WO2018037158A1

WO2018037158A1 - A stator module for an axial magnetic bearing

Info

Publication number: WO2018037158A1
Application number: PCT/FI2017/050585
Authority: WO
Inventors: Rafal Jastrzebski; Antti SUIKKI
Original assignee: Lappeenrannan Teknillinen Yliopisto
Priority date: 2016-08-24
Filing date: 2017-08-22
Publication date: 2018-03-01
Also published as: FI20165629A; FI128371B

Abstract

A stator module for an axial magnetic bearing comprises a one or more coils (101) for generating a magnetic flux and a magnetic core structure (102) for constituting a part of a magnetic circuit for the magnetic flux. The magnetic core structure comprises magnetic core elements (103-110) supported by a frame structure (111) of the stator module. Each magnetic core element comprises a first section for conducting a part of the magnetic flux to a rotor, a second section for conducting the part of the magnetic flux from the rotor, and a third section for conducting the part of the magnetic flux from the second section to the first section. As there are many magnetic core elements, each magnetic core element can be arranged to comprise stacked sheets for conducting the magnetic flux along the sheets.

Description

A stator module for an axial magnetic bearing

Field of the disclosure

The disclosure relates generally to magnetic levitation. More particularly, the disclosure relates to a stator module suitable for operating as a part of an axial magnetic bearing. Furthermore, the disclosure relates to an axial magnetic bearing.

Background

Magnetic levitation systems, such as e.g. active magnetic bearings "AMB", are commonly known in the art. Magnetic levitation systems are commonly utilized for supporting a rotating or oscillating object. Using magnetic levitation in rotating machinery results in for example: reduction of friction, oil-free operation, lower maintenance costs, and/or higher reliability when compared to traditional rotating machines with mechanical bearings. An axial magnetic bearing comprises typically a rotor and a stator where the stator is configured to direct a controllable axial magnetic force to the rotor. The stator comprises one or more stator modules each comprising a coil for generating a magnetic flux and a magnetic core element for constituting a magnetic circuit for the magnetic flux together with the rotor and with the air-gaps between the magnetic core element and the rotor. In a case where the stator comprises two stator modules of the kind mentioned above, the stator modules are typically arranged so that the axial magnetic forces directed by the stator modules to the rotor are opposite to each other. In a case where the stator comprises only one stator module of the kind mentioned above, the axial magnetic bearing may comprise for example a permanent magnet system for directing an axial magnetic force to the rotor so that the axial magnetic force directed to the rotor by the permanent magnet system is opposite to the axial magnetic force directed to the rotor by the stator module.

An axial magnetic bearing of the kind described above is however not free from challenges. One of the challenges is related to the magnetic core element which is usually a rotationally symmetric element that comprises an annular groove for the coil of the axial magnetic bearing. Typically, the magnetic core element is made of solid steel because it is challenging to construct a laminated structure, i.e. a stacked sheet structure, so that the magnetic flux is conducted along the sheets and not through the sheets. An inherent drawback of a magnetic core element made of solid steel is that a changing magnetic flux induces eddy currents which, in turn, tend to suppress the changes of the magnetic flux. This phenomenon limits the operational bandwidth of the axial magnetic bearing. It is also possible that a magnetic core element is made of e.g. ferrite or other material that is electrically less conductive than solid steel but these materials are typically more expensive than steel. Furthermore, the relative magnetic permeability of these materials is typically smaller than that of steel and thus there can be a need to increase the physical size of the magnetic core element and thereby also the physical size of the axial magnetic bearing. The increased physical size may complicate e.g. the integration of the axial magnetic bearing with radial magnetic bearings. Furthermore, in some cases, a magnetic core element made of material whose relative magnetic permeability is smaller than that of steel may lead to a situation in which the axial magnetic bearing needs to be provided with permanent magnets that would not be needed if the magnetic core element were made of steel.

Summary

The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In this document, the word "geometric" when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric line or axis, a geometric plane, a non-planar geometric surface, a geometric room, or any other geometric entity that is one, two, or three dimensional. In accordance with the invention, there is provided a new stator module suitable for operating as a part of an axial magnetic bearing. A stator module according to the invention comprises:

- one or more coils for conducting one or more electric currents so as to generate a magnetic flux, and

- a magnetic core structure for constituting a magnetic circuit for the magnetic flux together with the rotor of the axial magnetic bearing and with air-gaps between the magnetic core structure and the rotor.

The magnetic core structure comprises a plurality of magnetic core elements supported by a frame structure of the stator module. Each of the magnetic core elements comprises a first section for conducting a part of the magnetic flux to the rotor, a second section for conducting the part of the magnetic flux from the rotor, and a third section for conducting the part of the magnetic flux from the second section to the first section. Each of the magnetic core elements is arranged to comprise stacked sheets for conducting the appropriate part of the magnetic flux along the sheets. The sheets are made of ferromagnetic material and there are layers of electrical insulator between the sheets. The use of the laminated structure, i.e. the stacked sheets, is facilitated by that fact that there are many magnetic core elements instead of a single ring-shaped magnetic core element. In accordance with the invention, there is provided also a new axial magnetic bearing that comprises:

- a rotor,

- at least one stator module according to the invention for directing an axial magnetic force to the rotor, - equipment for generating a position signal indicative of an axial position of the rotor, and - a controller for receiving the position signal and for controlling electric current of the coil of the stator module on the basis of a deviation of the axial position of the rotor module from a reference axial position.

The axial magnetic bearing may comprise two stator modules for directing mutually opposite axial magnetic forces to the rotor. However, it is worth noting that the axial magnetic bearing may comprise only one stator module for supporting the rotor module against axial loading having a constant direction, e.g. against downwards directed loading including the gravity force. It is also possible that the axial magnetic bearing comprises, in addition to the stator module, one or more permanent magnets so that the stator module and the one or more permanent magnets are arranged to direct mutually opposite axial magnetic forces to the rotor.

A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in connection with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of un-recited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figures 1 a and 1 b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention, figures 2a and 2b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention, figures 3a and 3b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention, figure 4 illustrates an axial magnetic bearing according to an exemplifying and non- limiting embodiment of the invention, and figure 5 shows a perspective view of a stator module according to an exemplifying and non-limiting embodiment of the invention.

Description of exemplifying and non-limiting embodiments The specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given below are not exhaustive unless otherwise explicitly stated.

Figures 1 a and 1 b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention. The stator module is suitable for operating as a part of an axial magnetic bearing. Figure 1 a shows a front view of the stator module so that the viewing direction is parallel with the geometric rotational axis 1 18 of a rotor 1 17 of the axial magnetic bearing. The geometric rotational axis 1 18 is parallel with the z-axis of a coordinate system 190. The rotor 1 17 can be for example a part of a rotor of a turbo electric machine such as e.g. a turbo generator or a turbo compressor. Figure 1 b shows a section view of the stator module so that the section is taken along a line A-A shown in figure 1 a and the section plane is parallel with the xz-plane of the coordinate system 190. The stator module comprises an annular coil 101 for conducting electric current so as to generate a magnetic flux. The conductor turns of the coil 101 are arranged to surround the rotor 1 17 so as to generate a circumferential current density. In figure 1 b, exemplifying flux lines of the magnetic flux are depicted with dashed lines 127 and 128. The stator module comprises a magnetic core structure 102. As illustrated in figure 1 b, the magnetic core structure 102, the rotor 1 17, and the air-gaps between the magnetic core structure and the rotor constitute a magnetic circuit for the magnetic flux so that an axial magnetic force is directed to the rotor 1 17.

The magnetic core structure 102 comprises magnetic core elements 103, 104, 105, 106, 107, 108, 109, and 1 10 which are mechanically supported by a frame structure 1 1 1 of the stator module. As shown in figure 1 a, the magnetic core elements 103- 1 10 are placed equidistantly on a circumference of a geometric circle. Each of the magnetic core elements 103-1 10 constitutes a groove for a part of the coil as illustrated in figures 1 a and 1 b. Each of the magnetic core elements 103-1 10 comprises a first section for conducting a part of the magnetic flux to the rotor, a second section for conducting the part of the magnetic flux from the rotor, and a third section for conducting the part of the magnetic flux from the second section to the first section. In figure 1 b, the first section of the magnetic core element 103 is denoted with a reference 1 12, the second section of the magnetic core element 103 is denoted with a reference 1 13, and the third section of the magnetic core element 103 is denoted with a reference 1 14. As illustrated in figures 1 a and 1 b, the magnetic core elements 103-1 10 comprise stacked sheets for conducting the magnetic flux along the sheets. In this exemplifying stator module, the sheets are stacked in a substantially circumferential direction as illustrated in figure 1 a. For example, the sheets of the magnetic core elements 103 and 107 are stacked in the y-direction of the coordinate system 190, and the sheets of the magnetic core elements 105 and 109 are stacked in the x-direction of the coordinate system 190. As illustrated in figure 1 b, each of the sheets is substantially planar and U-shaped so that the sheet comprises a first part constituting a part of the first section 1 12 of the magnetic core element, a second part constituting a part of the second section 1 13 of the magnetic core element, and a third part constituting a part of the third section 1 14 of the magnetic core element. In this exemplifying stator module, the first and second sections of each magnetic core element protrude, in the axial direction of the axial magnetic bearing, from the third section of the magnetic core element. It is also possible that the first and/or second sections are not axial for example in cases where there is a need to arrange more room for the coil of the stator module.

In the exemplifying stator module illustrated in figures 1 a and 1 b, the first and second sections of the magnetic core elements 103-1 10 are curved so that, when the stator module is seen in the axial direction corresponding to figure 1 a, circumferential outlines of the air-gap surfaces of the magnetic core structure are arches of a geometric circle whose center coincides with the geometric rotational axis 1 18 of the rotor. In figure 1 a, two of the air-gap surfaces of the magnetic core structure 102 are denoted with references 1 15 and 1 16.

Figures 2a and 2b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention. The stator module is suitable for operating as a part of an axial magnetic bearing. Figure 2a shows a front view of the stator module so that the viewing direction is parallel with the geometric rotational axis 218 of a rotor 217 of the axial magnetic bearing. The geometric rotational axis 218 is parallel with the z-axis of a coordinate system 290. Figure 2b shows a view of a section taken along a line A-A shown in figure 2a. The section plane is parallel with the xz-plane of the coordinate system 290. The stator module comprises a coil 201 for conducting electric current so as to generate a magnetic flux. The conductor turns of the coil 201 are arranged to surround the rotor 217 so as to generate a circumferential current density. The stator module comprises a magnetic core structure 202. In the same way as illustrated in figure 1 b, the magnetic core structure 202, the rotor 217, and the air-gaps between the magnetic core structure and the rotor constitute a magnetic circuit for the magnetic flux so that an axial magnetic force is directed to the rotor 217.

The magnetic core structure 202 comprises magnetic core elements 203, 204, 205, 206, 207, 208, 209, and 210 which are mechanically supported by a frame structure 21 1 of the stator module. Each of the magnetic core elements 203-210 constitutes a groove for a part of the coil 201 as illustrated in figures 2a and 2b. Each of the magnetic core elements 203-210 comprises a first section for conducting a part of the magnetic flux to the rotor, a second section for conducting the part of the magnetic flux from the rotor, and a third section for conducting the part of the magnetic flux from the second section to the first section. In figure 2b, the first section of the magnetic core element 203 is denoted with a reference 212, the second section of the magnetic core element 203 is denoted with a reference 213, and the third section of the magnetic core element 203 is denoted with a reference 214. The magnetic core elements 203-210 comprise stacked sheets for conducting the magnetic flux along the sheets. In this exemplifying stator module, each sheet is bent so that the sheet comprises a first part constituting a part of the first section of the magnetic core element under consideration, a second part constituting a part of the second section of the magnetic core element, and a third part constituting a part of the third section of the magnetic core element. Thus, in this exemplifying case, the sheets are bent to be substantially U-shaped as illustrated in figure 2b. In this exemplifying stator module, the first and second sections of each magnetic core element protrude, in the axial direction of the axial magnetic bearing, from the third section of the magnetic core element. It is also possible that the first and/or second sections are not axial for example in cases where there is a need to arrange more room for the coil of the stator module.

Figures 3a and 3b illustrate a stator module according to an exemplifying and non- limiting embodiment of the invention. The stator module is suitable for operating as a part of an axial magnetic bearing. Figure 3a shows a front view of the stator module so that the viewing direction is parallel with the geometric rotational axis 318 of a rotor 317 of the axial magnetic bearing. The geometric rotational axis 318 is parallel with the z-axis of a coordinate system 390. Figure 3b shows a view of a section taken along a line A-A shown in figure 3a. The section plane is parallel with the xz-plane of the coordinate system 390. The stator module comprises coils for conducting electric currents each of which generates a part of the total magnetic flux flowing through the rotor 317. In figure 3a, five of the coils are denoted with references 301 a, 301 b, 301 c, 301 d, and 301 e. The stator module comprises a magnetic core structure 302. As illustrated in figure 3b, the magnetic core structure 302, the rotor 317, and the air-gaps between the magnetic core structure and the rotor constitute a magnetic circuit for the magnetic flux so that an axial magnetic force is directed to the rotor 317. In figure 3b, an exemplifying flux line of the magnetic flux is depicted with a dashed line 327.

The magnetic core structure 302 comprises magnetic core elements which are mechanically supported by a frame structure 31 1 of the stator module. In figure 3a, four of the magnetic core elements are denoted with references 303, 304, 305, and 306. Each of the magnetic core elements constitutes a substantially radial groove for two of the coils. For example, the magnetic core element 304 constitutes a substantially radial groove for the coils 301 c and 301 d. Each of the magnetic core elements comprises a first section for conducting a part of the magnetic flux to the rotor, a second section for conducting the part of the magnetic flux from the rotor, and a third section for conducting the part of the magnetic flux from the second section to the first section. In figure 3b, the first section of the magnetic core element 304 is denoted with a reference 312, the second section of the magnetic core element 304 is denoted with a reference 313, and the third section of the magnetic core element 304 is denoted with a reference 314. As illustrated in figures 3a and 3b, the magnetic core elements comprise stacked sheets for conducting the magnetic flux along the sheets. In this exemplifying stator module, the sheets of each magnetic core element are stacked in a substantially radial direction as illustrated in figure 3a. For example, the sheets of the magnetic core element 304 are stacked in the y-direction of the coordinate system 390, and the sheets of the magnetic core element 306 are stacked in the x-direction of the coordinate system 390. As illustrated in figure 3b, each of the sheets is substantially planar and U- shaped so that the sheet comprises a first part constituting a part of the first section 312 of the magnetic core element, a second part constituting a part of the second section 313 of the magnetic core element, and a third part constituting a part of the third section 314 of the magnetic core element. Figure 4 illustrates an axial magnetic bearing according to an exemplifying and non- limiting embodiment of the invention. The axial direction is the z-direction of a coordinate system 490. The axial magnetic bearing comprises a rotor 417 that comprises a disc 418. The rotor can be for example a rotor of an electric machine where the axis of rotation is parallel with the z-axis of the coordinate system 490. The axial magnetic bearing comprises equipment for generating a position signal P_z indicative of the axial position of the rotor module. In the exemplifying case illustrated in figure 4, the equipment for generating the position signal P_z comprises a sensor 421 and a circuitry 422 for generating the position signal on the basis of the output signal of the sensor. The sensor 421 can be, for example but not necessarily, an inductive sensor where the inductance is dependent on the distance from the sensor 421 to a conical surface of the rotor, and the circuitry 422 can be configured to form the position signal P_z on the basis of the inductance. The equipment for generating the position signal P_z comprises advantageously also another sensor facing towards another conical surface of the rotor, where the other conical surface tapers in the negative z-direction of the coordinate system 490. In this exemplifying case, the circuitry 422 can be configured to form the position signal Pz on the basis of the difference between the inductances of the sensors. The other sensor and the other conical surface of the rotor are not shown in figure 4.

The axial magnetic bearing comprises stator modules 419 and 420 according to an exemplifying embodiment of the invention. The stator modules 419 and 420 are arranged to direct mutually opposite axial magnetic forces to the disc 418 of the rotor 417. In figure 4, the stator modules 419 and 420 are shown as section views where the section plane is parallel with the xz-plane of the coordinate system 490. Exemplifying flux lines of the magnetic fluxes generated by the stator modules are depicted with dashed lines in figure 4. The axial magnetic bearing comprises a controller 426 for receiving the position signal P_z and for controlling electric currents of the stator modules 419 and 420 on the basis of a deviation of the axial position of the rotor 417 from the reference axial position. The controller 426 comprises a control section 425 and controllable output stages 423 and 424 for supplying electric currents to the coils of the stator modules 419 and 420. The control section 425 is configured to control the output stages 423 and 424 so that the position signal P_z is driven to the reference value of the position signal. The control section 425 can be implemented with one or more analogue circuits and/or with one or more digital circuits each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit "ASIC", or a configurable hardware processor such as, for example, a field programmable gate array "FPGA".

Figure 5 shows a perspective view of a stator module 550 that is suitable for operating as a part of an axial magnetic bearing. The coil of the stator module 550 is not shown in figure 5. Figure 5 does not depict how the magnetic core elements of the stator module 550 are composed of stacked sheets. The magnetic core elements can be composed of stacked sheets for example in the same way as the magnetic core elements 103-1 10 shown in figure 1 a are composed of stacked sheets. The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.

Claims

What is claimed is:

1 . A stator module for an axial magnetic bearing, the stator module comprising:

- one or more coils (101 , 201 , 301 a-301 e) for conducting one or more electric currents so as to generate a magnetic flux, and - a magnetic core structure (102, 202, 302) for constituting a magnetic circuit for the magnetic flux together with a rotor of the axial magnetic bearing and with air-gaps between the magnetic core structure and the rotor, characterized in that the magnetic core structure comprises a plurality of magnetic core elements (103-1 10, 203-210, 303-306) supported by a frame structure (1 1 1 , 21 1 , 31 1 ) of the stator module, each of the magnetic core elements comprising a first section (1 12, 212, 312) for conducting a part of the magnetic flux to the rotor, a second section (1 13, 213, 313) for conducting the part of the magnetic flux from the rotor, and a third section (1 14, 214, 314) for conducting the part of the magnetic flux from the second section to the first section, wherein each of the magnetic core elements comprises stacked sheets for conducting the part of the magnetic flux along the sheets.

2. A stator module according to claim 1 , wherein the coil (101 , 201 ) comprises coil turns surrounding the rotor so as to generate a substantially circumferential current density and each of the magnetic core elements (103-1 10, 203-210) constitutes a substantially circumferential groove for a part of the coil.

3. A stator module according to claim 2, wherein the sheets are stacked in a substantially circumferential direction, and each of the sheets is substantially planar and U-shaped so that the sheet comprises a first part constituting a part of the first section (1 12) of the magnetic core element, a second part constituting a part of the second section (1 13) of the magnetic core element, and a third part constituting a part of the third section (1 14) of the magnetic core element.

4. A stator module according to claim 3, wherein the first and second sections of the magnetic core elements (1 12, 1 13) have curved shapes so that, when the stator module is seen in an axial direction of the axial magnetic bearing, circumferential outlines of air-gap surfaces (1 15, 1 16) of the magnetic core structure are arches whose center of curvature coincides with a geometric rotational axis of the rotor.

5. A stator module according to claim 2, wherein each of the sheets is bent so that parts of the sheets constituting the first and second sections (212, 213) of the magnetic core element are stacked substantially in a radial direction and parts of the sheets constituting the third section (214) of the magnetic core element are stacked substantially in an axial direction.

6. A stator module according to claim 1 , wherein the magnetic core elements (303-306) constitute substantially radial grooves for the coils (301 a-301 e), the sheets of each of the magnetic core elements are stacked in a substantially radial direction, and each of the sheets is substantially planar and U-shaped so that the sheet comprises a first part constituting a part of the first section (312) of the magnetic core element, a second part constituting a part of the second section (313) of the magnetic core element, and a third part constituting a part of the third section (314) of the magnetic core element.

7. A stator module according to any of claims 1 -6, wherein the first and second sections (1 12, 1 13, 212, 213, ) of the magnetic core element protrude, in an axial direction of the axial magnetic bearing, from the third section of the magnetic core element.

8. A stator module according to any of claims 1 -7, wherein the magnetic core elements are placed equidistantly on a circumference of a geometric circle.

9. An axial magnetic bearing comprising:

- a rotor (417),

- at least one stator module (419) according to any of claims 1 -8 for directing an axial magnetic force to the rotor,

- equipment (421 , 422) for generating a position signal indicative of an axial position of the rotor, and - a controller (426) for receiving the position signal and for controlling electric current of the coil of the stator module on the basis of a deviation of the axial position of the rotor module from a reference axial position.

10. An axial magnetic bearing according to claim 9, wherein the axial magnetic bearing comprises another stator module (420) for directing another axial magnetic force to the rotor and the controller is configured to control electric current of a coil of the other stator module on the basis of the deviation of the axial position of the rotor from the reference axial position, the axial magnetic forces directed by the stator modules to the rotor being opposite to each other.