Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-08-24
2003-08-19
Waks, Joseph (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06608418
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to turbo-generators, and more particularly to bearing support and control systems for permanent magnet generator based turbo-generators.
BACKGROUND OF THE INVENTION
In airborne applications that require the generation of both cool and conditioned pressurized air as well as the generation of electric power, the use of a turbo-generator driven by engine bleed air provides a compact, contained system that does not adversely affect the performance of the aircraft as compared to, e.g., other generator systems. In a turbo-generator, a shaft is rotatably supported in a housing and serves to connect a permanent magnet generator and a turbine. The pressurized supply air is supplied to and expanded in the turbine where it achieves a very low temperature in order to provide cooling. The compressed air acting on the turbine rotates the shaft which, in turn, drives the permanent magnet generator (PMG). The rotation of the PMG rotor generates a rotating magnetic field in the stator windings. This produces an electric power output whose voltage and frequency are directly related to the speed of rotation of the rotor.
To support the shaft which connects the PMG and the turbine, a turbo-generator typically employs three bearings. Two of the bearings are radial bearings which prevent the shaft from shifting radially. The third bearing is a thrust bearing which holds the shaft in a fixed axial position. If the bearings permit more than just slight amounts of free play, the shaft will shift under influence of the strong permanent magnets of the generator rotor or when the turbine is loaded and will allow the rotor or the blade tips to contact the encircling housing.
In prior turbo-generators, bearings, e.g. ball bearings, hydrodynamic fluid film bearings (commonly called air bearings), etc., have been used to locate the shaft radially and axially since such bearings provide minimal free play. Air bearings, however, require extremely small clearances, require supply air for cooling, and are slightly damaged at each start up and shut down as a result of lack of support at low speeds. Because of the small clearance in the air bearings, dirt or other combination increases the susceptibility for damage and, in addition, such clearances result in relatively high friction and heat. As a result, bearing replacement is a frequent cost item and, if failure occurs during operation, the PMG and/or turbine may be damaged. A shaft which is supported by air bearings must be removed axially from the housing for repair or replacement and this usually requires that the turbo-generator be removed from the aircraft and sent to a remote repair depot.
Magnetic bearings are used for supporting shafts in various types of machinery. In radial magnetic bearings, several electromagnets are spaced angularly around a shaft and, when energized, produce opposing magnetic forces which cause the shaft to levitate in free space within the housing. Sensors detect the actual position of the shaft and vary the energization of the electromagnets in such a manner as to keep the shaft centered precisely on a predetermined axis. One such system is described in U.S. Pat. No. 5,310,311, entitled Air Cycle Machine With Magnetic Bearings, assigned to the assignee of the present application, the teachings and disclosure of which are hereby incorporated in their entireties by reference thereto. While the initial cost of a magnetic bearing system may be somewhat higher than that of an air bearing system, magnetic bearings permit more easily attainable machining tolerances and larger clearances, require no air for cooling, experience a relatively long service life, and are capable of supporting shafts either at rest or operating at speeds of 100,000 RPM or higher.
Despite the clear advantages provided by magnetic bearings, their use has been precluded for PMG-based turbo-generators designed for airborne operation. In these applications, and particularly in fighter aircraft operation, the turbo-generator assembly is highly compact and operates at high rotational speeds in an effort to reduce size and weight. Further, the permanent magnets used in the PMG have a high magnetic density to enable the maximum electrical output generation during operation with minimal weight. As a result of these factors, the environment within the turbo-generator contains a high degree of magnetic flux. Because the control of magnetic bearings relies on precise magnetic flux variation to correct the smallest shaft position variation, use of these devices in the highly magnetic environment of a PMG-based turbo-generator has been precluded. Such is particularly true for fighter aircraft applications in which extremely high dynamic forces caused by aircraft maneuvers are typical.
BRIEF SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a new and improved turbo-generator machine which utilizes magnetic bearings to precisely support the PMG/turbine shaft for rotation at high speeds in a high vibration, high shock, and high temperature environment such as typically exists in a jet aircraft.
A further object of the invention is to provide a turbo-generator having magnetic bearings and a housing which are uniquely assembled as a clamshell structure permitting relatively quick and easy removal of the shaft from the bearings and the housing for purposes of repairing or replacing the shaft and/or other components of the turbo-generator.
The invention also resides in the use of magnetic force for biasing the shaft radially in opposition to the side forces created by the magnetic flux acting between the permanent magnets and the stator of the PMG. This is aided by the coordination of design parameters such as the ratio between the magnetic bearing air gap and the PMG rotor/stator air gap. Further, precise control of the magnetic bearings is enabled by minimizing the axial magnetic leakage flux from the PMG that is allowed to disrupt the magnetic bearing control and sensing. In one embodiment this is accomplished by controlling the ratio between axial and radial cross section of the PMG stator and the shaft. Further, the system of the present invention allows the magnetic center of the PMG to float within the stator housing, i.e. no mechanical centering is required.
In one embodiment the turbo-generator is a radial inflow turbine with a permanent magnet generator. The turbine and generator are mounted on a single shaft that is supported by magnetic bearings. The turbo-generator includes rotor containment for the high-energy rotors. The magnetic bearing center section uses an axial split feature to allow replacement of the shaft and wheel assembly using common hand tools, thereby providing for high maintainability. This dramatically reduces overhaul time if service is required. The turbo-generator may be used in an exemplary system that provides both cold air for a radar poly alpha olephin (PAO) cooling loop and electrical power for the radar. The system is self-contained except for the bleed air connection, a start-up/shut-down electric power connection and an air exhaust.
In one embodiment of the present invention, a turbo-generator for an aircraft comprises a housing and a shaft disposed in the housing and having a central axis. A permanent magnet generator is mounted on the shaft. Further, a turbine mounted on the shaft in axially spaced relation with the permanent magnet generator. This turbine includes an inlet for receiving gas from an aircraft engine, and is driven by the gas to drive the permanent magnet generator via the shaft. The gas received by the turbine expands therein, is cooled as a result of expanding, and is exhausted as chilled gas by the turbine. Axially spaced bearings for radially supporting the shaft for rotation in the housing are also included. Each of the bearings comprises a number of electromagnets mounted within the housing and spaced angularly around the shaft. The electromagnets include selectively energizable electrical coils for producing magnet
Andres Mike
Coons Terry
Temple Terry
Leydig , Voit & Mayer, Ltd.
Smiths Aerospace, Inc.
Waks Joseph
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