Device for magnetically suspending a rotor

The device for magnetically suspending a rotor relative to a structure comprises at least one axial active magnetic bearing with a main electromagnet coil disposed in a stator and having load-bearing surfaces placed facing an armature extending essentially perpendicularly to the axis of the rotor. A device for detecting the axial position of the rotor is associated with circuits for servo-controlling the current carried by the main electromagnet coil. A compensation coil is disposed close to the main electromagnet coil, but in a position that is situated away from the load-bearing surfaces of the stator that co-operate with the rotor armature. The compensation coil is connected in series with the main electromagnet coil, but carries current in the opposite direction to that carried by said main electromagnet coil. The device constitutes an axial abutment of optimized capacity without magnetic leakage.

This application claims priority to French application No. 06 50121 filed Jan. 13, 2006.

The present invention relates to a device for magnetically suspending a rotor relative to a structure, the device comprising at least one axial active magnetic bearing with a main electromagnet coil disposed in a first stator secured to the structure and having load-bearing surfaces placed facing a first rotor armature extending substantially perpendicularly to the axis of the rotor and presenting a free peripheral end, a device for detecting the axial position of the rotor, and circuits for servo-controlling the current flowing in the main electromagnet coil.

BACKGROUND OF THE INVENTION

A device of this type is already known, e.g. as shown inFIG. 7, that comprises an axial bearing120comprising a stator122of ferromagnetic material presenting an annular housing concentric with the axis of the rotor10for receiving a coil123. The active surfaces125,126of the stator122situated on either side of the housing in which the coil123is placed co-operate with a disk-shaped armature11that is secured to the rotor10and that is essentially perpendicular to the axis of the rotor10. A sensor35, which may be of the inductive, optical, or capacitive type, is also associated with the axial bearing120to detect the position of the rotor armature11relative to the stator122and deliver a signal to servo-control circuits (not shown) that power the coil123in order to create a magnetic field such that the active surfaces125,126of the stator122can exert a force of attraction on the armature11so as to maintain it in an axial position that is stable. An axial bearing of the same type may also be disposed symmetrically relative to the armature11so as to exert a force of attraction on the second plane face of the armature11that is perpendicular to the axis of the rotor10.

The structure20on which the stator122is mounted may also serve as a support for a radial magnetic bearing30disposed close to the axial bearing120.

The radial active magnetic bearing30may comprise a stator31of laminated ferromagnetic material which is mounted on the structure20and includes electromagnetic windings32connected by connection wires33to power-supply and servo-control circuits (not shown). The radial magnetic bearing30further comprises an annular armature34likewise made of laminated magnetic material that is fitted on the rotor10and is thus concentric with the rotor10. A detector35detects the radial position of the rotor10and may be placed on a support secured to the structure20in the vicinity of the radial bearing30in order to detect the radial position of the reference surface36at the periphery of the rotor10that faces the detector35. The signals from the detector35, which may be of the inductive, capacitive, or optical type, are applied to circuits for servo-controlling the current supplied to the electromagnet windings32. In the example ofFIG. 7, the detector35, which is of the inductive type, serves to detect the position of the rotor10both in an axial direction and in two mutually perpendicular radial directions. The reference magnetic surface36is sandwiched in the axial direction between two surfaces of non-magnetic material.

In the device ofFIG. 7, which uses only one coil123within the axial magnetic bearing120, for a rotor armature11of given outside diameter, a maximum load-bearing surface area is obtained between the active surfaces125and126situated on either side of the housing for the coil123and the plane face of the rotor armature11which is situated facing these active faces.

However, the coil123creates circuits101,102of non-zero magnetic flux circulation through the stators of the radial magnetic bearing30and of the position detector35, through the rotor armatures34,36,11, and through the shaft10.

More particularly, the circuit101leads to the radial bearing30being magnetized, thereby leading to a loss in its capacity and creating coupling between the radial force and the axial force.

The circuit102leads to magnetization of the position sensors, leading to a loss of sensitivity thereof and creating coupling between measurements and the axial force.

The device shown inFIG. 7thus presents the major drawback of creating significant amounts of magnetic leakage.

In order to remedy that problem and avoid magnetizing the surroundings of the magnetic abutment constituting the axial bearing, the solution shown inFIG. 8has been proposed, in which figure those elements of the rotor10, of the structure20, and of the radial magnetic bearing30that are unchanged carry the same references and are not described again.

In the solution proposed with the device ofFIG. 8, the axial magnetic bearing220has a stator222with two annular housings concentric with the axis of the rotor10for receiving coils223and228.

By using an even number of coils223,228and by causing current to flow in the coil228in the opposite direction to the current flowing in the coil223, it is possible to ensure that each closed outline201,202surrounding the coils223,228perceives magnetic excitation that is zero.

The solution shown inFIG. 8thus makes it possible to avoid the surroundings of the axial bearing220being magnetized by the magnetic excitation created by the coils of said axial bearing. This avoids magnetization interfering with the radial bearing30, with the position sensor35, or with the entire surroundings of the axial bearing.

Nevertheless, the fact of using two coils223,228situated in two open housings reduces the active surface areas225,226,227that co-operate with the armature11, thereby leading to a loss of load-carrying area for a disk-shaped armature11of given diameter. Unfortunately, in various applications, given the high speed of rotation of the rotor, it is not possible to increase the diameter of the rotor armature in the axial bearing beyond certain limits, so that implementing multiple coils within an axial bearing becomes problematic because of the residual active surface areas no longer being large enough, thereby limiting the capacity of the axial bearing.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention seeks to remedy the above-mentioned drawbacks and to provide a magnetic abutment of diameter that is minimized for given capacity, while avoiding creating magnetic leakage looped in the machine and presenting a risk of leading to side effects harming proper operation of other devices situated in the vicinity of an axial magnetic bearing of a device for magnetically suspending a rotor.

In accordance with the invention, these objects are achieved by a device for magnetically suspending a rotor relative to a structure, the device comprising at least one axial active magnetic bearing with a main electromagnet coil disposed in a first stator secured to the structure and having load-bearing surfaces placed facing a first rotor armature extending substantially perpendicularly to the axis of the rotor and presenting a free peripheral end, a device for detecting the axial position of the rotor, and circuits for servo-controlling the current flowing in the main electromagnet coil, the device further comprising a compensation coil that is disposed close to the main electromagnet coil, but in a position that is situated away from the load-bearing surfaces of the first stator that co-operate with the first rotor armature, the compensation coil being connected in series with the main electromagnet coil and carrying current in a direction opposite to that carried by said main electromagnet coil.

In a first particular embodiment, the compensation coil is disposed on the peripherally outer wall of the first stator and is situated beyond the free peripheral end of the first rotor armature.

In another particular embodiment, the compensation coil is disposed on the inner peripheral wall of the first stator and is situated facing the base of the first rotor armature that is connected to the rotor.

In yet another particular embodiment, the compensation coil is disposed on a substantially radial wall of the first stator on its side opposite from the first rotor armature.

The compensation coil and the main coil of the axial active magnetic bearing are concentric.

Advantageously, the device of the invention further includes at least one radial active magnetic bearing disposed in the vicinity of the axial active magnetic bearing, the radial active magnetic bearing comprising a second stator secured to the structure and provided with electromagnet windings, and a cylindrical second rotor armature of axis coinciding with the axis of the rotor, a device for detecting the radial position of the rotor, and circuits for servo-controlling the current carried by said electromagnet windings of said second stator.

More particularly, in an aspect of the invention, the device for detecting the radial position of the rotor includes at least one sensor interposed between the second stator and the first stator.

The device may further include an electric motor for driving the rotor in the vicinity of the axial active magnetic bearing.

In general, the device of the invention makes it possible to provide a magnetic suspension that includes an axial magnetic abutment having large capacity and without magnetic leakage.

MORE DETAILED DESCRIPTION

FIG. 1shows an embodiment of a device for magnetically suspending a rotor10, the device essentially comprising an axial active magnetic bearing40and a radial active magnetic bearing30that enable the rotor10to be supported without contact relative to a stationary structure20.

FIG. 1shows only one radial magnetic bearing30disposed in the vicinity of the axial magnetic bearing40. Nevertheless, a second radial bearing70, that may optionally be of the same type as the radial bearing30, is normally implemented in the vicinity of another portion of the rotor10at a certain distance from the radial bearing30in order to ensure that the rotor is held completely in the radial direction (seeFIGS. 3 and 4).

A second axial magnetic bearing analogous to axial bearing40may also be installed in the vicinity of the axial bearing40symmetrically relative to the rotor armature11that is constituted in the form of a disk perpendicular to the axis of the rotor10. Under such circumstances, the first axial bearing40co-operates with a first front face111of the rotor armature11while the second axial bearing50co-operates with a second front face112of the rotor armature11(FIG. 3).

In another possible embodiment, a complete magnetic suspension comprises, in the vicinity of each of the ends of the rotor10, an analogous assembly made up of a respective radial bearing30or70and a respective axial bearing40or60, the disposition simply being symmetrical about the midpoint of the rotor10, i.e. a first bearing assembly comprises an axial bearing40co-operating with a front face111of a first stator armature11, the axial bearing40being situated in the vicinity of the first radial bearing30, while a second bearing assembly comprises an axial bearing60co-operating with a front face111A of a second rotor armature11A, the axial bearing60being situated in the vicinity of the second radial bearing70between said bearing and the second rotor armature11A (FIG. 4). In the embodiment ofFIG. 4, the outside front faces112,112A of the rotor armatures11,11A are not used, however the action of the axial bearings40,60exerted on the faces111,111A of the rotor armatures11,11A makes it possible to exert opposing forces in both directions along the axis of the rotor, in a manner like that in which two axial bearings40,50co-operate with the two opposite front faces111,112of a single rotor armature11(FIG. 3).

With reference again toFIG. 1, there can be seen a bearing assembly constituting a preferred embodiment of the invention.

The axial active bearing40comprises a main electromagnet coil43placed in a stator42of ferromagnetic material secured to the structure20. The stator42defines active surfaces or load-carrying surfaces45,46that are situated on either side of the housing for the main coil43and placed facing the front face111of the rotor armature11at a small distance therefrom, defining an airgap. The rotor armature11secured to the rotor10is essentially perpendicular to the axis of the rotor10and presents a free peripheral end110remote from its zone connected to the rotor10.

A compensation coil47is disposed on the stator42in such a manner as to be concentric with the main coil43, and fairly close to said main coil43, while nevertheless being situated in a position that is to be found in the radial direction away from the free peripheral end110of the rotor armature11.

The compensation coil47is connected in series with the main coil43, but it is wound or connected in the opposite direction to the main coil43so as to carry current in the direction opposite to the current carried by the main coil43. This serves to cancel the magnetic excitation on any path outside the axial magnetic bearing40and thus to avoid creating interfering magnetization in surrounding parts.

By way of example,FIG. 1shows the current traveling in the main coil43as a vector whose tip is coming out of the plane of the figure while the current carried by the compensation coil47is represented by a vector whose tip is going into the plane of the figure. The closed circuits1and2ofFIG. 1represent paths outside the axial abutment40in which magnetic flux circulation is zero.

As a result, although the active surfaces45,46of the stator42situated facing the rotor armature11remain maximized in area for a given radial size of the radial armature11, there is no parasitic magnetization of the radial magnetic bearing30or of the position detector35that are located close to the axial bearing40.

A protective plate48may be fitted on the stator42outside the compensation coil47in order to protect it.

The radial magnetic bearing30and the position detector35may conserve the same structure as that described above with reference toFIGS. 7 and 8, such that this structure is not described again. In particular, the position detector35can detect both the axial position of the rotor and the radial positions of the rotor along two mutually perpendicular axes. Nevertheless, it would also be possible to use radial detectors and an axial detector that are distinct.

The radial magnetic bearing70ofFIGS. 3 and 4may itself optionally be similar to the radial magnetic bearing30. In the examples shown inFIGS. 3 and 4, the cylindrical rotor armature74, the stator71, and the windings72are similar to the corresponding elements34,31, and32of the radial magnetic bearing30, and the position detector75may likewise be similar to the detector35for radial detection purposes.

FIG. 2is a schematic showing the circuits50for powering and servo-controlling the coils of the various bearings, while taking account of the position signals Px, Py, Pz delivered by the position detector35.

The main coil43and the compensation coil47of the axial bearing40that are connected in series are powered from servo-control circuits151having the signal Pz from the position detector35applied thereto, that gives information about the axial position of the rotor10.

Similarly, the windings32of the radial bearing30are powered from servo-control circuits152having the signals Px, Py from the position detector35applied thereto that provide information about the radial position of the rotor10. Naturally, the signals coming from additional detectors could also be applied to the circuits151or152in various known configurations.

In the examples ofFIGS. 3 and 4, the axial bearings50and60can be made in a manner similar to the axial bearing40and they can thus comprise a respective main coil53,63together with a respective compensation coil57,67, e.g. situated radially beyond the respective peripheral wall110,110A of the respective rotor armature11,11A, the axial position servo-control being provided using the position detector35. The coils53and57or63and67can be powered like the coils43and47from the servo-control circuits151.

As can be seen inFIGS. 3 and 4, an electric motor80having a rotor armature84and a laminated stator81provided with windings82may be disposed on the rotor10, e.g. between the radial bearings30and70. The fact that the axial magnetic suspension device of the invention does not have any magnetic leakage reduces the risk of the operation of the motor80being disturbed, as well as any risk of disturbing the operation of the radial bearings30,70or of the detectors35,75.

The embodiment ofFIG. 1constitutes a preferred embodiment. Nevertheless, other variant embodiments are possible, such as those shown inFIGS. 5 and 6.

The compensation coil can be placed close to the main coil33away from the load-bearing surfaces of the magnetic structure co-operating with the first rotor armature, in positions that are different from that shown inFIG. 1.

InFIG. 5, there can be seen a compensation coil147which is disposed on an inside peripheral wall of the first stator42outside the load-bearing surfaces45,46. The compensation coil147is situated facing the base of the rotor armature11in the vicinity of the peripheral surface of the rotor10in the smallest-diameter zone of the armature11, i.e. the zone with reduced load-carrying capacity.

FIG. 6shows another variant embodiment in which the compensation coil247is disposed on an essentially radial wall of the stator42on its side opposite from the load-bearing surfaces45,46that face the rotor armature11.

Providing it defines an equivalent number of ampere-turns, the compensation coil147or247does not necessarily have the same shape as the main coil43and, for example, it may be flatter.

In the variant ofFIGS. 5 and 6, the compensation coil147or247is still connected in series with the main coil43, but is wound or connected in the opposite direction to the main coil43, like the coils47and43inFIGS. 1 and 2, such that the sum of the ampere-turns of the coils43and147or43and247is substantially zero.

Naturally, various modifications and additions could be applied to the embodiments described above.

Thus, the invention also applies when an active magnetic bearing is of the position self-detection type without a separate position detector being associated therewith.