MXPA99006969A - A device in the stator of a rotating electric machine - Google Patents

Abstract

In a device for increasing the mechanical rigidity and natural frequency of the stator in a rotating electric machine, which stator is provided with stator teeth between the slots (2) holding the winding (12), stator teeth (4, 6), the free ends of which are situated at the air gap between stator and rotor, have at least one yoke (14) formed in one piece with adjacent stator teeth, arranged across each stator slot in order to mechanically secure the stator teeth in tangential direction.

Description

A DEVICE IN THE STATOR OF A ROTATING ELECTRICAL MACHINE FIELD OF THE INVENTION The present invention relates to a device for increasing the mechanical stiffness and natural frequency of the stator in a rotating electric machine, wherein the stator is provided with teeth that lie between the grooves that hold the winding, and the free ends of the The teeth are located in the intermediate air space between the stator and the rotor. The invention also relates to that rotating electrical machine.

BACKGROUND OF THE INVENTION High-voltage electric AC machines, such as generators, in a power station to generate electric power, dual-feed machines, machines with external poles, synchronous machines and the converter in cascades REF. : 30870 static, current, asynchronous, have until now been designed for voltages that are in the range of 15 to 30 kV, and 30 kV has been considered as an upper limit. This generally means that a generator must be connected to the power grid through a transformer that raises the voltage up to the level of the power grid, that is, to a value that is in the range of about 130 to 400 kV. In U.S. Patent No. 5,036,165, a conductor is described wherein the insulation is provided with an inner layer and an outer layer of pyrolyzed, semiconductive fiberglass. It is also known to provide such insulation to conductors that are in a dynamoelectric machine, as described, for example, in U.S. Patent No. 5,066,881, wherein a layer of pyrolyzed, semiconductor glass fiber is placed in contact with the two parallel rods that form the conductor, and the insulation in the stator slots, is surrounded by an outer layer of pyrolyzed, semiconducting fiberglass. The pyrolyzed glass fiber material is described as convenient since it retains its resistivity even after impregnation treatment. A solution to this problem is described in Swedish patent application No. 9602083-9 which describes an arrangement in which separate spacers, such as slot wedges, are inserted in the space between the free ends of the adjacent teeth. of the stator. A disadvantage of this solution is that the slot wedges are easily loosened as a result of vibrations in the stator and of different thermal expansion in the stator teeth and wedges for the grooves. No. 4,443,725 also discloses a special system of wedges for grooves to be inserted into the stator slots. The purpose of this wedge system is, however, not to increase the stiffness of the stator teeth, but rather to keep the electrical conductors in place in the stator slots.

DESCRIPTION OF THE INVENTION The aim of the present invention is to provide a new solution to the problem of the vibrations that occur in the stator teeth, in the type of alternating current machines under discussion, and that is not affected by the disadvantages of the previous solution. This object is achieved with a device of the type described in the introduction, having the characteristic features defined in claim 1, and with a machine as claimed in claim 10. By providing at least one yoke that increases rigidity, a Through the stator slots and forming a part with the adjacent teeth of the stator, the risk of loosening the yoke securing the stator teeth in the tangential direction is eliminated. According to a preferred embodiment of the device according to the invention, the yoke is formed in the upper part of the groove. This location of the yoke provides the best mechanical reinforcement of the stator teeth. However, in accordance with other advantageous embodiments of the device according to the invention, the yoke can be formed through the slot, at some distance from the top thereof, or alternatively a plurality of yokes can be formed through the slot , at different distances from the top of the slot. In accordance with still another advantageous embodiment of the invention, to allow the yoke to absorb the loads arising in the tangential direction, the width in the direction of the groove should not be less than a lower limit of typically 2 to 3 m. By closing the stator slot in this manner, the electrical properties of the machine can be adversely affected by causing the yoke or yokes to result in increased leakage in the slots, resulting in an increased need for excitation in the load, as a result, that is, an increased loss in the inductor winding. The increased leak in the slot also influences the transient reactance of the machine. Closing the slots also reduces the harmonic components of the slots in the flow through the air gap, so that only a fraction, typically 5 to 15%, of the harmonic components obtained with an open slot remains. In this way, substantially all noise caused magnetically by the harmonic components of the grooves disappears. To reduce the increase in excitation losses, with stator slots, closed, in accordance with the advantageous embodiments of the device according to the invention, the yoke is constructed in such a way that its magnetic properties deviate from the magnetic properties of the devices. stator teeth. The yoke can thus be constructed in such a way that the relative magnetic permeability in the material of the yoke is close to the value of 1, for example by drilling the yoke, or by working the material of the yoke, for example with laser, to decrease its magnetic permeability relative, preferably up to a value that is in the vicinity of 1. The invention is intended to be used first with a rotating electrical machine in which the stator windings are extracted through the slots in the stator and in the where the windings are wound from a high voltage cable, of a type comprising a core with a plurality of wire parts, an inner semiconductor layer surrounding the core, an insulating layer surrounding the inner semiconductor layer, and a external semiconductor layer surrounding the insulating layer. When using insulated, high-voltage electrical conductors, in the following cables called high-voltage cables, with a solid insulation similar to that used in cables to transmit electrical power (for example, XLPE cables), the machine voltage can be increased to Such levels can be connected directly to the power grid without an intermediate transformer. Therefore the transformer can be eliminated. In this type of machine, the slots in which the cables are placed in the stator, are generally deeper than those of conventional technology, since thicker insulation is required, due to the higher voltage and the greater number of turns in the winding. This increases the problems of natural mechanical vibrations that occur in the stator teeth, between the stator slots. In a stator with deep grooves, harmful vibrations, generated by electromagnetic forces and as a result of resonance phenomena, typically occur with a frequency of twice the frequency of the network. The advantages of the device according to the invention are then mentioned particularly for this type of machines. With the machine according to the invention, the windings are preferably composed of cables of a type having an extruded insulation, solid, such as those used today for the distribution of energy, for example XLPE cables or cables with EPR insulation. These cables are flexible, which is an important property in this context, since the technology for the device according to the invention, is mainly based on winding systems in which the winding is formed from cable that is bent during its assembly. The flexibility of an XLPE cable normally corresponds to a radius of curvature of approximately 20 cm for a cable with a diameter of 30 mm, and a radius of curvature of approximately 65 cm for a cable of 80 mm in diameter. In the present application the term "flexible" is used to indicate that the winding is flexible up to a radius of curvature that is in the order of four times the diameter of the cable, preferably eight to twelve times the diameter of the cable. The cable may preferably have a diameter that is in the range of 20 to 200 mm and a conductive area that is in the range of 80 to 3000 mm2. In the machines according to the present invention, the windings are constructed to retain their properties even when they are bent and when subjected to thermal stresses during operation. It is vital that the layers retain their adhesion to each other, in this context. The material properties of the layers are decisive here, particularly their elasticity and relative thermal expansion coefficients. In an XLPE cable, for example, the insulating layer consists of low density polyethylene, crosslinked, and the semiconductor layers consist of polyethylene with particles of carbon black and metal particles, mixed therein. Changes in volume, as a result of temperature fluctuations, are completely absorbed as changes in the cable radius and, thanks to the comparatively slight difference between the coefficients of thermal expansion in the layers, in relation to the elasticity of the These materials, the radial expansion can take place without losing the adhesion between the layers. The combinations of materials presented above should be considered as examples only. Other combinations that satisfy the specified conditions and also the condition of being semiconductive, that is, having a resistivity that is in the range of 10"1 to 106 ohm-cm, for example, 1 to 500 ohm-cm, or of 10 to 200 ohm-cm, naturally fall also within the scope of the invention.

The insulation layer may consist, for example, of a solid thermoplastic material such as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polimet ilpent eno (PMP), crosslinked materials such as cross-linked polyethylene (XLPE), or rubber such as ethylene-propylene rubber (EPR) or silicone rubber. The semiconductor layers, internal and external, may be of the same basic material, but with particles of conductive material, such as carbon black powder or metallic powder, mixed therein. The mechanical properties of these materials, particularly their coefficients of thermal expansion, are affected relatively little due to whether or not there is a mixture of carbon black powder or metallic powder, at least in the proportions required to achieve the necessary conductivity in accordance with the invention. The insulating layer and the semiconductor layers thus have substantially the same coefficients of thermal expansion. Ethylene-vinyl acetate / nitrile rubber copolymers, butyl-grafted polyethylene, ethylene-butyl acrylate copolymers and ethylene-ethyl acrylate copolymers may also constitute suitable polymers for the semiconductor layers. Even when different types of material are used as a base in the different layers, it is desirable that their coefficients of thermal expansion be substantially the same. This is the case with the combination of materials listed above. The materials listed above have a relatively good elasticity, with an E modulus of E < 500 MPa, preferably less than 200 MPa. The elasticity is sufficient so that any minor differences between the coefficients of thermal expansion of the materials in the layers are absorbed in the radial direction of the elasticity, so that no ruptures appear, or any other damage, and in such a way that the layers do not detach from each other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as the strength of the weakest of the materials. The conductivity of the two semiconductor layers is sufficient to substantially equalize the potential along each layer. The conductivity of the semiconductor outer layer is large enough to enclose the electric field in the cable, but small enough so as not to cause significant losses due to currents induced in the longitudinal direction of the layer. Thus, each of the two semiconductor layers essentially constitutes an equipotential surface and the winding, with these layers, will substantially enclose the electric field therein. Of course, there is no objection to the fact that one or more additional semiconductor layers are accommodated in the insulation layer. The invention will now be described in greater detail, with reference to the accompanying drawings in which Figure 1 shows a slot division in the stator, with an open slot, Figure 2 a slot division designed in accordance with the present invention, Figure 3 shows an alternative embodiment according to the invention, and Figure 4 shows a cross section through the high voltage cable used according to the invention. Figure 1 shows a slot division of the iron sheet core in the stator, comprising a slot 2 and a portion of the stator teeth 4, 6 on each side of the slot 2. The slot 2 is accommodated to receive the winding wires 12 that are distributed axially through the stator and slots 2 are normally deeper in this type of AC machine, than in conventional machines. This contracts the disadvantages of the stator having low natural frequencies and of the oscillations that occur easily in the stator 4., 6, as mentioned above. As can be seen in Figure 1, the slots 2 in this type of machine, contrary to conventional generators, resembles a bicycle chain with protuberances 10 between each cable 12 in the teeth 4, 6 located between the slots 2, of such that the cable is secured radially. This type of slot is often referred to as "semi-closed", to differentiate it from conventional, rectangular, open slots, with perfectly straight sides completely out of the air gap. The slot 2 is open towards the intermediate air space, in the upper part of the slot, to the left in figure 1. The opposite end of the slot is called the bottom of the slot. To handle the problems of the natural vibrations in the stator, discussed above, and to increase the tangential rigidity, the yokes 14 are provided through the slots, in accordance with the invention, see figure 2. In the embodiment shown in this figure a yoke is accommodated in the upper part of the groove and another yoke is accommodated approximately to the center of the groove 2. The most efficient location of the yoke, from the mechanical aspect, is in the upper part of the groove. However, it may be a good idea to provide several yokes in the case of deep grooves, and in certain cases it may be desirable to have no yoke at the top of the groove, but only at points further down the groove. The yoke (or yokes) 14 is made in one piece with the adjacent teeth 4, 6 of the stator. The tangential stability achieved by the yokes 14 increases the natural frequency and provides a considerably increased stiffness in each individual tooth, as well as an increased bending stiffness in the whole stator body. Another important advantage is that the electromagnetic forces, tangential, in the intermediate air space, which are derived from the poles of the rotor, are evenly distributed between the teeth. To ensure that the yoke or yokes 14 will provide sufficient mechanical reinforcement, their width d in the direction of the groove 2 should not normally fall below a limit of typically 2 to 3 mm. As discussed above, the yokes cause an increased leak in the grooves. The increased leakage flow limits the short-circuit currents in the case of some short circuit, and eliminates, or at least reduces, harmonic components of the grooves, in the flow of the intermediate air space. However, increased leakage in the grooves causes increased excitation losses. For this reason the yoke or yokes 14 should preferably be constructed in such a way that their magnetic properties deviate from the magnetic properties of stator teeth 4, 6. The yoke is preferably constructed in such a way that the relative magnetic permeability in the material of the yoke is in the vicinity of the value 1. This can be achieved by piercing the yokes, as shown in Figure 3 with the number 16 However, it must be ensured that the perforations compromise the stabilizing influence of the yoke or yokes. Alternatively, the magnetic permeability in the yoke material can be reduced through proper treatment of the material, for example, by laser treatment. Figure 4 shows a cross section through a high voltage cable 29 used in the rotating electrical machine according to the present invention. The high voltage cable 29 is composed of a number of wire parts 31 having a circular cross section and, for example, made of copper. These wire parts 31 are accommodated in the central part of the high voltage cable 29 and around the wire parts 31 there is a first semiconductor layer 32. Around the first semiconductor layer 32 is an insulating layer 33., for example an XLPE insulation, and around the insulating layer 33 there is a second semiconductor layer 34.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates. Having described the invention as above, the content of the following is claimed as property:

Claims (18)

1. A device for increasing the mechanical stiffness and natural frequency of the stator, in a rotating electric machine, wherein the stator is provided with teeth between the grooves that hold the winding, the free ends of the teeth are located in the intermediate air gap which is located between the stator and the rotor, the device is characterized in that at least one yoke designed to increase the stiffness, and formed in a single piece with the adjacent teeth of the stator, is accommodated through each slot of the stator to ensure mechanically stator teeth in the tangential direction.

2. A device according to claim 1, characterized in that the yoke is formed in the upper part of the slot.

3. A device according to claim 1, characterized in that the yoke is formed through the slot, at a distance from the top of the slot.

4. A device according to claim 1, characterized in that the width of the yoke in the direction of the groove is not less than a lower limit typically of 2 to 3 mm.

5. A device according to the rei indication 1, characterized in that a plurality of yokes is formed through the slot, at different distances from the top of the slot.

6. A device as claimed in any one of claims 1 to 5, characterized in that the yoke is constructed in such a way that its magnetic properties deviate from the magnetic properties of the teeth of the stator.

7. A device in accordance with that claimed in rei indication 6, characterized in that the yoke is constructed in such a way that the relative magnetic permeability in the yoke material is close to the value 1.

8. A device according to claim as claimed in claim 6 or claim 7, characterized in that the yoke is perforated.

9. A device according to claim as claimed in any of claims 6 to 8, characterized in that the material in the yoke is machined to decrease its magnetic permeability.

10. A rotating electric machine having windings removed in the slots in the stator, characterized in that the windings are wound from high voltage cable, and because the machine is provided with a device in accordance with that claimed in any of the rei indications of 1 to 9.

11. A machine according to claim 10, characterized in that the high voltage cable is of a type comprising a core with a plurality of wire parts, an inner semiconductor layer surrounding the core, an insulating layer surrounding the inner semiconductor layer , and an outer semiconductor layer surrounding the insulation layer.

12. A machine according to claim 11, characterized in that the high voltage cable has a diameter that is within the range of 20 to 200 mm and a conductive area that is within the range of 80 to 3000 mm2.

13. A machine according to claim 1 in any of claims 10 to 12, characterized in that the winding is flexible and because the layers adhere to one another.

14. A machine as claimed in any of claims 10 to 13, characterized in that those layers consist of materials with such an elasticity and such a relationship between the coefficients of thermal expansion of the materials, that the changes in volume of the layers, caused by temperature fluctuations, during operation, are absorbed by the elasticity of the materials, so that the layers retain their adhesion to each other, to the values of temperature fluctuations that occur during operation .

15. A machine according to claim 6 or claim 7, characterized in that the materials in those layers have high elasticity, preferably with an E-modulus less than 500 MPa, and most preferably less than 200 MPa.

16. A machine according to any of claims 10 to 15, characterized in that the coefficients of thermal expansion for the materials in the layers are substantially of the same magnitude.

17. A machine according to claim as claimed in any of claims 10 to 16, characterized in that the adhesion between the layers is at least the same magnitude as the strength of the weakest of the materials.

18. A machine according to claim 1 in any of claims 10 to 17, characterized in that each of the semiconductor layers essentially constitutes an equipotential surface.