WO2014007851A1

WO2014007851A1 - Active magnetic bearing assembly and arrangement of magnets therefor

Info

Publication number: WO2014007851A1
Application number: PCT/US2013/029706
Authority: WO
Inventors: Shuo Cheng; Dezheng Wu
Original assignee: Abb Research Ltd.
Priority date: 2012-07-03
Filing date: 2013-03-07
Publication date: 2014-01-09

Abstract

The subject matter described herein includes an active magnetic bearing assembly. The active magnetic bearing assembly includes a rotor configured to rotate with the shaft of a rotating machine. The active magnetic bearing assembly includes a stator that circumferentially surrounds the rotor and remains stationary. The assembly further includes a plurality of magnets coupled to the stator and circumferentially placed from each other about the stator. Each magnet has an inner portion and an outer portion. The inner portion is radially closer to the rotor than the outer portion. The inner and outer portions are of different material or shape from each other.

Description

ACTIVE MAGNETIC BEARING ASSEMBLY AND ARRANGEMENT OF

MAGNETS THEREFOR

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Patent Application Serial No. 61/667,934 filed July 3, 2012; the disclosure of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

The subject matter described herein relates to magnetic bearing assemblies. More particularly, the subject matter described herein relates to an active magnetic bearing assembly and an arrangement of magnets for such an assembly.

BACKGROUND

An active magnetic bearing assembly includes a rotor, a stator, position sensors, a controller, and a power amplifier. The rotor is supported or levitated by electromagnetic force so that there is no contact between the stator and the rotor. Active magnetic bearings are used in rotating machinery, such as electric rotary machines or turbo machinery to provide non-contact levitation or support. In order to have good dynamic response and force-current linearity, a magnetic bias flux is needed between the stator and the rotor. Typically, the bias flux is generated by stator coils or by permanent magnets in the stator. When permanent magnets are used to provide the bias flux, the active magnetic bearing can be configured in such a way that the bias flux in the rotor is homopolar, meaning that the magnetic polarity of the assembly does not change during operation. The permanent magnets create magnetic flux in the radial and axial directions of the magnetic bearing assembly.

One problem with current magnetic bearing assembly configurations where permanent magnets are used is that magnetic flux density is not axially uniform in the air gap between the rotor and the stator, where the axial direction is the direction in which the shaft extends. The non-uniformity in magnetic flux density is caused by the anisotropic permeability of the materials used in the stator. The highest flux density can be two or more times the lowest flux density in the air gap. The uneven distribution of the bias flux reduces the specific load capacity of the active magnetic bearing, which increases the dimensions of the active magnetic bearing. Increasing the dimensions of the active magnetic bearing is undesirable because the increased dimensions may change the rotodynamic performance of the shaft and lower the maximum speed of the shaft.

Accordingly, in light of these difficulties, there exists a need for an active magnetic bearing assembly with an arrangement of magnets to increase the uniformity of magnetic flux density in the air gap between the rotor and the stator.

SUMMARY

The subject matter described herein includes an active magnetic bearing assembly. The active magnetic bearing assembly includes a rotor configured to rotate with the shaft of a rotating machine. The active magnetic bearing assembly includes a stator that circumferentially surrounds the rotor and remains stationary. The assembly further includes a plurality of magnets coupled to the stator and circumferentially spaced from each other about the stator. Each magnet has an inner portion and an outer portion. The inner portion is radially closer to the rotor than the outer portion. The inner and outer portions are of different material or shape from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1A is a sectional view taken in an axial direction and Figure 1 B is a sectional view, taken along the plane B-B in Fig. 1A, of an active magnetic bearing assembly in accordance with a first embodiment of the subject matter described herein;

Figure 2A is a sectional view taken in an axial direction and Figure 2B is a sectional view, taken along the plane B-B in Fig. 2A, of an active magnetic bearing assembly in accordance with a second embodiment of the subject matter described herein;

Figure 3A is a sectional view taken in an axial direction and Figure 3B is a sectional view, taken along the plane B-B in Fig. 3A, of an active magnetic bearing assembly in accordance with a third embodiment of the subject matter described herein; and

Figure 4A is a sectional view taken in an axial direction and Figure 4B is a sectional view, taken along the plane B-B in Fig. 4A, of an active magnetic bearing assembly in accordance with a fourth embodiment of the subject matter described herein.

DETAILED DESCRIPTION

Figures 1A and 1 B are sectional views of an active magnetic bearing assembly according to an embodiment of the subject matter described herein. In the illustrated example, the magnetic bearing assembly includes a rotor 1 that rotates with and surrounds a shaft 2. A stator 3 remains stationary. A plurality of permanent magnets 4A and 4B is located on stator 3. A plurality of control coils 5 magnetically levitates or supports rotor 1. The stator 3 includes a plurality of poles 6.

As illustrated in Figure 1 A, a bias return ring 7 serves as a return path for magnetic flux. For example, magnetic flux lines, such as flux line 8, extend from the north pole of permanent magnet 4B, through stator 3, through air gap 9 between stator 3 and rotor 1 , through rotor 1 , back through air gap 9, through flux return ring 7, and back to the south pole of permanent magnet 4B. Due to the anisotropic permeability of the material that makes up stator 3, magnetic flux density caused by permanent magnet 4B alone would be uneven in the axial direction, which is represented by line XY in air gap 9, with the magnetic flux density being greater at the Y end of line XY, which degrades the performance of the magnetic bearing assembly.

In order to increase the uniformity in magnetic flux density, an active magnetic bearing assembly may include permanent magnets, each having an inner portion and an outer portion, where the inner and outer portions are physically different and/or shaped differently from each other in the axial direction. In Figures 1A and 1 B, the outer portion or magnet 4A is axially longer than the inner portion or magnet 4B. Because magnet 4A is axially longer than magnet 4B, the magnetic flux density in the axial direction caused by magnet 4A is different from that caused by magnet 4B. The differences in magnetic flux density caused by the two magnets or two magnetic portions results in a more even axial distribution of magnetic flux density in air gap 9.

As shown in Figures 1 A and 1 B, each permanent magnet 4A and 4B is located between stator 3 and bias return ring 7. The pairs or sets of permanent magnets 4A and 4B are also circumferentially spaced from each adjacent pair by an angle of 90 degrees. Each permanent magnet 4A and 4B has an arcuate structure that is centered about one of the four poles 6 of stator 3. Each permanent magnet 4A is positioned within a recess formed in an outer surface of stator 3, and each permanent magnet 4B is sandwiched between stator 3 and bias return ring 7. Stator 3 and bias return ring 7 may each be made from a laminated magnetic material, such as laminated silicon steel. In an alternate implementation, one or both of stator 3 and bias return ring 7 may be made from a solid magnetic material.

Figures 2A and 2B illustrate an active magnetic bearing assembly according to a second embodiment of the subject matter described herein. In Figures 2A and 2B, the active magnetic bearing assembly includes unitary magnets located at stator pole 6 and disposed between stator 3 and bias return ring 7. Each permanent magnet is shaped such that its outer surface is longer than its inner surface in the axial direction. That is, outer surface 4C of each permanent magnet is longer in the axial direction than inner surface 4D, and a slope or taper 10 exists between outer surface 4C and inner surface 4D. Again the result of this taper is to increase the uniformity of magnetic flux density in air gap 9.

Like the embodiment illustrated in Figures 1A and 1 B, in the active magnetic bearing assembly illustrated in Figures 2A and 2B, the permanent magnets are circumferentially separated from each other by an angle of 90 degrees, and each permanent magnet has an arcuate structure that is centered about one of the four poles 6 of stator 3. Taper 10 of each permanent magnet abuts a corresponding taper 11 formed in an outer surface of stator 3. Each permanent magnet is sandwiched between stator 3 and bias return ring 7.

Figures 3A and 3B illustrate an active magnetic bearing according to a third embodiment of the subject matter described herein. In Figures 3A and 3B, the outer and inner portions of each permanent magnet comprise separate permanent magnets 4A and 4B. However, unlike the embodiment illustrated in Figures 1A and 1 B, outer magnets 4A illustrated in Figures 3A and 3B may have the same axial length as inner magnets 4B. In addition, outer magnet 4A of each permanent magnet may be formed of a material with a different remanent flux density (B_R) than inner portion 4B. In this example, each outer magnet 4A has a higher remanent flux density than each inner magnet 4B.

Like the previous embodiments, in Figures 3A and 3B each set of permanent magnets 4A and 4B is circumferentially separated from the other sets by an angle of 90 degrees, and each permanent magnet 4A and 4B has an arcuate structure centered about a pole 6 of stator 3. Each set of permanent magnets 4A and 4B is sandwiched between stator 3 and bias return ring 7. It should also be noted that inner permanent magnets 4B have lower remanence than that of the outer permanent magnets 4A. The difference in remanence may be the result of inner permanent magnets 4B being physically different from outer permanent magnets 4A. Inner permanent magnets 4B may have different shapes and/or be made of different materials from outer permanent magnets 4A. For example, in Figure 3A, inner permanent magnet 4B is thicker in the radial direction than outer permanent magnets 4A.

Figures 4A and 4B illustrate an active magnetic bearing assembly according to another embodiment of the subject matter described herein where outer magnet 4A of each permanent magnet is axially longer than inner magnet 4B. Unlike the embodiment illustrated in Figures 1A and 1 B, in the embodiment illustrated in Figures 4A and 4B, each inner magnet 4B is axially centered about its respective outer portion 4B in view of the inclusion of two stators 3A and 3B, rather than a single stator and a bias return ring.

Like the previous embodiments, in the embodiment illustrated in Figures 4A and 4B, each set of permanent magnets 4A and 4B is circumferentially separated from the other sets by an angle of 90 degrees. Each permanent magnet 4A and 4B comprises an arcuate structure centered about one of the four poles 6 of stator 3A. Outer permanent magnets 4A are located in a recess formed in an outer surface of stators 3A and 3B Inner permanent magnets 4B are sandwiched between stators 3A and 3B. In the embodiments described above, each magnet is described as being a permanent magnet. However, the subject matter described herein is not limited to using permanent magnets. In any of the embodiments described herein, any one or more of the magnets can be replaced with electromagnets without departing from the scope of the subject matter described herein.

In the embodiments described above where magnets 4A and 4B are described as being separate magnets, it is understood that magnets 4A and 4B can be part of a unitary structure the same shape such that magnets 4A and 4B are portions of the same magnet. Similarly, in embodiments where magnets 4A and 4B are shown as being portions of the same permanent magnet, it is understood that magnets 4A and 4B may be physically distinct or separate from each other such that magnets 4A and 4B are separate magnets. In some embodiments, the outer magnet 4A of each set of permanent magnets may have a longer axial length than the inner magnet 4B in addition to having a higher remanent flux density than the inner portion 4B. In some embodiments, the outer surface of either or both of the outer and inner magnets 4A and 4B may be longer in the axial direction than the inner surface such that a slope or taper exists between the outer and inner surfaces of the outer magnet 4A and/or the inner magnet 4B.

It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.

Claims

CLAIMS What is claimed is:

1. A magnetic bearing assembly comprising:

a rotor configured to rotate with a shaft of a rotating machine; a first stator circumferentially surrounding the rotor and configured to remain stationary;

a plurality of control coils for magnetically supporting the rotor; and

a plurality of magnets coupled to the first stator and circumferentially spaced from each other around the first stator, each of the magnets having an inner portion and an outer portion, the inner portion being radially closer to the rotor than the outer portion, the inner and outer portions being of different material or shape from each other.

2. The magnetic bearing assembly of claim 1 wherein the outer portion of each magnet is longer than the inner portion in an axial direction.

3. The magnetic bearing assembly of claim 1 wherein each magnet includes an outer surface that is axially longer than an inner surface and a taper from the outer surface to the inner surface.

4. The magnetic bearing assembly of claim 1 wherein the outer portion of each magnet is configured to produce a higher flux density than the inner portion of each magnet.

5. The magnetic bearing assembly of claim 1 wherein each of the magnets comprises a permanent magnet.

6. The magnetic bearing assembly of claim 1 wherein each of the magnets comprises an electromagnet.

7. The magnetic bearing assembly of claim 1 wherein the outer portion of each magnet is physically distinct from the inner portion of each magnet such that the inner and outer portions of each magnet form distinct magnets.

8. The magnetic bearing assembly of claim 1 wherein each magnet is of unitary construction such that the inner and outer portions of each magnet are part of the same magnet.

9. The magnetic bearing assembly of claim 1 wherein inner portion of each magnet has a lower remanence that the outer portion.

10. The magnetic bearing assembly of claim 1 comprising a bias return ring coupled to the first stator, wherein each of the magnets is located between the first stator and the bias return ring.

1 1. The magnetic bearing assembly of claim 1 comprising a second stator circumferentially surrounding the rotor and axially spaced from the first stator, wherein each of the magnets is located between the first and second stators.

12. A magnetic bearing assembly comprising:

a plurality of control coils for magnetically supporting the rotor; and

a plurality of permanent magnets coupled to the first stator and circumferentially spaced from each other around the first stator, each of the permanent magnets having an inner portion and an outer portion, the inner portion being radially closer to the rotor than the outer portion, the inner portion having a lower remanence that the outer portion.

13. The magnetic bearing assembly of claim 12 wherein the outer portion of each permanent magnet is longer than the inner portion in an axial direction.

14. The magnetic bearing assembly of claim 12 wherein each permanent magnet includes an outer surface that is axially longer than an inner surface and a taper from the outer surface to the inner surface.

15. The magnetic bearing assembly of claim 12 wherein the outer portion of each permanent magnet is configured to produce a higher magnetic flux density in the axial direction than the inner portion of each permanent magnet.

16. The magnetic bearing assembly of claim 12 wherein the outer portion of each permanent magnet is physically distinct from the inner portion of each permanent magnet such that the inner and outer portions of each permanent magnet form distinct permanent magnets.

17. The magnetic bearing assembly of claim 12 wherein each permanent magnet is of unitary construction such that the inner and outer portions of each permanent magnet are part of the same permanent magnet.

18. The magnetic bearing assembly of claim 12 comprising a bias return ring coupled to the first stator, wherein each of the permanent magnets is located between the first stator and the bias return ring.

19. The magnetic bearing assembly of claim 12 comprising a second stator circumferentially surrounding the rotor and axially spaced from the first stator, wherein each of the permanent magnets is located between the first and second stators.