Magnetic bearing cell with rotor and stator

The invention relates to a rotationally symmetrical.sup.1 magnetic bearing cell (1) with a rotor (2) arranged to rotate about the central axis (10) of the cell (1) and having a shaft (4) and at least two axially magnetised permanent magnetic rings (5, 6, 30) fitted an axial distance apart on the shaft and with a stator (3) having pole components (12, 13, 16, 17) and two annular coils (14, 15) associated with the rotor end faces; to improve the radial rigidity without increasing active control complexity, it is proposed that pole components (26, 27, 38) of magnetically conductive material be provided for the peripheral regions of the two axially outer permanent magnet rings (5, 6 or 5, 30) to direct the magnetic flux generated by the annular coils (14, 15) into two substantially independent magnetic flux circuits (28, 29). FNT .sup.1 Translator's: note The German text reads "aufgebautet". This is grammatically not correct. Instead the German text should read "aufgebaute". The latter has been assumed for the translation.

MAGNETIC BEARING CELL WITH ROTOR AND STATOR 
The invention relates to a rotationally symmetrical magnetic bearing cell 
with a rotor arranged to rotate about the central axis of the cell having 
a shaft and at least two axially magnetised permanent magnetic rings 
fitted an axial distance apart on the shaft, as well as a stator having 
pole components and two annular coils associated with the rotor end faces 
whereby the rotor and stator components are arranged in such a way with 
respect to each other that the permanent magnets generate a magnetic flux 
which surrounds the central axis toroidally embracing the annular coils. 
A magnetic bearing cell having these characteristics is known from DE-C 34 
09 047. Its rotor has two axially magnetised permanent magnetic rings 
which generate the magnetic flux toroidally about the central axis 
embracing the annular coils. The magnetic flux penetrates the permanent 
magnetic rings of the rotor which are axially arranged behind each other 
as well as the peripheral pole components so that a high radial rigidity 
is attained. The bearing is unstable in the axial direction and therefore 
requires an active control with respect to this direction. For this 
purpose annular coils are associated with each of the rotor end faces. 
Each of these annular coils also generates a toroidal magnetic flux which 
surrounds the central axis and which superimposes itself on the magnetic 
flux generated by the permanent magnetic rings. The active control is 
preferably selected in such a manner that the annular coils do not carry a 
current when the rotor has attained its nominal position. If the rotor 
deviates from its nominal position in the axial direction, then a current 
is applied to the annular coils. The magnitude of the current and its 
direction depend on the magnitude and the direction of the deviation. 
Depending on the direction of the current, the toroidal magnetic flux 
generated by the annular coils has the same or the opposite direction with 
respect to the magnetic flux produced by the permanent magnetic rings. 
SUMMARY OF THE INVENTION 
In order to improve radial rigidity of a magnetic bearing of the 
aforementioned kind, it is proposed to increase the number of permanent 
magnetic rings on the rotor and to provide in addition on the stator at 
least one additional permanent magnetic ring which is also axially 
magnetised (refer to DE-A 41 06 063). These measures increase the flux 
produced by the permanent magnets, i.e. increase the axial forces and thus 
give rise to the desired increase in radial rigidity. In order to be able 
to perform their control task, the annular coils must be capable of 
generating control fluxes which compensate the increased axial forces. The 
pre-requisite for this is an increased cross section for the annular coils 
as well as increased complexity regarding current generation and active 
control for the coils. 
It is the task of the present invention to design a magnetic bearing cell 
of the aforementioned kind in such a way, that in spite of an improvement 
in radial rigidity by increasing the number of permanent magnetic rings, 
active control complexity is not increased. 
According to the present invention this task is solved by associating pole 
components of magnetically conductive material with the peripheral regions 
of the two axially outer permanent magnetic rings to direct the magnetic 
flux generated by the annular cells into two substantially independent 
magnetic flux circuits. 
The proposed measures favour the formation of control fluxes directly 
associated with the annular coils. These cause the magnetic fluxes which 
are generated by the annular coils and which are substantially independent 
of each other to only penetrate the respective pole components surrounding 
the annular coils, a part of the permanent magnetic ring in the vicinity 
as well as the respective axial and peripheral slit between rotor and 
stator. Greater resistances are thus not present in these magnetic flux 
circuits. The magnetic fluxes which chiefly contribute to the axial 
control of the magnetic field need no longer penetrate the other still 
present components (permanent magnets, attenuation discs) and slits. Thus 
the complexity required for the annular coils with regard to size and 
power supply does thus no longer depend on the number of permanent 
magnetic rings present.

The magnetic bearing cell 1 shown in the drawing figure embraces rotor 2 
and stator 3. 
Components of the rotor 2 are shaft 4 and the permanent magnetic rings 5 
and 6 attached to shaft 4. Inner hub rings 8, 9 are provided for 
attachment of the permanent magnetic rings 5, 6 to shaft 4. The toroidal 
magnetic flux surrounding central axis 10 which is produced by the 
permanent magnetic rings 5, 6 is indicated by arrow 11. 
Stator 3 embraces with respect to central axis 10 rotationally 
symmetrically designed pole components 12, 13, the common cross section of 
which is substantially C-shaped. Annular coils 14, 15 are located in face 
region C. The inner sections 16, 17 form pole surfaces 18, 19 which face 
the permanent magnetic rings 5, 6 of rotor 2. A sensor arrangement 21 as 
well as an electronic controller 22 which is shown as a block are provided 
to control the current flow through annular coils 14, 15. 
Annular disc 24 which is made of non-magnetisable material of high 
electrical conductivity, like copper, for example, engages in slit 23 
between the permanent magnetic rings 5 and 6. Annular disc 24 has a 
peripheral cylindrical section 25, which rests against the insides of 
components 12, 13. In the case of substantially axially directed relative 
movements, eddy currents are generated in the annular disc 24 and also in 
the cylindrical section 25, which produce the desired damping effect. The 
cylindrical section 25 has a centering function and moreover it promotes 
the removal of heat caused by the eddy currents. 
Further stator components 26, 27 are associated with the peripheral regions 
of permanent magnetic rings 5, 6 which are formed as stator rings 
surrounding the permanent magnet rings 5, 6. They are made of a 
magnetically well conducting material and have the effect that annular 
coils 14, 15 generate--in contrast to magnetic flux circuit 11 produced by 
the permanent magnetic rings 5, 6--two magnetic flux circuits 28, 29. 
In the design example according to drawing FIG. 2, three rotating and 
axially magnetised permanent magnetic rings 5, 6 and 30 are provided. The 
slit 31 between permanent magnetic rings 6 and 30 is engaged by an annular 
disc 32, which carries a further, resting permanent magnetic ring 33. The 
radial dimensions of this stator permanent magnetic ring 33 correspond to 
the dimensions of rotor permanent magnetic rings 6 and 30. Moreover, the 
disc 32 is made of non-magnetisable material (stainless steel, for 
example) and also has the peripheral cylindrical section 34, which rests 
against the inside of component 13 for the purpose of centering. If, in 
addition, the material has a high electrical conductance, then it will 
contribute to an improvement of the damping characteristics. 
Magnet rings 5, 6, 30, 33 are magnetised in the axial direction in such a 
way that attracting forces act between them. Together with pole components 
12, 13, 16, 17, they form a magnetic circuit (arrow 11) which is 
responsible for the radial rigidity. Outer reinforcement rings 35, 36, 37 
embrace the rotating permanent magnetic rings and protect these against 
being damaged by the influence of the relatively high centrifugal forces 
during the rotation. 
The outer permanent magnetic rings 5 and 30 are associated at their 
peripheral regions with stator rings 26, 27 which cause the desired 
splitting of the magnetic field generated by annular coils 14, 15 into two 
separate magnetic fluxes (arrows 28, 29). If in the areas of the stator 
rings 28, 29, Belleville spring washers 38 or packs of Belleville spring 
washers are necessary for mutual tensioning of the various ring components 
25, 26, 27, 34, then it is expedient to provide Belleville spring washers 
made of a magnetically well conducting material. These may replace a 
stator ring 28 or 29, or may be additionally present.