self excited design

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A FIELD BASED, SELF-EXCITED COMPULSATOR

POWER SUPPLY FOR A 9 MJ RAILGUN

DEMONSTRATOR

By:

A. W. WallsS. B. Pratap

G. W. BrunsonK. G. CookJ. D. Herbst

S. M. ManifoldB. M. RechR.F. Thelen

R. C. Thompson

W. G. Brinkman

Center for ElectromechanicsThe University of Texas at Austin

PRC, Mail Code R7000Austin, TX 78712(512) 471-4496

PR 100

Fifth EML Conference, Destin, FL, April 2-5, 1990.

IEEE Transactions on Magnetics, vol. 27, No. 1, January 1991, pp. 335-349

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in other works must be obtained from the IEEE.


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IEEETRANSACTIONS ON MAGNETICS, VOL. 27,NO. 1, JANUARY 199 1AFIELD&AsED, SELF-EXCITEDCOMPULSATORPOWER UPPLY ORASMJRAILGUNDEMONSTRATOR

W.A Walls, S.B. Pratap, W.G. Brinkman, K.G. ook, J.D. Herbst,S.M. Manifold, B.M. Rech, R.F. Thelen , pnd R.C. ThompsonCenter for ElectromechanicsThe University of Texas a t Austin10100BurnetRd., Bldg. 133Austin,TX 78758-4497

335

Abstract: Fabrication efforts have begun on afield-based compulsator for firing 9 MJ projectiles froma railgun launcher. The machine is designed t o store200 MJ kinetic energy and fire a salvo of nine round s inthree minutes a t velocities between 2.5 and 4.0 k d s .Prime power required t o meet this firing schedule is1,865kW and will be supplied bya gas turbine engine. Itis also possible t o fire a burst of two shots in rapidsuccession, if desired. Operating speed of the machineis 8,250 pm and it has design ratings of 3.2 MA peakcurrent and 20 GW peak power into a 9 MJ railgun load.A two-pole configuration is used for pulse lengthconsiderations and selectively passive compensation isemployed to producea relatively flat pulse a nd limit peakprojectile acceleration t o about 980,000 d s 2 (100kgees).Other distinguishing features include an air coremagnetic circuit, separate rotor armatu re windings forself-excitation and railgun firing, ambient temperaturefield coils, and excitation field magnetic energy recoverycapability.

Th e r o t o r is made of fiber reinforced epoxycomposite rings an d is supported by high streng th metalstub shafts which are shielded from the excitation fieldby water cooled copper sleeves. Both rotor armat urewindings are formed from aluminum litz wire tominimize conductor mass and eddy current losses. Thestationary compensating winding is supported by alaminated stainless steel stator structure t o which thealuminum field coils ar e attached. Hydrostatic oi l filmbearings support the ro tor and are sealed against theevacuated rotor cavity with segmented carbon ring seals.Separate brush mechanisms are employed t o collectexcitation and railgun currents. A unique transpositionof bru sh ring conductors in the m ain discharge circuitis used t o ensure uniform current division betweenbrushes. The compulsator mounts are designedt o allowthe machine t o rotate against linear dampers duringdischarge t o minimize peak torque transmission t o th eskid base.

A detailed description of the machine as designed,and i ts auxiliary and control systems, is provided in th ispaper. Fabrication and assembly methods are reviewedand the current stat us of the project is discussed. Inconjunction with this project, a lightweight railgun isbeing developed and is discussed in a companion paperpresented at the 5 th EML conference. [l]

Compulsators are well suited as power suppliesfor railguns because a single element stores the requiredenergy, generates th e electrical pulse, and preconditionsit before delivery t o the gun . High system efficiency isachieved because the compulsator recovers the inductiveenergy in the g un before projectile exit. Burst firing isachievable because sufEcient energy can be stored in therotor for several shots. This allows prime power t o be

averaged over several shots, which can reduce systempeak power requirements substantially. Othe rimporta nt a dvanta ges of compulsators include anaturally occurring current zero at projectile exit, pulseshaping capability, and high energy and power densitiesrelative t o other power supply options. For th isparticular mission, t o satisfy weight and sizeconstraints, a two pole, self-excited, air-core machinewas selected . To accommodate relatively low projectileacceleration limits (by railgun standards), a selectivelypassive compensation scheme is incorporated in thedesign and results i n a flat current wave form from thecompulsator into a railgun load (fig. 1). A cross sectionof the machine is shown in figure 2and a list ofimportant machine parameters are given in table 1.

'c/ \0.00 0.8 1.6 2 4 3.2 4.0 4.8 5.6 6.4 73 8.0

TIME (ms)3901.0421

Figure 1. Compulsator output pulse shape duringa 2.5km/s, 9 MJ ailgun shotAn overal l ci rcui t schemat ic for thecompulsatorhailgun circuit is shown in figure 3. Asmall capacitor, used t o initiate self-excitation, ischarged and the compulsator r o t o r is brought to fullspeed. Self-excitation proceeds by lowering the fieldbrushes and discharging the capacitor into the fieldcoiVexcitation arm atu re circuit. The resulting ac signalfrom the excitation armature is rectified and used t obuild additional field curren t. About0.25 s before fullfield current (42 kA ) is reached, the main dischargebrushes are lowered in preparation for railgun firing.Switching of the gun circuit is accomplished with tenparallel solid-state devices and the projectile launchoccurs in the ensuing 6 ms. Field energy recovery is

then begun by properly controlling the rectifier bridge.System performance projections for variousprojectile masses and generator speeds are provided intable 2.

0018-9464/91/0100-0335$01.001991 IEEE

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336MAIN STATOREND PLATE

Ta

JOBEm w s iBEARING

COMPENSATING

MAINBRUSI

" I 1 - 1 - 1CONNECTION

EN DSTATORCASING3901 0422

THRUSTEND

Figure 2. Task C compulsator cross section view

le 1. Self-excited, air core compulsatorcharacteristics

pERA:M p GUNCIRCUIT

ELD EXCITAnONCIRCUIT

FIELD2 IrA

3901 0423

Figure 3 . Detailed compulsator and railgun electricalschematic

HES

.FIELDEXCITATIONBRUSHES

Table 2. Compulsator driven gun performance forseveral 9-MJ projectiles

11 Proiectile I Muzzle I Peak Gun I PeakVelocity Current Acceleration( k d s ) (MA) (kgees)

2.002.88 2.5

KevDesim IsswKey design issues for the compulsator include thefollowing interrelated areas:

armature end-turn geometry,field coil shielding and end-turn design,r o to r shaft material selection, andr o t o r dynamics.

The chosen two-pole design subjects the full r o to rvolume t o the excitation flux. This causes extreme eddy-current heating in any solid metal components in therotor. This heating can be minimized by materialselection, reducing the time the excitation field ispresent, and by internally shielding conductive rotorcomponents. Al l of these techniques are used t o allowthe use of a high strength metal shaft f o r th ecompulsator r o t o r . A 17 kV excitation armaturewinding (separate from the main armature winding) isincluded in the rotor t o decrease field self-excitation timeand the field is rapidly quenched by regenerating thefield inductive energy back into the rotor. Shaf t eddycurrent heating is minimized by water cooled coppersleeves which shield the stub shafts from the excitationflux. Quick field charging is also enhanced by breakingup conductive paths in the stator structure which wouldotherwise experience circulating currents opposing fieldrise. Unfortunately, these sta tor conductive paths couldbe useful in shielding r o t o r armature discharge flux

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ShaftsElectrical connections and outpu t conductors forboth a rmatu re windings a re included in the connectionend shaft. Transitio n litz wire conductors are routedbetween the shaft and the flywheel and make crimpjoints t o solid copper coaxial output conductors.Excitation and main armature output conductors areconcentrically arranged and carry the respectivecurrents t o brush dip rings at the outboard end of theconnection end shaft. Copper coated aluminum brus hrings are used t o minimize mass and inertia of thebrush rings, which directly affect torsional stresses inthe shaft during discharge deceleration. A n iron-nickelalloy, InvarTM, s used as the struc tural portion of th eshaft fo r its low coefficient of thermal expansion. As thecopper conductors are resistively heated by a multiple-shot burst, the InvarTM revents excessive shaft thermalgrowth and minimizes bearing clearance changes overthe nine-sh ot sequence. The thrust-en d shaft,attachment t o the turbine drive , supports the thrust-bearing runner and is fabricated from a high strengthstainless steel alloy.

Both stub shafts are fitted with water cooledmagnesium-zirconium-chromium (MZC) copper alloyeddy curren t shields. Eddy currents generated in th eshields oppose the excitation field and exclude it fromth e sha ft components. The shields, however, doexperience substantial losses and require active coolingfor a multishot firing sequence. A0.76V s flow rate of anambient tempera ture water-glycol solution is introducedthrough a pressurized gallery sealed with carbon ringseals on each end of the rotor. To prevent the heate dshields from damaging composite structures in therotor, a ceramic sleeve is located around each shield toserve as a thermal barrier. The ceramic is placed in astat e of compression prior t o installation by a graphite-epoxy banding which is thermally interference fit.

The armature winding used to provide field coilself-excitation current is located between the rotor shaftand flywheel. It is a two-pole winding that i s 90 phaseshifted from the main armature winding to avoid EMcoupling. Each pole has 12 turns an d the two poles areconnected in series so that peak voltage generation isindreased and field coil charging times are reduced.The aluminum litz wire conductor is composed of twosquare conductors t o allow forming of the end turns inthe same plane a s the winding. Winding conductors aresized t o allow nine consecutive charging cycles withoutactive cooling. Temperature rise per cycle is 15Cin th econductors and 10C when averaged with thesurrounding epoxy used to encapsulate the winding.

from interactin g with the field coil. Without them, thefield-coil conductors will experience additional inducedcurren ts and electrom agnetic (EM) loading from theircoupling with t he discharge fields.Another key design issue results from a trade-offbetween main armature voltage generation, EMdischarge loading, and rotor length. From a rotordynamics viewpoint, a shorter rotor is more rigid andtherefore, more likely to operate subcritically. Due t o thehigh armature current and the nine shot burst

requirement, the stranded armature conductors arenecessarily large (2.5-cm thick x 22.8-cm wide).Accomplishing the en d tu rns for these conductors takesa large fraction of the rotor active length. By placing thefield coil end turns outside the armature end turns,generating the required 6-kV voltage can be achievedwith a minimum rotor active length. Unfortunately, therequired minimum rotor length results in a rotor to oflexible t o allow operation below the first rotor flexuralcritical speed. Bearing stiffness and damping ar etherefore optimized t o allow operation approximately30% above the critical speed so tha t 9-MJ shots from fullspeed will not drop the rotor speed into the range of thecritical frequency.Interaction of the rad ial excitation fields with th earmature end turns is another major consequence oflocating the field coil end turns outside the rotor endturns. Large axial loads on both the rotor and field coil

result from this interaction and must be balanced oneach end of the machine t o avoid overloading the thrustbearing. A symmetric armature design is required t oensure an axial force balance during discharge.

The compulsator rotor is primarily a fiberreinforced composite structure built from concentricannula r rings. Use of composites is required since rotormater i a l s must be bo th nonmagnet i c andnonconductive, but these lightweight materials alsoallow energy storage a t a high specific value. Size of therotor is d ic tat ed by shear s t resses a t t hearmature/flywheel interface at discharge, voltagegeneration, and energy storage requirements. The rotorconsists of six primary components including thrus t en dand connection end stub shafts, a compositeintermediate shaft stru cture, the excitation armatu rewinding, a composite flywheel structure, and the mainarmatu re with it s banding. All armatur e conductorsare stranded and transposed aluminum litz wireswhich are vacuum impregnated with a glass clothreinforced epoxy for struc tur al integrity. Assembly ofthe rotor is accomplished by starting with the stu b shaftand installing additional layers, many of which areinterference fit onto the substructure. A section view ofthe rotor is shown in figure 4.

/DRIVECOUPLING

THRUST ENDASHAFT FLYWHEEL CONNECTIONUPPORTRING END SHAFT

Figure 4. Rotor cross section

3901 0424


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JzkiwhdThe rotor flywheel is made up of seven compositerings using both graphite and S-glass reinforcement.These rings provide the bulk of the energy storage andserve t o support the main armature winding. Eachflywheel ring is installed with interference using ahydraulic assisted press fit method wherein hydraulicpressure is used t o expand the ring so that it can be slidonto the assembly. Epoxy is used as the working fluid sotha t an interfacial bond is established between rings.

h B .The primary armature winding is a two pole, sixturn per pole configuration with the two poles arrangedin parallel so that each conductor carries half of thepeak curren t generated by the machine. End-to-endsymmetry in the winding is required t o balance axialloads created by the interaction of the end tur n curren tswith the excitation field, and is accomplished byfabricati ng the winding i n two layers wound fromopposite ends of the flywheel. The aluminum litz wireexperiences a 12OC per shot temperature rise, therefore,no cooling is required for a nine-shot burst. Pole star tand finish leads are located on the faces of the flywheeland supported by hoop wound graphite plates throughshear stresses i n a bonded interface. These leads arejoined t o the copper litz wire transition conductors at the

52 position located above the flywheel extension region(fig. 4).Constraining the armature centrifugal loadingand keeping it in contact with the flywheel duringmotoring are functions of the armature banding. Ahigh strength, high modulus graphite fiber compositetube is used for the banding, which is installed with aheavy interference fit after the armature winding hasbeen epoxy impregn ated. About 5.17 MPa radialinterface pressure between the armature and flywheel ismaintained at full speed t o enhance the bond shearstrength and overcome radially outward EM loading inlocalized area s within the armatu re conductors. Thegraphite banding is axially segmented every 7.6 cm toreduce eddy current losses.

Selectively passive compensation of the rotorarmature is provided by a stationary compensatingwinding located on the bore of a laminated statorstructure. This structure also serves t o support the twopole, solenoidal type field coil and transfer the statordischarge torque t o the casing of the machine.Compensating end plates are attached t o either end ofthe laminated structure with 48 stainless steel tie bolts.Main stator end plates house the shaft radial bearingsand are attached to the stator casing as well. The thrustbearing housing is mounted off the main end plate onone end of the machine and the brush mechanisms areattached to the end plate on the opposite end. Due t o thelength of the brush end rotor shaft, an additional radialbearing is located between the main discharge and thefield-excitation brushes . The machine is mounted withbearings on two fixed pedestals t o allow the stator t orotate approximately 10" against linear dampers duringdischarge to minimize peak torque transmitted t o theskid base.Design of the compulsator stator is driven bymanagement of EM loading arising from field coil self-loading, compensating winding currents, transient

fields generated by rotor armature conductors, and theinteractions between all of these effects. As the rotorrotates during the generation of a pulse, the loading isdistributed differently among stator components. Whenthe armature and compensating windings are aligned(minimum inductance position), the armature currentsare well compensated by image currents in thecompensating winding. Practically all stator dischargeloading is absorbed by the compensating winding at th isposition. A t maximum inductance, when the twowindings are 90" out of phase, the armature fluxproduces eddy currents in all other stator conductivestructures. To minimize field charging time and losses,shielding of the field coil from the armature dischargeflux is not complete. This allows the field coil t o coupleflux from the discharge current and produces additionalfield current. The interaction of the armatu re flux withthe current in the field coil also generates additionalload on th e field coil conductors.

Electromagnetic analysis for the compulsator wasperformed using the CEM-UT developed, three-dimensional, finite element based transient codes.These codes predict flux distributions due to knowncurrents, eddy currents in surrounding structures, andcalculate the forces produced on both conductors andstructures.

The stat or structure must be axially laminated t ominimize the formation of eddy currents duringoperation. Eddy current losses in the stator structureare acceptable with 1.21 mm thick laminates of 30 1stainless steel. The sheets are bonded together with a 0.2mm thick film adhesive and then axially preloadedthrough the compensating end plates with 48 tie bars.To prevent the tie bars from forming conductive loops,they are insulated from the laminations and thecompensating end plates.- Win-

The compensating winding is located as close t othe rotor armature as possible t o provide maximumcoupling and therefore reduce the minimum inductanceof the machine. Bonded onto the bore of the laminatedstato r structure , thi s two-pole winding is composed of 26shorted tur ns per pole. Each conductor is roughly 2.5cm square and made of aluminum litz wire. Imagecurrents flow in the compensating winding only when itcouples armature discharge flux. No coupling existswhen the two windings are 90" out of phase. This can beseen in figure 5which shows representative current vs.time traces for several of the shorted loops in thewinding.field Coil

A solenoidal arrangement of the field coil waschosen to provide a more uniform distribution of radialflux over the voltage generating region of the armature.A total of 5.8-MA.turns provide a 2.4 T radial field a t thearmature conductors. Due to the two-pole geometry, thisfield app ears through the en tire machine andrepresents 40 MJ of inductive energy storage. Thisenergy is recovered back into the rotor after each shot.Resistive energy losses of 6 MJ result in a temperaturerise of the room-temperature coil of only 3C percharging cycle, therefore no cooling is required.




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0 1.0 2.0 3.0 4.0 5.0TIME (ms)

3901 0125

Figure 5. Compe nsating winding curr ent profilesduring a 9 MJ shot

The lightweight aluminum conductors arefabricated by waterjet cutting 1.9 cm thick plates inspiral fashion t o form four turn s. After bendingoperations, the plates ar e stacked and joined to provide70 turns per coil half. Each coil half is then epoxyimpregnated for structural and electrical insulationpurposes. Upon installat ion around the laminatedstator structure, the two coil halves are connectedelectrically in series. Support for the field coil is derivedfrom the laminated stator structure, th e stat or casing,and the main stato r end plate.

Support of the r o t o r is derived from pressurizedhydrostatic oil film radial and thrust bearings.Hydrostatic bearings were selected for their highstiffness and damping characteristics, and becausethese parameters can be easily adjusted as requiredduring initial testing. Supply pressure t o all bearings is20.6 MPa and total flow rate is 9.15 Us. Each of the tworadial bearings provide a radial stiffness of 7 x 109 N/mand a damping rate of 5.65 N . d s at the design speed of8,250 rpm.Due t o the high peripheral speed of the rotor, t hepressure in t he stato r cavity must be reduced t o about 5t o r r to avoid excessive windage loss. Sealing of the statorcavity against the bearing sump region is accomplishedwith three segmented carbon ring seals. The galleriescreated by the three seals are either pressurized withnitrogen gas or actively scavenged. Similar seals areused t o seal inlet and exhaust cooling water for the shafteddy current shields.

BrushMe&jmismSThe main brush mechanisms transfer the 6-mslong, 3.2 MA peak current discharge pulse from theshaft brush slip rings t o the railgun busbars. Eachterminal utilizes 330 brushes which are eachpneumatically actuated onto the slip ring with 65 N.Mechanism design is similar t o that developed for

pulsed homopolar generators, but the short pulseduration creates problems with current distributionalong the l ength of the slip rings. To ensure a uniformcurrent distributor between individual brushes, they a reconnected t o a set of transposed conductors before

making attachment t o the brush housings. Althoughthe integral of current squared and time is well belowdemonstrated levels for this type of brush, the peakcurrent per brush creates higher EM loading on thestraps. A robust str ap design is used t o preventexcessive stresses and deflections from occurrring whenthey ar e carrying the discharge current.Field excitation current i s carried by 21 brushesper terminal or 1.97 kA per brush, since the currentcarrying time is much longer than in the maindischarge circuit. Normal field charging andregeneration takes about 1.6 s; but i n certain shut downmodes where the field circuit is used t o electrically brakethe rotor, these brushes must carry reduced current for45 s.

Podver .. .Prime power for the compulsator will be suppliedby a General Electric LM500 turbine through a speed-increasing gearbox and slip clutch arrangement.During discharge of the compulsator, deceleration of thepower turbine r o t o r is softened by the slip clutch, whichwill be set t o slip below the turbine maximum torquespecification. Backup auxiliary power will also beavailable from power take-off points on the drivegearbox. Primary auxiliary power is supplied by a Prattand Whitney PT6 class gas turbine and reductiongearbox. Compulsator bearing supply and scavengepumps, LM500 hydraulic start system, water coolingpumps, and other miscellaneous systems will be drivenby the PT6 turbine.A lightweight aluminum skid will serve as amount for the turbines, auxiliaries, and fuel and lube oilreservoirs. Onboard control of the system will beaccomplished by a programmable logic controllerlocated on th e auxiliaries skid. This controller will havethe capability t o fully operate the system, but a remotelink for operator override control and d ata transmissionis also provided.

Design of the compulsator for the 9 MJ RangeGun has been delayed significantly by the desire t oprovide the best demonstration of the technology andseveral unforeseen technical requirements. Limitingpeak projectile acceleration has required use of a newcompensation technique. While experiments conductedat CEM-UT and in Culham Labs [23 have shown theconcept of selectively passive compensation o be viable, ithas had major implications on the EM and mechanicaldesign of the machine. Eliminat ion of all cryogenicrequirements was an important step in crediblydemonstrating the compulsator for high energy fielddeployable railguns. While the mass and size of themachine are larger than would be the case with acryogenically cooled version, the present design is asignificant advancement in compact railgun powersupply technology. Presently; this machine is underconstruction and is scheduled for initial testing in late1990.

[l ] J.H. Price, J.L. Bacon, and R.C. Zowarka, Jr.,Lightweigh!, Lar ge Calib er Rai lgu nDevelopment, t o be presented at the 5th EMLConference, April 2-5, 1990, Eglin AFB, Florida.




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[21 C.R. Spi kin gs , "Ex per ime nta l Verification ofSelective Passive compensation," Proceeding of 7thIEEE Pulsed Power Conference, Monterrey, CA,June 1989.

113 W.A. Walls, et al., "Design of a Self-Excited, Air-Core Compulsator for a Skid Mounted, RepetitiveFire 9 M J Railgun System," IEEE Transactions onMarnetics, vol25, no. 1,Janu ary 1989.M.L. Spann, S.B. Pratap, W.A. Walls, and J.D.Herbst, "The Conceptual Design of a LightweightC o m p u l s a t o r - D r i v e n E l e c t r o m a g n e t i cAccelerator," DiTest of Technical Papers , 6th IEEEPulsed Power Conference, Arlington, VA, 1987, pp739-742.

[2]

[3] W.F. Weldon, M.D. Driga, and H.H. Woodson,U.SPatent 4,200,831.[4] M.D. Driga, S.B. Pr ata p, and W.F. Weldon."Design of Compensated Pulsed Alternators withCurrent Waveform Flexibility," Direst of TechnicalPaDers, 6th IEEE Pulsed Power Conference,Arlington, VA, 1987, pp 111-114.

M.L. Spann, S.B. Pratap, W.G. Brinkman, D.E.Perkins, and R.F. Thelen, "A Rapid Fire,Compulsator Driven Railgu n System," I E EETransactions on Magnetics, vol Mag-22, no. 6,November 1986, pp 1753-1756.

[5]

[6] ,M.D. We rs t, D.E. Pe rk ins, S.B. Pratap, M.L.Spann, and R.F. Thelen, "Testing of a Rapid FireCompensated Pulsed Alternator System," IEEETransactions on Magnetics, vol 25, no. 1, January1989, pp 599-604W.A. Walls, and S.M. Manifold, "Applications ofLightweight Composite Materials in PulsedRotating Electrical Generators," DiPest o fTechnical PaDers, 6th IEEE Pulsed PowerConference, Arling ton, VA, 1987, pp 103-106.M.D. Driga , S.B. Pratap , and W.F. Weldon, "NewTrends in Compulsator Design," I E E ETransactions on Mametics. vol 25, no. 1, January1989, pp 142-146.M.L. Spann, W.L. Bird, W.F. Weldon, and H.H.Woodson, "Detailed Design, Assembly, and Testingof an Active Rotary Flux Compressor," presentedat the 3rd IEEE Pulsed Power Conference,Albuquerque, NM, Jun e 1-3, 1981.

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