Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-10-27
2001-02-20
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S090500, C310S06800R, C310S051000, C318S615000, C318S606000, C318S623000, 36, 36
Reexamination Certificate
active
06191513
ABSTRACT:
The present invention relates generally to magnetic bearings. Examples of magnetic bearings are disclosed in prior U.S. Pat. Nos. 5,084,643; 5,202,824; and 5,666,014 to Chen (one of the inventors), which are incorporated herein by reference.
To meet stability, control, and energy storage requirements for the next generation of satellites, combined momentum wheel and energy storage devices that are both efficient and compact are considered to be required. Since angular momentum is the product of wheel polar moment of inertia and rotational speed, a reduction in the wheel polar moment of inertia (i.e., the momentum wheel size and mass) must be countered by an increase in the speed if comparable reaction torques are to be provided for attitude control as well as energy storage. Since the momentum wheel mass is approximately proportional to the wheel diameter and the wheel polar moment of inertia is proportional to the diameter cubed, if the speed is increased by a factor of 10, the momentum wheel rim average diameter can be reduced by a factor of 2.15 (the cube root of 10), i.e., the wheel mass may be reduced by slightly more than half. Momentum wheel speeds using conventional ball bearings are limited to about 5,000 or 6,000 rpm for a life of about 10 years. Increasing the speed by a factor of 10 to about 50,000 to 60,000 rpm presents a challenge on bearing design. While improvements in bearing ceramic materials and lubricants should extend the potential operating speed range and life, extension of ball bearing capabilities to such high speeds is considered to be clearly beyond the capability of today's ball bearings, leaving magnetic bearings as the only currently viable alternative.
Active magnetic bearings have conventionally had stationary electromagnetic poles around the rotor. In rotation, the rotor surface material moves in and out of the magnetic flux of the protruding poles. The changing flux in the rotor surface material as it undergoes many fluctuations per revolution generates heat due to magnetic hysteresis and eddy currents. The eddy currents so generated not only cause power loss in the form of heat but also delay the control response of the electromagnets. To reduce the eddy current effect, the bearing cores have usually been made of silicon steel laminations. The eddy current heat generation when high speed rotors for satellites or other devices are operated in a vacuum can be a serious problem because it is difficult to dissipate the heat in a vacuum. This is because any heat generated on a rotor in a vacuum has to rely on thermal radiation for dissipation, which is ineffective until the rotor temperature reaches a level that may be detrimental to the momentum wheel material, or other thermal growth related problems may occur. Using permanent magnet biased homopolar active magnetic bearings which have extended pole edges in the circumferential direction can reduce the losses through reduced numbers of poles and the magnitude of the flux variations but do not completely solve the eddy current heating problem. The magnetic flux fluctuation frequency in high speed rotors is very high, resulting in high heat generation since these losses are proportional to frequency (speed) squared. This was especially evident in difficulties encountered with a magnetically suspended rotor system which was designed to operate to 75,000 rpm with homopolar bearings but was limited to approximately 20,000 rpm due to eddy current drag, as discussed in J. Kirk, “Performance of a Magnetically Suspended Flywheel Energy Storage System”, Proc. 4th International Symposium on Magnetic Bearings, Zurich, Switzerland, 1994, pp 547-552.
In order to solve the above described problems of heat generated on a high-speed flywheel rotor, i.e., for momentum wheel and energy storage, operating in a vacuum, it has been suggested that continuous ring pole permanent magnetic bearings be used. Since the magnetic flux of ring shaped poles is not disrupted during rotation, the hysteresis and eddy current core losses can be kept to a minimum. Two radial permanent magnet ring type bearings have been suggested for a flywheel energy storage power quality application, as J. Walton and H. Chen (two of the joint inventors of the present invention) have discussed in “Novel Magnetic Bearings for a Flywheel Energy Storage System”, presented at ISROMAC-6, Honolulu, Feb. 25-29, 1996. However, these bearings have stationary and rotating disks packed with many axially polarized permanent magnet rings; they are expensive to fabricate and have centrifugal stress concern at high speeds; they are not adequately stable; and their large axial negative stiffnesses require oversized active thrust magnetic bearings.
Another paper which may be of interest is “Magnetic Suspension System with Permanent Magnet Motion Control” by Oka, K. and Higuchi, T., Proc. 4th Int'l Symp. on Mag. Brg's, pp 317-320, 1994.
It is accordingly an object of the present invention to provide a stable magnetic bearing with minimum eddy current and hysteresis core losses and which is inexpensive to fabricate.
It is another object of the present invention to provide such a bearing which would allow gimbaling of a flywheel shaft for minute angular momentum corrections of a spacecraft such as a communications satellite.
It is a further object of the present invention to provide such a bearing which is compact axially for use for flywheels.
It is still another object of the present invention to provide such a bearing for use as a journal bearing but which also has axial bearing stiffness so that a separate thrust bearing is not required.
In order to provide such a stable magnetic bearing, in accordance with the present invention, the magnetic bearing is provided with a stator which magnetically interacts with the rotor and which is movable in response to feed-back of rotor position to utilize the magnetic interaction to effect movement of the rotor toward a predetermined rotor position. In order to provide a uniform magnetic field and thereby minimize eddy current and hysteresis core losses, the magnet is ring-shaped.
The above and other objects, features, and advantages of the present invention will be apparent in the following detailed description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawings wherein the same reference numerals denote the same or similar parts throughout the several views.
REFERENCES:
patent: 5084643 (1992-01-01), Chen
patent: 5202824 (1993-04-01), Chen
patent: 5666014 (1997-09-01), Chen
patent: 5752774 (1998-05-01), Heshmat et al.
patent: 5760510 (1998-06-01), Nomura et al.
patent: 5821656 (1998-10-01), Colby et al.
patent: 5825105 (1998-10-01), Barber et al.
patent: 5834867 (1998-11-01), Kikuchi et al.
patent: 5844339 (1998-12-01), Schroeder et al.
patent: 5856719 (1999-01-01), De Armas
patent: 5880546 (1999-03-01), Marroux et al.
C. Henrikson et al, “Magnetically Suspended Momentum Wheels for Spacecraft Stabilization,” AIAA Paper No. 74-128, AIAA 12thAerospace Sciences Meeting, Washintgon, D.C., 1974.
J. Kirk et al, “Performance of a Magnetically Suspended Flywheel Energy Storage System,” Fourth International Symposium on Magnetic Bearings, Zurich, Sw, 1994, pp 547-552.
H. Chen et al, “Novel Magnetic Bearings for a Flywheel Energy Storage System,” ISROMAC-6, Honolulu, 1996.
K. Oka et al, “Magnetic Suspension System with Permanent Magnet Motion Control,” Fourth International Symposium on Magnetic Bearings, Zurich, Sw., 1994, pp 131-137.
J. Walowit et al, “Analytical and Experimental Investigation of Magnetic Support Systems. Part I: Analysis,”ASME J. of Lubrication Technology, vol. 104, 1962, pp. 418-428.
K. Astrom et al, Chap. 13, “Perspectives on Adaptive Control,”Adaptive Control, Addison-Wesley Pub. Co. of New York, 1989, pp. 478-498.
H. Chen, “Design and Analysis of a Sensorless Magnetic Damper,” 95-GT-180, International Gas Turbine and Aeroengine Congress & Exposition, Houston, Tx, 1995.
Chen H. Ming
Locke Dennis H.
Walton II James F.
Dougherty Thomas M.
Lam Thanh
Mohawk Innovative Technology, Inc.
Simmons James C.
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