More-Electric Gas Turbine Engines

A new NASA Lewis Research Center and U.S. Army Research Laboratory (ARL) thrust, the more-electric commercial engine, is creating significant interest in industry. This engine would have an integral starter-generator on the gas generator shaft and would be fully supported by magnetic bearings. The NASA/Army emphasis is on a high-temperature magnetic bearing for future gas turbine engines. Magnetic bearings could increase the reliability and reduce the weight of such engines by eliminating the lubrication system. They could also increase the DN (diameter of the bearing times the rpm) limit on engine speed and allow active vibration cancellation systems to be used, resulting in a more efficient, more-electric engine.

The magnetic bearing, which is similar to an electric motor, has a laminated rotor and stator made of cobalt steel. Electrical wire coils wound around the stator form a series of electromagnets around the circumference that exert a force on the rotor. A probe senses the position of the rotor, and a feedback controller keeps it in the center of the cavity. The engine rotor, bearings, and case form a flexible structure with a large number of modes. The bearing feedback controller must be designed so that none of these modes become unstable. This controller could also be adapted to varying flight conditions. Lastly, this controller could be made to monitor the health of the system so that seal rubs could be avoided.

The cobalt steel used in this magnetic bearing has a curie point greater than 1700 °F, and the copper wire has a melting point beyond that. Practical limitations associated with the maximum magnetic field strength in cobalt steel and the stress in the rotating components limit the temperature to about 1200 °F.

The objective of this effort is to determine the limits in temperature and speed of a magnetic bearing operating in an engine environment. Our approach is to use our in-house experience in magnets, mechanical components, high-temperature materials, and surface lubrication to build and test a magnetic bearing in both a rig and an engine test either at Lewis or through cooperative programs in industrial facilities.

Last year, we made significant advances, and additional work is planned. We procured and tested a high-temperature capacitive displacement probe to 1200 °F. We built a high-temperature homopolar magnetic bearing. Our flexible casing rig is being converted to a high-temperature magnetic bearing rig, and testing should start next year. We plan to develop a high-temperature, compact wire insulation and to fiber reinforce the core lamination to operate at higher temperature and high speeds. In addition, we plan to modify our stability analysis and controller theory by including a nonlinear magnetic bearing model. We are developing a controller that has an expert system that can adapt the controller to changing flight conditions and diagnose the health of the system. Then, we will demonstrate the bearing on our rotordynamics rig and, finally, on an engine.

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Last updated April 30, 1997

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