Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-05-13
2004-10-19
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S090000
Reexamination Certificate
active
06806605
ABSTRACT:
BACKGROUND OF THE INVENTION
Magnetic bearings are in many cases the desired bearings for support of rotating objects especially where high-speeds, non-contamination with lubricant or bearing wear products, or long life is required. Magnetic bearings can be designed with various actively controlled degrees of freedom between one degree and five degrees. In some emerging applications, such as flywheel energy storage systems, minimizing the amount of required control is preferable for minimizing the system costs and extending the operating life. Single degree actively controlled magnetic bearing designs employ passive radial magnetic bearings to maintain radial stability. Such bearings typically have inherently low radial stiffness, however they allow very simple, reliable and long life control.
To date several different configurations of passive radial magnetic bearings have been developed and each has advantages as well as drawbacks. In applications requiring the highest speed capability, the most desirable bearing design would be one that does not use permanent magnets on the rotating portion. Permanent magnets such as NdFeB rare earth magnets and others have low tensile strengths of only about 10,000 psi. Magnets on the rotating portion would be subject to the high centrifugal loading and hence prone to fail. However, in some applications, such as those involving high vibration or higher radial and or tilt moment loading for example, it may be desirable or necessary to use a passive radial bearing design with rotating permanent magnets to achieve increased radial stiffness.
Passive radial magnetic bearing designs with magnets on both the rotor and stator portions generate the highest radial stiffness per amount of magnet material and bearing diameter, but permanent magnets on both the rotor and stator portions alone is not enough to insure the maximum radial stiffness; bearing design also affects the radial stiffness of the bearing. Some previous bearing designs have used both rotating and stationary permanent magnets but have also included steel pole rings to axially focus the magnetic flux at the axial airgaps. However, the steel pole rings allow flux redistribution at the surfaces during radial displacement of the rotor because of the high magnetic permeability of the steel pole rings. This results in a reduction in the radial stiffness of the bearings.
Other designs of passive radial magnetic bearings have achieved their maximum radial stiffness by using concentric axially magnetized permanent magnet rings attached to both the rotor and stator, with the ring magnets arranged with radially alternating polarities. The ring magnets on the rotor and stator cooperate to generate an axial attractive force as well as radial centering forces. Two or more concentric magnet rings are usually used so that maximum stiffness is achieved by all axial magnetic airgaps having surfaces bounded by permanent magnet material. For use in disk drives, such magnetic bearings use as many as four concentric magnetic pole rings for achieving the desired performance. A pivot bearing provides axial stability. Disk drives are small in diameter and operate at relatively low peripheral speed. Because the stresses in a rotating structure are a function of the peripheral speed, the ring magnets in these small bearings are subjected only to small stresses induced by centrifugal forces, so they can operate without risk of failure from those forces.
In high speed applications, such as for use in energy storage flywheels, a similar magnetic bearing design has been described that uses opposed rings of concentric axially magnetized ring magnets on the rotor and stator, with alternating polarity on radially adjacent rings. The ring magnets on the rotor are banded with high tensile strength carbon fiber/epoxy composites rings to support the ring magnets radially and bond the magnet to the inner support disk. However, even with the use of high strength banding around the magnets, the outer diameter dimension of the magnets, and hence the radial stiffness they can provide, is limited. To compensate, radial stiffness is increased by increasing the surface areas of the bearing magnets through use multiple axially tiered assemblies.
FIG. 1
is a schematic representation of this type of magnetic bearing of the prior art having a passive radial magnetic bearing
30
including bearing elements
33
attached to a rotor, and bearing elements
34
attached to a stator. The rotating bearing elements
33
include permanent magnets
35
attached to the shaft
31
through use of support disks
37
and are banded with high tensile strength bands
38
. The stationary bearing elements
34
include permanent magnets
36
attached to the stator housing
32
through use of spacers
39
. This type of bearing requires the use of multiple precision assemblies, and it is expensive both to produce the components and to assemble them. The near zero coefficient of thermal expansion of rare earth magnets makes assembly difficult as the ring magnets can not simply be cooled and placed inside the reinforcement bands for generation of high preload. Likewise, the bands, whether metal or composite, cannot be heated to high temperatures to generate a high interference assembly because the magnets are temperature limited. Further, high stiffness carbon fiber/epoxy bands are also temperature limited and they posses low coefficients of thermal expansion, which reduces thermal interference capability. These factors complicate the manufacturing processes for producing supporting structure that can exert radial precompression in the ring magnets and thereby reduces the maximum diameter that can practically be achieved in a magnetic bearing. In addition to the high cost of the use of multiple axially stacked bearing assemblies, the multiple assemblies take up shaft space and could potentially lead to unacceptably low resonant frequencies depending on the operating speed and geometry.
SUMMARY OF THE INVENTION
The invention provides a full levitation magnetic bearing system for vertically supporting a rotor for high speed rotation on a stator, and an improved passive radial magnetic bearing that combines optimal radial stiffness with high operating speed capability and easy, low cost manufacturing and assembly. The magnetic bearing has two or more concentric, axially magnetized permanent magnet poles on both the rotor and stator that cooperate to produce axial suspending forces and radial centering forces. With permanent magnets defining the surfaces of the axial airgap, the magnetic bearing achieves the highest radial stiffness per amount of magnet material and bearing diameter. The magnetic bearings are preferably made with a diameter large enough to generate the desired radial stiffness so that multiple axially tiered assemblies are not required. Because of the large bearing diameter, the peripheral speed encountered by the rotating ring magnets can become as high as 200 m/sec or higher. To prevent failure of the rotating magnets, the rings are constructed of individual arc segments, thereby allowing the bearing ring to grow in diameter without encountering a hoop direction tensile failure as would occur with a solid ring. The pieces are radially supported inside a containment cup on the axial end of the rotating body against centrifugal forces induced by high-speed rotation. The magnet pieces are subjected only to radial compressive stresses from their compression under centrifugal force against the inner diameter of the cup. Because magnets, such as rare earth magnets that are typically used for high energy magnetic products, have a compression strength nearly fifteen times higher in compression than tension, the magnet pieces are safely loaded in the support cup. The rotating ring magnets are preferably made up of a sufficient number of pieces so that the pieces are sufficiently small in circumferential length to reduce any loading from bending or frictional tension with the cup inner diameter. The high hoop stress imparted in the cup wall from th
Indigo Energy, Inc.
Mohandesi Iraj A.
Mullins Burton S.
Neary J. Michael
LandOfFree
Permanent magnetic bearing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Permanent magnetic bearing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Permanent magnetic bearing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3284148