TECHNICAL FIELD

The present invention relates to a device in a stator in a rotating electric machine comprising a compressing ring arranged at each end of the stator with several - one after the other - axially arranged and joined-together layers of a magnetic material, between the layers there being arranged a layer of electrically insulating material, in the compressing ring there being arranged channels for the passage of a coolant.

The invention is particularly well suited for use in large generators. In particular, it is well suited where the unwanted heat development is considerable, for example as a consequence of large losses caused by eddy currents induced by the magnetic leakage field.

BACKGROUND ART

Rotating electric machines comprise a rotor and a stator. The so-called turbogenerator is a common type of rotating electric machine. In this, the cylindrical stator is concentrically arranged around the rotor, thus forming an air gap between the rotor and the stator. The stator here comprises a cylindrical stator core, usually referred to as an iron core or sheet iron core. The sheet iron core further comprises a plurality of layers of magnetic material with alternating layers of elec- trically insulating material. The layers are arranged such that their plane is parallel with the cross section of the cylindrical stator core. They often consist of relatively thin plates of a magnetic material, which on their surfaces are coated with an electrically insulating coating. The plates may be sector-shaped, such that several adjacently disposed plates form a ring with the same cross section as that of the stator

core. Each such ring then comprises a central layer of the magnetic plate, which is surrounded on each side by a layer of the electrically insulating coating. By arranging a plurality of such rings, axially one after the other, a cylindrical laminated stator core is formed.

To retain the stator core thus laminated, a so-called compressing ring is arranged at each end of the cylindrical core. By compressing ring is meant here an annular member which is disposed at each end of the stator core and which is intended to transmit axial forces to the stator core to keep this together.

In addition to the stator core, the stator also comprises a winding. The winding consists of a number of coils, which are disposed in axial slots in the stator iron core. In each slot there are two insulated coil sides or coil halves, each one containing one or more conductors. Each conductor comprises a number of parallel-connected strands. Further, the different coils are joined together on the coil ends into a complete winding.

PROBLEMS

In prior art rotating electric machines, the compressing rings were usually designed as a homogeneous ring of steel. Such a design of the compressing ring entails a good dimensional stability and allows large axial forces to be transmitted to the stator core without the compressing ring being signifi- cantly deformed. However, the design also entails significant disadvantages. Especially in large rotating electric machines, very large magnetic leakage fluxes arise around the stator coil ends during operation. These leakage fluxes are parti¬ cularly awkward at low reactive power and especially during underexcited operation, that is, when the rotating electric machine is operated as a generator which derives reactive

power from the electricity distribution network to which it is connected.

The leakage flux from the stator coil ends finds its way axially inwards towards the end of the stator and causes cir¬ culating current paths, inter alia in the compressing rings. These current paths here result in eddy current losses and heating of the compressing ring and of the adjacent stator core.

To reduce the above-mentioned problems, it has been proposed to design the compressing rings laminated, with layers of magnetic material alternating with layers of an electrically insulating material. The current paths which arise in the compressing ring are limited to run in a thin layer, between two surrounding layers of electrically insulating material. In this way, the magnetic losses are reduced.

In order for these laminated compressing rings to obtain good physical properties, the different plates included in the compressing ring are joined together with a glue. This glueing is usually carried out with a so-called vacuum glueing method. The result is a laminated compressing ring with relatively good magnetic properties and with a rigidity with respect to bending which allows sufficiently great axial forces to be transmitted to the stator core without the ring being deformed.

Although these laminated compressing rings have better magne- tic properties than the previously used steel rings, the large magnetic leakage fluxes still lead to high losses in the com¬ pressing rings and to heating thereof. The negative effect of such heating has been reduced by cooling the compressing rings. According to the prior art, the cooling has been achieved with the aid of a coolant, usually hydrogen gas,

which is supplied to channels running axially through the compressing ring.

The decisive factor for how good a cooling capacity is obtained with a given coolant is the size of the total heat exchanging surface of the cooling channels. According to the prior art, however, the largest possible cooling surface has been greatly limited. According to the current technique, the cooling channels are in the form of tubular channels which are arranged axially through the ring. The surface of each channel is dependent on the diameter and the length of the channel . Since the length of the channel is given, equal to the length of the ring, the surface of a channel can be increased only by increasing the diameter of the channel. This, however, results in the volume of the channel also increasing. This leads to a reduction of the total amount of magnetic material in the compressing ring by a corresponding amount, which in turn deteriorates the magnetic properties of the compressing ring. The surface of the channel increases linearly with increasing diameter, whereas the volume increases quadratically with increasing diameter. Expressed in simple terms, each increase of the diameter of the channel thus entails a greater deterio¬ ration with respect to the magnetic properties of the com¬ pressing ring than the improvement which is achieved regarding the cooling capacity. Therefore, according to the prior art so far it has been necessary to greatly limit the heat-exchanging surface of each channel.

This has been possible only by cooling the generators with hydrogen gas at a high pressure, usually at an overpressure of about 5 bar. At this pressure, hydrogen gas, compared with air at atmospheric pressure and with the same temperature increase of the coolant, is able to remove a loss quantity about six times as large. In addition, at this high pressure hydrogen gas has a considerably higher heat transfer coefficient than air. The temperature difference between the cooled surface and

the gas is therefore, for the same loss density, considerably lower with hydrogen gas than with air.

Cooling with pressurized hydrogen gas, however, entails considerable problems. To start with, hydrogen cooling requires a cooling system which is completely separated from the surrounding atmosphere. Further, the generator must be designed to withstand the overpressure prevailing inside the generator casing. In addition, the small size of the hydrogen molecules makes exceptional demands on the density of con¬ duits, couplings and packings in the system. Thus, the cooling system is complicated to design and expensive to manufacture and maintain.

Even if the cooling system is well designed, there is always a risk of the hydrogen gas leaking out. At best, such leakage only causes an interruption in the operation of the rotating electric machine. The interruptions in the operation may involve very considerable economic losses, for example if the rotating electric machine is a power plant generator with a generator power of 500 MVA or more. In addition to economic losses, however, hydrogen leakage may also cause considerably worse damage, since the gas is explosive.

Hydrogen cooling of compressing rings at stator ends of rota¬ ting electric machines thus entails considerable problems as regards expensive and complicated cooling systems as well as the risk of costly interruptions in the operation and dangerous leakage.

The object of the present invention is therefore a device in a stator in a rotating electric machine, which greatly reduces the above-mentioned problems by allowing the compressing rings to be cooled with air as coolant.

THE SOLUTION

The object is achieved according to the present invention with a device of the kind mentioned in the introductory part of the description, which is characterized in that the channels are radially arranged in the compressing ring.

Since the cooling channels are radially arranged, their heat- exchanging surface may be dimensioned independently of the volume of the respective channel. The loss of magnetic material may thus be kept at a minimum. Each channel may consist of a space between two adjacently disposed plate layers. This results in a cooling channel, the surface of which is equal to twice the cross-section area of the com- pressing ring but the volume of which may be kept arbitrarily small by choosing a small space between the plate layers. In practice, of course, a certain space is required between the plate layers, each channel being given a certain minimum volume. However, also in practice, radial channels according to the invention allows a considerable improvement of the ratio of heat-exchanging surface to lost quantity of magnetic material compared with the prior art. The invention entails such an improvement of this ratio that the total cooling surface may be made six times larger, while maintaining the magnetic properties, compared with the prior art.

In this way it has also been possible to replace the previously used coolant pressurized hydrogen gas with air under atmospheric pressure. As indicated above, this means considerable simplifications and savings of cost. The reasons is that air cooling allows the cooling systems to be designed open and without complicated pressure-proof seals. All the problems associated with coolant leakage are completely eliminated.

According to the invention, the cooling channels may consist of voids which are limited by two adjacently arranged layers of magnetic and/or insulating material and by two elongated spacers essentially radially disposed between the two layers. The spacers ensure that a suitable distance is maintained between the two layers which axially limit the channel, also when the compressing ring is subjected to retaining axial forces. The radially arranged spaces also serve as guide means for the coolant, such that a laminar flow is counteracted and a turbulent flow, which is advantageous from the point of view of heat transfer, is favoured.

Further, at least some of the spacers may consist of a plura¬ lity of layers of a magnetic material, joined together one after the other and axially arranged in relation to the com¬ pressing ring, between which layers are arranged layers of an electrically insulating material. The spacers may thus be designed according to the same principle as the rest of the compressing ring. This results in very good magnetic proper- ties in all the parts of the compressing ring which are not voids . Spacers thus designed may advantageously be arranged in those parts of the compressing ring which are subjected to the greatest magnetic leakage fluxes. Since the leakage fluxes are normally greatest nearest the stator ends, the cooling channels in the compressing ring which are nearest the stator ends are advantageously provided with such laminated spacers.

Some of the spacers may also be formed from an essentially homogeneous, non-layered material. It is thus possible, for those cooling channels which are arranged in regions which are subjected to minor leakage fluxes, to design the spacers of less costly material, such as, for example, ordinary carbon steel or non-magnetic stainless austenitic steel . Since the spacers can be made of an essentially homogeneous material, they also become less expensive and simpler to manufacture, since fewer work operations are required. The two methods of

designing the spacers allow the magnetic performance of the compressing ring to be optimized in relation to the material and manufacturing cost of the compressing ring.

Further, the spacers may be secured to the layers of magnetic material, arranged nearest to the spacers, on one side each of the spacers. They may, for example, be welded, glued or both welded and glued to the plates which are arranged on separate sides of the spacers. This results in a compressing ring which, in addition to the axial retaining forces, can also withstand such shear forces which may arise as a result of electromagnetic phenomena in the stator, especially during transient operations.

The device according to the invention may further be provided with a plurality of clamp joints, which are intended to press the compressing rings in a direction towards each other and each of which comprises an elastic element, which is arranged outside the periphery of the stator core and a branch arranged at each end of the elastic element and extending radially along those sides of the compressing rings which are facing away from each other. According to the prior art, the com¬ pressing rings have been pressed in a direction towards each other with the aid of screw joints which are in the form of a tension screw extending through axial holes through the compressing rings and the stator core and which at their ends are provided with nuts. By screwing the nuts in a direction towards the stator core, the two compressing rings have been pressed in a direction towards each other. However, this embodiment entails a considerable disadvantage since it requires axial through-holes to be provided in the compressing rings and the stator core. Such holes, of course, require a considerable reduction of magnetic material, which in turn causes deteriorated magnetic properties of the whole stator. Further, they entail a risk of a short circuit arising between the plates if the insulation of the bolts is damaged.

The clamp joint according to the invention is completely arranged outside the stator core and the compressing rings. This results in good retaining property without the magnetic properties of the stator being deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplifying embodiment of the invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic side view, partially cut-away, of a device according to one embodiment of the invention.

Figure 2 is a perspective view of part of the device in Figure 1.

Figure 3 shows a cross section through part of a stator lamination.

Figure 4 shows a plan view of a stator lamination with spacers .

Figure 5 is a perspective end view of a spacer.

Figure 6 is a perspective end view of another spacer.

Figure 7 is a cross section through part of a compressing ring according to one embodiment of the invention.

Figures 8, 9 and 10 show, on an enlarged space, separate parts of the compressing ring shown in Figure 7.

Figure 1 shows part of a rotating electric machine, more particularly of a generator. The generator comprises a stator with a cylindrical stator core 1. (The stator also comprises other members such as winding and body. For the sake of

clarity, these are not shown. ) At each end of the stator core, a compressing ring 2 is arranged. Centrally through the stator core 1 and the compressing rings 2, there runs an axial hole 3 through which the rotor (not shown) of the generator extends.

Figure 2 shows one of the compressing rings 2, shown in Figure 1, in more detail. The compressing ring 2 is partially built up of a large number of stator laminations 5. Figure 3 shows that each stator lamination 5 consists of a core 5a and a surface layer 5b. The core 5a consists of a magnetic material, for example silicon-alloyed electroplate, and the surface layer consists of an electrically insulating surface coating, for example phenol-based electroplate insulating coating with a filler.

Figure 4 shows that the general shape of a stator lamination 5 constitutes a sector of a ring. When several stator lamina¬ tions are placed next to each other, a ring is thus formed. The compressing ring 2, in Figure 2, comprises a plurality of axially arranged layers of such rings. In this way, the com¬ pressing ring 2 comprises several axially arranged annular layers of magnetic material 5a. Between these layers 5a, layers 5b of insulating material are arranged. The insulating layer thus consists of the surface coating 5b of the stator laminations 5.

Figure 2 further shows that the compressing ring 2 has a general ring form and that several slots 6, running axially, are arranged radially from the inner envelope surface 3 of the compressing ring. In these axial slots 6, the winding coils (not shown) of the stator are arranged. Further, the com¬ pressing ring 2 is provided, at its external envelope surface 7, with axially running recesses 8. The axial recesses 8 are intended to cooperate with corresponding projecting elements (not shown) in a surrounding frame (not shown) for fixing the stator.

Figure 2 (see also Figure 7) further shows that that end surface 9 of the compressing ring 2, which faces away from the stator core 1, has a generally conical shape. This end surface 9 is divided into four sections with different inclination in relation to a radial plane. The outermost section 9a is parallel to the radial plane. The section 9b, arranged inside the outermost section 9a, exhibits an angle of about 15° to the radial plane. The next two sections 9c and 9c exhibit angles of about 35° and 70°, respectively, to the radial plane. Inside section 9d a section 9e is arranged. This section 9d is perpendicular to the radial plane and thus constitutes the inner cylindrical envelope surface of the compressing ring. The different angles of the sections 9b, 9c, 9d with the radial plane are achieved by displacing the inner ends of the stator laminations 5, included in each respective section, radially with respect to each other (see Figures 8, 9, 10) . In this way, the stator laminations 5 in the various sections 9a, 9b, 9c, 9d exhibit free areas of different sizes. In Figures 8, 9 and 10, it is clear that each stator lamina- tion 5 in section 9b exhibits a larger free area than those in section 9d. With this conical shape of the compressing ring 2, a suitable distribution of the large magnetic leakage fluxes which prevail at the stator end during operation, especially during underexcited operation, is obtained.

Figures 2 and 7 further show the radial cooling channels 10. In Figure 2 it is also indicated, by means of flow arrows, how the coolant, which consists of air, flows out of the cooling channels in the compressing ring 2. It should be noted that the coolant can also be driven in the opposite direction, that is, radially from outside and in through the channels 10. The cooling channels 10 are arranged radially in ten different layers of the compressing ring 2. Each cooling channel 10 iε formed from a space between two rings of stator laminations 5, arranged axially one after the other, and between two spacers 11. Figure 4 shows a stator lamination 5 on which several

spacers 11 are radially arranged. The spacers 11 consist of elongated elements and are radially arranged between two adjacently located layers of stator laminations 5.

As in Figure 5, the spacers 11 may be composed of a plurality of elongated sheets of the same material and surface coating as the stator laminations 5, the sheets being glued one above the other. These laminated spaces Ila are thus composed in the same way as the rest of the compressing ring 2, with several layers of magnetic material alternating with layers of an electrically insulating coating. The laminated spacers Ila are thus given the same good magnetic properties as the rest of the compressing ring. In the laminated spacers Ila, the ends of the different stator laminations 5 may be displaced in relation to each other. In this way, a certain inclination in the direction of the free end surface of the spacers Ila, which are exposed to the magnetic leakage fluxes, is obtained. Normally, this inclination of the end surfaces of the spacers Ila is arranged so as to correspond to the inclination of that section 9b, 9c, 9d of the compressing ring 2 in which it is included. This laminated shape of the spacers Ila is used in those cooling channels 10 which are arranged in the outer layers 9b, 9c, 9d of the compressing ring.

As in Figure 6, the spacers 11 may also consist of one single essentially homogeneous element lib. These non-laminated spacers lib are made, for example, from a non-magnetic stain¬ less austenitic carbon steel. The non-laminated spacers lib, which are less expensive than the laminated spacers Ila, are arranged in the inner section 9e of the compressing ring.

Both the laminated Ila and the non-laminated lib spacers are glued and welded to the adjacent stator laminations 5. This results in the torsional rigidity of the compressing ring which is required for taking up the shear forces which may arise in the stator, especially during transient operations.

Alternatively, the spacers Ila, lib may either be only glued or welded to the adjacent stator laminations 5.

To obtain good rigidity in the entire compressing ring 2, this is vacuum-glued with a strong epoxy resin. To hold the stator core 1 and the compressing rings 2 together, the two com¬ pressing rings 2 are pressed against each other with the aid of several clamp joints 4. Each clamp joint 4 comprises an elongated elastic element in the form of a set bolt 4a, which is arranged axially along the periphery of the stator core 1 and the compressing rings 2. At both ends of the set bolt 4a, a branch 4b with a through-hole is fitted onto the set bolt. The branch 4b makes contact with the end surface of the respective compressing ring 2 and extends radially inwards towards the centre of the stator. Axially outside each branch 4b, a nut 4c is threaded onto that end of the set bolt 4a which projects through the branch. By tightening the nuts 4c facing each other, axial forces arise which press the com¬ pressing rings 2 against each other. In this way, good ability to withstand the shear forces which may arise in the stator during severe operations is imparted to the stator core 1 with compressing rings 2.

During operation of the generator, cooling air is added from outside axially in a direction towards the two end portions of the stator with the compressing rings 2. The air can be supplied, for example, with the aid of separate fans or with fan blades (not shown) , which are arranged on the rotor shaft (not shown) . When the cooling air hits the compressing ring 2, the air is diverted and distributed. The major part of the cooling air is conducted towards the centre of the compressing ring 2 and passes in between the compressing ring 2 and the rotor. At the same time a distribution of the air along the whole compressing ring 2 takes place. The cooling air is then distributed such that a suitable flow arises in each radially aranged cooling channel 10 through the compressing ring 2. In

the cooling channels 10, an interchange of heat takes place along the surfaces of the stator laminations 5 and the spacers Ila, lib which define the cooling channels 10. Because of the radial shape of the cooling channels 10, the total heat- exchanging surface becomes large in relation to the quantity of magnetic material which is lost because of the channels. Therefore, it is possible to utilize ordinary air under atmospheric pressure as coolant.

It is, of course, possible to utilize the invention also with other types of coolant, for example hydrogen gas, water or oil. As regards the geometrical shape of the compressing ring with the angles of inclination of the different sections in relation to the radial plane, this may, of course, be varied in view of special demands. The set bolts in the clamp joints may be replaced with other elastic elements such as spiral or cup springs.