Scope of the invention The invention relates an arrangement for reducing vibration in an electrical machine according to the Preamble in Claim 1 , an electrical machine according to the Preamble in Claim 9, and a method for reducing vibration in an electrical machine according to the Preamble in Claim 10.

Background art The fundamental wave of magnetic flux creates a rotating force field in the air gap of an electrical machine, between the rotor and the stator. The spatial wavelength of this harmonic force field is 360/2p degrees, in which p is the number of pole pairs, and the frequency of the force field in the stator coordinates is 2F, in which F is the input frequency. In an ideal situation the force field is balanced and does not cause any net force to the stator and rotor. Irrespective of this, the stator deformations caused by this force field are so large that they are transferred almost as such to the frame of the electrical machine. A forced excitation directed to the frame of the electrical machine is noticed as vibration in the frame and the sheet fields, and spreads, especially in connection with large machines, as noise in the frame and in air. The rotating force wave is a required part of electrical machine operation and torque creation, and cannot be substantially reduced. On the other hand, the amplitude of the force wave is so great that even a thick stator back tends to deform. This happens in spite of the fact that the elastic properties of the stator core in a plane perpendicular to the shaft are close to the corresponding values for steel. On the other hand, the defor- mations of the stator have a quasi-static nature because the frequency of the characteristic form corresponding to the force wave— for example, 380 Hz— is clearly higher than the excitation frequency— for example, 120 Hz. The excitation frequency is the supply frequency multiplied by two. The vibration problem at the second multiple of the supply frequency is particularly substantial in large two-pole electrical machines. Higher numbers of poles shorten the spatial wavelength of the force field, which reduces the deformations of the stator.

The stator usually comprises thin plates that are attached together using back beams and welding, for example. The outermost part of the stator forms the stator back, and there are grooves in the internal part of the stator in which the windings are placed. Stator deformation can be reduced by making the stator back thicker or by using external reinforcements to stiffen the stator. However, this will increase the number of stages of manufacture, the manufacturing costs, the weight of the machine, and its space require- ment.

Another method of solving the problem is isolating the stator from the frame. The objective of isolation is to reduce the transmission of stator deformations to the frame and the subsequent emergence of vibration and noise. A common method for the construction of machines is to suspend the source of vibration in a flexible manner. In the case of a stator, this is impeded by the fact that the stator support must be able to bear quasi- static and dynamic loads in various operating situations. Such loads include gravitational force, nominal torque, and short circuit torque. Furthermore, the stator core and the rigidity of its suspension have a substantial effect on the characteristic frequencies and, thus, the vibration behavior of the entire motor. A general solution for large two-pole turbogenerators is to isolate the stator.

Description of invention

The purpose of the present invention is to create an arrangement and method for reducing vibration in an electrical machine.

In order to achieve this, the invention is characterized by the features specified in the characteristics section of Claims 1, 9, and 10. Some other preferred embodiments of the invention have the characteristics specified in the dependent claims.

In an arrangement according to the invention for reducing vibration in an electrical machine, the electrical machine comprises a rotor and a stator located at the distance of an air gap from the rotor. The stator is made of sheets and composed of axially successive core modules. The stator comprises a radial cooling channel between two core modules. The outer surface of the stator core modules comprises back beams which are as long as the stator. In the outer surface of at least one stator core module, at least one spring element is fitted between the stator back and the back beam. The spring element is fastened to the back beam.

An electrical machine according to the invention comprises a rotor and a stator located at the distance of an air gap from the rotor. The stator is made of sheets and composed of axially successive core modules. The stator comprises a radial cooling channel between two core modules. The outer surface of the stator core modules comprises back beams which are as long as the stator. In the outer surface of at least one stator core module, at least one spring element is fitted between the stator back and the back beam. The spring element is fastened to the back beam.

In a method according to the invention for reducing vibration in an electrical machine, the electrical machine comprises a rotor and a stator located at the distance of an air gap from the rotor. In the method, the stator is made of sheets and composed of axially successive core modules. A radial cooling channel between two core modules is formed in the stator. Back beams, which are as long as the stator, are installed on the outer surface of the stator core modules. In the outer surface of at least one stator core module, at least one spring element is fitted between the stator back and the back beam and is fas- tened to the back beam.

The solution according to the invention is used for reducing vibrations and noise of electronic origin. The solution in accordance with the invention significantly reduces the transmission of stator deformations, caused by the fundamental flux of the electrical machine, to vibrations in the frame and, as a result, to noise in the frame and air. In the solution according to the invention, the stator, the source of vibration, is isolated from the back beam. Required flexibility between the stator back and the back beams can be created by using spring elements between the stator back and the back beam. The spring element is unbending in the circumferential and axial directions but flexible in the radial direction. The flexibility of the spring element in the radial direction creates the vibrational isolation. A spring element is, for example, a leaf spring comprising one or more steel strips. A plate spring is an example of a single-strip leaf spring. A single-strip leaf spring has at least one primarily axial face which is used as the fastening surface when fitting the spring leaf to the outer surface of the stator core module. The leaf spring can also be, for example, symmetrical with the vertical symmetry axis which is perpendicular to the axial direction. In this case, the leaf spring has two primarily axial faces, both of which are fastened to the outer surface of the stator core module.

The back beam and one-strip leaf spring can also be machined from one blank piece. This helps to gain higher strength than when assembling them from two pieces. The advantage of the solution according to the invention is that the joint is relatively rigid in both the tangential and axial directions. Tangential rigidity is required especially in fault situations such as short circuits and high-speed reclosing. Axial rigidity is required to maintain the axial compression strength between the stator core modules and to control the axial characteristic form of the stator. The solution in accordance with the invention does not significantly increase the outer diameter of the stator, because the thickness of the spring element itself and the gap between the stator back and the spring element are very small. The weight of the stator does not increase remarkably either, because the weight of the spring element and its fastening is very small in comparison to the weight of the stator. The invention is suitable for use with all radially cooled electrical machines in which cooling is arranged through radial air ducts. When installed to the stator core module, the spring element and its fastening do not in any way reduce the width of the air ducts between the modules. The solution is suitable for both symmetrically and asymmetrically cooled electrical machines. According to an embodiment of the invention, three spring elements are fastened in the circumferential direction to the outer surface of the stator core module. When the angle center point is in the middle of the stator, the angle between the spring elements is preferably 90...150°. Three fastening points is a suitable number for a two-pole electrical machine in which the deformation of the back of the stator core caused by the distribution of force corresponds to a rotating ellipse as the time proceeds. With three fastening points the net force targeted to the stator core module is small. According to an embodiment of the invention, the spring elements of at least two axially successive stator core modules are attached to different back beams.

According to another embodiment of the invention, the stator is also isolated from the pressure ring of the stator end. The end plate of the stator is supported with rod-shaped supports, such as screws, to the pressure ring so that there is a gap between the end plate and the pressure ring and the rod-shaped supports are flexible in a plane perpendicular to the stator shaft.

The arrangement and the method according to the invention are beneficial in large two- pole electrical machines in which the invention improves especially the management of second-order vibrations. The invention can also be used to reduce higher- frequency impulses originating in the stator from being transmitted to the frame and becoming audible. The solution in accordance with the invention is suitable for two-pole machines as well as asynchronous or synchronous machines with more poles.

Figures in the drawings

In the following, the invention will be described in more detail with the help of certain embodiments by referring to the enclosed drawings, where:

- Figure 1 is a partial illustration of an electrical machine viewed from the end;

- Figure 2 illustrates the stator viewed from above; - Figure 3 illustrates the fastening of the spring element to the back beam;

- Figure 4 illustrates the leaf springs;

- Figure 5 illustrates a leaf spring with several strips; - Figure 6 illustrates the fastening of the stator end to the pressure ring.

Detailed description

The electrical machine 1 illustrated in the figures is a motor or a generator.

Figures 1 and 2 illustrate the arrangement to reduce vibration in an electrical machine. The electrical machine 1 comprises a rotor 3 and a stator 2 located at the distance of an air gap 4 from the rotor. The stator 2 is formed of successive axial stator core modules 2a-d. The stator core module 2a-d is made of thin sheets stacked on top of one another. The stator core modules 2a-d are joined together by using back beams 7 and welding. The back beams 7 are fastened to the frame (not shown) of the electrical machine as known to the man skilled in the art. A duct pin 10 is placed between the stator core modules 2a-d to separate them and to form radial r channel 5 for a cooling air flow. The stator 2 is cooled through the radial cooling channels 5. The outermost part of the stator forms the stator back 9, and there are grooves in the internal part of the stator in which the windings (not shown in the figures) are placed.

The outer surface 6 of the stator core modules 2a-d comprises back beams 7 whose length L is the same as the stator 2. The back beams 7 are located at intervals on the ou- ter surface 6 of the stator 2. The majority of the torque of electrical machine 1 is transferred via the stator back 9 to the back beams 7 which transfer the torque to the pressure rings 15 of the end. The torque is further transferred through the end plate fastening to the body of the electrical machine. A spring element, leaf spring 8b, is fitted to the outer surface 6 of the stator core module 2a-d, between the stator back 9 and the back beam 7. As the spring element is fitted onto the outer surface 6 of the core module there is no need to machine the stator core. The spring element 8b forms a flexible joint between the stator core module 2a-d and the back beams 7. The isolation of vibration in the stator back 9 is based on the flexibil- ity of the spring element 8b in the radial direction r. A small gap 14 is left between the back 9 of the stator 2 comprising the stator core modules 2a-d and the back beam 7. The spring element 8a-c is attached to the back beam 7 by welding. In Figure 3 the spring element 8b is fastened to the back beam 7 with a welding joint 12b realized with outline welding. The spring element 8b is embedded inside the back beam 7. Thereby the diameter of the stator 2 and the back beam 7 does not grow. For example, three spring elements 8a-c are fastened in the circumferential direction to one stator core module 2a-d. When the angle center point is in the middle of the stator, the angle between the spring elements is preferably 90...150°. In a balanced solution the spring elements are essentially equally spaced.

The stator core module 2a-d is fastened to the three back beams 7 of the stator 2 by means of the spring elements. The amount of three fastening points with such an equal spacing is beneficial in a two-pole electrical machine in which the deformation of the back 9 of the stator core 2a-d caused by power distribution corresponds to a rotating ellipse as the time proceeds. In this case the total force caused by the fastening and targeted to the stator core module is small and the center point of the stator core module 2a-d stays in its place. If there are, for example, six back beams, one stator core module 2a-d is fastened through the spring elements 8a-c to three back beams 7.

The spring elements 8a-c of the axially successive stator core modules 2a-d are attached to different back beams 7. In Figure 2 the spring elements 8a-c of the second 2b and the fourth 2d stator core module 2a-d are attached to the same back beam 7, that is, in every second stator core module 2a-d the spring elements 8a-c are fastened to the same back beam 7.

Figures 4a)-c) illustrate two different one-strip leaf springs, plate springs 8a-b. In Figure 4a) a plate spring 8a is viewed from the side, and in Figure 4b) from above. The plate spring 8a is a plate bent from one point, and the created angle divides the plate 8a into two parts 13a-b. The plate spring 8a has two essentially axial a faces 1 la-b. The plate spring 8a is fastened to the outer surface 6 of the stator core module 2a-d from the axial a face 1 la of the first part 13a with the welding joint 12. The plate spring 8a is fastened from the second part 13b of the plate to the lower surface of the back beam 7 facing the stator 2. In Figure 4c) the plate spring 8b is a plate bent from two points, and the two created angles divide the plate into three parts 12a-c. The plate spring 8a has two essentially axial faces l la-b. The plate spring 8b is fastened to the outer surface 6 of the stator core module 2a-d from the axial a face l la-b of the first part 13a and the third part 13c with the welding joint 12a. The plate spring 8b is fastened from the second part 13b of the plate to the lower surface of the back beam 7 facing the stator 2.

The plate spring 8b in Figure 4c) is symmetrical with the vertical symmetry axis s which is perpendicular to the axial direction. The plate spring 8b is installed to the lo- wer surface of the back beam 7 so that the symmetry axis s of the plate spring 8b is on the axial a central axis of the back beam 7.

The shape of the plate spring 8a-b is preferably elongated. The axial a width of the plate spring 8a-b is 10...40% from the circumferential c length of the plate spring.

Figure 5 illustrates a leaf spring 8c which is formed of several strips 20. In Figure 5 all strips 20 are equally long, but their lengths can also vary, for example, shorten when entering in the circumferential direction from one edge of the back beam 7 to the other. In Figure 4 and 5 the spring element 8a-c is fastened to the lower surface of the back beam 7. This fastening is more preferable to manufacture than the fastening of Figures 1-3, in which the spring element 8b is embedded in the back beam 7.

Each stator core module 2a-d of the stator 2 in an electrical machine 1 creates some tor- que. If there is a tight compression between the stator core modules 2a-d and the stator core modules are connected to each other, the stator core modules 2a-d do not need to be fastened from several points to the back beams 7 with the spring elements. It is not necessary to fasten every single stator core module to the back beams. Thereby the connections made with the duct pins 10 between the stator core modules 2a-d transfer loads, and force is transmitted from the one stator core module 2a-d to the other. In this case each stator core module 2a-d does not need to be isolated from the frame of electrical machine 1. In this case, for example, in every second stator core module 2a-d the spring elements 8a-c are fitted between the stator back 9 and back beams 7, or, for ex- ample, 40-60% of the stator core modules 2a-d are equipped with the spring elements 8a-c.

The vibration isolation of the electrical machine 1 can be improved by isolating the sta- tor 2 from the pressure rings 15 of the end, Figure 6. The end plate 16 of the stator 2 supports the stator 2 formed of stator core modules 2a-d in the axial direction a, and is fastened to the outmost stator core module 2a of the stator 2. The back beam 7 is fastened to the end plate 16 by welding. The end plate 16 of the stator 2 is supported with rod-shaped supports 17 to the pressure ring 15. In Figure 6 the rod-shaped support is a screw. With the rod-shaped supports 17 it is possible to create a gap 18 between the end plate 16 and the pressure ring 15. The vibration isolation of the stator end plate is based on the flexibility of rod-shaped supports 17 in the radial direction.

The threaded part 19 of the screw is short in relation to the free length of the screw, the threaded part being, for example, 10...35% of the free length of the screw.

Figure 6 illustrates one end of the stator. It is possible to implement the solution simultaneously in both ends of the stator. The solution illustrated in the figures substantially reduces the frame and bearing vibrations caused by the main flux of electrical machines. The solution also reduces the frame vibrations and bearing vibrations caused by other flux components.

Part list: 1 electrical machine; 2 stator; 2a-d stator core module; 3 rotor; 4 air gap; 5 ra- dial cooling channel; 6 outer surface; 7 back beam; 8a, 8b, 8c leaf spring; 9 stator back; 10 duct pin; 1 la-b face; 12a-b welding joint; 13a, b, c plate part; 14 gap; 15 pressure ring; 16 end plate; 17 rod-shaped support; 18 gap; 19 threaded part; 20 strip; a axial direction; c circumferential direction; L length; r radial direction; s symmetry axis.