METHOD AND APPARATUS FOR PROFILING MAGNETIC FLUX

Abstract: An electric machine according to the invention comprises a rotor means (10) and a stator means (20) surrounding the rotor means having radially extending slots (21). Slot lock components (14) are provided in at least some of said slots. The slot lock extends in said slot essentially to the periphery of the rotor bore of the stator means. The relative permeability ( mu r) and the saturated flux density of the slot lock means is essentially good. In the inventive method the density of magnetic flux effecting on the rotor means surface is made more uniform by slot lock components described above.

Description: Method and apparatus for profiling magnetic flux Field of the invention The present invention relates to a method in an electric machine in accordance with the preamble of claim 1 , and more precisely to a method utilizing a slot lock means disposed within a stator slot. The invention relates further to an electric machine according to the preamble of claim 5 comprising special slot lock means. The invention relates also to a slot lock according to the preamble of claim 13 for use in an electric machine. Background of the invention In electric machines, and especially in so called solid rotor machines a loss exists on the surface of the rotor, said loss being caused by harmonics of a magnetic flux effecting in an air gap between the rotor and a stator. The loss causes decrease in the efficiency and overheating problems. This shape of the flux density of prior art electric motors and the uneven distribution thereof is illustrated schematically in figure la. This recognized problem has been tried to get solved for instance by providing specific stator teeth in accordance with schematic figure lb to stator plates of solid rotor machines. These will even the flux on the rotor surface, decreasing thus the generated loss term and the effect of permeance harmonics. The effect of the teeth is illustrated in more detail in table of figure lc comparing the stator solutions of figures la and lb. This kind of stator teeth have been disclosed e.g. by publication "Properties of high-speed solid-rotor induction machines", Juha Pyrhonen, Panu Kurronen, Research Report EN A- 19, Lappeenranta University of Technology, Department of Energy Technology, 1993, which is incorporated herein by this reference. A patent application PCT/FI97/00338 (= WO publication 97/45942) which was a non- public disclosure at the time of filing this application, discloses as one embodiment thereof a more advanced solution in which the toothing of stator plates of a solid rotor machine has been combined with an effect provided by rotor bars provided in the solid rotor means. This has found to have an advantageous effect to said flux density distribution and thus to the loss existing on the rotor surface. This PCT application is also incorporated herein by this reference. The tooth solution disclosed by the above references has been found per se to operate and they have assisted in decreasing the loss of the solid rotor machines. However, the teeth have also some disadvantages. They are substantially small from the size thereof and thus hard and expensive to manufacture to the stator plates. Problems have occurred during the manufacture e.g. when selecting proper tools and also in the durability thereof. The spacing between the teeth is made very narrow, which makes the provision of coilings to the stator substantially more difficult. The air gap between the rotor and the tip of the tooth is small. In addition, it has been found that the solution has a disadvantageous effect to the amount of so called gas friction. Therefore the solution is not kept as the most optimal in all cases, for instance due to the hardness of the manufacture thereof and the overall high costs of the solution, nor even applicable to all different motor solutions. The above referenced Research report EN A- 19 discloses also a use of so called slot lock. According to this solution, which is based on an essentially different solution from the basic principles thereof than what the stator teeth are, a separate slot lock is positioned in the slots of the stator plates, said slot lock usually being of suitable fiber glass laminate. According to one alternative iron powder may be added to the glass fiber laminate (e.g. an iron powder glass fiber laminate sold by Isovolta with a trade name "MAGNOVAL"). The prior art slot lock has an essentially flat, strip like shape, the thickness thereof in the radial direction of the machine being e.g. about 2.5 mm and the width about 9.5 mm. The prior art radially flat slot lock extends in the "coil space" of the slot, i.e. only within a portion of the slot. In other words, it extends from the coils only essentially little towards the rotor, whereby the slot remains essentially open in the proximity to the rotor. Therefore the referenced publication states that it is not possible to receive the corresponding advantages in view of uniformity of the magnetic flux and efficiency coefficient by means of this solution than what can be achieved by the above referenced stator teeth. The publication, however, keeps the known essentially flat slot lock as a solution which is cheaper and can be implemented more easily. Thus it might be in some occasions a more economical solution, when considering the entire costs, than the teeth providing per se better uniformity of the flux. The problem is, however, the essentially weaker electric properties relative to the other solutions, and lack of suitable manufacturing technology and attachment technology. A solution having essentially corresponding basic principle and thus also similar problems than the above is also disclosed by EP publication No. 0 398 746 A3. Summary of the invention It is an object of the present invention to overcome the disadvantages of the prior art and to provide a new type of solution for providing a more uniform magnetic flux in an electric machine, whereby it is possible to improve the coefficient of efficiency of the machine. A further object of the present invention is to provide a method and an apparatus utilizing a slot lock provided from improved material and properties and having a new type of basic idea. Another further object of the present invention is to provide a method and an apparatus by means of which manufacture of an electric machine having improved efficiency is made more ready and economical to accomplish. Another further object of the present invention to provide a method and a means realizing it, by means of which it is possible to utilize already commercially available and normally shaped standard stator plates when manufacturing a stator means provided from plates and having improved properties. Another further object of the present invention to provide an method and means for realizing it, by means of which it is possible to affect to the distributed inductance within the stator slot. The invention is based on that surprising idea that there are materials which fulfill certain conditions, which materials can be used as stator lock material such that the stator slot is filled essentially until the periphery of the rotor bore, and thereby to provide an essential improvement in properties of an electric machine, even though normal stator slot shapes are used, whereby an essential advantage is obtained for instance in the easiness of manufacture and costs. More precisely, the method according to the present invention is mainly characterized by what is disclosed in appended claims 1 to 4 and especially by the characterizing portion of claim 1. The apparatus according to the present invention is mainly characterized by what is disclosed in appended claims 5 to 12, and especially by the characterizing portion of claim 5. The slot lock according to the present invention is mainly characterized by what is disclosed in appended claims 13 to 18, and especially by the characterizing portion of claim 13. According to a preferred embodiment of the present invention an electric machine comprises a rotor means and a stator means surrounding the rotor means, said stator means being provided with radially extending slots. A slot lock component is provided in at least some of said slots, said components being characterized by extending in said slot essentially to the periphery of the rotor bore of the stator means and that the relative permeability (μ r ) of the slot lock component is essentially good, and greater than 5, and that the saturated flux density is at least 0.2 tesla. It is preferred that the resistivity of the material is as high as possible, and especially preferred is a situation in which the resistivity of the material corresponds essentially the resistivity of isolators. In addition, it is preferred that the material provides sufficient strength against heat. According to the inventive method a stator lock is used in a stator means slot of an electric machine. The magnetic flux affecting on the surface of a rotor means is made more uniform by means of such a slot lock which is arranged to extend within said slot essentially to the periphery of the rotor bore of the stator means and the material of which is selected such that the relative permeability (μ r ) is essentially good, and greater than 5, and that the saturated flux density is at least 0.2 tesla. According to the present invention, a single slot lock to be used in an electric machine is manufactured from material having an essentially good relative permeability (μ r ), which is always greater than 5, and a saturated flux density which is at least 0.2 T. Significant advantages are obtained by means of the present invention. For instance, the coefficient of efficiency of a solid rotor machine is improved essentially when compared to the prior art solutions while the heating problem of the motor is reduced significantly. By means of the invention it is possible to decrease the size and weight of the motor when compared to prior art motors having corresponding performance. The motor according to the invention can operate in a substantially wide range of rotational speeds. It enables also a larger air gap. The inventive construction is simple to implement and manufacture, and the invention provides essential advantage in form of reduced manufacturing costs. In the following the present invention and the other objects and advantages thereof will be described in exemplifying manner with reference to the annexed drawing, in which similar reference characters throughout the various figures refer to similar features. It should be understood that the following exemplifying description is not meant to restrict the invention to the specific forms presented in this connection but rather the invention is meant to cover all modifications, similarities and alternatives which are included in the spirit and scope of the present invention, as is defined by the appended claims. Brief description of the drawings Figures la and lb disclose a simplified schematic and partially sectioned side presentation for distribution of a magnetic flux in two different prior art embodiments, and figure lc is a schematic presentation for the relative distribution of flux densities for the embodiments of figures la and lb. Figure 2 discloses one embodiment for a slot lock in accordance with the present invention. Figure 3 discloses one further embodiment for a slot lock in accordance with the present invention. Figure 4 discloses a diagram for results of simulations accomplished to the prior art arrangements and arrangements in accordance with the invention. Figures 5a - 5c illustrate the influence of the slot lock to distributed or leakage inductance of a machine. Detailed description of the drawings Figure la discloses an enlarged partial view of a shape of one stator slot 21 ' utilized in prior art. The distribution of the flux density in the electric air gap l s provided by this is illustrated by lines 26'. Figure lb discloses a tooth 22 extending in a rotor bore 24 from the stator plate 20 in both sides of the stator slot 21. The advantageous influence of the teeth to the flux density distribution is evident from lines 26 illustrating the flux. As can be noted from the diagram of figure lc, the flux distribution (b) provided by teeth- like shaped stator slot tooth 22 is essentially more uniform than what can be provided by the conventional solution (a). The stator means 20 provided with the slots shaped in a manner disclosed by figure lb, and more precisely the permeance harmonics thereof, do not heat much the rotor 10, if at all, i.e. the influence of permeance harmonics is minimized. However, this solutions does have the disadvantages presented above in the preamble. Figure 2 discloses a solution according to the invention replacing the solutions of figures la and b, by means of which it is possible to obtain at least as uniform distribution as by figure lb solution, but with a manner which is essentially simpler and more easy to implement. As is disclosed by figure 2, a normal air gap 8 is provided between the rotor 10 and the stator plate 20. A coiling 6 is provided in a per se known manner within the stator slot 21. A slot lock 14 according to the invention is also placed in the slot 21, said lock, extending essentially up to the circumference 11 of the rotor bore, and being essentially parallel with said circumference 11. The slot lock 14 is manufactured from material having a relative permeability μ r that is essentially good, and in any case greater than 5. Preferably the permeability is within a range from 8 to 17, but it can be even greater than this. In addition, the saturated flux density of the material is also essentially good, preferably at least 0.2 Tesla, and especially preferred value thereof is greater than about 0.4 T, or even more, such as 0.8 T. An example of a suitable material is a material of a PREMAG product family by Premix Oy. PREMAG TP 2803 is presented as an example of these, the characteristics thereof being disclosed in detail in "INFO" brochure, January 1995 by Premix Oy, which is incorporated herein by this reference. This material is based on PBT (Poly Butyne Terephala- te), and it is a specially designed iron powder which can be machined to the final shape thereof by per se known standard moulding technologies. In view of the invention, this material provides good magnetic properties. The relative permeability of this material is about 17 - 16.5 in frequency range f = 1 - 100 kHz, and still 10 in frequency range f = 1000 kHz. The saturated flux density for the material is 0.93 T. The resistivity is in the order < 1 Ωcm. Materials having in all conditions a permeability over 8, and whose properties also otherwise correspond the above disclosed, are considered as especially advantageous, and thus this material suits especially well for use in the invention. A slot lock realizing the above conditions can be provided e.g. by methods known from powder technology or powder metallurgy from so called gas or water atomized metal powder, which technology has been presented in more detail e.g. in journal Metallitekniik- ka 3/95 in article "Good products by powder metallurgy" in pages 32-34 (author J. Liima- tainen). This article explaining one possible technology to be utilized in this invention is also included herein by this reference. Other possible methods for use when manufacturing the slot lock are e.g. extrusion, such as extrusion of quartz composite into a mould, casting or sintering. What is essential in the selection of an appropriate manufacturing technology is that the technology enables provision and shaping of the slot lock component such that the desired requirements for magnetic, electric and mechanic properties are obtained by the produced slot lock. In addition, slot isolators 16 isolating the coils 6 from the slot lock 14 and the stator plates are disclosed. These can be formed from a isolating material which is well known by the skilled person, such as from paper or plastic strips or from suitable isolator coating. In some instances it may be appropriate to place a suitable isolator 16 between the slot lock 14 and the surface of the stator slot wall. The isolator is required from instance in cases where the resistivity of the selected slot lock material is low, e.g. of order 1 Ωcm. Standard plates can be used as stator plates, which are remarkably less expensive than e.g. the plates of figure lb and the availability of which is good. Figure 3 discloses otherwise similar arrangement to figure 2, but in this the slot lock extends towards the rotor 10 by a dome like shape, i.e. is slightly out from the circumferential surface 11 of the bore of stator plate 20. This form has been found, during simulations, to provide additional advantage in the uniformity of the magnetic flux. Figure 4 discloses as a line diagram results for a simulation performed to a 500 kW solid rotor machine with four different stator arrangements in accordance with figures la, lb, 2 and 3. From these figures la and lb correspond prior art solutions and the stators of figures 2 and 3 are provided with slot lock components in accordance with the invention. The horizontal axis of the table illustrates the surface 10 of the rotor for the length of one slot pitch and the vertical axis illustrates the relative change in the flux (distribution of the flux density) on the rotor surface. Curve la presents a stator corresponding the arrangement of figure la, i.e. a conventional stator slot. Curve lb presents a stator corresponding the arrangement of figure lb, i.e. a stator plate provided with 0.75 mm high teething. As can be seen, these results correspond well the results shown by figure lc, even though these results were obtained to a substantially smaller, i.e. 12 kW test machine. Curve 2 presents the flux density distribution for an arrangement corresponding figure 2. Curve 3 corresponds the situation of figure 3. It can be noted that a substantially uniform relative flux density can be obtained by this arrangement comprising a dome-shaped slot lock from the material in accordance with the invention. The slot lock in accordance with the invention provides equalization of the flux as a consequence to the reason that the magnetic properties thereof even the shape of the flux existing on the rotor surface at the point over the slot, as can be seen from the table of figure 4. The semi-magnetic material positioned in the slot opening, which has also been suitably shaped, provides a guidance of the main flux on the rotor surface also to an area below the slot opening. Other parameters, such as length of the air gap of the machine, have also to be selected in an appropriate manner depending on the used material and shape of the slot lock. According to one preferred feature of the slot lock according to the invention it is possible to adjust distributed inductance of the stator by the slot lock. This is especially preferred in solid rotor machines, since a ripple occurring in the stator current of a frequency transformer supply tends to cause an amplitude ripple having a corresponding frequency, which causes a loss when penetrating into the machine. An increase in the distributed inductance of the stator slots increases the transient inductance of the machine whereby the current ripple is reduced. From the exemplifying shapes of figures 5a-c a high distributed inductance exists in the situation of figure 5a and a small distributed inductance exists in the situations of figures 5b and c. Thus the invention provides an apparatus and a method by means of which an improved and a readily accomplishable solution is provided for equalizing the flux effecting on a rotor means surface. At present it is believed that remarkable advantage is obtained, for instance, in the coefficient of efficiency in machines provided with so called solid rotors and plate stators, and especially in bigger machines, such as machines having a stator bore from Θ 100 to 1000 mm. However, the invention is not by any means restricted to these. It is to be noted that the above examples of the embodiments of the invention are not limiting the scope of the invention defined by the claims. By means of the above description and drawings it is e.g. obvious for the skilled person to use a method and an apparatus according to the invention in which the shape of the slot lock differs essentially from the illustrated. In view of the invention the properties of the slot lock material are more essential than the shape thereof, even though the shaping of the slot lock has found to have an advantageous additional effect in the uniformity of the magnetic flux and in the amount of distributed inductance.

Claims: Claims 1. A method in an electric machine, characterized in that the magnetic flux affecting on the surface of a rotor means is made more uniform by means of such a slot lock provided within a slot of a stator means which is arranged to extend within said slot essentially to the periphery of the rotor bore of the stator means and the material of which is selected such that the relative permeability (μ r ) is essentially good, and greater than 5, and that the saturated flux density is at least 0.2 T. 2. A method according to claim 1, characterized in that the slot lock material is selected such that the relative permeability (μ r ) is within a range from 8 to 17 and the saturated flux density is at least 0.4 T, and preferably at least 0.8 T. 3. A method according to claim 1 or 2, characterized in that the slot lock is manufactured by means of powder metallurgy, extrusion, casting or sintering. 4. A method according to any of the preceding claims, characterizedin that it further comprises adjustment of the distributed inductance of the stator slot by means of the slot lock shape. 5. An electric machine comprising a rotor means (10) and a stator means (20) surrounding the rotor means, said stator means being provided with radially extending slots (21) and slot lock components (14) provided in at least some of said slots, each of the slot lock components (14) extending in said slot essentially to the periphery (11) of the rotor bore of the stator means (20), characterized in that the relative permeability (μ τ ) of the slot lock component material is essentially good and greater than 5, and that the saturated flux density thereof is at least 0.2 T. 6. An electric machine according to claim 5, characterized in that the relative permeability (μ r ) of the slot lock material is within a range from 8 to 17 and that the saturated flux density of the slot lock material is at least 0.4 T, and preferably at least 0.8 T. 7. An electric machine according to claim 5, characterized in that the material of the slot lock components (14) is metal powder having appropriate magnetic characteristics and that the slot lock components are manufactured by a technique utilizing powder metallurgy. 8. An electric machine according to any of claims 5 -7, characterized in that the slot lock component (14) fills substantially the whole stator slot area (21) formed between the coils (6) in the slot, stator walls and stator bore periphery (11). 9. An electric machine according to any of claims 5 - 8, characterized in that the surface of the slot lock components (14) facing towards the rotor means (10) forms a shape which protrudes from the periphery of the stator bore (11) towards the rotor means, and preferably a crown like shape. 10. An electric machine according to any of claims 5 -9, characterized in that said stator means (20) is a standard assembly provided by plates having standard form and said rotor means (10) is a solid rotor. 11. An electric machine according to any of claims 5 - 10, characterized in that the resistivity of the slot lock component material is substantially high, the resistivity preferably corresponding the resistivity of insulators. 12. An electric machine according to any of claims 5-11, characterized in that an insulator means (16) is provided between the slot lock components (14) and the coiling (6) of the stator means (20) and/or slots (21) of the stator means (20), the insulator preferably being a plastic or paper strip or an insulator coating. 13. A slot lock (14) used in a slot extending radially in a stator means (20) surrounding a rotor means (10) of an electric machine, said slot lock arranged to extend in said slot essentially to the periphery (11) of the rotor bore of the stator means (20), characterize d in that the relative permeability (μ r ) of the material of said slot lock component is essentially good, and greater than 5, and that the saturated flux density thereof is at least 0.2 T. 14. A slot lock according to claim 13, characterized in that the relative permeability (μ r ) of the slot lock material is within a range from 8 to 17 and that the saturated flux density of the slot lock material is at least 0.4 T, and preferably at least 0.8 T. 15. A slot lock according to claim 13 or 14, c h a r a c t e r i z e d in that the material of the slot lock (14) is metal powder having appropriate magnetic characteristics and that the slot lock components are manufactured by a technique utilizing powder metallurgy. 16. A slot lock according to any of claims 13 - 15, c h a r a c t e r i z e d in that the slot lock component (14) is arranged to fill substantially that area of the stator slot (21) which remains between the coils (6) in the slot, stator walls and stator bore periphery (11). 17. A slot lock according to any of claims 13 - 16, characterized in that the surface of the slot lock (14) facing towards the rotor means (10) is arranged to form a shape which protrudes from the periphery of the stator bore (11) towards the rotor means, and preferably a crown like shape. 18. A slot lock according to any of claims 13 - 17, characterized in that the resistivity of the slot lock material is substantially high, the resistivity preferably corresponding the resistivity of insulators.

Inventors: ALAMAEKI JARMO, HUPPUNEN JUSSI, PYRHOENEN JUHA, ALAMAEKI, JARMO, HUPPUNEN, JUSSI, PYRHOENEN, JUHA
Applicants: ALAMAEKI JARMO, HUPPUNEN JUSSI, PYRHOENEN JUHA, ALAMAEKI, JARMO, HUPPUNEN, JUSSI, PYRHOENEN, JUHA
Application year: 1998
Main classification: H02K3
IPC codes: H02K3