Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-08-20
2004-06-08
Le, Dang (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C384S582000, C074S572200, C074S574300
Reexamination Certificate
active
06747378
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a damping system for the bearings of an energy storage system. More precisely, the invention relates to a dual stiffness damping system for a flywheel assembly that remains relatively flexible during normal operating conditions, so as to reduce rigid body critical speed of the flywheel assembly, but that arrests potentially deleterious relative deflections between rotor and stator assemblies during extreme external loads and/or vibrations, e.g., due to an earthquake.
DESCRIPTION OF THE RELATED ART
Evacuated energy storage systems, which internally produce and store kinetic energy in high speed rotors, or flywheels, have been developed as an alternative to batteries and other means of storing energy for at least 30 years. Evacuated energy storage systems typically comprise an energy-storing rotor, which includes an outer rim commonly made of high-strength, low-density composite fibers to maximize energy storage density, and a high-powered, high-strength generator that turns the rotor at high rotational velocities. To reduce energy loss through air friction, flywheel systems often, if not exclusively, are contained in an evacuated chamber, which is evacuated by a drag pump.
Drag pumps extract air from the energy storage system to create the vacuum necessary to reduce air friction losses. Typically, drag pumps can produce a vacuum of about 10
−5
Torr. Loss of vacuum in an energy storage system, however, would produce higher temperatures due to additional frictional energy losses, which manifest as heat losses. If such an unevacuated energy storage system were equipped to monitor temperatures, which is common, the energy storage system likely would be shutting down constantly. Consequently, maintaining a vacuum is crucial to the continued operation of an evacuated energy storage system.
Flywheel rotors typically are rotatably supported on and guided by bearings that permit free motion between a moving part, e.g., the flywheel rotor shaft, and a fixed part, e.g., the stator assembly. Bearings typically minimize energy loss associated with friction and, correspondingly, minimize wear and tear on moving and fixed parts.
Two common bearing types known to the art are roller-type and fluid-type bearings. Mechanical bearings of the roller- or ball-type transfer loads imparted to the bearing by the moving part to a fixed support and, typically, are made of metal, alloys or ceramic materials. Mechanical bearings of the hydrostatic fluid-type transfer loads, instead, to a high-pressure fluid film that (i) separates moving from stationary parts and (ii) provides lubrication to the moving part.
Bearings generally are mounted in dampers for the purpose of, inter alia, (i) damping vibrations caused by, e.g., the rotation of the shaft, misalignment and/or eccentricity of the rotor with respect to the stator, and/or external vibrations; (ii) transferring heat away from the bearings; and (iii) reducing the load on the bearings. By accomplishing these three purposes, dampers produce longer bearing life and greatly facilitate magnetic levitation.
Typically, bearing dampers are flexible. Flexibility substantially reduces the rigid body critical speed of the flywheel rotor to a low frequency, which can be crossed safely, e.g., while the flywheel assembly powers up to its normal, design operating speed, with corresponding low energy. Indeed, as a rule, dampers should be relatively flexible with correspondingly low axial, radial, and transverse stiffness. For example, it is undesirable for a damper to affect the lift system of a rotor that is supported by magnetic bearings. Accordingly, axial stiffness should remain as low as possible. In another example, low radial stiffness reduces the dynamic force acting on the bearing, which can extend the bearing's service life. However low radial stiffness also enables radial displacement of the rotor assembly with respect to the stator assembly. In yet another example, stiffer dampers produce stiffer flywheel assemblies, which are more susceptible to problems associated with imbalances.
Accordingly, an ideal damper produces (i) relatively low damping when a flywheel assembly is operating at high speeds; (ii) relatively high damping when a flywheel assembly is operating at low speeds; and (iii) maximum damping at or near critical velocity. Indeed, critical velocity is a function of and proportional to damper stiffness. The less stiff the damper is, the lower the critical velocity is. Accordingly, less overall damping is required than if a stiffer damper were used. Still, at critical velocity, a maximum amount of the overall damping is required. The opposite is also true, i.e., in relative terms, the greater the damper stiffness is, the higher the critical velocity is. As a result, the flywheel assembly requires more overall damping and, moreover, a maximum amount of the overall damping is required at or near critical velocity.
One problem with flexible dampers, however, is that, in contrast with more rigid dampers, flexible dampers permit relatively large deflections, displacements, and/or movements, which, under normal, i.e., design, operating conditions, is acceptable. However, under abnormal or a typically operating conditions, e.g., during a period of extreme external loads and/or vibrations such as from an earthquake or other large dynamic force, flexible dampers of the prior art are unsuitable. Indeed, excessive deflection during periods of extreme external vibrations can cause flywheel assembly moving parts to contact stationary parts with potentially catastrophic consequences.
For example, during an earthquake, the stator assembly accelerates as a function of the acceleration and attenuation of the earthquake but the rotor, which is magnetically levitated, resists acceleration. Accordingly, the acceleration forces cause the stator assembly to displace with respect to the rotor. If the stator assembly displaces enough, it could come into contact with the rotor, further causing one or more of the following:
(i) breakage;
(ii) local overheating, which can destroy the material properties of the rotor, stator and/or other component parts of the energy storage system;
(iii) damage to the rotor further causing a misbalance, which makes the rotor dynamically unstable; and/or
(iv) damage to the rotor increasing the clearance between the rotor and stator assemblies, which additional clearance could exceed the capability of the drag pump to effectively evacuate the energy storage system, causing a loss of vacuum.
This produces a dilemma. The drag pump operates more efficiently and more effectively the closer the component parts of the energy storage system, e.g., the stator and rotor assemblies, are with respect to one another. Ideally, one desires zero tolerance between the rotor and the stator assemblies. However, in practical application, which accounts for typically displacement due to internal vibrations and ambient conditions, a clearance of about 0.015 inches (15 mils) with a tolerance of about +/−2 mils is preferred. Accordingly, drag pumps must be able to evacuate the flywheel assembly based on a maximum clearance of about 17 mils.
However, a typical operating conditions demand greater clearances to provide a greater factor of safety. Accordingly, the clearance would have to be greater than 17 mils. Furthermore, a larger drag pump might be needed. Moreover, the flywheel assembly likely would be larger.
SUMMARY OF THE INVENTION
Thus, it would be desirable to produce a bearing damping system that dampens vibrations, i.e., reduces the amplitude of the vibrations, induced by the rotation of a shaft, deflection of the shaft, and/or by the misalignment, or eccentricity, of the shaft that occur during normal operating conditions. Moreover, it would be desirable to produce a bearing damper system that is flexible over a short relative deflection distance of the stator assembly relative to the rotor assembly, but whose stiffness increases dramatically thereafter to further arrest deflect
Beacon Power Corporation
Conlin David G.
Edwards & Angell LLP
Le Dang
Penny, Jr. John J.
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