FDX--A Fast Discharge Homopolar Generator
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During the winter of 1973 and spring of 1974 a study was undertaken by the Energy Storage Group* of The University of Texas at Austin to determine the fundamental limitations to the discharge times of homopolar generators. The limit was ultimately characterized as the magnetic diffusion time for the rotating and stationary conductors in the generator. This study was initially funded by the Texas Atomic Energy Research Foundation (TAERF) and later by the Electric Power Research Institute (EPRI). As a result of the study, a Fast Discharge Experiment (FDX) was proposed and funded first by EPRI and later by the U.S. Department of Energy (DOE). FDX is a small (365 kJ), counterrotating disk type homopolar generator designed to explore the limits to homopolar generator discharge times. The initial discharge experiments began in the fall of 1977. The FDX rotors are forged aluminum alloy with flame sprayed copper slip rings. Solid copper graphite brushes are used with a 95% packing factor on the slip rings. The high magnetic field required for fast discharge (3.6 T average) is provided by discharging the CEM 5.0 MJ homopolar generator into a four-tum, graphite-reinforced, room temperature copper coil. Since the field is pulsed and FDX rotors cannot be self-motored, they are brought up to speed with two 37 kW air turbines. The two aluminum rotors are 30 cm in diameter and of a rimmed, modified constant stress configuration. They are designed for a maximum operating speed of 28,000 r/min at which point they each store 182.5 kJ and develop 104 V. The aluminum discharge coax is approximately 38 cm in diameter and is designed to carry the 1.88 MA anticipated from a half speed (14,000 r/min) short circuit discharge which would stop the rotors in 1.0 ms. It is predicted that the machine will ring on its own internal impedance for approximately five cycles in this mode. The discharge coax is shorted by four very fast making switchea described in a companion paper. Additional impedance can be introduced into the discharge circuit by extending the switch coaxes to allow full speed1.4 MA discharges in approximately 3.5 ms.