Analyzer (DMA) tests. Also documented in this paper will be the results from specialized viscoelasticity tests performed on representative composite windings to determine preload loss versus time, temperature, and stress. Also, in collaboration with the Army Research Laboratory, a nested cylinder code is being developed to predict viscoelasticity induced preload losses.Center for Electromechanic

Hotz, T.J.

Kitzmiller, J.R.

Penney, C.E.

Werst, M.D.

Texas ScholarWorks

Abstract - The Cannon Caliber Electromagnetic Gun(CCEMG) program at The University of Texas at Austin Centerfor Electromechanics incorporates two solid state switch mod-ules.  The 95 MW rectifier/inverter bridge (RIB) generates thefield inside of the compulsator (CPA) and the 825 kA gun switchmodule (GSM) is the main closing switch between the CPA andthe launcher.  Since commissioning the CCEMG system, nearly400 discharges have been performed using these switches.  Thesetests produced a significant amount of information which will bevery useful for the design of future solid state switching systemson other electromagnetic launch programs.   This paper will pre-sent the basic design, testing and lessons learned from the com-missioning of the CCEMG system switches.INTRODUCTIONNew electromagnetic gun systems being designed at TheUniversity of Texas at Austin Center for Electromechanics(UT-CEM) use switch technology developed during theCCEMG program.  For this program, a four-pole air corecompulsator was used to drive a 2.25 m augmented railguncapable of rapid firing 15 projectiles at a velocity of 1,850m/s.  The program was completed in Fall 1996 having per-formed over 400 tests including six single shot tests anddemonstration of multishot dry fire capability [1]. During theCCEMG program, UT-CEM gained much knowledge in threedifferent areas: compulsators, railguns, and high voltage highcurrent switches.  This paper will concentrate on the problemsencountered and lessons learned from the switching systemsof the CCEMG program.The CCEMG system incorporates five different switchdesigns, each with unique operating requirements [2],[3].  Anoverall system schematic is shown in Fig. 1.  The five includethe field initiation module (FIM), rectifier/inverter bridge(RIB), gun switch module (GSM), explosive opening switch(EOS) and a muzzle arc suppression switch.  Only two of theswitches will be covered in the paper, the RIB and GSM.  TheEOS and FIM histories have been well established at UT-CEM ([3][4]) and are not addressed in this paper.Additionally, the muzzle arc suppression switch was designedbut never built or tested and therefore will not be covered inthis paper.The RIB is a single phase full wave bridge responsible forfield coil excitation and regenerative field coil dump.  Thefour legs of the bridge each consist of six thyristors arrangedin series/parallel configuration.  Once the FIM has been dis-charged into the field coil, the RIB starts rectification until thedesired level of excitation for particular test is achieved.  Thelevel is predetermined from simulation and entered as an inputparameter to the main control system.  The switch wasd signed with a goal of a maximum of 25 kA in the field coil;however, 30.5 kA was achieved on CCEMG run #116.  Theother function of the RIB is inversion.  Inversion is necessaryto meet program goals of a 15 shot burst at 5 Hz.  Since a dis-charge only uses some of the energy stored in the field coil,the remaining energy can be used to help accelerate the rotorbetween salvos.  Thus, after a discharge, the RIB is gated asan inverter and converts the stored electrical energy into kinet-ic energy.The other switching system discussed in this paper is theGSM.  The GSM consists of 40 parallel 4.5 kV thyristorsdesigned to deliver 825 kA in pulsed duty mode.  The GSMprovided the path to deliver the current from the compulsatorto the railgun during a discharge.  The GSM was designed,built and tested as a snubberless system [2].  A normal dis-charge would last approximately 2.5 ms and is considered softduty for these thyristors.  Preliminary analysis determined thatthe 4.5 kV limit would not be exceeded during normal opera-tion of the CCEMG system even during turn-off of thedevices.RECTIFIER/INVERTER BRIDGEMechanical PackagingThe complete RIB is packaged in one enclosure.  The sin-gle unit design allowed for the smallest volume and the bestSingle Shot Switch Performance on the Cannon CaliberElectromagnetic Gun ProgramT.J. Hotz, C.E. Penney, J.R. Kitzmiller, and M.D. WerstThe University of Texas at Austin, Center for Electromechanics, Austin, TexasManuscript received May 1, 1998.T.J. Hotz may be contacted at 512-471-4496, fax 512-471-0781,t.hotz@mail.utexas.eduThis research was supported by U.S. Army ARDEC and the U.S. MarineCorps under contract no. DAAA21-92-C-0062.  Funding was provided byThe University of Texas.matching of impedance between the parallel current paths.The R/I bridge contains four identical quadrants of six thyris-tors each.  The six thyristors in a quadrant have an RC snub-ber circuit located above the thyristors.  The ac input power isfed through flexible hexapolar cable into the center nodes ofthe thyristor stacks.  The rectified dc output leaves from thetop of the bridge assembly and returns from the field coil onthe bottom. The bridge layout is shown in Fig. 2 and anassembly photo is shown in Fig. 3.  The thyristor selected for the bridge is a custom version ofthe Powerex TD20, rated at 3,800 V.  The Powerex TD20 wasselected for performance and packaging reasons.  This unitwas available in a thin ceramic case, which reduced the overall stack height.  The bridge is assembled in two identicalhalves.  Each half is pressed together with a gang clampingsystem, meaning the preload is maintained using spring wash-ers. Each half uses a pair of titanium flanges that have threeself-aligning tilt pads held together with a single steel rod.  Gate DriveOne of the more critical areas of design for power electron-ic circuitry is proper device gating.  Proper gating techniquesare very important for reliably turning the thyristor on as wellas for longevity of the device.  Some of the earliest designwork done on the CCEMG power electronics was to investi-gate what type of gate drive circuit would be optimal for thissystem [2].  This early study found the best method of gatedrive for this system was a hard drive gate pulse with a 50 µsback porch for the moderate to high di/dt and dv/dt require-ments.The basic gate drive circuit used for both the bridge andGSM is shown in Fig 4.  For the hard gate drive pulse, a 60 V,3.5 A is generated at the output of the pulse transformer. A sin-gle pulse transformer, with eight secondary turns, gates eachbridge quadrant, ensuring that all thyristors are turned on at3010.0099EOSRectifier/ Inverter BridgeRailgunField Initiation ModuleSolid State Closing SwitchStatorCompulsatorRotorMOV Voltage ClampGun Discharge SubsystemBrush/ Slip RingFig. 1.  Overall CCEMG system schematic.DC (+)AC I/ORC SNUBBER COMPONENTSAC I/O BUSDC I/ODC (-)3010.0106FRONT VIEWTOP VIEWFig. 2.  Mechanical layout of RIB, top and front views.the same time.  The IGBT has strict gating requirements muchlike a thyristor.  To satisfy these requirements, a complemen-tary FET pair was used.  The gate drive circuits have turnedthe thyristor on with 100% reliability as long as there was asufficient forward drop across the device (at least 40 V).  Thetiming of the gate pulses with respect to actual CPA voltageduring a run is shown in Fig. 5.  (Note that the gate pulsesshown are the output of the pulse transformer and are optical-ly isolated therefore not the actual gate pulse used to drive thethyristor).Current MonitorsTo monitor the actual current distribution between thyris-tors during operation, Rogowski coils are used to instrumenteach thyristor.  The 24 Rogowski coil signals are conditionedand monitored by the bridge current sharing monitor (BCM).Since the bridge operates in quadrants, all thyristors in a quad-rant are gated at the same time.  Likewise, the BCM handlesthe 24 Rogowski signals as four groups of six signals each.After integration of the raw Rogowski signal, the average ofeach six channel quadrant is formed.  This establishes a quad-rant average value for each of the four quadrants.  The mini-mum acceptable sharing threshold is then added and subtract-ed from each average signal.  This process yields a rangeagainst which each of the 24 individual integrated Rogowskisignals is compared.  Any signal from an individual Rogowskifalling outside of its associated range will generate an errorcondition.   Fig. 3.  Assembly of RIB.HFBR-240226374.7kΩ0.1µF10ΩDSGDSG12 J1+15V+5V12J2100Ω1Ω10-20ΩC1E1C2E2G1E1G2E250 ACM50DY-12E+120VD23010.0100Out to 1GBT via RG 174Fig. 4.  Schematic of gate drive circuit.S 3010.1855.1In addition to average current, positive and negative currentflow is indicated for each channel using a similar rangingtechnique.  The positive and negative levels are fixed conduc-tion thresholds for each of the forward and reverse directions.All three of the resultant signals (high, low, and negative cur-rent) are latched and displayed by quadrant on an array ofLEDs.  The BCM suffered from a couple of minor problems.  TheCCEMG system was able to modify or work around theseflaws.  First, the BCM was designed to be an active controlcomponent, comparing current levels at all times.  The prob-lem with this system is that comparing current levels duringturn on and off of the thyristors often produced errors indicat-ing that the system is out of balance.  It was learned that thecurrent sharing could be compared about 100 µs after gatingthe thyristors. Thyristor turn off produced no error in theCCEMG system; however, the potential for nonuniform shar-ing does exist.  The solution used on the CCEMG systeminvolved not using the BCM signals during the interval fromthyristor gating to 100 µs after gating.The second problem with the BCM involved the compari-son levels.  The BCM collects six signals, integrates them,sums them, and then divides by six to form the average signal.Adding a set value and subtracting a set value then forms arange of operation.  At this point, each thyristor signal is com-pared to this range.  The BCM as described can work quitewell at any given current level.  The wide operating range ofthe bridge rectifier causes the problem.  A different approachwould be to use a percentage of the current level to determinethe size of the window instead of an absolute number.The third problem with the operation of the BCM wasnoise.  Two issues contributed to noise being a problem in theBCM system.  One was small signal levels from the Rogowskicoils at low current levels.  The solution to low signal levelsis a more sensitive Rogowski coil coupled with higher ampli-fier limits.  The other problem on the CCEMG was a physicalpackaging problem.  The CCEMG system was designed underaggressive size/weight requirements that resulted in a verytightly packaged bridge rectifier.  The physical layout of thebridge has opposite polarity thyristor sets in the same stack-up.  The close proximity caused opposite polarity rectifyingsignals to be induced into the BCM system during large di/dtevents.  The point of entry of the erroneous signals is two-fold:  coupling at the Rogowski coil itself, and in the twistedshield pair wiring inside the bridge enclosure.  Reducing thecoupling proved to be a difficult task, in the given bridgeenclosure.  Shielding of the Rogowski coils was achieved bywrapping the coils with copper foil and attaching the coppershielding to ground.  In testing after shielding, signal couplingwas reduced to a reasonable level; however, it was not elimi-nated.  GUN SWITCH MODULEEarly testing during the CCEMG program called for thesystem to reach program goals for gun current and projectilevelocity on a single shot test.  The goal called for a dischargegenerating a gun current of 825 kA and a muzzle velocity of1,850 m/s using a 185 g projectile.  Once single shot testingbegan, it only took six shots to reach this goal.  For all of thedischarges, both dry and live fire, leading up to the sixth sin-gl  shot gun shot the GSM performed flawlessly.  However,on shot #6 one of the 40 parallel thyristors broke down caus-ing a short across the GSM.  Although damage was minimal,the CCEMG team took a good hard look at the cause of thefault and subsequent design changes.-3000 -2000-1000010002000Compulsator voltage (V)3530252015Time (ms)543210Gate pulse voltage (V) CPAV Gate_A1 Gate_B1CCEMG TEST#116-54 Volts49 Volts64 Volts -68 Volts3010.0166Fig. 5.  Gate pulses and compulsator voltage.In order to better understand the reasons for the fault, a thor-ough understanding of the entire system sequencing is need-ed.  As mentioned earlier, the discharge sequence begins witha capacitive discharge of the FIM into the field coil.  Once theFIM has completed its dump and the desired threshold is ver-ified, the control system initiates the rectification mode of theRIB.  The RIB ramps the field coil current up to the desiredvalue entered into the control system prior to system opera-tion.  Once the desired field coil current level has beenreached, the control system switches the RIB to freewheelmode and waits for two freewheel cycles before the GSM canbe gated.  After two freewheel cycles, the 40 thyristors of theGSM are gated simultaneously and the current is transferredfrom the compulsator to the railgun.  For safe operation andmaximum efficiency, the GSM is gated at the compulsatorvoltage reaches approximately 100 V in the forward direction.The gun current pulse lasts for approximately 2.5 ms and thethyristors turn off as the current in the GSM goes throughzero.Snubberless operation of the GSM would allow the pro-gram to meet weight and volume goals of the switching sys-tems.  Therefore, a high priority was put on making the snub-berless design work.  In many applications using thyristors, asnubber is used to lessen the voltage contribution from a fastturn off of the device.  The voltage generated directly corre-lated to the induction in the circuit and the change in currentas current passes through zero.  This can be seen as:V = L di/dtEarly analysis calculated the sum of the compulsator voltageat turn off and the L di/dt contribution would generate at peakvoltage of 3,750 V, resulting in a 20% margin of safety for thedesign using 40 thyristors in parallel [2].  The di/dt at turn offwas calculated by simulation but had to be verified experi-mentally.  The GSM was built and an iron core compulsatorwas used to test switch operation.  The test sequence workedthe current up in conservative steps with a watchful eye onturn off characteristics.  Two tests were performed at a peakcurrent of 833 and 847 kA and the di/dt on these two test wasmeasured at 457 and 444 MA/s, respectively.  Applying theimpedance and di/dt measured on this test to the CCEMG sys-tem verified the 20% margin of safety for the switch.With so much consideration and experimental verificationgiven the design of the GSM, why was there a breakdown onshot #6?  Post analysis of the data from tests #5 and 6 and sub-sequent analysis of the circuit point to potential weaknesses inthe system.  First of all, the circuit inductance was being mea-sured experimentally on these gun shots to be about 1 µHhigher than the simulation yielded.  Several voltage measure-ments were being taken during these tests to pin down theextra inductance.  Additionally, the di/dt went unchecked dur-ing these tests and turns out to be 30% greater than anticipat-ed.  These factors led to a greater than anticipated turn offvoltage contribution which is believed to have approached therating of the thyristors.  The peak voltage was measured atonly -3,394 V (Fig. 6), but the sample rate of the oscilloscopewas insufficient to record the true peak.  So, the circuit modelwas simulated to calculate the L di/dt contribution and result-ed in a 2,724 V spike generated at turn off using the new cir-cuit inductance and 605 MA/s di/dt measured on shot #5(Fig. 7).  The compulsator voltage at turn off was measured tobe 1,924 V.  Because of circuit characteristics, the compul-sator voltage was at its negative peak at turn off thus addingto the problem.  Summing the two numbers would result in avoltage spike of 4,648 V which exceeds the 4,500 V rating ofthe GSM.-2000-1000010002000Voltage (V)0.2010.2000.1990.1980.1970.1960.195Time (s)-600-400-2000200400600Current (x103A)-600-400-2000200400600di/dt (x106 A/s) CPA Voltage Gun Current di/dtCCEMG Shot #5di/dt = 605 MA/s3910.0932.1-600-400-2000200400600Current (x103A)0.1860.1840.1820.1800.1780.1760.1740.1720.1700.168Time (s)-3000-2000-10000100020003000Voltage (V) Gun Current CPA VoltageGun Current = 660.4 kACPA Voltage = -3,394 V3910.0931.1Fig. 6. Discharge data from CCEMG shot #6. Fig. 7. Discharge data from CCEMG shot #5.This new information meant that a snubber had to be addedto the switch to lower the turn off contribution during highcurrent tests.  Additional information from the thyristor man-ufacturer indicated that the 4,500 V rating of the device hadlittle or no safety margin.  Consequently, a simple RC-snub-ber was designed and built.  Prior to installation, lower currenttesting would be performed to experimentally verify the turnoff contribution at a high sampling rate with new recordingdevices.  By the end of the program the snubber had neverbeen connected to the GSM because the system was neveroperated at a level which would approach the voltage rating ofthe devices.CONCLUSIONSAlthough the final program goals for the CCEMG programwere not realized, UT-CEM gained invaluable knowledgeabout solid state switching systems on high voltage, high cur-rent applications.  First, if Rogowskis are to be used for faultanalysis and current sharing comparisons, then the differentlegs of the bridge should be package with the maximum pos-sible distance between them to prevent flux coupling duringturn on and turn off.  Additionally, Rogowski design shouldinclude a means for shielding the coil and take careful consid-eration of signal to noise ratios for maximum effectiveness.Current monitors should ignore the sharing properties of thethyristors during the turn on period.  The usefulness of currentmonitors at low current levels should be carefully considered.Several important lessons were learned from the GSM aswell.  Unless advancements are made in thyristor technology,the larger switching systems of the future will require snub-bers to prevent breakdowns.  Additionally, all impedancesshould be well characterized both numerically and experi-mentally.  Testing of components for future systems should gothrough extensive destructive testing to verify manufacturer'sclaims of device ratings.  Additionally, circuit parametersshould be closely monitored and designed in order to preventpeak system voltages from coinciding with the turn off time ofthe thyristors.ACKNOWLEDGMENTSThe authors would like to acknowledge the numerous peo-ple from U.S. Army ARDEC, U.S. Marine Corps, UDLP, andUT-CEM who contributed to making the CCEMG system areality. Funding for this effort has been provided by theUniversity of Texas at Austin.  This work was sponsored byU.S. Army ARDEC, U.S. Marine Corps, and The Universityof Texas at Austin.REFERENCES[1] M.D. Werst, et al, "Continued testing of the cannon caliber elec-tromagnetic gun system (CCEMG)," submitted to the 9th EMLSymposium, Edinburgh, Scotland, May, 1998.[2] D.J. Wehrlen, R.A. Lee, and R.F. Thelen, "Power electronicsand controls for air core compulsator," IEEE Transactions onMagnetics, Vol. 31 No. 1, Jan. 1995.[3] C.E. Penney and T.J. Hotz, "Design and testing of the powerelectronics for the cannon caliber electromagnetic gun system,"IEEE Transactions on Magnetics, Vol. 33 No. 1, Jan. 1997.[4] R.A. Lee, D.J. Wehrlen and C.E. Penney, "Design and testing ofa coaxial opening switch and solid state closing switch for thecannon caliber electromagnetic launcher system (CCEMG),"10th IEEE Pulsed Power Conference, Albuquerque, NM, July10-13, 1995.

Single Shot Switch Performance on the Cannon Caliber Electromagnetic Gun Program

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Single Shot Switch Performance on the Cannon Caliber Electromagnetic Gun Program

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