阅读说明：本技术 磁轴承装置中的杂散磁通补偿 (Stray flux compensation in magnetic bearing devices ) 是由 菲利普·布勒 于 2018-07-05 设计创作，主要内容包括：一种用于使转子(22)磁悬置以用于绕旋转轴(A)旋转的磁轴承装置包括放大器装置、第一主线圈(p)和第二主线圈(n)。为了补偿当主线圈被提供有来自放大器装置的电流时引起的杂散磁通，补偿线圈(c)以如下这样的极性连接在主线圈的公共节点与放大器装置之间：使得流过补偿线圈的电流将减少由主线圈(p，n)引起的杂散磁通。(A magnetic bearing device for magnetically suspending a rotor (22) for rotation about a rotational axis (a) comprises an amplifier device, a first main coil (p) and a second main coil (n). In order to compensate for stray magnetic flux caused when the main coil is supplied with current from the amplifier device, a compensation coil (c) is connected between the common node of the main coils and the amplifier device with such polarity that: so that the current flowing through the compensation coil will reduce the stray flux caused by the main coil (p, n).)

1. A magnetic bearing device for magnetically suspending a rotor (22) for rotation about an axis of rotation (A),

the magnetic bearing device comprising an amplifier device (10), a first main coil (p) extending around the rotational axis (A) and a second main coil (n) extending around the rotational axis (A),

one terminal (p1) of the first main winding (p) is connected to a first output (11) of the amplifier device (10);

one terminal (n1) of the second primary winding (n) is connected to a second output (12) of the amplifier device (10);

a further terminal (p2) of the first primary winding (p) is connected to a further terminal (n2) of the second primary winding (n) at a common node (14);

the main coil (p, n) is supplied with a current (i) by the amplifier device (10)_p，i_n) The stray magnetic flux is caused when the magnetic flux is generated,

characterized in that the magnetic bearing means comprise at least one compensation coil (c; cp, cn) connected between the third output terminal (13) of the amplifier means (10) and the common node (14),

characterized in that the compensation coil (c; cp, cn) extends around the axis of rotation (A) and

characterized in that the first main coil (p), the second main coil (n) and the compensation coil (c) are connected to the amplifier device with such polarities as: causing any current (i) flowing between the amplifier device (10) and the common node (14) through the compensation coil (c)_c) Inducing a magnetic field that reduces the stray magnetic flux induced by the main coil (p, n).

2. A magnetic bearing device according to claim 1, wherein the first main coil (p), the second main coil (n) and the compensation coil (c) have substantially the same number of turns around the rotation axis (a).

3. The magnetic bearing device according to claim 1 or 2, further comprising a housing (40), the first main coil (p), the second main coil (n) and the compensation coil (c) being arranged within the housing (40).

4. Magnetic bearing device according to any one of the preceding claims, wherein the magnetic bearing device comprises an active axial magnetic bearing comprising a first axial stator assembly (31) containing the first main coil (p) and a second axial stator assembly (32) containing the second main coil (n),

the first axial stator assembly (31) and the rotor (22) defining a first control flux path;

the second axial stator assembly (32) and the rotor (22) defining a second control flux path; and is

The compensation coil (c) is arranged outside the first and second control flux paths.

5. The magnetic bearing device of claim 4, further comprising an active radial magnetic bearing comprising a radial stator assembly (33) containing a plurality of radial coils (r), the compensation coils being arranged adjacent to the radial stator assembly (33), radially surrounding the radial coils (r).

6. The magnetic bearing device according to any of the preceding claims, comprising at least two compensation coils (cp, cn) connected in a series configuration between the third output (13) of the amplifier device (10) and the common node (14), each compensation coil (cp, cn) extending around the rotational axis (a).

7. A magnetic bearing arrangement according to claim 6, wherein the compensation coils (cp, cn) together have substantially the same number of turns around the rotational axis (A) as each of the first and second main coils (p, n).

8. A magnetic bearing device according to claim 6 or 7, wherein the magnetic bearing device comprises an active axial magnetic bearing comprising a first axial stator assembly (31) containing the first main coil (p) and a second axial stator assembly (32) containing the second main coil (n),

the first axial stator assembly (31) and the rotor (22) defining a first control flux path;

the second axial stator assembly (32) and the rotor (22) defining a second control flux path,

wherein one of the compensation coils (cp) is arranged adjacent to the first axial stator assembly and the other of the compensation coils (cn) is arranged adjacent to the second axial stator assembly.

9. A method of operating a magnetic bearing device according to any one of the preceding claims, the method comprising operating the amplifier device (10) to provide a first current (i) through the first main coil (p)_p) A second current (i) through the second main coil (n)_n) And a third current (i) through the compensation coil (c)_c)。

Technical Field

The present invention relates to a magnetic bearing device and a method of operating a magnetic bearing device.

Background

Magnetic bearings are commonly used to support a rotor having a shaft in a non-contact manner. One type of magnetic bearing is an axial bearing that inhibits movement of the rotor in an axial direction relative to its rotational axis. To this end, the rotor typically comprises a disc shaped portion and the two stator assemblies are arranged to face two opposite sides of the disc shaped portion. Each stator assembly includes a coil extending around an axis. By providing a current to the coils, each coil generates a magnetic field. Since the stator assembly and the rotor comprise magnetically conductive materials, the magnetic field causes a magnetic flux through the respective stator assembly and the rotor that passes through an air gap between the stator assembly and the rotor and thereby generates an attractive magnetic force that pulls the rotor toward the respective stator assembly. The axial position of the rotor relative to the stator assembly can be controlled by employing a position sensor for measuring the axial position of the rotor and controlling the current through the two coils in accordance with the sensor output. The stator assembly is typically surrounded by a housing, which may be made of a magnetically conductive material.

The magnetic field generated by the coil also causes stray magnetic flux through the shaft and through the housing surrounding the stator assembly. Such stray magnetic flux is generally undesirable because it may adversely affect other components of the machine, particularly the position sensors, the radial magnetic bearings, the auxiliary bearings and the seals. Furthermore, if the stray field extends to an area outside the housing, the stray flux may create disturbances in the external system. Stray flux may also negatively affect the operation of the motor if the rotor is driven by the motor.

To reduce stray magnetic flux, US 5,084,644 suggests providing additional compensation coils associated with each stator assembly. The compensation coils are arranged at positions where they do not exert a significant force on the rotor. The bucking coils generate a magnetic field to oppose and substantially cancel stray magnetic flux generated by the primary coils of the respective stator assemblies. To this end, with a balancing resistor in series with a compensation coil, the compensation coil is connected in parallel to the main coil for controlling the current in the compensation coil in the following manner: the product of the current through the bucking coil multiplied by its number of turns is equal to the corresponding product of the primary coil during steady state conditions, while the direction of the current through the bucking coil is opposite to the direction of the current of the primary coil. This arrangement requires one compensation coil per main coil, i.e. a total of two compensation coils for the entire axial bearing.

It is proposed in DE 102010013675 a1 to provide two compensation coils per main coil, one of the compensation coils being arranged radially between the main coil and the rotor shaft, while the other compensation coil is arranged radially further to the outside than the main coil. The two compensation coils are connected in series with the main coil.

US 7,635,937B 2 suggests to integrate the compensation coils into the stator and to connect them in series with the main coils.

US 5,682,071 discloses a magnetic bearing device comprising a constant current source. The constant current source feeds a constant total current to two coils in parallel, the two coils being arranged on opposite sides of the rotor. The sensing device detects the movement of the rotor and informs the controller, which responds by controlling the respective current to each coil to maintain the rotor at the desired position. The other end of each coil is connected to an arrangement of switches and freewheeling diodes. This document does not mention stray flux compensation.

WO 2005/121580 a1 discloses a magnetic bearing device in which two coils placed on opposite sides of a rotor are connected to an amplifier device in a series configuration. A bias current is fed from the amplifier means through the series connected coils. A control current is fed from the amplifier means to a common node between the series-connected coils to increase the current in one coil and decrease the current in the other coil. This document also does not mention stray flux compensation.

Disclosure of Invention

It is an object of the present invention to provide a magnetic bearing device exhibiting reduced stray magnetic flux while having a simple arrangement.

This object is achieved by a magnetic bearing device as defined in claim 1. Further embodiments of the invention are specified in the dependent claims.

The present invention provides a magnetic bearing device for magnetically suspending a rotor for rotation about an axis of rotation. The magnetic bearing device includes an amplifier device, a first main coil extending about the rotational axis, and a second main coil extending about the rotational axis. Each of the main coils has two terminals. One terminal of the first main winding is connected to a first output terminal of the amplifier device, and one terminal of the second main winding is connected to a second output terminal of the amplifier device. The other terminals of the main coils are connected to each other to form a common node. The main coil causes stray magnetic flux when current is supplied by the amplifier device. To reduce stray magnetic flux, the magnetic bearing device comprises a compensation coil connected between the third output of the amplifier device and the common node, the compensation coil also extending around the rotational axis. In other words, the common node is connected to the third output of the amplifier device via the compensation coil. The first main coil, the second main coil and the compensation coil are connected to the amplifier device with such winding directions and polarities as: such that any current flowing between the amplifier device and the common node through the bucking coil causes a magnetic field that reduces stray magnetic flux caused by the main coil.

The terminals of each of the main coils and the terminals of the compensation coil may be designated as "first" and "second" terminals. In this context, the term "winding direction" should be understood to designate the direction of the DC current around the axis of rotation when the DC current flows from the first terminal to the second terminal of the coil. The winding direction is "positive" when the direction of the current is right-handed around the axis of rotation, and "negative" when the direction of the current is left-handed. Further, herein, the following notation is used to define the current direction: the value of the DC current has a positive sign if current flows from the amplifier and through the coil into the common node. If the current flows in the opposite direction, it has a negative sign.

The winding direction and polarity of the coil should be chosen as follows: for all coils, the DC current should have the same direction (sense) around the rotation axis whenever the DC current has a positive value (i.e. whenever the DC current flows from the respective output of the amplifier device through the respective coil to the common node). Since the sum of the (signed) values of the currents flowing into the common node according to kirchhoff's current law must be zero, the current flowing through the compensation coil will always have a value which is the negative of the sum of the currents through the main coil. Thus, the current through the bucking coil will automatically induce a magnetic field that cancels the magnetic field induced by the current in the main coil, thereby reducing stray flux.

In particular, if the main coil and the compensation coil all have the same winding direction around the rotational axis, the main coil and the compensation coil should all be connected to the amplifier device with the same polarity. In other words, if all coils have the same winding direction, the first terminal of each coil should be connected to the amplifier device, while the second terminal should be connected to a common node. If the winding directions of the coils are not exactly the same, the polarity with which the coils should be connected between the amplifier device and the common node can be selected accordingly easily.

Advantageously, the first main coil, the second main coil and the compensation coil have substantially the same number of turns around the axis of rotation. In this way the sum of the magnetomotive forces of the two main coils and the compensation coil will be substantially zero. In this context, the magnetomotive force is given by the product of the (signed) value of the current per coil and the (signed) number of windings, the positive sign of which represents the positive winding direction and the negative sign of which represents the negative winding direction.

If the magnetic bearing device comprises a housing, it is advantageous if the housing encloses both the main coils and the compensation coils. This is especially the case when the housing comprises a magnetically conductive material, since the housing will typically form part of a stray magnetic flux path for the magnetic stray field caused by the main coil. The housing will at the same time also form part of the compensating flux path of the counteracting magnetic field caused by the compensating coil.

The invention is advantageously implemented in an active axial magnetic bearing. In this case, the magnetic bearing device includes a first axial stator assembly containing a first main coil and a second axial stator assembly containing a second main coil. The rotor and the two axial stator assemblies comprise magnetically conductive material. Each axial stator assembly and rotor together define a control magnetic flux path extending across a gap between the respective axial stator assembly and rotor. The first axial stator assembly is arranged relative to the rotor in the following manner: causing a first current through the first main coil to generate a first control magnetic flux in a first control magnetic flux path that induces an attractive magnetic force between the first axial stator assembly and the rotor in a first axial direction; and the second axial stator assembly is arranged relative to the rotor in the following manner: such that a second current through the second main coil generates a second control magnetic flux in a second control magnetic flux path that induces an attractive magnetic force between the second axial stator assembly and the rotor in a second axial direction opposite the first axial direction. For example, the rotor may include a disk portion, and then the axial stator assembly may face a different axial side of the disk portion. However, other shapes of the rotor are possible. The compensation coil is arranged outside the control flux path of the main coil. In this way, the third current through the compensation coil does not substantially induce axial magnetic forces on the rotor when it generates the compensation flux.

The compensation coils can be arranged in various positions of the magnetic bearing device according to the availability of space. For example, the bucking coil can be disposed adjacent one of the axial stator assemblies. If the magnetic bearing device comprises an active radial magnetic bearing in addition to an active axial magnetic bearing, which comprises a radial stator assembly comprising a plurality of radial coils, it is advantageous to arrange the compensation coils adjacent to the radial stator assembly radially surrounding the radial coils, since in some magnetic bearing devices sufficient space for the compensation coils can be found in this position.

The compensation coil may be conceptually divided into two or more sub-coils, which are connected in series in the following manner: so that the current passing through the sub-coils flows in the same direction (sense) around the rotation axis. Since each sub-coil has a reduced number of turns, it is easier to allocate sufficient space for the sub-coil than a single compensation coil. In other words, the magnetic bearing device may comprise at least two compensation coils connected in a series configuration between the third output of the amplifier device and the common node, each compensation coil extending around the rotational axis, the compensation coils being connected in the following manner: so that the current through the compensation coil flows in the same direction (sense) around the rotation axis. Advantageously, the compensation coils together have substantially the same number of turns around the axis of rotation as each of the first and second main coils.

More specifically, in an axial magnetic bearing, one of the compensation coils may be disposed adjacent to a first axial stator assembly and the other of the compensation coils may be disposed adjacent to a second axial stator assembly, thereby creating a substantially symmetrical arrangement. In particular, if the rotor comprises a disc shaped portion and if the axial stator assembly faces different sides of the disc shaped portion, the compensation coils may also be arranged at different sides of the disc shaped portion.

A corresponding method of operating a magnetic bearing device includes operating an amplifier device to provide a first current through a first main winding, a second current through a second main winding, and a third current through a compensation winding. If the coils are connected with the proper winding direction and polarity, the third current will automatically induce a magnetic field that cancels the magnetic field induced by the current in the main coil, thereby reducing stray flux.

Drawings

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings, which are for the purpose of illustrating the presently preferred embodiments of the invention and are not for the purpose of limiting the presently preferred embodiments of the invention. In the drawings, there is shown in the drawings,

fig. 1 shows a schematic circuit diagram of a magnetic bearing device according to a first embodiment of the present invention;

fig. 2 shows a schematic central longitudinal section of a magnetic bearing device according to a first embodiment in a first mode of operation;

fig. 3 shows a schematic central longitudinal section of the magnetic bearing device according to the first embodiment in a second mode of operation;

fig. 4 shows a schematic central longitudinal section of a magnetic bearing device according to a second embodiment along the section plane IV-IV of fig. 5;

FIG. 5 shows a schematic cross-section of a magnetic bearing device according to a second embodiment along the sectional plane V-V of FIG. 4, the out-of-plane position of the compensation coils being indicated by dashed lines;

fig. 6 shows a schematic circuit diagram of a magnetic bearing device according to a third embodiment of the present invention; and

fig. 7 shows a schematic central longitudinal section of a magnetic bearing device according to a third embodiment.

Detailed Description

Fig. 1 shows a schematic circuit diagram of a magnetic bearing device according to a first embodiment of the present invention. The magnetic bearing device comprises an amplifier 10 having three outputs 11, 12 and 13. The magnetic bearing device further comprises a first main coil p and a second main coil n. The first main coil p defines a first terminal p1 at one end thereof and a second terminal p2 at the other end thereof. Likewise, the second primary coil n defines a first terminal n1 and a second terminal n 2. The first terminal p1 of the first main winding p is connected to the first output 11 of the amplifier 10. The first terminal n1 of the second primary winding n is connected to the second output 12 of the amplifier 10. The second terminal p2 of the first main winding p and the second terminal n2 of the second main winding n are connected to each other to form a common node 14. A compensation coil c having a first terminal c1 and a second terminal c2 is connected between the third output terminal 13 of the amplifier 10 and the common node 14. As explained in more detail below in connection with fig. 2, all three coils p, n and c have the same winding direction around the rotational axis of the rotor. The winding direction is indicated by dots in fig. 1. If a DC current flows through the respective coil from the terminal indicated by the dot to the other terminal, the current will flow in the same direction around the rotation axis (i.e. in the same direction around the rotation axis).

The amplifier 10 converts the first current i_pIs fed from its first output 11 to the first main winding p, a second current i_nIs fed from its second output 12 to a second main winding n and supplies a third current i_cFed from its third output 13 to the compensation coil c. The position sensor 50 determines the position of the rotor and informs a controller integrated with the amplifier 10. Based on the position signal, the controller controls the current i_p、i_nAnd i_cTo maintain the rotor in the desired position. Using the notation defined above, the current i_p、i_nAnd i_cThe following conditions are satisfied:

i_p+i_n+i_c＝0。

the arrangement of the first main coil p, the second main coil n and the compensation coil c within the axial bearing is shown in a highly schematic manner in fig. 2. The rotor 20 is suspended to rotate about a rotational axis a. The rotor 20 comprises an elongate shaft portion 21 and a disc portion 22, both portions comprising a magnetically conductive material. The first axial stator assembly 31 faces a first axial side of the disc shaped portion 22 and the second axial stator assembly 32 faces a second axial side of the disc shaped portion 22. The first axial stator assembly 31 includes a first primary coil p and the second axial stator assembly 32 includes a second primary coil n. Each axial stator assembly forms a yoke in the form of an annular disc in which an annular groove is formed, the groove being open towards the disc-shaped portion of the rotor, the coils being arranged in the annular groove, and the yoke forming two annular end faces which face the disc-shaped portion 22 of the rotor 20 at different radial positions. The yoke is made of a magnetic conductive material.

The stator assemblies 31, 32 are surrounded by a housing 40, which also comprises a magnetically conductive material. The axial stator assemblies 31, 32 and the housing 40 are stationary. To this end, the magnetic bearing device will typically include additional components for holding the stator assembly and other parts in place. For simplicity, these components are not shown in fig. 2. They are generally non-magnetic and therefore do not carry a significant amount of magnetic flux.

The current flowing through the first main coil p generates a first magnetic field guided by the first axial stator assembly 31 and the magnetically conductive material in the disc rotor portion 22 forming a first magnetically controlled magnetic flux Φ_p. Likewise, the current flowing through the second main coil n generates a second magnetic field directed by the second axial stator assembly 32 and the magnetically conductive material in the disk rotor portion 22 to form a second magnetron flux Φ_n. Due to the air gap between each axial stator assembly 31, 32 and the disc shaped portion 22 of the rotor 20, each stator assembly generates an attractive force for pulling the disc shaped portion 22 towards the respective axial stator assembly. In other words, each axial stator assembly acts as an electromagnet that pulls the disc shaped portion 22 in its direction.

The compensation coil c is arranged to be driven by the current i_cThe induced magnetic field does not induce any magnetic field on the disk-shaped portion 22 of the rotor 20In the position of large axial forces. This is achieved by arranging the compensation coil c in the following positions: magnetic flux (compensation magnetic flux Φ) caused by compensation coil c_c) Is not in magnetic control magnetic flux phi_pAnd phi_nOverlap in the magnetic flux paths. In addition, the compensation flux phi_cShould not extend across the axial air gap with disc portion 22. In the illustrative example of FIG. 2, the compensation flux Φ_cPasses primarily through the housing 40 and the shaft portion 21 of the rotor 20, but never across the air gap with the disk portion 22.

Total stray magnetic flux Φ caused by currents in the three coils p, n and c_StrayProportional to the sum of the magnetomotive forces of the three coils, the proportionality constant depending on geometry, material, etc. The total stray flux can be expressed as follows:

Φ_strayConstant (N)_pi_p+N_ni_n+N_ci_c)，

Wherein N is_p、N_nAnd N_cThe number of turns of the coils p, n, and c is designated, respectively. N if the winding direction about the axis of rotation is left-handed_p、N_nAnd N_cIs positive and N if the winding direction is right-handed_p、N_nAnd N_cThe sign of (a) is negative.

If all three coils have the same winding direction and the same number of turns N ═ N_p＝N_n＝N_cThen stray magnetic flux phi_StrayWill be zero:

Φ_strayConstant (N)_pi_p+N_ni_n+N_ci_c) Constant N (i)_p+i_n+i_c)＝0。

If the number of turns of the bucking coil is different from the number of turns of the main coil, the bucking coil will still serve to reduce stray flux caused by the main coil.

The proposed arrangement of the coils has several advantages. Only one single compensation coil is required. Only three leads are required between the amplifier and the coil within the magnetic bearing.

Fig. 2 shows a first possible mode of operation of the amplifier arrangementFormula (II) is shown. In this mode of operation, a bias current is provided at the third output 13 of the amplifier to flow through the compensation coil c to the common node 14 (or vice versa). The bias current is split at the common node 14 to flow in parallel through both primary windings p and n. The amplifier controls the first output 11 and the second output 12 in the following way: so that part i of the bias current carried by the first main winding p_pAnd a portion i carried by the second main coil n_nIs varied to achieve position control in the bearing.

In this operating mode, the current i_p、i_nAnd i_cWill flow around the axis of rotation in the direction shown in figure 2: as indicated by the dots and crosses in fig. 2, assume a current i in the first main coil p_pFlowing around the axis of rotation in the direction of the right hand, as long as none of the currents increases beyond the bias current i_cThen a current i through the second main winding n_nWill also flow in the direction of the right hand. Therefore, as shown by the arrow in fig. 2, the control magnetic flux Φ_pAnd phi_nBoth having the same direction around the two main coils. Current i in the compensation coil_cWill automatically flow in the opposite direction around the axis of rotation, giving rise to a compensating flux phi which counteracts the stray flux caused by the main coil_c。

For this mode of operation, the amplifier may be configured, for example, as in US 5,682,071. In particular, the amplifier may comprise a constant current source providing a constant bias current to the output 13, while each of the outputs 11 and 12 is connected to a switch and a freewheeling diode. The switches are used to selectively connect the respective output terminals to a defined potential level to vary the current flowing through the respective coils, while the freewheeling diodes cause current to flow between the respective output terminals and the constant current source when the switches are open. However, other configurations of the amplifier are possible, and the bias current flowing through the output terminal 13 does not necessarily have to be constant.

In this mode of operation, the load inductance seen by the amplifier is not increased by the compensation coil, since the compensation coil is only in the substantially constant bias current path.

FIG. 3 shows an enlargementA second possible mode of operation of the device. In this operating mode, the bias current flows from the first output 11 of the amplifier through the two primary windings p, n to the second output 12 of the amplifier (or the bias current flows from the second output 12 of the amplifier through the two primary windings n, p to the first output 11 of the amplifier). Variable control current i_cIs provided between the third output terminal 13 and the common node 14 via the compensation coil c. According to its direction, controlling the current i_cIncreasing bias current in one main winding and decreasing bias current in the other main winding, or controlling current i_cThe bias current in one main winding is decreased and the bias current in the other main winding is increased.

As long as there is no control current, i.e. as long as only a bias current is applied, the sum of the signed values of the currents through the main windings is zero and, as shown in fig. 3, the current i_pAnd i_nFlow in opposite directions about the axis of rotation. Thus, stray magnetic fluxes caused by these currents will cancel. The stray flux will only be present if a control current is supplied to the main coils p and n. The sum i of the stray flux and the signed value of the current in the main coil_p+i_nAnd (4) in proportion. The sum being exactly the control current i_cNegative value of (d):

i_c＝-(i_p+i_n)。

thus, the stray flux caused by the current in the main coil is caused by the control current i through the compensation coil_cThe resulting counteracting magnetic field compensates.

A suitable amplifier arrangement of this configuration is disclosed in WO 2005/122580 (see fig. 19). In this document, a bias current is fed between first and second outputs of the amplifier means through the main windings connected in series, and a variable control current flows between the third output and the common node, which variable control current increases the current in one winding above the bias current and decreases the current in the other winding below the bias current, or decreases the current in one winding below the bias current and increases the current in the other winding above the bias current.

Additional advantages of the second embodiment are: the bias current used to create the pre-magnetization of the bearing does not flow through the compensation coil. Thus, ohmic losses in the compensation coil are minimized. Since the bucking coil serves to reduce the field of the primary coil and thus tends to reduce the overall inductance of the system, the load inductance seen by amplifier inspection is not increased by the bucking coil.

Depending on where sufficient space is available, the compensation coils may be positioned at various different locations of the magnetic bearing device. Ideally, the bucking coils are placed close to the shaft portion of the rotor to keep the diameter of the bucking coils as small as possible, thereby reducing the amount of copper required for the bucking coils. It is desirable to place the bucking coils close to the axial bearing to keep the amount of cable to a minimum. A very suitable location for the compensation coils is radially between the shaft portion of the rotor and one of the axial stator components. Another, albeit somewhat less preferred, position is radially about, or axially adjacent, one of the axial stator assemblies.

Fig. 4 and 5 show another suitable location. In this embodiment, the magnetic bearing device further comprises a radial bearing arranged in the same housing 40 as the axial bearing. The radial bearing comprises a radial stator assembly 33 carrying a plurality of radial coils r. The radial stator assembly 33 comprises an annular portion that surrounds the pole portion carrying the radial coil r. As shown in fig. 4, the bucking coil may be positioned axially adjacent the annular portion, radially surrounding the radial coil. In fig. 5, the compensation coils are arranged out of plane. Thus, its position is indicated by the dashed line.

Fig. 6 shows a third embodiment. In this embodiment, two series connected compensation coils cp and cn are provided, i.e. the single compensation coil of the previously described embodiment is conceptually divided into two series connected sub-coils. Each of the compensation coils cp and cn has a number of turns corresponding to half of the number of turns of the main coils p and n. The two compensation coils cp, cn have the same winding direction and polarity, i.e. a first terminal of coil cp is connected to the third output 13 of the amplifier 10, a second terminal of coil cp is connected to a first terminal of coil cn, and a second terminal of coil cn is connected to the common node 14. In this way, the two compensation coils cp, cn effectively function as a single compensation coil having a number of turns corresponding to the sum of the number of turns of the two coils cp, cn.

Fig. 7 shows a possible arrangement of two compensation coils of the third embodiment. The compensation coil cp is arranged adjacent to the first axial stator assembly 31 and the further compensation coil cn is arranged adjacent to the second axial stator assembly 32. Since each compensation coil has only half the number of turns that a single compensation coil has, less space is required at the location of each compensation coil. The compensation coils are symmetrically arranged on different sides of the disk shaped portion 22 of the rotor 20 so as to minimize any effect the compensation coils may have on the control path of the axial stator assembly. Of course, other arrangements of the compensation coils are possible.

Although the present invention has been described with reference to the exemplary embodiments, it is to be understood that the present invention is not limited to these embodiments, and various modifications may be made without departing from the scope of the present invention.

List of reference numerals

10 amplifier

11 first output terminal

12 second output terminal

13 third output terminal

14 common node

20 rotor

21 axle part

22 disc-shaped part

31 first axial stator assembly

32 second axial stator assembly

33 radial stator assembly

40 casing

50 position sensor

p first main coil

p1 first terminal

p2 second terminal

n second main coil

n1 first terminal

n2 second terminal

c compensation coil

c1 first terminal

c2 second terminal

cp first compensation coil

cn second compensation coil

r radial coil

Φ_pMagnetic flux through the first main coil

Φ_nMagnetic flux through the second main coil

Φ_cMagnetic flux through the compensation coil

i_pCurrent in the first main coil

i_nCurrent in the second main coil

i_cCurrent in compensation coil