Electromagnetic Thrust Bearing for a Homopolar Machine: Experimental Implementation

Development of large, inertial pulsed power supplies is essential to the success of the controlled thermonuclear 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 report discusses the experimental implementation of an electromagnetic thrust bearing on the 0.5MJ homopolar machine presently in operation at The University of Texas at Austin. A control system describing the electromagnetic thrust bearing is formulated and the system stability is determined. The circuitry is then described which produces the required differential field current and monitors the position of the rotor. The latter is done by a position sensing unit consisting of an LED and a phototransistor. The former is done with an operational amplifier circuit that generates equal and opposite voltages which drive the two SCR field supplies in a manner to create the required differential current. For a static rotor, test results show that a restoring force is generated and that the rotor can be stabilized against the axial forces.