Design of a 20 GW, Self-Excited, Air-Core Compensated Pulsed Alternator Railgun Power Supply
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A compact compensated pulsed alternator (compulsator) has been designed as a power supply for a 9 MJ kinetic energy railgun in conjunction with the Electromagnetic Gun Weapons System Program sponsored by the U.S. Army and the Defense Advanced Research Projects Agency. The prototype machine, which is presently in fabrication, is to operate at 8,600 rpm and has design ratings of 3.2 MA peak current and 20 GW peak power into the matched railgun load. Rotor kinetic energy at full speed is 210 MJ of which 30 MJ is delivered during the 6 ms discharge pulse. A two-pole configuration is used for pulse length considerations and selectively passive compensation is employed to produce a relatively flat pulse as required by the railgun to limit maximum projectile acceleration. Distinguishing features include an air core magnetic circuit, separate rotor armature windings for self-excitation and railgun firing, ambient temperature aluminum field coils, and excitation field magnetic energy recovery capability. The rotor is made by interference assembly of fiber reinforced epoxy composite rings and is supported by high strength metal stub shafts which are shielded from the excitation field by water cooled copper sleeves. Evacuation of the rotor cavity is required to reduce windage losses since the peripheral velocity of the rotor is about 500 m/s at full speed. Both rotor armature windings are formed from aluminum litz wire to minimize conductor mass and eddy current losses. A stationary compensating winding, used to control the internal impedance of the generator during a discharge, is supported by a laminated stainless steel stator structure to which the ambient temperature aluminum field coils are attached. Hydrostatic oil film bearings support the rotor and are sealed against the evacuated rotor cavity with segmented carbon ring seals. Separate brush mechanisms are employed to collect excitation and railgun currents. A unique transposition of brushring conductors in the main discharge circuit is used to ensure uniform current division between brushes. Self-excitation to a nominal 2.4 T air gap flux density is accomplished in the 800 ms prior to discharge by rectifying the 17 kV ac excitation armature output with a combination of series and parallel diodes. After the discharge, the diodes are triggered to regenerate the magnetic energy stored by the field coils back into the rotor. The compulsator mounts are designed to allow the machine to rotate against linear dampers during discharge to minimize peak torque transmission to the machine mount.