Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-05-23
2004-03-02
Lam, Thanh (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06700258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic thrust bearing and more particularly a magnetic thrust bearing that uses permanent bias flux with a simplified construction to allow for highly efficient force generation, high speed rotation capability and low cost construction.
2. Description of Related Art
Magnetic thrust bearings were originally constructed by using a single ferromagnetic disk attached to a rotating shaft. The thrust disk is then acted upon by electromagnets with a C-shaped cross-section located above and below the disk. This offers a very simple and low cost
construction but has a very low efficiency along with requiring complex nonlinear control.
The next advancement uses the same mechanical construction, however the electronics employs a large constant current to each coil to generate a bias flux. A small control current is added on top of the bias currents to control the bearing. The result of using a bias flux is simplified control because the relationship of force to control current becomes linearized. Linearization is provided because the force is proportional to the square of the flux density. This functions by adding the control flux to one coil's bias flux and at the same time subtracting the same control flux from the other. The force generated is then directly related to the difference of the squares of the net top and net bottom fluxes, and this varies linearly with control current. The drawback of this bearing configuration is the steady-state electrical inefficiency from having to electrically maintain the bias currents.
FIG. 1
shows the configuration
30
. The thrust disk
32
is attached to the shaft
31
and acted upon by an upper C-core ring
34
and a lower C-core ring
33
. An upper coil
36
and lower coil
35
are used to generate magnetic flux. A bias current is applied to each coil
35
,
36
to generate bias fluxes
37
and
38
. A control current is then applied in superposition to the bias currents in each coil
35
,
36
which generates control fluxes
39
and
40
. In
FIG. 1
, the upper control and bias fluxes
40
,
38
add and the lower control and bias fluxes
39
,
37
subtract. The net result is force exerted upward on the disk
32
that varies linearly with the control current. A non-dimensionalized example on the linearization is as follows. If the bias fluxes have an arbitrary value of
5
and a control flux is superposed with a value of 1, the flux on the top side of the disk becomes 6 and on the bottom side becomes 4. The net force is then (6{circumflex over ( )}2−4{circumflex over ( )}2) or 20. Because of the bias flux, the relation of force to control current becomes both linearized and amplified. With a control flux of 2, the resulting force would then be double, 40. Without the bias flux, control flux would only be applied to one core at a time to generate force and a control flux of 1 and 2 would result in forces of only 2 and 4. Two amplifiers would also be required for operation.
An improved design of magnetic thrust bearings places permanent magnets in series with the electromagnets so that the bias flux is generated without use of electric power. U.S. Pat. Nos. 3,937,148 and 5,003,211 show variations using this concept. This design increases the steady-state electrical efficiency, however the permeability of high energy permanent magnets is very low. Therefore, the electromagnets require much more control current to generate the same control flux because of the higher reluctance of the magnetic circuits.
FIG. 2
shows the configuration
50
. The thrust disk
52
is attached to the shaft
51
and is acted upon by an upper C-core ring
54
and a lower C-core ring
53
. Permanent magnets
57
and
58
generate the bias fluxes
59
and
60
. Opposed control currents in coils
55
and
56
generate the control fluxes
61
and
62
. As before, the control and bias fluxes are additive in one core
54
and subtractive in the other
53
, resulting in an upward force on the disk
32
. Unfortunately, magnets
57
and
58
have permeability comparable to an airgap. Therefore, the required control current to generate the equivalent control fluxes
61
and
62
as in the configuration
30
control fluxes
39
and
40
is much higher.
A further improvement is to use permanent magnets for generating bias flux but the permanent magnet flux paths are made non-coincident with the path of the electromagnet flux. The permanent magnets are not in series with the electromagnets but instead share only a portion of the same paths that include the airgaps. The result is a greatly improved design that allows for both linear and highly efficient control. U.S. Pat. No. 3,890,019 is one configuration and this is shown in FIG.
3
. The thrust disk
72
is attached to the shaft
71
and is acted upon by a single external C-core yoke ring
73
. A single coil
74
is used to generate the control flux
79
. Permanent magnets
75
and
76
generate the bias fluxes
77
and
78
. Superposition of the control and bias fluxes
79
,
77
,
78
cause an upward force on the disk
72
. The only drawback with this configuration is that it does not achieve the highest possible force capability or efficiency because of ill-defined large airgaps in the permanent magnetic flux paths
77
and
78
.
U.S. Pat. No. 3,865,442 is a more efficient design using the same concept of non-coincident control and bias flux paths.
FIG. 4
shows the configuration
130
. Three thrust disks
132
,
133
,
134
are attached to the shaft
131
. The thrust disks
132
,
133
,
134
are acted upon by a single external C-core ring
135
with a control coil
136
for producing control flux
141
. Permanent magnets
137
and
138
attached to the shaft
131
generate the bias fluxes
139
and
140
. The drawbacks of this design are the use of rotating permanent magnets, which limit the high speed rotation capability due to their low strength, and the complexity. The use of three thrust disks is also undesirable.
U.S. Pat. No. 3,955,858 discloses an improved thrust bearing design in which the permanent magnet is stationery. The configuration
90
is shown in FIG.
5
. Two thrust disks
92
and
93
are attached to the shaft
91
and are acted upon by stator rings
94
and
95
. A radially magnetized permanent magnet
96
generates the bias flux
99
. The control flux
100
is generated by the control coils
97
and
98
. As shown, superposition of the fluxes results in an upward force on the disks
92
and
93
. The design unfortunately has a more complicated than desired construction, including a radially magnetized permanent magnet and two thrust disks. U.S. Pat. No. 5,315,197 describes the same configuration but also discloses a modified version, allowing for use of only one thrust disk. The drawback to this design is the inclusion of a radial airgap in the magnetic circuit. The radial airgap causes generation of radially destabilizing forces. A similar configuration, U.S. Pat. No. 5,514,924, adds multiple radial control coils to the same design.
U.S. Pat. No. 5,250,865 shows further improved thrust bearing configuration by only requiring one thrust disk and all permanent magnets are stationery. Unfortunately, the invention is complicated and requires use of four permanent magnets with eight airgaps. The bearing also requires assembly of multiple precision pieces for generation of the five flux paths.
More recently, U.S. Pat. No. 5,804,899 discloses a magnetic bearing with a biased thrust actuator. This invention is same thrust bearing as disclosed in U.S. Pat. No. 5,317,197 but only with a large structure added and some separate permanent providing some radial centering force. A radially magnetized permanent magnet and two thrust disk portions are again required.
There still exists a need for a high force, high efficiency magnetic thrust bearing that can allow for high speed rotation and also has a simple, low cost construction
SUMMARY OF THE INVENTION
The invention is an improved magnetic thrust bearing that uses permanen
Huynh Co Si
McMullen Patrick T.
Calnetix
Fulbright & Jaworski LLP
Lam Thanh
LandOfFree
Magnetic thrust bearing with permanent bias flux does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic thrust bearing with permanent bias flux, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic thrust bearing with permanent bias flux will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3254316