Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-11-02
2004-03-09
Le, Dang (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S181000, C310S090000
Reexamination Certificate
active
06703735
ABSTRACT:
This invention pertains to active magnetic thrust bearings and more particularly an active magnetic thrust bearing that acts in cooperation with only a single axial side of a rotor, using an efficient permanent magnet bias for linearized and highly amplified control. Compared with prior art active magnetic thrust bearings that use permanent magnet bias on two or more axial surfaces, the invention offers greatly simplified construction and assembly and reduced costs.
BACKGROUND OF THE INVENTION
Existing designs of active magnetic thrust bearings have suffered from problems including nonlinear control, inefficient force generation, and complex construction with actuation on two or more axial surf of a rotor. Many designs that use efficient force generation from permanent magnet bias, use one Or more thrust disks attached to the rotating object, and a stator that must be assembled to enclose a disk or to be enclosed by multiple disks. This type of construction is undesirable because it requires high tolerances on multiple piece assemblies, is expensive in terms of the multiple precision pieces and their assembly and because of the difficulties in assembly and disassembly of the rotor and bearing system It would be preferable to have an active magnetic thrust bearing that could operate on a single axial side of the rotor, facilitating much simpler and lower cost construction
A prior art single sided active magnetic thrust bearing
30
using an unbiased electromagnet, shown in
FIG. 1
, includes a rotor or thrust disk
32
attached to a shaft
31
, and a stator
39
constructed of a ferromagnetic yoke
33
located adjacent to the disk
32
. The yoke
33
has an electromagnetic coil
34
and two ring poles
35
and
36
that form an axial air gap
37
between the stator yoke
33
and the thrust disk
32
. Current through the electromagnetic coil
34
produces a controllable magnetic flux
38
that attracts the thrust disk
32
toward the stator
33
.
Unfortunately, the force to current response is nonlinear, which makes control of the magnetic bearing
30
difficult. The force generated is also small for the amount of current in the coil
34
. Many turns of the coil
34
could be used to create a high intensity of flux
38
with minimal current, however this increases the inductance of the coil
34
and slows the response time, making it unsuitable for use in magnetic levitation bearings, This magnetic bearing
30
also generates a very high unstable tilting moment because a small change in the distance between the poles
35
,
36
and the disk
32
causes a large change in the axial force. The magnetic bearing
30
also produces force only in the vertical direction. A positive current or negative current in the coil
34
both cause an upward force. To increase the force generation per control current and to make the response linear, a large bias current can be continuously run through the coil
34
. The control current is then superposed on top of that current to provide a controllable force. The problem with this technique is that the magnetic bearing requires constant power consumption, and the unstable tilt moment generated is very large making full levitation systems more difficult with a nonlinear force-to-position response. Establishing a large bias flux through appreciable air gaps also requires a very large bias current and or number of coil turns.
A single sided active magnetic thrust bearing configuration using a permanent magnet in series with an electromagnet for generating bias flux of prior art is shown in FIG.
2
. In this design, a permanent magnet is used to create the bias flux for the bearing. The active magnetic thrust bearing
40
is comprised of a thrust disk
42
attached to a shaft
41
, and a cooperating ferromagnetic yoke
43
of an electromagnet
52
that is fastened to a fixed stator
51
and closely spaced from the disk
42
. An electromagnetic coil
44
in the electromagnet
52
, for generation of a control flux
49
, is wound between inner and outer annular ring poles
46
and
47
of the yoke
43
. A permanent magnet
45
generates a bias flux
50
without requiring electric power to the coil
44
. The control and bias flux
49
,
50
exit and enter the stator through the ring poles
46
and
47
. The spacing between the thrust disk
42
and the poles
46
,
47
of the yoke
43
constitute an axial air gap
48
between the fixed yoke
43
and the rotating thrust disc
42
.
Although the permanent magnet
45
can generate a high bias flux
50
without requiring power and the flux can be established over larger air gaps
48
, this design of magnetic bearing
40
has several deficiencies. The permanent magnet has a very low magnetic permeability, similar to an air gap. Therefore the control flux
49
created by the coil
44
must drive through a much larger effective air gap, so the amount of control flux generated per amount of coil current is significantly reduced. The force efficiency of the magnetic bearing is lower than desired. Also, operation with a control flux opposite in direction to the bias flux for causing a reduction in anal force can be difficult since the coil must work against the permanent magnet.
Other types of active magnetic thrust bearings that have linear response and efficient force generation have been developed. These thrust bearing use permanent magnets to generate a bias flux and electromagnetic coils to generate the control flux, However in these designs, the bearing is designed such that the coil need not drive the control flux through the permanent magnet. The control flux and the bias flux have non-coincident paths, but they share the portions of their paths including the axial air gaps where the fluxes add or subtract for highly amplified force generation. Because the control flux need not pass through the high reluctance permanent magnet, the amount of control flux per coil current is much greater. Several designs using this principal have been developed. Unfortunately, all such designs work by using two axial sides of the rotor and two or more axial surfaces. The control flux provides a highly efficient force response because the control flux adds with the bias flux on one axial side of the rotor and at the same time is subtracted from the bias flux on the opposite side. A reverse in the control current causes a reverse in the direction of the generated force. The problem with these magnetic bearings is that they require a complicated structure in which the stator must axially enclose a single thrust disk or the stator itself is enclosed by two or more disks. The multiple precision pieces are expensive and assembly and disassembly of machines using these bearings is difficult. The stator is essentially locked around the rotor when assembled. This can hinder magnetic bearing implementation in many applications.
Therefore, a need existed has long for a high force, high efficiency magnetic thrust bearing with a simple construction that can act in cooperation with a single axial side of a rotor.
SUMMARY OF THE INVENTION
Accordingly, this invention provides an active magnetic thrust bearing that acts in cooperation with only a single axial side of a rotor that is rotatable about an axis of rotation, while also having an efficient permanent magnet bias for linearized and highly amplified control. The active magnetic bearing uses two concentric ring poles that axially face a ferromagnetic axial surface of the rotor, creating two annular axial air gaps. A permanent magnet in the stator drives a bias flux through a first path including one ring pole, its air gap, the rotor, the second air gap and the second ring pole. The permanent magnet also drives flux through a second path in the stator, by-passing the rotor. The second path has a comparable reluctance to that level of flux produced by the permanent magnet.
An electromagnetic coil in the stator is wound coaxially with the axis of rotation. The coil drives a control flux in a circuit including the second path, both ring poles and axial air gaps. The bias and control fluxes a
Elkassabgi Heba Y.
Indigo Energy, Inc.
Le Dang
Neary J. Michael
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