PWM Techniques

7/29/2019 PWM Techniques

1/27

Power ElectronicsPower Electronics

Chapter 6

PWM Techniques


2/27

P

ower

Ele

ctr

on

ics

2

The most widely used control techniqueThe most widely used control technique

in power electronicsin power electronics

Pulse Width Modulation (PWM)(Chopping control)

DC/DC AC/AC

DC/AC AC/DC


3/27

P

ower

Ele

ctr

on

ics

3

OutlineOutline6.1 Basic principles6.1 Basic principles

6.2 Some major PWM techniques in DC/AC inverters6.2 Some major PWM techniques in DC/AC inverters

6.3 PWM techniques with feedback control6.3 PWM techniques with feedback control

6.4 PWM rectifiers6.4 PWM rectifiers


4/27

P

ower

Ele

ctr

on

ics

4

6.1 Basic principles of PWM6.1 Basic principles of PWMSimilar response to different shape of impulse inputSimilar response to different shape of impulse input

The equalThe equal--area theorem:area theorem:

Responses tend to be identical when input signalsResponses tend to be identical when input signals

have same area and time durations of input impulseshave same area and time durations of input impulses

become very small.become very small.


5/27

P

ower

Ele

ctr

on

ics

5

Basic principles of PWMBasic principles of PWMApplication of the equalApplication of the equal--area theoremarea theorem

This is sinusoidalThis is sinusoidal

PWM (SPWM)PWM (SPWM)

The equalThe equal--areaarea

theorem can be appliedtheorem can be appliedto realize any shape ofto realize any shape of

waveformswaveforms


6/27

P

ower

Ele

ctr

on

ics

6

A list of PWM techniquesA list of PWM techniquesTriangularTriangular--wave samplingwave sampling

Natural samplingNatural sampling

Uniform samplingUniform sampling

CalculationCalculation

Calculation based on equalCalculation based on equal--area criterionarea criterion

Selective harmonics eliminationSelective harmonics elimination

Hysteric controlHysteric control

Space Vector Modulation (SVM, or SVPWM)Space Vector Modulation (SVM, or SVPWM)

Random PWMRandom PWM


7/27

P

ower

Ele

ctr

on

ics

7

6.2 Some major PWM techniques6.2 Some major PWM techniques

Natural samplingNatural sampling

Uniform samplingUniform samplingSelective harmonics eliminationSelective harmonics elimination

Some practical issuesSome practical issues

Synchronous modulation and asynchronous modulationSynchronous modulation and asynchronous modulation

Harmonics in the PWM inverter output voltagesHarmonics in the PWM inverter output voltages

Ways to improve DC input voltage utilization and reduceWays to improve DC input voltage utilization and reduce

switching frequencyswitching frequency Connection of multiple PWM invertersConnection of multiple PWM inverters


8/27

P

ower

Ele

ctr

on

ics

8

TriangularTriangular--wave natural samplingwave natural samplingUniUni--polar PWM in singlepolar PWM in single--phase VSIphase VSI

UniUni--polar sampling is used topolar sampling is used torealizerealize uniuni--polar PWM.polar PWM.

Controlsignal

CarrierMudulation

Carrier

Ud

+

V1

V2

V3

V4

VD1

VD2

VD3

VD4

uo

R L

ur

uc


9/27

P

ower

Ele

ctr

on

ics

9

TriangularTriangular--wave natural samplingwave natural samplingBiBi--polar PWM in singlepolar PWM in single--phase VSIphase VSI

Control

signal

CarrierMudulation

Carrier

Ud

+

V1

V2

V3

V4

VD1

VD2

VD3

VD4

uo

R L

ur

uc

BiBi--polar sampling is used topolar sampling is used to

realize birealize bi--polar PWM.polar PWM.


10/27

P

ower

Ele

ctr

on

ics

10

TriangularTriangular--wave natural samplingwave natural samplingIn 3In 3--phase VSIphase VSI

ThreeThree

--phase bridge inverterphase bridge inverter

can only realize bican only realize bi--bolarbolarPWMPWM

therefore should be controlledtherefore should be controlled

by bipolar sampling.by bipolar sampling.


11/27

P

ower

Ele

ctr

on

ics

11

TriangularTriangular--wave uniform samplingwave uniform sampling

Easier to realizeEasier to realize

by computerby computer--controlcontrol

Modulation factorModulation factor


12/27

P

ower

Ele

ctr

on

ics

12

Selective harmonics eliminationSelective harmonics elimination

PWM (SHEPWM)PWM (SHEPWM)


13/27

P

ower

Ele

ctr

on

ics

13

Frequency relationship between triangularFrequency relationship between triangular--

wave carrier and control signalwave carrier and control signal

Asynchronous ModulationAsynchronous Modulation

Synchronous ModulationSynchronous Modulation


14/27

P

ower

Ele

ctr

on

ics

14

Harmonics in the PWM inverterHarmonics in the PWM inverter

output voltagesoutput voltages

No lower orderNo lower order

harmonicsharmonics

The lowest frequencyThe lowest frequencyharmonics isharmonics is wwcc andand

adjacent harmonics.adjacent harmonics.

wwcc has the highesthas the highest

harmonic content.harmonic content.

Spectrum of 1Spectrum of 1--phasephase

bridge PWM inverterbridge PWM inverter

output voltageoutput voltage

1

0

0 2+-1 2 3

4+- 0 2+- 4+-0 1+- 3+- 5+-

Magnitu

de(%)

(nc+k

r)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

k

n

a=1.0a=0.8a=0.5

a=0


15/27

P

ower

Ele

ctr

on

ics

15

Harmonics in the PWM inverterHarmonics in the PWM inverter

output voltagesoutput voltages

No lower orderNo lower order

harmonicsharmonics

No harmonics atNo harmonics at cc..The lowestThe lowest

frequency andfrequency and

highest contenthighest content

harmonics areharmonics arecc22rr and 2and 2ccrr..

Spectrum of 3Spectrum of 3--phasephase

bridge PWM inverterbridge PWM inverter

output voltageoutput voltage

1

0

0 2+-1 2 3

4+- 0 2+- 4+-0 1+- 3+- 5+-

agntue

0.2

0.4

0.6

0.8

1.0

1.2

k

n

a=1.0a=0.8a=0.5

a=0

(nc+k

r)


16/27

P

ower

Ele

ctr

on

ics

16

Ways to improve uti lization of DC inputWays to improve uti lization of DC input

voltage and reduce switching frequencyvoltage and reduce switching frequencyUse trapezoidal waveform as modulating signalUse trapezoidal waveform as modulating signal

instead of sinusoidalinstead of sinusoidal


17/27

P

ower

Ele

ctr

on

ics

17

Ways to improve utilization of DC inputWays to improve uti lization of DC input

voltage and reduce switching frequencyvoltage and reduce switching frequency

Use 3k order harmonicsUse 3k order harmonics

bias in the modulatingbias in the modulating

signalsignal

uc

ur1

u

Ot

ur

ur1

u

O t

ur3

uc

urU1

urV1

urW1

u

uUN'

Ud

-Ud

Ot

O

urU

urV

urW uc

O t

O

O

O

O

t

t

t

t

t

uVN'

uWN'

uUV

u

1

-1

1

-1

-0.5

uP

2Ud

2

Ud


18/27

P

ower

Ele

ctr

on

ics

18

Connection of multiple PWM invertersConnection of multiple PWM inverters

PurposesPurposes

Expand output power ratingExpand output power rating

Reduce harmonicsReduce harmonics


19/27

P

ower

Ele

ctr

on

ics

19

6.3 PWM techniques with6.3 PWM techniques with

feedback controlfeedback controlCurrent hysteric controlCurrent hysteric control

Voltage hysteric controlVoltage hysteric control

TriangularTriangular--wave comparison (sampling) withwave comparison (sampling) withfeedback controlfeedback control


20/27

P

ower

Ele

ctr

on

ics

20

Current hysteretic controlCurrent hysteretic controlIn SingleIn Single--phase VSIphase VSI


21/27

P

ower

Ele

ctr

on

ics

21

Current hysteretic controlCurrent hysteretic control

In 3In 3--phase VSIphase VSI


22/27

P

ower

Ele

ctr

on

ics

22

Voltage hysteretic controlVoltage hysteretic control

Filter

+

-

u

u*

u

2

Ud

2

Ud


23/27

P

ower

Ele

ctr

on

ics

23

TriangularTriangular--wave comparison (sampling)wave comparison (sampling)

with feedback controlwith feedback control


24/27

P

ower

Ele

ctr

on

ics

24

6.4 PWM rectifiers6.4 PWM rectifiersOperation PrinciplesOperation Principles

a) Rectification modea) Rectification mode b) Inversion modeb) Inversion mode

c) Reactive powerc) Reactive power

compensation modecompensation mode

d) Current leading byd) Current leading by


25/27

P

ower

Ele

ctr

on

ics

25

PWM rectifiersPWM rectif iers

ThreeThree--phase circuitphase circuit


26/27

P

ower

Ele

ctr

on

ics

26

PWM rectifiersPWM rectif iersIndirect current controlIndirect current control

PI+-

Load

Triangular-wave

sin(t+2k/3)(k=0,1,2)

cos(t+2k/3)

(k=0,1,2)

u*d

ud

+

-

+

-

-

+

id

uR

uL

XL

Ru

A,B,C

ud

R L

ua,u

b,u

c


27/27

P

ower

Ele

ctro

nic

s

27

PWM rectifiersPWM rectif iersDirect current control

PWM Techniques

Documents

Transcript of PWM Techniques