WO1997024536A1

WO1997024536A1 - Radial constrained backup bushings for re-centering of magnetic bearings

Info

Publication number: WO1997024536A1
Application number: PCT/US1996/020396
Authority: WO
Inventors: Robin M. Miller
Original assignee: United Technologies Automotive, Inc.
Priority date: 1995-12-29
Filing date: 1996-12-16
Publication date: 1997-07-10

Abstract

Bearing systems, such as magnetic bearings, are widely used today in a variety of applications. All bearing system are, however, subject to failure. Therefore, backup systems are often utilized in conjunction with a main bearing system in order to provide backup to the main bearings if the main bearings should fail. The backup bushing system of the present application is utilized with a flywheel assembly aligned and rotating about the axis of a stationary shaft and includes a first bushing disposed substantially stationary with respect to the axis and having a first contact portion; and a second bushing spaced from the first bushing and disposed substantially stationary with respect to the axis and having a second contact portion. If the flywheel assembly is moved off-center with respect to the axis the flywheel assembly will engage the first and second contact portions of the first and second bushings. Upon engagement, the bushings operate to contain the rotation of the flywheel assembly between the contact portions thereby substantially maintaining the alignment of the flywheel assembly with respect to the axis and preventing 'whirl' from occurring.

Description

RADIAL CONSTRAINED BACKUP-BUSHINGS FOR RE-CENTERING OF MAGNETIC BEARINGS

Background

1. Technical Field

The present application relates to backup bushings and especially to radially constrained backup bushings for use in conjunction with magnetic bearings.

2. Background of Related Art

The flywheel, a balanced mass spinning around a constant axis that stores energy as rotational kinetic energy is one of humankind's earliest devices, serving as the basis for both the potter's wheel and the grindstone.

Today, flywheel energy storage (FES) systems which store electric energy as kinetic energy and generate electric energy from the stored kinetic energy are being utilized for a number of applications. FES systems are currently being utilized in both mobile applications such as automotive and space applications, as well as stationary applications such as utility load-leveling systems, uninterrupted power supplies, and as storage capacity for solar and wind power systems. As is traditional, the term stationary refers to a system which is positioned primarily in a given geographic location as contrasted to a mobile system which is able to readily move between a variety of geographic locations. FES systems generally include several principal components; namely a flywheel having a rotor and a hub, a motor/generator as well as magnetic bearings. Typically the system will also include a structural housing, a vacuum pump, electrical power input/output and electronic controls for the magnetic bearings.

It is known in the art to construct the flywheel rotor of high specific strength (i.e. strength/density) composite materials in order to optimize the flywheel's performance. The motor/generator is utilized to transfer electric power into the system to store it as kinetic energy when the system is acting as a motor and is also utilized to generate electric energy from the stored kinetic energy to transfer the electric energy out of the system when the system is operating as a generator. High-performance FES systems operate in a vacuum to minimize windage losses, aerodynamic heating and rotor instability. These high-performance systems therefore include a structural housing which also serves as a containment vessel to enclose any debris resulting from the failure ofthe rotor. Current FES systems also use magnetic bearings for supporting, or suspending the rotating flywheel within the housing.

Typically, the magnetic bearings utilized are either active or passive. In a typical active system the flywheel is suspended by magnetic forces created by the magnetic bearings. These forces, along with the loads that act on the flywheel, are controlled and balanced by position or proximity sensors and electronic feedback circuits working together to control the stability ofthe flywheel by introducing magnetic flux forces by controlling the currents in electromagnetic windings within the bearing assembly.

Passive magnetic bearings, on the other hand, use powerful permanent magnets with specific geometries to support and stabilize the spinning flywheel without resorting to feedback control. Passive bearings help minimize parasitic losses, are cost effective and are generally utilized in stationary systems where complex control logic is typically not needed during normal operation. On the other hand, active bearings allow for more dynamic stability than passive bearings and are useful in mobile applications, such as in automobiles where compensation for road shocks and rotor balance to avoid flywheel instability is important.

Both passive and active magnetic bearings work very well in situations of low energy loss and low vibration when they are properly placed, or centered, and operating. However, if the flywheel is forced off- center or if there is an interruption in the power source the bearings may not be able to restore themselves and can abruptly fail. In order to minimize damage within the system due to failure, many flywheels utilize rolling element backup bearings placed along the inner diameter of the flywheel such that if the flywheel becomes misaligned such that it is no longer operating on the magnetic bearings the backup bearings will help prevent extensive damage to the entire system. When the magnetic bearings fail an event known in the art as "whirl" may occur. "Whirl" is when the flywheel spins around within the clearance required for the magnetic bearings thereby causing extremely high rotating forces on the stationary components in the system and on traditional backup bearings. "Whirl" can shorten the life of the backup bearings and creates additional stress on the flywheel which can cause the flywheel to be damaged and break. Although traditional backup bearings help during emergency failure, they do not prevent the "whirl" phenomena and therefore do not spin the flywheel down to rest without possible damage to the system. In addition, for large, stationary systems with rotating flywheel weights over about 500 lbs the cost of these rolling element backup bearings can be prohibitive, especially since the market for these large, stationary systems is cost driven.

A need therefore exists for a cost effective device which would allow the flywheel to safely spin down to rest if there is an interruption in the power source to the magnetic bearings without allowing the system to

"whirl".

The present application provides for a cost effective backup bushing system which allows the flywheel to safely spin down to rest if there is an interruption in the power source to the magnetic bearings without allowing "whirl" to occur. The present bushing system may also be utilized to re- center the flywheel assembly.

Summary

A backup bushing system for use with a flywheel assembly aligned and rotating about the axis of a stationary shaft is provided, the backup bushing system comprising: a first bushing disposed substantially stationary with respect to the axis and having a first contact portion; and a second bushing spaced from the first bushing and disposed substantially stationary with respect to the axis and having a second contact portion. If the flywheel assembly is moved off-center with respect to the axis the flywheel assembly will engage the first and second contact portions ofthe first and second bushings. Upon engagement, the bushings operate to contain the rotation of the flywheel assembly between the contact portions thereby substantially maintaining the alignment of the flywheel assembly with respect to the axis and preventing "whirl" from occurring. Brief Description of the Drawings

Various embodiments are described herein with reference to the drawings, wherein:

Fig. 1 is a cross-sectional view of a Flywheel Energy Storage (FES) system utilizing one embodiment of a backup bushing assembly according to the present application out of engagement with the flywheel assembly; and

Fig. 2 is a cross-sectional view of the embodiment of Fig. 1, in engagement with the flywheel assembly.

The figures are meant to further illustrate the present application and not to limit the scope thereof.

Detailed Description of the Preferred Embodiments

Referring now to Fig. 1 there is illustrated a cross-sectional view of one embodiment of a Flywheel Energy Storage (FES) system 10 according to the present application. System 10 consists of an outer vacuum housing 12, an inner containment ring 14, a rotating flywheel assembly 16, a stationary shaft 17 having an axis "Y", a motor/generator 24 and main bearings 26a,b,c. System 10 is preferably designed to store a total energy of 15 kilowatt-hours (kWh) at a maximum speed of approximately 16,000 revolutions per minute (rpm). The usable energy storage at constant power is 12.5 kWh over an operating range from about 4,000 rpm to approximately 16,000 rpm. The rated power ofthe system is 12.5 kilowatts (kW) throughout the operating range, with approximately 30kW available for several seconds at the maximum speed. In the present embodiment FES system 10 is a stationary system having a rotating flywheel assembly weighing approximately 1000 lbs. with the flywheel assembly 16 having a diameter of approximately 36 inches and an overall height of approximately 45 inches.

Housing 12 encloses vacuum chamber 13 in which the flywheel assembly 16 operates. Inner containment ring 14 helps contain any debris resulting from failure ofthe flywheel assembly 16, if failure should occur.

Inner containment ring 14 which is located about flywheel assembly 16 is preferably made of a high tensile strength material, such as steel, in order to withstand the momentum of impact from any debris resulting from flywheel failure. Although the present embodiment utilizes a housing 12 in conjunction with a containment ring 14, a number of various containment systems may be utilized by one of skill in the art. The design of any containment system requires knowledge of: 1) the failure mode of the flywheel, 2) the kinematics of the failed pieces, and 3) response of the containment system. By evaluating each of these criteria one of skill in the art can design a suitable containment system for a particular FES system.

In the present embodiment the motor/generator 24 is a brushless permanent magnet motor which is air-cooled so as to minimize cost. The motor/generator 24 spins the flywheel assembly 16 up to speed to transfer electric power into the system to store it as kinetic energy when the system is acting as a motor and also operates to generate electric energy from the stored kinetic energy to transfer the electric energy out of the system by coupling to the flywheel assembly when the system is operating as a generator, as is known in the art. In the present embodiment, motor/generator 24 is designed to be a 12.5kw, 100% duty factor, 480V permanent magnet, three-phase motor with 30kw maximum input and output. Alternate motor/generators may be utilized depending upon the particular application for the FES system. With continued reference to Fig. 1, flywheel assembly 16 consists of a composite rotor 18, a hub 20, a cylinder 21 which contains a back-iron (not shown) ofthe motor/generator 24 and a hub 22 containing the main bearings' rotors (not shown). Although the present embodiment utilizes a composite rotor, other types of rotors including metallic rotors may also be utilized. In the present embodiment an E-glass composite rotor is preferred because due to the large size of the present system a low cost, durable material which is easy to manufacture is desired. In addition, a composite rotor is preferred because optimal energy storage can be accomplished by maximizing the ratio of energy to mass, termed the energy density, and the greatest energy density is found using the highest specific tensile strength flywheel material and also because metallic rotors tend to shatter into sharp piercing fragments if failure occurs while composite rotors do not. Lightweight composite materials, consisting of fibers in a matrix, typically have very high material strengths relative to their mass densities. Rotor 18 is preferably made of an E-glass composite material, but alternatively may be made of any material which has a high specific strength. Regardless of the material utilized, some factors which determine the design of the flywheel assembly include the desired energy storage capacity, cost, number of cycles and size constraints, etc. all of which are related to the application for which the FES system will be utilized. In the present embodiment the flywheel assembly would be used for stationary applications such as peak load management of electric power. Flywheel assembly 16 is supported by main bearings 26a,b,c as it rotates about stationary shaft 17. In the present embodiment main bearings 26a,b,c are preferably active, magnetic bearings and include an axial magnetic bearing as well as a pair of radial magnetic bearings. Magnetic bearings are the preferred method of providing support to the flywheel assembly 16 because they have low frictional losses, high speed capability and are compatible with a vacuum environment (i.e., do not require lubrication). The axial magnetic bearing suspends the flywheel assembly 16 within housing 12 while the radial magnetic bearings align the flywheel assembly 16 about shaft 17, as is known in the art. In the present embodiment the radial bearings can take approximately 300 lbs of load. Sensors 28 are connected through a path to each active bearing axis for each bearing 26a,b,c, for a total of five axis (radial bearings x and y axis, axial bearing z axis) for the three bearings. Sensors 28 are proximity sensors which provide electronic feedback in order to control the stability of the flywheel assembly by digitally controlling the bearing properties and operation in order to properly place the bearings and the flywheel relative to each other.

In the present embodiment magnetic bearings 26a,b,c are permanent magnet biased active bearings operating constantly at all rotating speeds, although other magnetic bearings which would be known to those skilled in the art may be utilized depending upon the particular application. Magnetic bearings 26a, b, c work well in most operating situations but do have load limits and may become unstable or even fail due to many reasons including, but not limited to, loss of power. There is therefore provided in the present application backup-bushing system 30 which provides backup to magnetic bearings 26a, b, c in situations where the magnetic bearings 26a, b, c become unstable or fail.

Backup bushing system 30 includes a first, conical bushing 32 disposed about shaft 17 and a second, conical bushing 34, spaced from bushing 32 and disposed about shaft 17. Both bushings 32, 34 are attached to stationary shaft 17 at opposite ends thereof by bolts, clamps or the like although bushings 32, 34 may be attached to shaft 17 in any manner as long as the bushings are stationary with respect to the shaft. In the present embodiment bushings 32, 34 are preferably made of brass or bronze and are approximately 22" and 18" in diameter, although alternate sizes and materials such as soft iron or nylon may be utilized, as long as acceptable bushing life is achieved. Alternatively, bearings may be utilized in place of the bushings, if cost is not a factor. Whatever types of elements are utilized for backup, the bearings or bushings should have low friction losses, be capable of operating in a vacuum (i.e. are "dry" lubricated), be durable and should preferably be cost effective.

Bushings 32, 34 preferably each have a conical contact portion 32a, 34b such that if the flywheel assembly 16 rests on the bushings with gravity, the flywheel assembly would be centered within the two contact portions 32a, 34b. Contact portions 32a, 34b are preferably disposed at an angle θ from the center axis "Y" of shaft 17 so as to create a greater re-centering force when contacting rotating flywheel assembly 16. The narrower angle θ is, the greater the centering force on flywheel assembly 16. In the present embodiment θ is between approximately 30° - 45°, with an angle of approximately 45° being illustrated in Figs. 1 and 2, the 45° angle creating approximately 1000 lbs of re-centering force when contact portions 32a, 34b contact flywheel assembly 16.

Although a conical shape is shown in the present embodiment other shapes which would allow for re-alignment of the flywheel assembly 16, such as an oval shape may be utilized provided that the flywheel assembly has a corresponding shape for low friction engagement with the backup bearing assembly, the shape helps drive the flywheel assembly to rest and the re-centering force is sufficient to keep the flywheel assembly 16 aligned as it is spinning down to rest. Flywheel assembly 16 preferably includes a conical shaped contact surface 35a,b on either end corresponding to the shape of contact portions 32a, 34b such that upon contact with contact portions 32a, 34b the motion of the flywheel assembly 16 is contained between the contact portions 32a, 34b as the flywheel assembly is driven down to rest on bushing 32. Contact surfaces 35a,b are preferably smooth surfaces with low friction so as to limit the heat and wear on the contact surfaces.

Referring now to Fig. 1 which shows the flywheel assembly during operation and out of contact with bushings 32, 34 in conjunction with Fig. 2 which shows the flywheel assembly in contact with and resting on bushing 32, the operation of the bushing system 30 will now be described. In operation, if flywheel assembly 16 experiences mechanical failure and moves off-center causing magnetic bearings 26a,b,c to become misaligned, or if the magnetic bearings are not properly aligned for any reason, including electric failure ofthe bearings, the flywheel assembly will contact conical contact portions 32a, 34b of bushings 32, 34, respectively. As the flywheel assembly 16 contacts conical contact portions 32a, 34b, the conical shape of the bushings and the weight ofthe flywheel assembly combine to keep the motion of the flywheel assembly 16 contained between the contact portions

32a, 34b thereby keeping the flywheel assembly aligned with respect to shaft 17, as well as driving the flywheel assembly in the direction of arrow "B", to rest on bushing 32.

Because the motion of flywheel assembly 16 is weight loaded by gravity against the conical contact portions 32a, 34b the flywheel assembly is prevented from experiencing "whirl" and is able to remain substantially centered about shaft 17, until coming to rest on bushing 32. The backup bushing assembly ofthe present embodiment thereby acts as an inexpensive fail-safe if there is an interruption in power. In addition, the present embodiment is easy to manufacture, is cost effective, durable and may also be utilized to re-center flywheel assembly 16 as described hereinbelow. If it is desired to re-center the flywheel assembly 16, the repulsive magnetic field created by the axial magnetic bearing, can be reversed in order to create a magnetic attraction thereby pulling the flywheel assembly 16 into contact with the conical contact portions 32a, 32b of bushings 32, 34 in order to re-center the flywheel assembly. As the flywheel assembly 16 contacts the conical contact portions 32a, 34b, the conical shape of the bushings and the weight of the flywheel assembly combine to keep the motion of the flywheel assembly 16 contained between the contact portions 32a, 34b whose shape acts to re-center the flywheel assembly with respect to shaft 17. Once the flywheel assembly has been re-centered the axially magnetic bearing 26a can be returned to a repulsive magnetic bearing, thereby once again levitating the flywheel assembly 16.

It will be understood that various modifications may be made to the embodiments disclosed herein. For example, the bushing assembly may be bearings instead of bushings, the magnetic bearings may instead be air bearings, and the backup bushing assembly may be used with other systems which utilize magnetic or air bearings, other than a flywheel energy storage system. In addition, although the stationary shaft of the present application is shown as being vertical, the shaft may alternatively be disposed horizontally or at an angle. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit ofthe claims appended hereto.

I Claim:

Claims

1. A backup bushing system for use with a flywheel assembly aligned and rotating about an axis, the backup bushing system comprising: a first bushing disposed substantially stationary with respect to the axis and having a first contact portion; a second bushing spaced from the first bushing and disposed substantially stationary with respect to the axis and having a second contact portion; wherein upon engagement of the flywheel assembly with the first and second contact portions of the first and second bushings the rotation of the flywheel assembly is contained between the contact portions thereby substantially maintaining the alignment of the flywheel assembly with respect to the axis.

2. The backup bushing system of Claim 1, wherein the flywheel assembly is supported by main bearings as it rotates about the axis.

3. The backup bushing system of Claim 2, wherein the main bearings are magnetic bearings.

4. The backup bushing system of Claim 1, wherein the flywheel assembly includes a stationary shaft, said stationary shaft defining the axis about which the flywheel assembly rotates.

5. The backup bushing system of Claim 4, wherein said first and second bushings are attached to said stationary shaft at opposite ends thereof, said first and second bushings being substantially stationary with respect to said shaft.

6. The backup bushing system of Claim 1, wherein said first and second bushings have a conical shape.

7. The backup bushing system of Claim 1, wherein said first and second contact portions are conical.

8. The backup bushing system of Claim 7, wherein said first and second contact portions are disposed at an angle from the axis defined by the stationary shaft, wherein said angle creates a re-centering force when contacting the rotating flywheel assembly.

9. The backup bushing system of Claim 8, wherein said angle is approximately 30 to approximately 45 degrees.

10. The backup bushing system of Claim 7, wherein said flywheel assembly includes a contact surface disposed on either end of said flywheel with respect to said shaft, each of said flywheel contact surfaces corresponding to the shape of the first and second contact portions, wherein said first and second contact portions contact said flywheel contact surface so that the motion ofthe flywheel assembly is contained as the flywheel assembly is driven to rest on said first bushing.