Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2003-03-25
2004-08-03
Nguyen, Tran (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06770995
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnetic bearings for levitating or suspending a rotatable component. More specifically, the invention relates to a magnetic bearing that provides radial positioning of a rotatable component on a passive basis to facilitate rotation of the component about a predetermined axis.
BACKGROUND OF THE INVENTION
Magnetic bearings are commonly used to levitate or suspend rotatable components, e.g., flywheels, and thereby facilitate rotation of the component about a predetermined axis. Magnetic bearings provide substantial advantages in relation to mechanical bearings. For example, magnetic bearings facilitate substantially friction-free operation, and thus function without most of parasitic energy losses that occur in virtually all mechanical bearings.
Magnetic bearings are classified as “active” or “passive.” Active magnetic bearings usually comprise one or more electromagnets that create return forces. A typical active magnetic bearing also comprises one or more position sensors that operate in conjunction with a servo control system. The servo control system varies the current passing through the electromagnets in a manner that causes the return forces to suspend and align the rotatable member along a desired axis of rotation.
Passive magnetic bearings typically comprise one or more permanent magnets fixed to the rotating or static components of the bearing. The permanent magnets produce attractive or repulsive forces that bias the rotating component toward or along a desired axis of rotation. Passive magnetic bearings, in general, are lighter, smaller, less complex, less expensive, and more reliable than active bearings of similar capability. A passive magnetic bearing, however, cannot provide stable positioning of the rotatable member in the radial and axial directions, i.e., with respect a set of orthogonal axes one of which extends along the desired axis of rotation. Passive magnetic bearings, therefore, are typically used in conjunction with one or more active bearings.
So-called “centering bearings” represent a particular type of passive magnetic bearing. Centering bearings exert a radial force on a rotatable member that biases the rotatable member toward a desired axis of rotation. One possible embodiment of a conventional centering bearing
100
is depicted in cross-section in
FIGS. 5 and 6
.
The bearing
100
comprises a first stator disk
102
and a second stator disk
104
. The bearing
100
further comprises a rotor disk
106
. The rotor disk
106
is fixedly coupled to a shaft
109
that supports a rotatable component such as a flywheel.
The stator disks
102
,
104
and the rotor disk
106
are each formed from a soft ferromagnetic material. The stator disk
102
includes a major surface
102
a
having a plurality of concentric raised portions, or teeth
102
b
, formed thereon. The teeth
102
b
each form a continuous ring, i.e., the teeth
102
b
each extend through a continuous arc of 360 degrees. The stator disk
104
likewise includes a major surface
104
a
having a plurality of concentric teeth
104
b
formed thereon.
The rotor disk
106
has a first surface
106
a
and a second surface
106
b
. The first surface
106
a
has a plurality of concentric teeth
106
c
formed thereon. The second surface
106
b
likewise has a plurality of concentric teeth
106
d
formed thereon. The geometry, i.e., the size and shape, of each tooth
106
c
substantially matches that of a corresponding tooth
102
b
on the stator disk
102
. The geometry of each tooth
106
d
substantially matches that of a corresponding tooth
104
b
on the stator disk
104
.
The rotor disk
106
is positioned between the stator disks
102
,
104
, as shown in FIG.
5
. More particularly, the rotor disk
106
is positioned so that the first surface
106
a
faces the surface
102
a
of the stator disk
102
across an axial gap
114
. The second surface
106
b
likewise faces the surface
104
a
of the stator disk
104
across an axial gap
116
.
The bearing
100
further comprises a ring-shaped permanent magnet
110
having a north pole
110
a
and a south pole
110
b
. The magnet
110
is fixed to a non-magnetizable mounting surface
108
. In addition, the magnet
110
is fixedly coupled to the stator disks
102
,
104
so that the north pole
110
a
is positioned proximate the stator disk
104
, and the south pole
110
b
is positioned proximate the stator disk
102
.
The noted arrangement of the magnet
110
, stator disks
102
,
104
, and rotor disk
106
produces a magnetic-flux circuit within the bearing
100
. The primary direction of flow of the magnetic flux is denoted by arrows
112
included in
FIG. 5
(the arrows
112
are not depicted in the lower portion of
FIG. 5
, for clarity). The magnetic flux flows from the north pole
110
a
into the stator disk
104
. The magnetic flux travels through the stator disk
104
, and is at least partially focused in the teeth
104
b
. The magnetic flux flows from the teeth
104
b
, across the gap
116
, and into to the teeth
106
d.
The magnetic flux flows through the rotor disk
106
, and is at least partially focused in the teeth
106
c
. The magnetic flux flows from the teeth
106
c
, across the gap
114
, and into the teeth
102
b
on the stator disk
102
. The magnetic flux subsequently flows through the stator disk
102
and into south pole
110
b
of the magnet
110
, thereby completing the magnetic circuit.
The noted flow of magnetic flux through the magnetic bearing
100
, in conjunction with the geometry and arrangement of the stator disks
102
,
104
and the rotor disk
106
, produces a centering effect on the shaft
109
. More particularly, the magnetic flux causes the teeth
102
b
on the first stator disk
102
to substantially align with the teeth
106
c
on the rotor disk
106
. The magnetic flux likewise causes the teeth
104
b
on the second stator disk
104
to substantially align with the teeth
106
d
on the rotor disk
106
. This phenomenon is based on the principle that the magnetic flux seeks a path of minimum reluctance.
Minimum reluctance in the flux circuit is achieved when the gaps
114
,
116
are minimized, i.e., when the distances that the flux must travel to reach the first stator disk
102
from the surface
106
a
of the rotor, or to reach the rotor
106
from the surface
104
a
of the stator disk
104
, are minimized. Minimization of the gap
114
occurs when the teeth
102
b
are substantially aligned with the teeth
106
c
. Minimization of the gap
116
likewise occurs when the teeth
104
b
are substantially aligned with the teeth
106
d
(as shown in FIG.
5
).
Hence, the magnetic flux flowing through the bearing
100
, in attempting to define a flow path of minimal reluctance, produces a magnetomotive force that urges each of the teeth
106
c
,
106
d
into substantial alignment with a corresponding tooth
102
b
,
104
b
. Aligning the teeth
102
b
,
104
b
,
106
c
,
106
d
suspends the shaft
109
and substantially aligns the shaft
109
with a predetermined axis extending in the “z” direction, thereby permitting the shaft
109
to rotate about that axis (the noted axis is denoted “C
1
,” and the direction of rotation is indicated by the arrow
126
in FIG.
5
). The resistance of the shaft
109
to radial displacement away from the predetermined axis is commonly referred to as the “stiffness” of the bearing
100
, and is proportionate to the above-noted magnetomotive produced by the flow of magnetic flux through the teeth
102
b
,
104
b
,
106
c
,
106
d.
The magnetic-flux circuit in the bearing
100
is subject to various losses. In other words, only a portion of the magnetic flux available from the permanent magnet
110
is available to suspend and align the shaft
109
. The teeth
102
b
,
104
b
,
106
c
,
106
d
represent one source of flux loss. In particular, a portion of the magnetic flux that enters each tooth
102
b
,
104
b
,
106
c
,
106
d
escapes into the space between adjacent teeth
102
b
,
104
b
,
106
c
,
106
d.
For example,
Aguirrechea J.
Nguyen Tran
Woodcock & Washburn LLP
LandOfFree
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