Electromagnetic Thrust Bearing for a Homopolar Machine--Theoretical Analysis
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Development of large, inertial pulsed power supplies is essential to the success of the controlled thennonuclear research program. A very attractive candidate for this supply is the homopolar motorgenerator which can produce the large current pulses (~lOOKA) necessary for the confinement and compression magnetic fields needed on the large fusion feasibility experiments. For a radial current flow, axial magnetic field configuration, the machine rotor is unstable to movement in the axial direction. Therefore a large mechanical thrust bearing is normally used which adds to the frictional losses and reduces the machine's efficiency. However, these losses could be reduced substantially by replacing the mechanical bearing with an electromagnetic thrust bearing. This is based on the principle of controlling the axial motion of the rotor by differential control of the two halves of the field. This report describes the theoretical analysis of such a system based on Maxwell's equations and the co-energy principle. The static force acting on the rotor and the inductance of the field coil are calculated as a function of the field current, the rotor displacement from magnetic center, and the differential current. Additionally the equation of motion of the rotor including the feedback control system which drives the differential current in the field coils is calculated. Results are obtained for two sets of initial conditions: 1) the rotor is initially displaced and 2) an initial impulse- like force is applied to the rotor in the axial direction. It is shown that for initial displacements less than lo- 3m and/or impulse forces less than 3Nt-sec stability can be achieved for the 0.5MJ homopolar machine presently in operation.