A method for arranging an electric machine on bearings, and a bearing assembly of an electric machine The present invention relates to a trunnion bearing assembly for the safety bearing of a high-speed electric machine, which high-speed electric machine comprises a rotor arranged to be rotatable, a stator, and at least one contactless bearing for the bearing during the opera- tion of the high-speed electric machine, which trunnion bearing assem- bly comprises at least three rotationally symmetrical bearing means which are mounted on bearings in relation to their centre of rotation, and that said at least three rotationally symmetrical bearing means are placed around a shaft substantially symmetrically so that the centre of rotation of the bearing means is placed on the circumference of sub- stantially the same circle in such a way that the diameter of the largest imaginary circle inside the bearing means is greater than the diameter of the shaft at the bearing means. The invention also relates to a high- speed electric machine which comprises a trunnion bearing assembly for the safety bearing of the electric machine, a rotor arranged to be rotatable, a stator, and at least one contactless bearing for the bearing during the operation of the high-speed electric machine, which trunnion bearing assembly comprises at least three rotationally symmetrical bearing means which are mounted on bearings in relation to their cen- tre of rotation, and that said at least three rotationally symmetrical bearing means are placed around a shaft substantially symmetrically so that the centre of rotation of the bearing means is placed on the cir- cumference of substantially the same circle in such a way that the diameter of the largest imaginary circle inside the bearing means is greater than the diameter of the shaft at the bearing means. Further, the invention relates to a method for the safety bearing of a high-speed electric machine by means of a trunnion bearing assembly, which high- speed electric machine comprises a rotor arranged to be rotatable, a stator, and at least one contactless bearing for the bearing during the operation of the high-speed electric machine, in which method the trunnion bearing is arranged with at least three rotationally symmetrical bearing means which are mounted on bearings in relation to their cen- tre of rotation, and that said at least three rotationally symmetrical bearing means are placed around a shaft substantially symmetrically

so that the centre of rotation of the bearing means is placed on the cir- cumference of substantially the same circle in such a way that the diameter of the largest imaginary circle inside the bearing means is greater than the diameter of the shaft at the bearing means.

In particular, high-speed electric machines are known, which apply magnetic bearings to keep the rotating rotor of the electrical machine in a desired position during the rotation of the rotor. The operation of the magnetic bearing is based on force effects on the rotor, to be produced by magnetic fields. Passive bearing implemented by permanent mag- nets is normally used in the construction of instruments only. In indus- trial applications, an electrically controlled active magnetic bearing is used, in which a control device controls the current to be supplied to the electrical magnets in such a way that the force effective on the fer- romagnetic piece keeps it floating in a contactless manner. It is thus a so-called contactless bearing to minimize the friction and wearing between the rotor shaft and the bearing.

However, during its rotation, the rotor may be subjected to sudden force effects which are difficult to compensate for by magnetic bearings only. This involves the risk that the rotor may touch the stator, the magnetic bearing or another part of the electric machine. Particularly in high-speed motors, this impact may cause breaking of the electric machine. To prevent such damage, electric machines with magnetic bearings are normally provided with a trunnion bearing assembly to prevent the cause of damage or at least to significantly reduce the risk of causing damage. Furthermore, the trunnion bearing assembly pre- vents the impact between the stator and the rotor when the magnetic bearings are turned off, e. g. when the electric machine is stopped.

Also, the trunnion bearing assembly is responsible for the bearing of the rotor during the stopping of the rotating motion of the rotor in failure situations, such as power failures or damage to the magnetic bearing assembly. The trunnion bearing is typically a bearing based on contact, such as a slide bearing or a ball bearing.

The trunnion bearing assembly should be provided with a suitable clearance to the rotor during the normal operation. Known trunnion

bearing arrangements in association with magnetic bearings include structures of the slide bearing type, either unlubricated or provided with the supply of a lubricant. One example of such a bearing arrangement is disclosed in US patent 5,355, 040. It is also possible to use conven- tional roller bearings, or a combination of the roller and slide bearings, as presented e. g. in US patent 5,272, 403. From other fields of technol- ogy, it is known to use a support-roll type bearing for the bearing of solids of revolution. Examples of such devices to be mentioned include horizontal revolving furnaces, in which the loading direction is almost constant and the rotational speeds are low.

The implementation of a well-functioning trunnion bearing assembly involves several problems. Causes for the problems include e. g. the high rotational speeds which are commonly used in machines with magnetic bearings, as well as the clearance which is required between the trunnion bearing and the rotor for functional reasons and which is often distinctly greater than the clearance that is normally used e. g. in slide bearing arrangements. Furthermore, it is often preferable to pro- vide the fast revolving rotor with a large diameter, wherein also the peripheral speed of the bearing points becomes high (>100 m/s) and the allowable rotational speeds of the roller bearings with suitable dimensions remain low. When the dimensions of the roller bearings are increased, their structural clearances are also increased. This will cause further problems in setting the clearance for the device. When the dimensions of the roller bearings are increased, also their mass inertia and frictional couple is increased, which are both disadvanta- geous for the acceleration and operation of the bearing. Furthermore, when the machine with magnetic bearings is free from lubricants in other respects, it is undesirable to use lubricants in the trunnion bear- ings.

In radial bearing arrangements of the slide type, the friction between the rotor and the bearing will easily become high. The development of a high frictional force on the perimeter of the rotor will involve the rotor in a rotary motion which is determined according to the trunnion bear- ing. In this rotary motion, the centre of the rotor orbits a circular path which causes centrifugal forces on the rotor. In view of the operation of

the radial-type trunnion bearing assembly, it is advantageous if the rotational speed of the rotor on the path limited by the trunnion bearing remains as low as possible. If the rotational speed increases to a high level, it will often lead to radial forces which are so great that they are difficult to control.

US patent 5,752, 774 discloses an arrangement to eliminate the radial clearance between the rotor and the trunnion bearing, when the rotor shaft touches the trunnion bearing assembly. The aim of that invention was to eliminate problems occurring in the application of so-called con- ventional arrangements, such as impacts and the involvement of the rotor in such a rotary motion which does not take place around the centre of the rotor. In said publication, the aim is to reduce the effect of impacts by means of various attenuator arrangements. The involve- ment in a rotary motion, in turn, is prevented by eliminating the clear- ance between the bearing means and the rotor. The elimination of the clearance between the rotor and the trunnion bearing, implemented according to the basic principle of the publication, requires the use of bearing means fitted to be movable.

In high-speed motors, the high rotational speed of the rotor sets further special requirements on the rotor construction and on the bearing assembly, of which the most important ones will be discussed in the following. When the aim is to achieve as high a critical rotational speed of the first order as possible, the shaft must be made as short and with as large a diameter as possible. Thus, the diameters become so large that when the roller bearing is selected on these grounds, the allowable rotational speed of the trunnion bearing remains low and it is thus not suitable for high-speed applications. Furthermore, if both ends of the shaft are already in use, it is not advantageous to provide the ends of the rotor with an axial bearing.

The geometry of the radial bearing must be designed so that it will resist the shifting of the rotor to a moving state, in which it rotates both around its own central shaft and simultaneously said central shaft orbits a circular path, causing high centrifugal forces.

It is an aim of the present invention to eliminate the above-mentioned drawbacks to a great extent and to improve the state of art. The pre- sent invention is based on the idea that the bearing means are rota- tionally symmetrical pieces mounted on bearings at their centres. To put it more precisely, the bearing structure according to the invention is primarily characterized in that at least some of the centres of rotation of the bearing means are arranged in substantially constant locations in the trunnion bearing assembly, and at least some of the bearing means are arranged to limit the movement of the shaft in the axial direction, and that the bearing means limiting the movement in the axial direction comprise at least a first set of bearing means in such a way that the centre of rotation of the first set of bearing means is placed in substan- tially the same first plane. The high-speed electric machine according to the invention is primarily characterized in that at least some of the centres of rotation of the bearing means are arranged in substantially constant locations in the trunnion bearing assembly, and that at least some of the bearing means are arranged to limit the movement of the shaft in the axial direction, and that the bearing means limiting the movement in the axial direction comprise at least a first set of bearing means in such a way that the centre of rotation of the first set of bear- ing means is placed in substantially the same first plane. Furthermore, the method according to the invention is primarily characterized in that in the method, at least some of the centres of rotation of the bearing means are arranged in substantially constant locations in the trunnion bearing assembly, and at least some of the bearing means are used to limit the movement of the shaft in the axial direction, and that of the bearing means limiting the movement in the axial direction, at least a first set of bearing means is provided, whose centre of rotation is located substantially in the same first plane.

The present invention shows remarkable advantages over solutions of prior art. By means of the arrangement according to the invention, the axial bearing can be placed relatively freely, which makes it possible, for example, to provide an advantageous location for the centre of thermal movements. The invention also provides freedom in view of the operational position of the electric machine. In addition, by the layout geometry of the bearings according to the invention, it is possible to

resist the shifting of the rotor to the moving state, in which it rotates around its own central shaft and the central shaft simultaneously orbits a circular path. Thus, centrifugal forces caused by the circular move- ment of the above-mentioned type will not develop to such an extent as in arrangements of prior art. The contact surfaces of the trunnion bearings are preferably made of a material of small friction, which will reduce e. g. the heating and wearing of the bearings. In an arrangement according to an advantageous embodiment of the invention, the con- tact surfaces are formed of rollers mounted on bearings in relation to the centres of rotation, which can also be used to reduce the friction.

Furthermore, the arrangement according to the invention makes it pos- sible that the same bearing assembly may comprise both axial bear- ings and radial bearings. Thus, the layout of the bearings becomes simpler, because no separate bearings will be needed for the trunnion bearing assembly in the axial direction and in the radial direction.

In the following, the invention will be described in more detail with ref- erence to the appended drawings, in which Fig. 1a shows a trunnion bearing according to an advantageous embodiment of the invention, seen from the end of the shaft and in a partly sectional view, Fig. 1 b shows the trunnion bearing of Fig. 1 a in a partial cross-sec- tion, seen from the side, Fig. 2a shows a trunnion bearing according to a second advanta- geous embodiment of the invention, seen from the end of the shaft and in a partly sectional view, Fig. 2b shows the trunnion bearing of Fig. 2a in a partial cross-sec- tion, seen from the side, Fig. 3a shows a trunnion bearing according to a third advantageous embodiment of the invention, seen from the end of the shaft and in a partly sectional view,

Fig. 3b shows the trunnion bearing of Fig. 3a in a partial cross-sec- tion, seen from the side, Fig. 4 shows, in a reduced partial cross-sectional view, an electric machine applying the trunnion bearing assembly according to the invention, Fig. 5 shows a principle view of a polygon limiting the path of movement of the centre of the rotor in an electric machine according to an advantageous embodiment of the invention, Fig. 6a shows a trunnion bearing according to a fourth advanta- geous embodiment of the invention, seen from the end of the shaft and in a partly sectional view, and Fig. 6b shows the trunnion bearing of Fig. 6a in a partial cross-sec- tion, seen from the side.

We shall next describe the structure of a trunnion bearing 1 according to an advantageous embodiment of the invention, implemented in con- nection with a high-speed electric machine, with reference to Figs. 1a and 1b. The rotor 3 of the electric machine 2 comprises a shaft 4. For operating situations, the rotor 3 is mounted on contactless bearings, such as magnetic bearings 5. Thus, the magnetic bearings 5 are con- trolled during the operation of the electric machine 2 in such a way that the shaft 4 is kept as stable as possible so that the rotor does not touch the stator 6. However, the rotor 3 may be subjected to force effects which cause the rotor 3 to be displaced from this centre. For this rea- son, the electric machine is provided with one or more trunnion bear- ings 1. The trunnion bearing shown in Figs. 1a and 1b is intended to eliminate the effect of deviations in the radial direction preferably as follows. The trunnion bearing 1 comprises bearing means 7, such as rollers, mounted on bearings in relation to the centres of rotation which remain in substantially constant positions. In the mounting of the bear- ing means 7, it is possible to use, for example, ball bearings or other arrangements of the roller bearing type, wherein the outer race of the bearing is used as the roller, and the bearing is arranged by means of

balls 8 or the like, as is shown in the cross-sectional view of Fig. 1b.

The bearing means 7 are preferably placed in such a way that the cen- tres of rotation of the bearing means are placed on the circumference of substantially the same circle. The diameter of this circle is selected so that the diameter of the largest imaginary circle inside the bearing means system of the trunnion bearing 1 is greater than the diameter D1 of the shaft at the trunnion bearing 1. The shaft 4 is thus in contact with a maximum of two bearing means 7. The centre of rotation of the bearing means 7 will remain in a substantially constant position, irre- spective of whether the shaft 4 is in contact with said bearing means or not. Furthermore, these bearing means 7 limit the movement of the rotational centre of the shaft in such a way that the largest path of the rotational centre of the shaft is not a geometrical circle but a polygon.

When the rotational movement is smaller than the polygon limited by the bearing means, the rotational movement can also be a circle, but the diameter of this circle is so small that the centrifugal force caused by the rotary motion is not harmfully great. In the trunnion bearing 1 of Fig. 1a, there are seven bearing means 7 placed at even intervals, wherein the rotational centres of the bearing means 7 are located on the circumference of the circle at intervals of about 51. 4°. However, it will be obvious that the invention is not limited solely to such trunnion bearings, but the number of bearing means 7 may vary in different applications.

The high rotational speed of the rotor 3 sets certain special require- ments on the structure of the trunnion bearing 1. The mass inertia and the frictional couple of the bearing means 7 should be reduced to a minimum. Thus, when the shaft 4 of the rotor 3 rotating at a high speed touches a bearing means 7 of the trunnion bearing 1, this bearing means will start to rotate. Thanks to the small mass inertia, the bearing means will relatively fast achieve a rotational speed which corresponds to the rotational speed of the shaft, wherein the thermal effect of the frictional force caused by the difference in speed between the shaft 4 and the bearing means 7 is relatively short in time. Furthermore, the low frictional couple will also reduce the heating of the shaft 4 and the bearing means 7, when the rotational speed of the bearing means 7 is lower than the rotational speed of the shaft 4 touching the bearing

means 7. Known bearing means 7 with very low friction include e. g. ball bearings, single-row angular ball bearings, and radially loaded cylindrical roller bearings with holders. However, it will be obvious that the present invention is not limited solely to apply the bearing means mentioned herein.

A small clearance must be left between the rotor 3 and the trunnion bearing, wherein the magnitude of this clearance will affect, for exam- ple, the number of bearing means 7 in the trunnion bearing 1. The number of bearing means is preferably selected so that the rotor 3 and its shaft 4 will not, in any situation during the operation of the high- speed machine, come to touch anything but the bearing means 7 of the trunnion bearing.

The trunnion bearing 1 can be integrated in a single unit which com- prises the necessary bearing means and a supporting structure 9 to keep the bearing means in place. This trunnion bearing 1 is coupled, for example, to the end part of the electric machine 2, wherein the trunnion bearing forms a through hole for the shaft 4. The shaft 4 can thus be coupled to a desired transmission mechanism (not shown) in a way known as such. The radial trunnion bearing 1 according to the invention is preferably arranged at each end of the shaft. However, it is obvious that the trunnion bearing according to the present invention can also be placed at another point on the shaft 4, or the number of trunnion bearings can also be one or greater than two.

Figures 2a and 2b show a trunnion bearing 1 according to a second advantageous embodiment of the invention. This axial trunnion bearing 1 is, to a great extent, similar to the radial trunnion bearing shown in Figs. 1a and 1b. Also this axial trunnion bearing 1 comprises bearing means, such as bearing means 7. These bearing means are mounted on bearings at their centre, which remains in a substantially constant location, for example by roller bearings. In this embodiment, the bear- ing means 7 are placed on the circumference of a circle in such a way that the centres of rotation are placed in the same plane which is sub- stantially perpendicular to the axial direction of the rotor 3. The shaft 4 is preferably provided with at least one gradation, wherein the shaft

comprises at least one planar surface A1 in addition to the ends. The diameter of the imaginary circle used in the layout of the bearing means 7 is thus smaller than the diameter D2 of the planar surface A1 formed in the shaft 4 of the rotor. The trunnion bearing 1 is placed close to said planar surface A1. Thus, when the rotor moves towards the trunnion bearing in the axial direction, the clearance between the bearing means of the trunnion bearing and said planar surface A1 is reduced, until the planar surface A1 of the shaft touches the bearing means. As a result of this contact, the rotating movement of the rotor causes that the bearing means start to rotate and prevent damage to the rotor. Also in this embodiment, it is essential that the bearing means used are rollers, or the like, mounted on bearings in relation to the centre of rotation and having a low mass inertia and frictional couple. For the axial bearing, these bearing means are placed sub- stantially in the same plane which is substantially perpendicular to the longitudinal direction of the shaft. It is obvious that the axial trunnion bearing 1 can, in some applications, also be placed at the end of the shaft, wherein the end of the shaft is used as the planar surface A1.

Furthermore, Figs. 3a and 3b show the structure of a trunnion bearing 1 according to a third advantageous embodiment of the invention. The trunnion bearing 1 is arranged to provide bearing both in the radial and the axial directions in situations, in which the magnetic bearings are not in operation or they cannot sufficiently prevent the movement of the shaft in the the radial and/or axial direction. The trunnion bearing 1 comprises a first set 7a of bearing means 7 as well as a second set 7b of bearing means 7. Also in this embodiment, these bearing means 7 are mounted on bearings at their centre of rotation, as presented above in this description. The first set 7a of bearing means is used for trun- nion bearing in the radial direction, wherein these bearing means are placed on the circumference of substantially the same circle in such a way that the largest diameter of the imaginary circle within the bearing means system is greater than the diameter D1 of the shaft 4 at said bearing point. Thus, not more than two bearing means 7 of the first set 7a are simultaneously in contact with the shaft 4.

In a corresponding manner, the second set 7b of bearing means 7 is used for the trunnion bearing in the axial direction. These bearing means are placed in such a way that the centres of rotation of the bearing means are placed substantially in the same plane which is substantially parallel to the longitudinal direction of the shaft 4. In the arrangement of Figs. 3a and 3b, the shaft 4 is provided with a groove or the like, which comprises two substantially planar surfaces A1, A2.

The distance D3 between these planar surfaces is slightly greater than the diameter D4 of the bearing means of the second set 7b. Thus, in the normal situation, a small clearance V is formed between the bear- ing means and the planar surfaces A1, A2. When the shaft 4 is dis- placed in the axial direction, the clearance V is reduced in the direction of the displacement, until the bearing means 7 of the second set 7b touch the plane A1, A2 in the direction of the displacement of the shaft.

Thus, the bearing means 7 prevent the displacement of the shaft fur- ther away from this specific point, and damage to the rotor can be pre- vented. This arrangement has, for example, the advantage that only one trunnion bearing assembly is needed in the axial direction.

The trunnion bearing 1 comprises a supporting structure 9 to which the bearing means of both the first set 7a and the second set 7b are cou- pled in a rotating manner, for example by arrangements of the roller bearing type. In an application of the trunnion bearing 1 according to this third advantageous embodiment of the invention, the bearing means 7 of the second set 7b are placed in such a way that their cen- tre is located on the circumference of substantially the same circle where the centres of rotation of the bearing means 7 of the first set 7a are located. Furthermore, all the bearing means are preferably placed at substantially even intervals. Thus, in the arrangement of Fig. 3a, in which each set 7a, 7b of bearing means comprises 5 bearing means, the angle between adjacent bearing means will be 36°.

Furthermore, Figs. 6a and 6b show the structure of a trunnion bearing 1 according to a fourth advantageous embodiment of the invention.

This structure differs from the structure of Figs. 3a and 3b primarily in that the second set 7b of bearing means arranged for bearing in the axial direction are not in the same plane, but the second set 7b of

bearing means is further divided into at least a first group and a second group. Thus, the bearing means of the first group are placed in a differ- ent plane than the bearing means of the second group. Furthermore, the bearing means of the first group can be placed on a different cir- cumference (at a different distance from the centre of the movement range of the shaft) than the bearing means of the second group. These different distances are indicated with references R1 and R2 in Fig. 6a.

In a corresponding manner, the distance between said planes is indi- cated with reference a in Fig. 6b. It is obvious that in the trunnion bearing 1 according to this advantageous embodiment, the bearing means of the axial direction can also be placed in more planes than said first and second planes. This embodiment is particularly advanta- geous in such applications in which the axial forces effective on the rotor change their direction at random, at a random frequency, when the trunnion bearing is in operation. Thus, the bearing means 7b of the axial direction do not change their direction of rotation when the direc- tion of the axial force is changed. Furthermore, by the trunnion bearing according to this embodiment, it is possible to implement random total thicknesses of the axial bearing, still using bearing means of standart dimensions.

Although it was presented above that the bearing means 7 are rollers, it will be obvious that other pieces which are symmetrical in relation to their centre of rotation can be used as well. Some examples to be mentioned include a cylinder which is thicker at its middle part than at its ends (a barrel), and a ball. In said pieces, the bearing can be arranged, for example, in such a way that the pieces are provided with a shaft which can be separately mounted on bearings in the support structure 9.

The advantageousness of the bearing means 7a, 7b of the roller type is based, for example, on the fact that their frictional couple and thereby also the circumferential load on the rotor forcing it to a rotating move- ment, is only slightly dependent on the load. In uniubricated slide bearing structures, however, the frictional force is approximately directly proportional to the load.

It is obvious that the bearing means of the axial direction do not need to be placed on the same circumference, but each bearing means 7b of the axial direction may be placed on a separate circumference. This applies also to the above-mentioned bearing structure 1 according to the fourth advantageous embodiment, where some or all of the bearing means of the first group may be placed on different circumferences than the other bearing means of the first group and, respectively, some or all of the bearing means of the second group may be placed on dif- ferent circumferences than the other bearing means of the second group.

Figure 4 shows yet another electric machine 2 applying the trunnion bearing assembly according to the invention. The electric machine 2 comprises a rotor 3 provided with a shaft 4 and a rotor winding 10.

Furthermore, the electric machine is provided with a stator 11 com- prising, for example, a stator winding 12. Trunnion bearings 1, 1'are coupled to the stator 11. The first trunnion bearing 1 is intended for use as a block stop against radial displacements of the shaft 4. In the sec- ond trunnion bearing 1', both the radial and the axial trunnion bearing are combined, wherein this second trunnion bearing 1'is used as a block stop against displacements of the shaft 4 in the radial and axial directions. The trunnion bearings 1, 1'are coupled between support structures 9,9'and the stator 11 or another frame element in the elec- tric machine (not shown). Further, Fig. 4 shows, in a reduced manner, magnetic bearings 5, by which the actual contactless bearing is to be provided.

The rotationally symmetrical pieces, or bearing means 7a, to be used for the radial bearing of the trunnion bearings 1, are placed substan- tially symmetrically around the rotor 3 (or its shaft 4) in such a way that they define a polygonal area as the range of movement for the centre of rotation of the rotor. The centre of this polygon is substantially at the centre of the rotor shaft 4 in a situation in which the rotor 3 is placed in its aimed position, substantially at the centre of the area defined by the stator. Thus, the distance between the rotor and the stator is optimal in view of possible impacts on the rotor. The imaginary sides of the poly- gon can be straight or curved. In Fig. 5, a broken line L illustrates a

theoretical polygon limiting an area, inside which the centre M of the shaft 4 may move in an advantageous embodiment. In this embodi- ment, the trunnion bearing 1, 1'comprises six bearing means 7a for radial bearing. From the figure, it can be seen that the sides of the polygon L (in this case a hexagon) are curved and that theoretically the shortest distance between the side and the centre of the polygon is determined on the basis of three radii R1, R2, R3 in such a way that this shortest distance is R3- (R1 + R2). Of these radii, the first one R1 indicates the radius of the rotor (or its shaft 4) at the bearing point. The second radius R2 is the radius of the bearing means. The third radius is the diameter of the imaginary circumference P of a circle, on which the centres O of the bearing means 7a are located. Although the sides of the polygon L can be curved, in practice, at high rotational speeds, the path of the centre of the rotor is limited to a polygon with straight sides.

The above-described principle of symmetry can also be applied in an application in which the trunnion bearing 1, 1'comprises both the bearing means 7a implementing the bearing in the radial direction and the bearing means 7b implementing the bearing in the axial direction.

Thus, the bearing means 7a implementing the bearing in the radial direction are placed substantially symmetrically on the same circumfer- ence. In a corresponding manner, the bearing means 7b implementing the bearing in the axial direction are placed substantially symmetrically on the same circumference, which does not, however, need to be the same as that of the bearing means 7a implementing the bearing in the radial direction. Consequently, the bearing means 7b to be used for bearing in the axial direction can be placed on the circumference of a circle whose diameter is unequal to the circumference of the imaginary circle for the placement of the bearing means 7a to be used for bearing in the radial direction.

It is obvious that the present invention is not limited solely to the above- presented embodiments but it can be modified within the scope of the appended claims.