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ACMotorDrives

PartA:InverterBasics

AEV5040:PowerElectronicsfor

ElectricVehiclesTermI,2011

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SynthesisofLow-FrequencyAC:BasicConcept

ExtensionofDCvoltagesynthesis InsteadofDC,gettheswitchingcycleaveragedvoltageoutputtotraceoutaslowsinusoid;

thatis,synthesizeaslowlyvaryingsinusoidalvoltagewitheachpointonwaveformbeingthe

averageofseveralswitchingcycles. Frequencyofsinusoidf1

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AverageRepresenta^onofthe

SwitchingPower-Pole

CopyrightNedMohan2008 3

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SpectralContent

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Harmonicsinoutput

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Single-PhaseInverters(likeUPS)

2legsfor1inverter(1UPS,interfaceto1supply);3legsfor3ACmotordrive,UPSleg=bi-posi^onalswitchingpowerpole

Cap(partofLPFstructure)neededforsensi^veloads

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Vcom

=

Vd

2

Similarcurrentport

arrangementasforDCdrives

Averagedrepresenta^on

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SinglePhaseInverterWaveforms

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vaN=

Vd

2+ v

an=

Vd

2+

vo

2

vbN=

Vd

2+ v

bn=

Vd

2

vo

2

nisa

hypothe^cal

neutral!

6

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1InvertersVoltages/Currents

CopyrightNedMohan2008 7

id=

voio

Vd

=

V0

Io

Vd

sin1

t sin 1

t1( )= 0.5

Vo

Vd

Iocos

1

Id

0.5V

o

Vd

Iocos 2

1

t1( )

id2

(t)

CurrentlagsvoltagebypfangleHarmonicsalsopresentfrom

switchingac^on

vo(t) = V

0sin

1t( ) & io (t) = I0 sin 1t1( )

PO= v

oio= v

did(no switching losses)

DCcomponentresponsiblefortheaverage

powertransfer+large2ndharmonic

component

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Waveformsonnextslide!

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InverterWaveforms

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Outputpulsates

at2fS;filtering

neededfor

sensi^veloads

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3Inverters(Synthesisof3-phasevoltages)

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Averaged

representa^on

3load

3legs(6pack)

van =

Vph sin(1t); vbn = Vph sin(1t120

0);vcn = Vph sin(1t 240

0);

(Desiredbalanced3(averaged)voltages)

vaN= V

com+ v

an; v

bN= V

com+ v

bn; v

cN= V

com+ v

cn;

(Switchesincludeoffsetcommon-modevoltage!)

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OffsetVoltageSuppression

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Superposi^on

Offsetvoltagesdonotappearacrosstheload

becausetheydonothaveaneffect!

3i =

0; i =

0

vcom

Vcom

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Typesof3Converters

Six-step SinePWM SV-PWM

12

NEXTTOPIC!