FIELD The invention relates to an electric machine of the axial flux type, comprising a circumferential stator structure, a rotor structure intended for magnetic interaction with the stator structure, and further a rotation axis for enabling rotation of the rotor relative to the stator structure, and the stator structure being double-sided comprising a first stator core, a second stator core and a winding in the stator cores.

BACKGROUND In this context, an electric machine refers particularly to an electric motor or a generator, not excluding other rotating electric machines. Double- sided axial flux electric machines are concerned. In axial flow electric ma¬ chines, the rotor magnetization and/or the stator winding is such that a mag¬ netic field in the direction of the rotation axis of the electric machine is gener- ated. In known double-sided axial flux machines the winding is such that the windings on the different stator cores are independent, i.e. separate, wherefore when one stator core and the winding thereon are examined, it is necessary to make a transition between winding wire portions belonging to the same winding wire and pointing in the direction of the perimeter of the stator core on the side of the perimeter of the stator core in the direction of the pe¬ rimeter for generating a loop. As a result of the manner of implementing the winding, the problem in known double-sided axial flux machines is thus that unnecessarily large amounts of winding wire are consumed, which increases both material and manufacturing costs. Publication AU4246572 describes a known implementation comprising a double-sided stator structure and a dou¬ ble-sided winding. Publication WO2004/047253, in turn, presents an imple¬ mentation, which in fact is not even double-sided, but the winding is on one stator core, i.e. said publication presents a so-called 2-level-winding provided on one stator core, wherein the winding overhangs of the winding, i.e. the turn¬ ing points of the winding loops on the side of the perimeter are slanted alter¬ nately in different directions on the side of the perimeter.

BRIEF DESCRIPTION The object of the invention is to provide a new type of electric ma- chine for alleviating the problems associated with known electric machines. This is achieved with the electric machine according to the invention, which is characterized in that the winding of the stator structure is such that the winding comprises a crossover point on the side of the perimeter of the stator structure, at which crossover point the winding proceeds from the first stator core to the second stator core, and that the winding comprises a return crossover point on the side of the perimeter of the stator structure, at a point different from the crossover point, at which return crossover point the winding returns from the second stator core to the first stator core. Preferred embodiments of the invention are described in the de¬ pendent claims and in the description. The invention is based on the winding halves on the different stator cores being of the same integrated winding because of the proceeding, i.e. continuing, of the winding from the side of the perimeter of the stator structure from the first stator core to the second and back, i.e. by means of the cross¬ over point comprised by the winding from the first stator core to the second stator core and by means of the return crossover point comprised by the wind¬ ing from the second stator core back to the first stator core. The winding pro¬ ceeds between the stator cores from the side of the perimeter connecting a first winding area in the first stator core with a second winding area in the sec¬ ond stator core into an integrated winding belonging to the same winding wire. Let it be stated that the terms associated with winding, 'proceeds, 'returns' and 'transition' do naturally not refer to a movement of the winding, but the routing of the winding, i.e. the continuing of the winding. The crossover point and, correspondingly, the return crossover point, refer to points of the winding, at which points the winding proceeds, i.e. continues, from the first sta¬ tor core to the second and, correspondingly, returns, i.e. continues, from the second stator core back to the first. The electric machine according to the invention brings forth a plural- ity of advantages. In the implementation according to the invention, the winding is implemented in a manner saving the amount of winding wire and the imple¬ mentation is also small-sized and low-loss. The invention also brings forth the fact that the summed-up length of the winding overhangs, i.e. the winding overhang (crossover point or return crossover point) on the side of the perime- ter and the winding overhang on the side of the inner circumference, is inde¬ pendent of the pole pitch of the electric machine, i.e. the distance between the magnetic poles, since, in the invention, the pole pitch is proportional to the dis¬ tance of the arc (distance along the circumference) between the crossover point and the return crossover point of the winding. Consequently, the advan¬ tage gained from the invention as saved winding wire is the greater the smaller the number of poles in the machine is, i.e. the larger the pole pitch of the ma¬ chine is. As is known, when the number of poles decreases, the pole pitch in¬ creases. But in known technology, the pole pitch is proportional to the length of the winding overhang (in which each winding wire connects winding points be¬ longing to the same winding wire on the same side of the same core, of which the first winding point ascends to the perimeter and the second returns to the inner circumference) passing on the perimeter of the stator core and, accord¬ ingly, a decrease in the number of poles, which thus means an increase in the pole pitch, causes a lengthening of the length of the winding overhang in the direction of the perimeter and passing on the perimeter. In the invention, the winding overhang on the side of the perimeter of the stator structure, i.e. the turning area of the winding loop, i.e. the top, is in the direction between the stator cores, since the crossover point and the return crossover point are wind¬ ing overhangs. The invention allows the length of the winding overhang to be shorter than in known implementations.

LIST OF FIGURES In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawings, in which Figure 1 shows a double-sided axial flux machine seen in a direction perpendicular to the rotation axis, Figure 2 shows a section along line A-A of Figure 1 , Figure 3 is a spread plane view of the winding of a three-phase mo¬ tor and rotor magnets along a circle, Figure 4 shows the winding of a three-phase motor, Figure 5 shows a winding group composed of six winding parts and having three functional pairs of two winding parts, Figure 6 shows a rotor with its magnetic areas, Figure 7 shows a cross-section of a rotor at line C-C of Figure 6, Figure 8 shows a stator core, Figure 9 shows stator cores, part of a winding, and a rotor, shown apart from each other, Figures 10 to 12 show the structures of Figure 9 interconnected, Figure 13 shows a double-sided axial flux machine according to a second embodiment of the invention machine seen in a direction perpendicular to the rotation axis, Figure 14 shows a section along line B-B of Figure 13, Figure 15 shows an implementation of the first embodiment of the invention, wherein the stator cores are slanted relative to each other, Figure 16 shows an implementation of the second embodiment of the invention, wherein the stator cores are slanted relative to each other, Figure 17 shows a winding composed of two winding parts and two transitional areas and belonging to the same integral aggregate.

DESCRIPTION OF EMBODIMENTS Two main embodiments and embodiments specifying them are pre- sented of the implementation of the invention. Particularly Figures 1 to 3, 7, 10 to 12 and 15 relate to the first main embodiment. Particularly Figures 13 to 14 and 15 relate to the second main embodiment: The other figures, i.e. Figures 4 to 6 and 8 describe both embodiments. In the first embodiment, a rotor struc¬ ture 200 is between stator cores 101 , 102, whereas in the second embodi- ment, a rotor structure 200, 200a, 200b is around the stator cores that are lo¬ cated in the middle. The common features of the different embodiments will be studied first. Thus, an electric machine of the axial flux type is concerned, com- prising a circumferential stator structure 100, a rotor structure 200 intended for magnetic interaction with the stator structure, and further a rotation axis 300 for enabling rotation of the rotor relative to the stator structure. As regards the rotation axis 300, it is pointed out at this stage that in the first embodiment, the rotation axis rotates together with the rotor 200, but in the second embodiment the rotation axis does not rotate with the rotor 200, but the rotation axis 300 provides a support, resting on which the rotor 200 rotates relative to the rotation axis 300. The bearing associated with the rotation axis is denoted by reference numeral 810 and the frame of the electric machine is denoted by reference numeral 820. In Figure 1 , i.e. in the first embodiment, the bearing 810 is between the rotation axis 300 and the frame 820. In Figure 13, i.e. in the second embodiment, the bearing 810 is between the rotation axis 300 and a rotor 200a, 200b, rotating relative thereto. The stator structure 100 is double-sided comprising a first stator core 101 , a second stator core 102 and a winding 400; 40, 41 to 43, 45, 50, 51 to 53 in the stator cores. The stator cores 101 , 102 are circumferential, particu¬ larly ring-shaped for rotational geometrical reasons. The circumferential wall of the stator core constitutes the stator core and inside of the circumferential wall of the stator core is a central area 700, into whose middle point the rotation axis 300 of the electric machine settles. As regards its ferromagnetic structure, the stator core is thus a disc with a central opening. The stator cores 101 , 102 are side by side. The stator structure and the winding are double-sided. In the first embodiment, the double-sidedness is such that the different stator cores 101 , 102 and their winding areas are side by side with the rotor in between such that the core 101 on the different side of the rotor and the winding areas therein are directed against the second core on the second side of the rotor and the winding areas therein. In the second embodiment, the stator cores are side by side in the middle and the winding areas in the cores are directed in mutually opposite directions towards the different sides of the surrounding double-sided rotor. Each stator core 101 , 102 is made from superimposed ferromag¬ netic disks, such as armature sheets, for example. The winding 400, 40, 41 to 43, 45, 50, 51 to 53 of the stator struc¬ ture is such that the winding comprises a crossover point 42 on the side of the perimeter of the stator structure 101 , 102, wherein the winding proceeds from the first stator core 101 to the second stator core 102. In addition, the winding comprises a return crossover point 52 on the side of the perimeter of the stator structure 101 , 102, at a different point than the crossover point 42, wherein the winding returns from the second stator core 102 to the first stator core 101. Said manner of winding enables a reduction in the consumption of winding wire since the winding makes a transition along a short path between the halves 41 , 43 of the winding part, such as winding part 40, i.e. from core 101 , to the second core 102, and not along the perimeter of the same core as in known solutions. Accordingly, the winding proceeds, i.e. continues, through the point 42 comprised by the winding from the first stator core 101 to the second stator core 102 and returns, i.e. continues, through the point 52 comprised by the winding from the second stator core 102 to the first stator core 101. Point 42 and 52 are connecting points comprised by the winding. Point 42 is on the side of the perimeter with the winding overhang of the winding in a multiturn winding, and, similarly, point 52 is on the side of the perimeter with the winding overhang of the winding in a multiturn winding. The winding is copper wire covered with enamel or of some other suitable material, for example. The winding wire may be of formed copper or round dynamo wire, for example. In the area between the crossover point 42 and the return crossover point 52, the winding comprises a transitional area 45 having a shorter dis- tance to the rotation axis than the distance of the crossover point 42 and the return crossover point 52 comprised by the winding from the rotation axis 300. This implementation further decreases the consumption of winding wire, since the transition between the winding parts 40, 50 takes place at a point wherein the distance in the direction of the circumference is shorter than on the perime- ter. More exactly, the winding 400, 40, 41 to 43, 45, 50, 51 to 53 is such that the winding proceeds on the first stator core 101 towards the perimeter of the stator structure as a winding area 41. Next, the winding proceeds from the side of the perimeter of the stator structure at the crossover point 42 from the first stator core 101 to the second stator core 102, proceeding then as a wind¬ ing area 43 on the second stator core 102 towards the inner circumference of the stator structure. Next, the winding proceeds by means of the transitional area 45 comprised thereby along a transitional distance to a point from where the winding proceeds as a winding area 51 on said second stator core towards the perimeter of the stator structure, after which at the return crossover point 52, which is at a different point than the crossover point 42, the winding returns from the second stator core 102 to the first stator core and proceeds as a wind¬ ing area 53 on the first stator core 101 towards the inner circumference of the stator structure. The winding part 40 comprises winding areas 41 to 43 and, corre¬ spondingly, the winding part 50 comprises winding areas 51 to 53. However, let it be pointed out as regards the term winding part that no separate parts 40, 50 are concerned, but an aggregate of the same winding wire, wherein the winding parts 40, 50 are interconnected in that portion 45 of the winding wire, i.e. the transitional area 45, which is between the winding parts 40, 50, i.e. more exactly between winding area 43 and 51. The crossover point, i.e. the winding area 42, connects the winding areas 41 and 43 that are on different cores, and the return crossover point 52, i.e. winding area 52, connects the winding areas 51 and 53 that are on different cores. The winding areas 41 to 43, 45, 51 to 53 belong to the same winding wire. The connection accom- pushed by the winding areas 42 and 52 does therefore not either refer tq the connection of separate winding wires; instead, the winding proceeds, i.e. continues, as a series of said successive winding areas when one winding turn is studied. There may be one or more turns, in each turn the winding wire has the same principle, i.e. the winding areas 41 to 43, 45, 51 to 53 in succession, in a multitum, also winding area 55. Connected by the winding area 45, these winding parts 40 and 50, together with said connecting winding area 45, constitute a functional winding part pair 40, 50 as regards electric operation, which in the case of an electric motor generates a variable magnetic field by means of an alternating current type of current supply and thus causes the rotational movement of the rotor structure 200 provided with magnetic means S, N. Correspondingly, in the case of an electric generator, the situation is such that the rotational movement of the rotor structure 200, provided with magnetic means S, N, generates a variable magnetic field, which generates alternating current in the winding part pair 40, 50. The winding may be single-turn, i.e. the same winding wire passes along the same winding path only once, or the winding may be multiturn, whereby the same winding wire passes along the same winding path a plurality of times thus generating a multiturn coil. Particularly in the case of a multiturn winding, the winding also comprises a second transitional area 55. The effect of the transitional area 45 is in a way eliminated by means of said second tran¬ sitional area 55, i.e. return to the starting point of the winding takes place, i.e. proceeding takes place from the end of the second winding part 50 on the first stator core 101 , i.e. the winding area 53, back to the point where the winding portion 41 of the first winding part begins. If the winding continued in the same direction of rotation seen in the circumferential direction, and did not return backwards towards the winding area 41 after the winding area 53, then the significance of said second transi¬ tional area is in that it enables transition to a point from which the ascent along the side of the stator core 101 towards the perimeter is started the next time. This being so, the winding would proceed as a wave winding forward, but ac- cordingly at the same time also from one stator core to another and back. The winding parts 40, 50 are hook-like, i.e. to some degree in the shape of the letter U or the letter V. Said V-shape may be the case for instance in the implementation shown in Figures 15 to 16, wherein the stator cores 101 , 102 and their windings are slanted relative to each other. The inclination be¬ tween the stator cores 101 , 102 is such that on the side of the perimeter at the winding, the distance between the stator cores 101 , 102 is shorter than on the side of the inner circumference, whereby the consumption of winding is smaller, since the length of the crossover point 42, as well as the length of the return crossover point 52, is short. If the stator cores 101 , 102 and their wind¬ ings are slanted relative to each other in accordance with Figures 15 and 16, then the rotor structure 200, 200a, 200b with the magnetic means S, N, i.e. permanent magnets, windings or iron core, are correspondingly slanted rela¬ tive to each other, thus following the inclination of the stator cores 101 , 102. Figures 6 and 7 illustrate the implementation of a magnetic structure S, N having an alternating polarity in the rotor. The magnetic structure com¬ prises a permanent magnetic structure comprising two opposite magnetic po¬ larities (S=South, N=North), wherein the order of the permanent magnets hav¬ ing an opposite polarity S, N alternates. These reference marks S and N refer to magnetic poles having opposite polarities (plus and minus). Instead of permanent magnets, windings provided with current feed can be used in the rotor. Another alternative is the use of an iron core in the rotor structure. Whether the structure S1 N is composed of permanent mag¬ nets, a winding or an iron core, the purpose of the magnetic structure S, N is, together with the stator winding 400, to provide means for generating magnetic interaction between the stator structure 101 , 102 and the rotor structure 200. If windings are used in the rotor, then current feed to the rotor winding, imple¬ mented with a slip ring or the like, for example, has to be arranged because of the rotation of the rotor. In addition to the double-sidedness of the stator structure 101 , 102, the magnetization structure S, N, which thus is in the rotor 200, is double-sided both in the first and second embodiment. As regards the second embodiment shown in Figure 13, it may be added that also the actual rotor structure 200 is double-sided comprising structural sides 200a and 200b. With reference to Figure 1 , in the first embodiment, the winding is on the side of the inner side surfaces of the stator cores 101 , 102, and thus the magnetization structure S, N is between the stator cores 101 , 102 on the out¬ ermost side surfaces of the central rotor. In an embodiment, the winding proceeds on both the first stator core and the second stator core in the radial direction of the electric machine. This is observed from the proceeding directions of the winding areas 41 , 43, 51 , 53. The stator core shown in Figure 8 comprises radial grooves 600 for the winding. Because of the openness of the grooving, the winding group shown in Figure 5 can be placed as a ready-wound whole package, i.e. a coil bundle, in between the two stator cores 101 , 102 shown in Figure 8 such that the sides of the winding on their side settle in the grooves 600 of the surroljnd- ing stator cores 101 , 102. Both the first stator core and the second stator core comprise grooves 600 for the winding. The grooves in the stator cores are in the radial direction of the electric machine. In order to achieve maximal saving in winding wire and in order for the transitional areas 45, 55 to be as short as possible, the winding is such that the transitional area comprised by the winding proceeds at the inner circumfer¬ ence of the stator structure, i.e. along the inner circumference or in the vicinity of the inner circumference. For corresponding reasons, the winding proceeds at the crossover point 42 from the first stator core 101 to the second stator core 102 in a perpendicular direction between the stator cores. For similar rea¬ sons, the winding proceeds at the return crossover point 52 from the second stator core ip2 to the first stator core 101 in a perpendicular direction between the stator cores. The implementation shown in Figures 3 and 4 is multi-phase, three- phase in the example of the figures. The winding of Figures 3 to 4 is composed of three winding wires or coils such that the ends of the first winding wire are denoted by reference marks U1 and U2, the ends of the second winding wire are denoted by reference marks W1 and W2, and the ends of the third winding wire are denoted by reference marks V1 and V2. The electric machine of Fig- ure 4 is four-pole, 12-groove and 3-phase. In Figure 4, NSSN denotes the fact that the N-type magnetic areas are the outermost. Symbol SNNS means that the S-type magnetic areas are the outermost. A magnet always comprises two polarities on different sides of the magnet, and therefore Figures 1, 7 and 13 do not show the middle areas of an NS-SN type of structure at the marks N for the sake of clarity, i.e. the S-type areas on the back surface of the N-type ar¬ eas. In Figure 4, symbol 870 refers to the boundary of the magnetic ar¬ eas having different polarities. It should be mentioned about the manner of drawing in Figure 3 that Figure 3 shows a spread plane view of the winding of a three-phase machine and rotor magnets along a circle. In addition, as regards Figure 3, it should be noted that the side surfaces 210, 211 of the rotor are shown in a position turned by 90 degrees in order for them to be seen in Figure 3. With reference to Figures 9 to 12, particularly Figures 10 to 12, the small size of the electric machine achieved with the structure according to the invention is observed. In Figures 9 to 12, in the electric machine shown by way of example, two winding groups 400 and 500 can be detected, i.e. two winding wires are in use. Both winding groups comprise six winding parts. Figure 5 shows one winding group 400. For example, the winding group 400 comprises winding parts 40, 50, 60, 70, 80 and 90, even though the order of the winding parts is such that a first subgroup 40a of three winding parts comprises the winding parts 40, 60 and 80, and their functional pairs are the winding parts 50, 70 and 90 of the second subgroup 40b. As was presented above, the first winding parts 40, 50 of the subgroups 40a, 40b constitute a functional pair, and therefore, correspondingly, the second winding parts 60, 70 of the part groups constitute a functional pair, and, correspondingly, the third winding parts 80, 90 of the subgroups 40a, 40b constitute a functional pair. The winding 400 of Figure 5 is created for instance such that the winding areas 41 to 43 of the first winding part 40 have first been traversed with a winding wire, and next the transitional area 45 and the winding areas 51 to 53 of the second winding part have been traversed. Next, return has oc¬ curred by means of the return transitional area 55 to the starting point. Said path is repeated a sufficient number of times in order to implement the desired number of turns, after which the functional pair constituted by the winding parts 60, 70 is next implemented the desired number of turns. And next, the func- tional pair constituted by the winding parts 80, 90 is implemented the desired number of turns. Figure 2 and 14 show the principle of the winding of a 4-pole electric machine as a diagram. Figures 2 and 14 illustrate 12 winding parts 140 to 190 and 240 to 290. In Figures 2 and 14, the winding is as a coil bundle. Areas 245, 255, drawn as a bundle, depict a transitional area, such as the transitional areas 45 or 55, which areas comprise the transitional areas between the dif- ferent winding parts comprised by several winding turns. In Figures 2 and 14, too, the winding parts constitute pairs of at least two winding parts, i.e. for in¬ stance winding parts 140 and 150 may be created from the same winding wire in succession, i.e. in the same way as the winding parts 40 and 50 above. The windings of the stator packs may be interconnected from the point shown by reference 444 in Figure 2, i.e. for instance between the cross¬ over points 42, 52 in the winding areas 40, 50. Said point 444 is a good point, since it is not in a rotational area, but neither so far on the perimeter that it would increase the dimension of the diameter of the stator aggregate. However, there are several alternatives for implementing the wind¬ ing, not only the ones presented in the figures and the ones presented in the present text relating thereto. Thus, the invention is not restricted only to given winding manners, nor is the invention restricted to any other detailed implementations described above. With reference to Figures 1 to 3, 7 and 10 to 12 in particular, in the first embodiment, the rotor structure 200 is between the stator cores 101 , 102. Ac¬ cordingly, between the stator cores is a rotational space 103 for the rotor struc¬ ture, and the rotor structure 200 is between the stator cores 101 , 102 on a ro¬ tation axis 300 being in the rotational space 103 rotatable with the rotation axis 300 relative to the stator structure 100. Consequently, the proceeding of the winding between the stator cores 101 , 102 takes place between the stator cores 101 , 102 on different sides relative to the rotational space 103, i.e. the crossover point 42 and the return crossover point 52 comprised by the winding cross the rotational space 103. With reference to Figures 13 to 14, in a second embodiment, the ro¬ tor structure 200, 200a, 200b is between the stator cores 101 , 102 located in the middle. The rotor structure is double-sided around the stator structure. The rotation axis 300 provides a support resting on which the rotor structure 200 is rotatable not only relative to the stator structure 101 , 102, but also relative to the rotation axis 300. Consequently, in the implementation of Figures 13 to 14, the rotation axis 300 itself does not rotate. Although the invention is described above with reference to the ex¬ ample in accordance with the accompanying drawings, it will be appreciated that the invention is not to be so limited, but it may be modified in a variety of ways within the scope of the appended claims.