Showing papers by "Gerald V. Brown published in 1992"

Performance tests of a cryogenic hybrid magnetic bearing for turbopumps

Abstract: Experiments were performed on a Hybrid Magnetic Bearing designed for cryogenic applications such as turbopumps. This bearing is considerably smaller and lighter than conventional magnetic bearings and is more efficient because it uses a permanent magnet to provide a bias flux. The tests were performed in a test rig that used liquid nitrogen to simulate cryogenic turbopump temperatures. The bearing was tested at room temperature and at liquid nitrogen temperature (-320 F). The maximum speed for the test rig was 14000 rpm. For a magnetic bearing stiffness of 20000 lb/in, the flexible rotor had two critical speeds. A static (nonrotating) bearing stiffness of 85000 lb/in was achieved. Magnetic bearing stiffness, permanent magnet stiffness, actuator gain, and actuator force interaction between two axes were evaluated, and controller/power amplifier characteristics were determined. The tests revealed that it is feasible to use this bearing in the cryogenic environment and to control the rotor dynamics of flexible rotors when passing through bending critical speeds. The tests also revealed that more effort should be placed on enhancing the controller to achieve higher bearing stiffness and on developing displacement sensors that reduce drift caused by temperature and reduce sensor electrical noise.

Force analysis of magnetic bearings with power-saving controls

Abstract: Most magnetic bearing control schemes use a bias current with a superimposed control current to linearize the relationship between the control current and the force it delivers For most operating conditions, the existence of the bias current requires more power than alternative methods that do not use conventional bias Two such methods are examined which diminish or eliminate bias current In the typical bias control scheme it is found that for a harmonic control force command into a voltage limited transconductance amplifier, the desired force output is obtained only up to certain combinations of force amplitude and frequency Above these values, the force amplitude is reduced and a phase lag occurs The power saving alternative control schemes typically exhibit such deficiencies at even lower command frequencies and amplitudes To assess the severity of these effects, a time history analysis of the force output is performed for the bias method and the alternative methods Results of the analysis show that the alternative approaches may be viable The various control methods examined were mathematically modeled using nondimensionalized variables to facilitate comparison of the various methods