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8/3/2019 AC Motor Drives Part A
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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
CopyrightNedMohan2008 4
Harmonicsinoutput
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Single-PhaseInverters(likeUPS)
2legsfor1inverter(1UPS,interfaceto1supply);3legsfor3ACmotordrive,UPSleg=bi-posi^onalswitchingpowerpole
Cap(partofLPFstructure)neededforsensi^veloads
CopyrightNedMohan2008 5
Vcom
=
Vd
2
Similarcurrentport
arrangementasforDCdrives
Averagedrepresenta^on
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SinglePhaseInverterWaveforms
CopyrightNedMohan2008
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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CopyrightNedMohan2008 8
Waveformsonnextslide!
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InverterWaveforms
CopyrightNedMohan2008 9
Outputpulsates
at2fS;filtering
neededfor
sensi^veloads
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3Inverters(Synthesisof3-phasevoltages)
CopyrightNedMohan2008 10
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
CopyrightNedMohan2008 11
Superposi^on
Offsetvoltagesdonotappearacrosstheload
becausetheydonothaveaneffect!
3i =
0; i =
0
vcom
Vcom
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Typesof3Converters
Six-step SinePWM SV-PWM
12
NEXTTOPIC!