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