ac igbt drives and their applications; dcs - nov 08
TRANSCRIPT
Western Mine Electrical AssociationNovember 20,2008
Rob MarnellGary Sorenson
AC vs. DC Motor ApplicationAC and DC Motors Require a Rotating Magnetic Field and a fixed magnetic field to Produce Torque and RotationDC Motors have the fixed field on the Motor Frame and the rotating field is created by brushes switching the field around the armature as the armature turns.AC Synchronous Motors have the fixed field on the rotor and the rotating field occurs in the stator with the AC current in the stator windings.
AC vs. DC Motor DC Motor The North and South
Poles attract each otherProduces a torque on the armatureArmature turns which puts new commutatorsegments under brushes and “rotates”the magnetic field inside armature so more torque can be produced
North
South
North
SouthField Flux
ArmatureFlux
TorqueProducing Force
AC Motor3-Phase AC Current Varies the A, B, C Field StrengthA
1 (S)
A2 (N
)
North
Sout
h
A 1 (
N)
A2 (S
)Nor
th
South
B1 (S)
B2 (N)North
South
B1 (N)
B2 (S)
North
South
C1 (N)
C2 (S)
NorthSouth
C1 (S)
C2 (N)
North
South
Generic AC PWM Inverter
Diodes, SCR’sIGBT’s
INVERTERFRONT-END
SCR’s, GTO’s, IGBT’s
SCR GTO IGBT
AC Drives In ExcavatorsStarted With Smaller Applications
ShovelsSmall DraglinesSwing Motion Only (700 HP/Motor)
Why not on large draglines?
1969 Brute Force Rules1969 Corvette LS5 427ci 435HPBig Muskie Rules the Strip Mine
1980’s1981 Corvette L81 350ci 180HP1981 First 395B AC shovel with Force Commutated SCR AC drives1983 Corvette? Why bother? Production was suspended.1983 680W AC Dragline IntroducedOther Highlights of the 1980’s
10% Unemployment in 198214% Inflation20% Mortgage Interest ratesAIDS Epidemic recognized by CDC in 1981
Force Commutated SCR Drive
Hailed as the End of DC brushes and commutationSCR Characteristics
0.7 Volts on gate turns on SCRCurrent flows until device is “reverse biased”On an AC drive application, device must be switched off and on multiple times per cycle with a DC bus as the supplyHow?
Forced Commutation
Advantages and Disadvantages of Force Commutated SCR DriveAdvantages Disadvantages
No brushes to change Extremely complex analog regulatorNo/poor regeneration. Used a chopper and resistor to burn off regeneration energy. (About 10% Energy penalty)Commutating capacitors failed. Fuse hungryPower Factor not good enough for a large DraglineLine HarmonicsSlow SCR switching speeds resulted in poor AC waveforms delivered to motorsMotor Encoders Required
1990’s1990 Corvette ZR-1 350ci 375HP (Now that’s progress)1990 395BII and 495B Drives with GTO Thyristors1993 2570WS with AC Swing using GTO Thyristors
GTO Thyristor AC DriveSolved the problems associated with SCR drive.
Regenerative Rectifier No Commutating CapacitorsInverters used fewer fuses
GTO Hardware overviewGate off Current is proportional to device current and measured in amps.
GTO THYRISTORBasically an SCR that can turn on AND off.Gate Turn Off current is high and equal to device current. High current gate drivers have lots of failures
20-150 Amps Gate Current in a 20 ga
wire? How does that work?
GTO’s Regenerating Rectifier
Basically a 4-Quadrant SCR drive similar to what is found on a static DC drive.Required sufficient AC voltage to prevent fuse loss.The result was balancing between poor power factor and blowing regen rectifier fuses with undesirable compromises either way
GTO DrivesAdvantages Disadvantages
Still no BrushesRegeneration saves electricityDigital Regulator reduced complexityNo Commutating CapacitorsInverter Faults Usually Don’t blow Fuses
Regen Rectifier blows a lot of fusesPower factor even worse than earlier Generation Drives. Could not be used for large draglines aloneGTO Drivers fail. GTO Gate requires tremendous amount of switching current.Line HarmonicsMotor Encoders Required
2000’s2001 Corvette Z06 385 HP2001 IGBT Drives Introduced on Excavators2008 Corvette Z06 505 HP2008 680W brought into 21st century with IGBT retrofit
IGBT Insulated Gate Bipolar TransistorFirst Devices in late 1980’s and early 1990’s were slow and troublesome.One problem was latch-up where the internal SCR would not turn off.Insulated Gate= low switching current and isolates power from control section
IGBT SCR
Insulated Gate
IGBT DrivesAdvantages Disadvantages
Still No BrushesDon’t Blow FusesPower Factor Problems ImprovedEfficiency Low Gate Current Solved Driver ProblemsHigh Switching Frequency Solves One Harmonics Issue
High switching frequency can create new harmonics issues if filters are not appliedMotor Encoders required
Power FactorP=V*I Cosine(Θ)
Θ= phase angle between voltage and current waveform
If current follows voltage wave form then lagging power factor. (voltage sag)If current is ahead of voltage waveform then leading power factor. (voltage boost)
Phase angle
Volts
Amps
Leading Power Factor
Phase angle
Volts
Amps
Lagging Power Factor
How to control power factorSynchronous Motor Excitation on MG setFixed Capacitors found on induction motorsSwitched Capacitors Synchronous Capacitors (DCS Voltage Stabilization)Stepless Reactive Power CompensatorActive IGBT Front End
Synchronous Motor Power FactorAdvantages Disadvantages
Free if you already have the MG setNo Harmonics
Really expensive if you don’t have the MG set alreadyRotating equipment and associated maintenance
Fixed CapacitorsAdvantages Disadvantages
Work great on fixed speed induction motorsDo not create harmonics
Varying loads require varying amounts of capacitance. Susceptible to damage from harmonics. Higher frequencies create higher currents in capacitor.
Switched CapacitorsAdvantages Disadvantages
Can compensate for variable loads
Add complexity with SCR switching VARS are added and removed in large “chunks”resulting in alternating over or under compensation as you step through the RPCFairly high capacitor, inductor, fuse, and SCR/diode usage
Stepless RPCAdvantages Disadvantages
Better at matching VAR needs from the load
Added ComplexityParticularly on Force Commutated SCR drive on Marion 301
Large Inductor used to cancel out fixed capacitors.
Inductor heats when the machine as at idle
IGBT Active Front EndIGBT Front End SCR Front End
Switching characteristics of IGBT’s allow extraction of current “leading” the voltage wave form
SCR’s Must turn on at the “back end” of the sine wave resulting in lagging power factor
Active Front EndAdvantages Disadvantages
Inherent RegenerationPower Factor Can be controlled leading and laggingCommon parts with invertersLow harmonics (3,5,7,9,11th
)
High Frequency HarmonicsFunction of the switching frequency.2kHz = 33rd harmonic3.6 kHz = 60th harmonic
Harmonics SHOULD be filtered
Risk of trail cable resonance otherwise
Why Upgrade? (Is it worth risking my job?)MG Sets AC Drives
Safe bet since the beginning of timeBrushesCommutation IssuesSpeed/Torque LimitsEfficiency = 82 %NoiseTraining Investment25+Year Life
New Kid on the BlockNo BrushesNo NeutralsNo CommutatorsPotential of Greater Dynamic Range with proper motor selectionEfficiency 92%Liquid Cooling can Greatly reduce noiseExpensive on an Existing DC machineEvery motor will require its own drive and encoder
The Case for AC drivesBrushes (wrong reason but some benefit)Efficiency (easy to evaluate the benefit) May improve if green house gas limits are imposed.Supply Frequency (50-60 Hz Conversion)Noise (MSHA limits)
Opportunity to drop noise levels by 20 dB (one set of ear plugs)
Power Factor- finally enables dragline applicationMaintenance Costs (Torsional Oscillations)Performance Opportunity
RequirementsSimplicity
Common partsModular design
20 Year Life (How will you know? Look at engineering)At best, past products were replaced on a 10 year cycle (not acceptable)175,000 hours Design life
Capacitor SelectionProperly Derated for ApplicationCabinet Design for Excavator Duty
CostInitial Cost of Motors and DrivesOperational Costs (efficiency)Maintenance Costs (brushes etc.)
Vulcan 680W DraglineFirst Totally AC IGBT Dragline in North America
Cab upgradeOriginal cab Updated cab
Regulator upgradeOriginal regulator IGBT regulator
Inverter UpgradeOriginal SCR inverter IGBT transistor module
680W IGBT One-Line Diagram
Are You Ready?In 2006 the Z06 Corvette finally delivered more power than the most powerful Corvette offered in 1969Have modern drives eclipsed the offerings delivered in 1969? Probably…in about 2006.
Could they be applied to Big Muskie?I will let you know in 20 years
680W Dragline UpgradeDrive Skid Active Front End
680W Dragline UpgradeInverters Assembled Drive Skid
680W Dragline UpgradeHoist Motor Rewind Motor Stator
680W DraglineInverters Test Inverter Testing
680W Dragline UpgradeInverter Testing More Testing
680W Dragline UpgradeOperators Cab
680W Dragline UpgradeCollector Rings Before Collector Rings After
680W Dragline UpgradeControl Room Hoist and Drag Motors
680W Dragline UpgradeRegulator Interface Swing Regulator Interface Hoist/Drag
680W Dragline OriginalBack of Regulator