmonic components around the switching frequency are absent

8-18

DC-Side Current in a Single-Phase Inverter

• Uni-polar voltage switching

8-19

Sinusoidal Synthesis by Voltage Shift

• Phase shift allows voltage cancellation to synthesize a 1-Phase sinusoidal output

8-20

Single-Phase Inverter

• Analysis at the fundamental frequency

8-21

Square-Wave and PWM Operation

• PWM results in much smaller ripple current

8-22

Push-Pull Inverter

• Low Voltage to higher output using square-wave operation

8-23

Three-Phase Inverter

• Three inverter legs; capacitor mid-point is fictitious

8-24

Three-Phase PWM

Waveforms

8-25

Three-Phase Inverter Harmonics

8-26

Three-Phase Inverter Output

• Linear and over-modulation ranges

8-27

Three-Phase Inverter: Square-Wave Mode

• Harmonics are of the fundamental frequency

8-28

Three-Phase Inverter: Fundamental Frequency

• Analysis at the fundamental frequency can be done using phasors

8-29

Square-Wave and PWM Operation

• PWM results in much smaller ripple current

8-30

DC-Side Current in a Three-Phase Inverter

• The current consists of a dc component and the switching-frequency related harmonics

8-31

Square-Wave Operation

• devices conducting are indicated

8-32

PWM Operation

• devices conducting are indicated

8-33

Short-Circuit States in PWM Operation

• top group or the bottom group results in short circuiting three terminals

8-34

Effect of Blanking Time

• Results in nonlinearity

8-35

Effect of Blanking Time

• Voltage jump when the current reverses direction

8-36

Effect of Blanking Time

• Effect on the output voltage

8-37

Programmed Harmonic Elimination

• Angles based on the desired output

8-38

Tolerance-Band Current Control

• Results in a variable frequency operation

8-39

Fixed-Frequency Operation

• Better control is possible using dq analysis

8-40

Transition from Inverter to Rectifier Mode

• Can analyze based on the fundamental-frequency components

8-41

Summary of DC-AC Inverters

• Functional representation in a block-diagram form