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Abstract: The Center for Electromechanics at The University of Texas at Austin (CEM-UT) has been involved in the development of rotating electrical machines ...
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William F. Weldon JackS. Josey Professor of Energy Studies Director, Center for Electromechanics The University of Texas at Austin Center for Electromechanics Balcones Research Center Austin, Texas 78712
Abstract: The Center for Electromechanics at The University of Texas at Austin (CEM-UT) has been involved in the development of rotating electrical machines for pulsed power applications since 1973 and in the development of electromagnetic launch technology since 1979. The CEM-UT single shot lab- oratory gun, a 90 mm round-bore railgun 10 m long, is reviewed. The extensive operating experience with the ferromagnetic compulsator built at CEM-UT in the mid-1980's is reviewed and two new com- pulsator projects are described. Finally, future trends in the evolution of power supplies for electro- magnetic railguilS are surveyed.
INTRODUCTION The Center for Electromechanics at The University of Texas at Austin has been involved in the development of specialized rotating electrical machines utilizing inertial energy storage for pulsed power applications since 1972 and in the development of electromagnetic (EM) launch technology since
THE ELECTROMAGNETIC RAILGUN The simplest of the EM launchers, the railgun, has enjoyed the greatest popularity and suc- ce s s, at least partially because of the availability of pulsed power supplies well suited to the nature of this dynamic load. The force produced on the armature is given by the well-known expression
where F = force in newtons L' = inductance gradient along the gun, dUdx I = current in the gun
The voltage required to drive the current in the gun is approximately
where
V = IR'x + VA + IL'u
dissipative speed elements voltage
(1)
envisioned as shown in figure 2, by considering that the path of the current through the rails and arma- ture results in an elevated magnetic field in the bore of the railgun which in turn results in an effective "magnetic pressure" acting outward on the current carrying elements. It is the action of this magnetic pressure on the armature that produces the desired acceleration. Although the armature area over which this pressure acts is constant throughout the launch, the area of rails subjected to high magnetic pressure constantly increases as the armature moves down the gun.
Figure 2. Magnetic pressure in bore of rail gun
If the railgun structure is not sufficiently stiff, the rails will move outward under this loading. While this outward rail motion obviously causes mechanical problems such as the fit of the projectile in the bore, a less obvious problem is that it presents an additional load on the power supply. Lateral movement of the rails results in an additional factor being introduced into equations (2) and (3), a transverse L' or dUdy designated as L'T in the modified power equation below.
P =I
R'x +IVA + I^2 L'ux + I^2 Uruy
where uX = velocity along the bore of the rail gun Uy = transverse separation velocity ofthe railgun rails
Of course, this additional power requirement does not produce useful or desirable work and, unfortu- nately, becomes a maximum at the same time the other power requirements are a maximum, thus increasing the peak power required of the power supply. CEM-UT has pioneered the development of stiff railgun structures designed to minimize trans- verse rail movement.[15] Past stiff railgun designs, although effective research tools, have been too bulky and heavy for practical field use (fig. 3).[16] Recent developments at CEM-UT have produced lightweight, stiffrailguns with exceptionally high values ofL' (fig. 4). POWER SUPPLY OPTIONS The short duration, but extremely high, power requirements of railguns make pulsed power supplies (which store energy at low power levels between shots, then deliver it at high power during the acceleration of a projectile) attractive. Effective energy storage mechanisms include inertial, magnetic, electrostatic, and electrochemical. Each method offers certain advantages and commonly used figures of merit include stored energy density and delivered power density. Table 1 compares the present state of the art for pulsed power supply technology. In practice, only the CPA is suited to directly driving the railgun as witnessed by the fact that only its optimal discharge time (per pulse energy density/power density) matches the pulse width requirement of a railgun. Homopolar generators and batteries develop insufficient power to directly drive a railgun and are typically used to charge an intermediate inductive store which, through an opening switch, develops the power required to drive the railgun. The energy decay time (UR time con- stant) for a typical inductive energy store is usually short; therefore, these devices are typically used only as intermediate energy storage devices.
BREECH CONNECTION
RAIL-INSULATOR ASSEMBLY
ALUMINA DISK
REC04LIIAHAGEMENT PLATE
STEEL STRUCTURAL SHELL
STEELUNER
1001 .0264.
Figure 3. 90 mm bore x 10 m long hydraulically preloaded railgun
Figure 4. Lightweight, high performance railgun
\
Figure 5. 9 MJ single shot EM gun system
(^0) <D <'i <'i
I() (^0) C\i (^) <'i - ,..... *I' 'f* "" (^) "" II '', \
~N-.. (^) ~C\i ''J^ \
::::.. (^) '
- ~ 1- ' ' z ~ '
(3 (^) a: (^0) ...J a:^ ~~, w ::I^ (j~' ~ (.)^ "! '9~,
(^) 1\t)'........ '
I() <D ci d
0.0 0.75 1.5 2.25 3.0 3.75 4.5 5.25 6. TIME (ms)
Figure 6. 8.1 MJ muzzle energy railgun shot- current and velocity profiles
The 9 MJ Single Shot Laboratory Gun facility was originally used to develop solid armatures for tactical applications, but is now being used to launch successively more complex integrated penetra- tor, sabot , armature packages. The highest launch energy achieved to date is 8.1 MJ (2.4 kg at 2.6 km/s) and launches are routinely being made in the 4 to 6 MJ range as required for the projectile development program.
fERROMAQNETIC COMPJJLSATORS A passive, ferromagnetic CPA completed in 1986 stores 38 MJ and is designed to deliver 1.2 MJ/pulse at 2,000 V and 950 kA (fig. 7). Operating at 240 Hz, the machine stores sufficient energy to produce a burst of ten pulses. It was originally designed to accelerate 80-g projectiles to 2 km/s at a ra te of fire of 60Hz in a 20-mm railgun. After exceeding these goals, it has found a variety of applica· tions as a versatile and reliable laboratory power supply. Most recently, it has been used to test high current solid state switches and to fire three-shot bursts in a 15 mm bore railgun at 10Hz.
Although ferromagnetic CPAs are heavier than the composite CPAs presently being built, they are quite attractive as low maintenance, highly versatile power supplies for fixed applications. An example of such an application is a system of four passive, ferromagnetic CPAs inv~tigated by CEM - UT for the RARDE EM Gun Range at Kirkcudbright. These machines, each 1.4 ni diameter. x 1.5 m long (slightly larger than the CPA in fig. 7), would produce the desired muzzle energy in an 8 m railgun barrel over the full performance range as shown in figure Sa. Furthermore, a single unit would be an attractive railgun power supply in its own right, producing muzzle energies well in excess of 2 MJ over an equally wide performance range (fig. 8b).
ADVANCED
! ~::l ~ 0 E 0 ~ u a: w ~0..
30 ,
i '!! ~ ~u ffi,
HAflOISON MISSION CO. HARDISON MISSION CO.
Figure 9. Hardison study -- mass and volume of advanced compulsators compared to capacitors
1001 .06.
Two advanced CPAs are presently under development at CEM-UT. The smaller machine is intended to drive a small caliber machine gun firing 32-g projectiles at 2 krn/s at a 10 Hz rate of fire [20] (fig. 10). The project weight goal of 1,000 kg has been beaten with the CPA weight of 750 kg. The CPA stores 8 MJ, delivers a peak power of 700 MW and incorporates such advanced features as self- excitation, ceramic shaft and bearings, all composite rotor, and lightweight, filament wound, aug- mented railgun. System testing is scheduled for spring of 1991.
The larger advanced CPA presently nearing completion is the power supply for the 9 MJ range gun (fig. 11) intended to produce 9 MJ of muzzle energy at velocities ranging from 2.5 to 4.0 km/s with a three shot per minute repetition rate from a stand-alone skid [21]. This CPA also makes extensive use of composite materials, is self-excited, and has provisions to reclaim the excitation energy after each pulse. This provision substantially increases the per-shot efficiency to about 33%. The machine shown in figure 12 stores 210 MJ and delivers up to 36 MJ per pulse at 5.8 kV and 3.2 MA. Two shots can be extracted from the stored inertial energy while power for the continuous firing requirement is provided by a General Electrical LM-500 gas turbine.
@ compulsator ® launcher ® SCR switch ® power train
@skid
@ launcher support frame ® torque management system
Figure 10. Small caliber compulsator powered railgun
Figure 12. 9 MJ range gun compulsator
For mobile applications, dual CPAs with counterrotating rotors or a single CPA with counterro- tating elements is required to eliminate reaction torque on the vehicle. While both of these configura- tions have been explored for HPGs, they have not yet been applied to CPAs. Work must continue to reduce required auxiliaries. Ceramic rolling element bearings being demonstrated on the small caliber CPA presently represent the limit of the state of the art. Larger ceramic bearings must be developed. It is believed that presently identified technology can provide size reductions between 2.5 and 4.0 com- pared to the advanced CPAs presently nearing completion.
Finally, as impressive as the performance of current and next generation railguns is, the prob- lem of transferring enormous currents across the gun bore/armature interface will continue to limit the longevity of such guns. The prospect of noncontacting EM launchers will continue to become more attractive as we learn more about them. CEM-UT is involved in optimizing the geometrical and elec- tromechanical parameters between the armature and stator of coaxial induction accelerators [23] and in developing the highly specialized power supplies required to make such accelerators practical [24].
REFERENCES [1] H. G. Rylander, H. H. Woodson, E. B. Becker, and R. E. Rowberg, "Homopolar Motor Generator Designs for Cheap Inertial Energy Storage," Conference on Technology of Controlled Thermonuclear Fusion Experiments and the Engineering Aspects 9f Fusion Reactors, Austin, TX, November 20-22, 1972. [2] W. F. Weldon, M. D. Driga, H. H. Woodson, and H. G. Rylander, "The Design, Fabrication, and Testing of a Five Megajoule Homopolar Motor-Generator," International Conference on Energy Storage, Compression, and Switching, Torino, Italy, November 5-7, 1974. [3] T. M. Bullion, M. D. Driga, J. H. Gully, H. G. Rylander, K M. Tolk, W. F. Weldon, H. H. Woodson, and R. C. Zowarka, Jr., "Testing and Analysis of a Fast Discharge Homopolar Machine (FOX)," Second IEEE International Pulsed Power Conference, Lubbock, TX, June 12-14, 1979. [4] T. M. Bullion, R. C. Zowarka, Jr., T. A. Aanstoos, W. F. Weldon, H. G. Rylander, and H. H. Woodson, "Five-Megajoule Homopolar Upgrade," Third IEEE International Pulsed Power Conference, Albuquerque, NM, June 1-3, 1981. [5] J. H. Gully, E. G. Estes, W. A. Walls, and W. F. Weldon, "Compact Homopolar Generator Development at CEM-UT," IEEE Transactions on Ma~roetics, Vol. Mag-20, No. 2, pp 203-206, March 1984.
