HIGH TEMPERATURE MAGNETIC THRUST BEARING: THEORY AND EXPERIMENT

Active magnetic bearings (AMBs) are being increasingly utilized in industrial applications due to their advantages over conventional bearings. They offer very low friction and wear, variable stiffness and damping, and greater tolerance of rotating mass imbalance. These unique features of AMBs have enabled design of robust rotating machinery at much higher speed with higher power concentration. The present work discusses the design of a high temperature magnetic bearing for operation at an axial thrust load of 4448N, speed 20000 rpm and temperature 538 C. Various disk profiles were considered to lower peak stresses due to centrifugal forces, including uniform (rectangular), linear tapered and hyperbolic. The predictions showed that the hyperbolic profile reduced stresses by 60% compared to the rectangular profile enabling rotor disks to operate at much higher speed. A test bearing was built with the hyperbolic disc profile. An Iron-Cobalt alloy, commercially known as Hyperco 27 was utilized for the thrust disc for its high yield strength 570MPa, high saturation flux density of 2.35T and high resistivity of 250 mu Omega-mm. Hyperco 50A was selected for the bearing stator, due to the lower load requirement and cost. Magnetic circuit design assumptions for the axial thrust AMB included (1) relative permeability of the magnetic material was nearly infinite, (2) fringing at gap edges as well as leakage flux were negligible, and (3) the field within the circuit was homogeneous. The initial circuit design was improved using finite element magnetic field analysis. The effective force acting on the hyperbolic rotor determined the required number of turns and current for the electromagnetic coils. Extensive structural finite element analyses suggested not to use an interference fit of the attached disk with the shaft. Rather, it was decided to utilize a sleeve and lock-nut mechanism. Inconel 718 was used for the shaft due to its slightly higher thermal expansion coefficient than Hyperco 27. The thrust AMB containment vessel included thermally-insulated radial and axial adjustment bolts to position and align the AMB inside the vessel. The AMB rotating assembly was spun using an electric motor. The magnetic force generated by the AMB at room temperature was similar to its predicted value, with a 0.85 derating factor. The magnetic force was temperature dependent and was reduced to 65% of its room temperature value, at 538 degrees C. The maximum operating speed reached thus far in the experimental study was 5000 rpm. The magnetic bearing force was nearly invariant with rotational speed at any given temperature (e.g., room and high), while the electric current was held constant. The design indicates that the novel magnetic thrust bearing should perform well at the target operating conditions of 4448N axial load at 538 degrees C (1000 lb-f at 1000 degrees F), and 20,000 rpm. This has been achieved thus far only up to 5000 rpm. The force appears to be very insensitive to motion induced eddy currents up to the present maximum speed of 5000 rpm. Future work will focus on reaching the full speed target of 20,000 rpm at 538 degrees C and 4,448 N loading.