Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-08-28
2004-03-23
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S090000, C310S074000
Reexamination Certificate
active
06710489
ABSTRACT:
BACKGROUND OF THE INVENTION
Flywheel power supplies have emerged as an alternative to electrochemical batteries for storing energy, with many advantages including higher reliability, longer life, lower or no maintenance, higher power capability and environmental friendliness. Flywheel power supplies store energy in a rotating flywheel that is supported by a low friction bearing system inside a chamber. The chamber is usually evacuated to reduce losses from aerodynamic drag. A motor/generator accelerates the flywheel for storing energy, and decelerates the flywheel for retrieving energy. Power electronics maintain the flow of energy in and out of the system and can instantaneously prevent power interruptions, or alternatively can manage peak loads.
One way to support a flywheel for rotation at high speeds is with rolling element mechanical bearings such as ball bearings. The life of mechanical bearings is strongly influenced by the loads that these bearings must carry. To extend the life of flywheel systems using mechanical bearings, a magnetic bearing can effectively be used in combination with the mechanical bearings for the purpose of reducing the load on the mechanical bearings. In this arrangement, the flywheel typically rotates about a vertical axis and the mechanical bearings provide radial support while the magnetic bearing carries much of the flywheel's weight axially. One such flywheel energy storage system is shown in FIG.
1
. The flywheel system
30
includes a steel flywheel
31
that rotates inside an evacuated vessel
32
. The interior of the vessel
32
is a chamber
41
maintained at a vacuum for reduction of aerodynamic drag. The flywheel
31
, in this design, has an integrated motor/generator
33
that accelerates and decelerates the flywheel through cooperation with the outer diameter of the flywheel
31
. The motor/generator
33
includes an outer laminated stator portion
34
, motor/generator coils
42
, stator flux return path
35
and a field coil
36
for operation. The flywheel
31
is supported for rotation on upper and lower rolling element mechanical bearings
37
and
39
. These bearings
37
,
39
are mounted in fixed upper and lower mounts
38
and
40
. Load on the bearings
37
,
39
is reduced through the use of an axial magnetic bearing
43
, shown as an annular electromagnet with electromagnetic coil
44
. The magnetic bearing
43
supports a majority of the weight of the flywheel
31
while allowing some desired amount of loading on the bearings
37
,
39
. The electromagnet is controlled either by using strain gauges, not shown, on the support structure that sense and control the bearing loading through use of a closed loop controller, or simply by use of a constant current power supply.
Unfortunately, this configuration of mechanical and magnetic bearing system is not optimal for allowing rotation to very high-speeds for efficiently storing large amounts of energy. There are several drawbacks. The rigid radial support provided by the mechanical bearings would cause the rigid body critical speeds to be encountered at a relatively high speed if the flywheel were operated to high speeds. Encountering these resonances at a high speed would impart severe loading on the bearings that would reduce their life and potentially be dangerous. Operation at high speeds, but below the rigid body critical speeds, causes high bearing loading from any flywheel imbalances because the flywheel is being forced to spin about a geometric center rather than the mass center.
Another major problem with operating a flywheel system to high speeds is that the loads on the bearings can significantly increase due to dimensional changes in the flywheel. The effect of high speed rotation is illustrated in FIG.
2
. The dimensions of a flywheel
50
are shown at
52
for zero speed and at
51
for high speed rotation. The effect of the centrifugal stress is that the outer diameter expectedly grows by a radial increment
53
from the radial and hoop stresses. This growth does not change the bearing loading. The secondary result from this growth is that the flywheel shrinks by an axial increment
54
from Poisson ratio contraction. For axially thick flywheels, the shrinkage can be as much as 0.050 inches. Such a large length change will not only drastically load the mechanical bearings against each other but can also cause them to fail before achieving full speed. The flywheel can also expand and contract axially from temperature changes in the flywheel or surrounding structure. Heating from high power motor/generators is one potential cause for added bearing loads.
The increased bearing loading drastically affects the life of mechanical bearings. Bearing life is generally a cubic function of the load, so that a doubling of the load will decrease the life by a factor of eight. Further compounding the shortening of life from the increased axial loading is that for angular contact bearings, axial loading can be as much as 35 times more fatiguing to the bearing than an equivalent size radial load. This sensitivity varies based on the contact angle, number of balls, ball diameter and the axial thrust load applied.
Besides problems of axial bearing loading that occurs between the bearings during operation, use of a mechanical strain gauges to measure axial loading at a single bearing is not as sensitive as desirable for removing almost the entire bearing axial load, especially if the flywheel support structure is rigid. Likewise, applying a constant current to the coil cannot provide sufficiently accurate axial load removal for maximum reliable operating life.
The construction of flywheel systems that support a flywheel with mechanical bearings can also suffer significant damage from shipping and handling. The system shown in
FIG. 1
has no power during shipping and hence the ball bearings must carry the full flywheel weight. The strain gage and load cell can become damaged and plastically deformed from impact loadings, especially if they were designed to be sensitive enough to maintain very low axial bearing loading. The bearings of this as well as other design systems could be easily damaged from impact loads such as simply setting the system down during transportation handling. The force generated from an impact can be several times the weight of the flywheel and can cause the balls to Brinnell indent the bearing races or cause the bearings to shift in position.
SUMMARY OF THE INVENTION
This invention provides a flywheel energy storage system that allows high-speed operation with use of mechanical rolling element bearings for flywheel support. The mechanical bearings provide radial support for a vertical axis flywheel but they allow it to be mechanically free or unrestrained in the axial direction. One or more magnetic bearings are used to carry the flywheel weight axially. The axial unconstraint by the mechanical bearings allows the flywheel to freely grow or shrink in axial length from Poisson Effect contraction that occurs when rotating to very high speeds and stress levels as well as from thermal expansions from motor/generator heating or other sources. Excessive axial loads applied from the bearings on each other are thereby prevented. The axial mechanical freedom also insures that the magnetic bearing carries all of the flywheel weight, thus dramatically extending the mechanical bearing lives. The life of rolling element bearings is generally a cubic function of the applied loads and axial loading on commonly used angular contact bearings is many times more fatiguing to the bearings than radially applied loads. Eliminating the axial loading from the flywheel greatly extends the bearing lives. Tandem multiple preloaded angular contact bearings can be used for the mechanical bearings. These bearing sets share the loads between several bearings, extending life, and are manufactured with the desirable minimum axial preload for longest term reliable operation. The axial preload is accurately built-in and does not change as the flywheel is rotated. Alternatively, the
Indigo Energy, Inc.
Neary J. Michael
Nguyen Tran
LandOfFree
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