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36.7Adjustable Frequency DrivesCPX9000 Drives363636363636363636363636363636363636363636363636363636363636How Does a VFD Convert Three-Phase AC to aVariable Output Voltage and Frequency?The six-pulse VFD: TheThe six-pulse VFD drivemajority of all conventional creates harmonic currentdrives that are built consist distortion. The harmonicof a six-pulse configuration. current that is created isThe figure below represents energy that can not be useda six-diode rectifier design that by customers and causesconverts three-phase utility external heat and losses to allpower to DC. The inverter components including othersection uses IGBTs to convert drives that are on the sameDC power to a simulated AC power distribution. See thesine wave that can vary in curve below showing afrequency from 0–320 Hz. 500 hp drive with 167A ofdamaging harmonic current.Six-Diode Rectifier DesignSix-Pulse DiodeBridge RectifierConverter SectionL2AL1AL3AInverter Section(+) DC(–) DCDynamic BrakingBus TransistorCapacitorsOutput TransistorsIGBT Section500 hp Six-Pulse Nonproductive Harmonic CurrentCurrentAmps10005000–500–10000.100 0.10625 0.1125Time in Seconds500 hp Six-Pulse Nonproductive Harmonic CurrentSix-Pulse CircuitCurrent harmonicsI 1 = 100% I 11 = 6.10% I 19 = 1.77%I 5 = 22.5% I 13 = 4.06% I 23 = 1.12%I 7 = 9.38% I 17 = 2.26% I 25 = 0.86%Power = 500 hpHarmonic current = 167 ampsACMotor0.11875 0.125Guidelines of Meeting IEEE Std. 519-1992Harmonic Distortion LimitsThe IEEE 519-1992Specification is a standardthat provides guidelines forcommercial and industrialusers that are implementingmedium and low voltageequipment.Maximum Harmonic Current Distortion in % of theFundamental (120V through 69,000V)Harmonic Order (Odd Harmonics)Isc/I L h