WO2008135628A1

WO2008135628A1 - Axial flux induction electrical machine

Info

Publication number: WO2008135628A1
Application number: PCT/FI2008/050098
Authority: WO
Inventors: Asko Parviainen; Ari Piispanen
Original assignee: Axco-Motors Oy
Priority date: 2007-05-03
Filing date: 2008-02-28
Publication date: 2008-11-13
Also published as: FI20075310A0; EP2143191A1; CN101682240A

Abstract

The invention relates to an axial flux induction electrical machine comprising a frame, a shaft (1) bearing-mounted to the frame, a disc-like rotor (2) supported to the shaft, a stator (4) comprising a stator winding (3) and supported by the frame on the first side of the rotor in axial direction. The disc- like rotor (2) comprises a non- ferromagnetic rotor frame (8) fabricated of a material with a high electrical conductivity and comprising a uniform inner periphery (9) and an outer periphery (10) and conductor bars (11) fabricated of the same material and galvanically connecting the peripheries (11). The conductor bars together with the inner and outer peripheries form in addition to the rotor frame also the cage winding of the rotor. Between the inner periphery and the outer periphery there is a plurality of ferromagnetic pieces (12) extending through the frame plate and being spaced apart from each other at an appropriate distance so that the radial conductor bars are appropriately located between the pieces. According to the invention, the ferromagnetic piece (12) has a surface plate (5, 15) on the surface against the stator, the resistivity of this surface plate being essentially higher than the resistivity of the ferromagnetic piece.

Description

AXIAL FLUX INDUCTION ELECTRICAL MACHINE

FIELD OF THE INVENTION

The invention relates to an axial flux induction electrical machine as defined in the preamble of claim 1. The invention is chiefly developed to function as a motor, but also different generator embodiments may come into question.

BACKGROUND OF THE INVENTION

An axial flux machine as such is not by nature very well applicable as a high-speed induction machine, because the characteristics of an induction machine are usually the best when there are only few poles (two, or four at the maximum) in the machine. However, a two-pole solution is often unsuitable for an axial flux machine, because the arrangement of the end-winding of the stator winding is not often a functional solution in a two-pole machine. Therefore, also a high-speed axial flux machine has usually to be designed at least as a four-pole configuration. In that case, both the magnetic flux in the stator yoke and the current of the machine in the windings have to flow a quarter of the inner and outer peripheries of the machine tangentially without producing torque.

There are known to be several types of axial flux machines. Their advantage is that the stator can be fabricated of strip by winding, the loss of material being thus very small. Another advantage of an axial flux machine is that the machine becomes very short . Permanent magnet axial flux machines in particular are found in practical embodiments .

Well-known technology in the field of the invention is presented in US Patent 3296475. The aforementioned patent describes an axial flux machine with a disc-like rotor fabricated by casting. The structure is simple and easy to construct, but its major disadvantage is its low durability already at moderately high rotation speeds. Therefore, the structure is suitable only for low rotation speeds, usually below 3000 rpm.

These disadvantages have been eliminated by the axial flux induction electrical machine of the patent application WO2006/021616, characterized in that the disc-like rotor frame comprises one or a plurality of circular plates, which have been machined of work-hardened metal sheet, such as rolled or otherwise work-hardened aluminium alloy sheet. The electrical conductivity of a sheet of this kind is good, being for instance as near as possible to that of pure aluminium, 35 MS/m, and usually varying between 15-28 MS/m, and its relative permeability being ~ 1. The appropriate aluminium alloys are both durable and have a good electrical conductivity. Pure aluminium conducts electricity better than aluminium alloys, but it is mechanically weak, and therefore its application in high-speed machines cannot be justified.

The above-described machines according to the patent application WO2006/021616 are durable, simple and compact constructions, which can reach rotation speeds of 30 000 rpm and more. Hence, the only drawback of these machines may be considered to be their relatively low efficiency, which is of the order of about 90%.

OBJECT OF THE INVENTION

It is an object of the present invention to eliminate the aforementioned drawbacks. The object of the invention in particular is to introduce a novel axial flux induction machine, which enables to improve the efficiency of the machine by several percentage units even up to or above 95%. Further, the object of the invention is, along with the improved efficiency, to increase the maximum rotation speed and power of the machine as well as to prevent excessive heating of the machine .

SUMMARY OF THE INVENTION

The axial flux induction electrical machine of the invention is characterized in what will be presented in claim 1.

An axial flux induction machine of the invention comprises a frame, a shaft bearing-mounted to the frame, a disc-like rotor supported by the shaft, and a stator comprising a stator winding and supported by the frame on the first side of the rotor in axial direction. The disc-like rotor comprises a circular, non-ferromagnetic rotor frame having an essentially uniform thickness, fabricated of a material with a high electrical conductivity, the rotor frame comprising uniform inner and outer peripheries and conductor bars fabricated of the same material, the conductor bars galvanically connecting the inner and outer peripheries, and the conductor bars together with the inner and outer peripheries forming in addition to the rotor frame also the cage winding of the rotor. Between the inner and outer peripheries of the rotor there is a plurality of ferromagnetic pieces projecting through the frame plate and being spaced apart from each other at an appropriate distance so that the radial conductor bars of the rotor are located appropriately between the pieces. In accordance with the invention, each ferromagnetic piece has a surface plate on the surface against the stator, the resistivity of this surface plate being essentially higher than the resistivity of the ferromagnetic piece. With this structure, eddy currents on the rotor surface can be very efficiently prevented.

In the axial flux induction electrical machine of the invention, appropriately composed aluminium alloy is applied both in the electrically conductive structure and in the actual frame structure of the machine. In the invention, application of a suitable copper alloy may similarly come into question. In this rotor, steel is used as a path for the magnetic flux, whereas in the commonly known technique steel parts comprise the load-bearing structures of the rotor. When using strong aluminium or copper in the entire rotor, that is, both in the rotor frame and in the short-circuit rings, a firm structure is achieved that withstands well also the centrifugal forces caused by the ferromagnetic parts in the rotor. Also in certain embodiments having only one stator and one rotor, ferromagnetic material can additionally be used as a load-bearing structure, in other words, a ferromagnetic element is arranged beside the non-ferromagnetic rotor frame on the opposite side of the stator.

The resistivity of commonly used ferromagnetic elements is of the order of 0.1 μΩm, and thus the resistivity of the surface plate is appropriately above 0.2 μΩm. The resistivity of the surface plate is preferably between 0.3 and 1.5 μΩm.

The material of the surface place may be selected individually for each case according to the desired resistivity and strength properties; the material can be for instance cobalt iron, martensitic steel or aluminium-iron alloy.

The best functional result is achieved by the surface plate of the invention when the thickness of the surface plate is larger than the air gap between the rotor and the stator; usually, the thickness must be clearly larger than this. The air gaps commonly used in axial flux induction machines are of the order of 1 mm, and thus the thickness of the surface plate is preferably of the order of 1-4 mm, by which the desired result of the invention, that is, essential improvement in efficiency, is achieved.

In an embodiment of the invention, the surface plate is essentially level with the surface of the rotor frame, in which case the surface plate closes the ferromagnetic piece out of sight inside the rotor frame.

In another embodiment of the invention, the ferromagnetic pieces extend from inside the rotor frame to a distance outside the surface of the frame so that the surface plates at the ends of the ferromagnetic pieces do not touch the rotor frame, but are at a distance from it.

In a preferable embodiment of the invention, the surface plate extends outside that surface of the ferromagnetic piece which is against the stator. Now the edges of the surface plates can be placed at a very small distance from each other and in any case notably closer to each other than the ferromagnetic pieces. In an embodiment of the invention, the surface plates are even in contact with each other, that is, they constitute a uniform ring of plates that covers those surfaces of all the ferromagnetic pieces which are against the stator.

The appropriately ferromagnetic pieces are pieces extending mainly in the direction of the radius of the rotor and essentially taking the form of a truncated narrow sector, that is, the inner ends of the pieces are narrower and the outer ends are wider so that conductor bars of an essentially uniform thickness are formed between these pieces, these bars connecting the outer and inner peripheries of the rotor . The ferromagnetic pieces of the rotor are preferably fabricated of common structural steel, for instance Fe52. The saturation flux density of this steel grade is high, and the steel grade is therefore suitable for carrying the magnetic flux through the rotor. It is obvious to a person skilled in the art that any material with a high permeability and high saturation flux density may come into question for carrying this magnetic flux through the rotor. The material may also be some appropriate composite material with the above-described electromagnetic characteristics. It is preferable for the ferromagnetic parts to have a low electrical conductivity. Usually, however, in steels with a low electrical conductivity, also the saturation flux density is low, and therefore a satisfactory compromise has to be found.

In an embodiment of the invention, an electrically highly conductive coating, such as copper coating, is arranged on one or both surfaces of the surface plate. The electrically highly conductive coating provides as unobstructed, that is, as resistanceless path as possible for eddy currents to minimize losses in the rotor.

The surface plate structure of the invention can be implemented in axial flux induction machines of different constructions, such as in machines with one stator or two stators. With respect to these different machine constructions, one is referred to the aforementioned patent application WO2006/021616.

The machine construction of the invention has significant advantages over the known technology. Based on the tests performed, with the machine construction of the invention the efficiency of an axial flux induction machine is improved by 2-4 percentage units, even more than 5 percentage units. This way, it has been possible to decrease the temperature rise of the machine when the power and rotation speed increase.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means of examples with reference to the attached drawings, in which

Figure 1 shows the cage winding used in a machine of the invention,

Figure 2 is a cut-away drawing showing one embodiment of the invention,

Figure 3 is a cut-away drawing showing a second embodiment of the invention,

Figure 4 is a cut-away drawing showing a third embodiment of the invention,

Figure 5 is a cut-away drawing showing a fourth embodiment of the invention, and

Figure 6 shows a detail of the machine of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates the rotor construction used in the motor embodiment of the invention, comprising two plates joined together. The plates have been machined for instance by precision stamping from work-hardened aluminium alloy sheet. The frame plate 8 formed of the cage winding comprises a uniform inner periphery 9 and a uniform outer periphery 10 and conductor bars 11 of the same material, the conductor bars galvanically connecting the peripheries. The conductor bars are bars of equal size extending in the direction of the rotor radius, located at even distances between the inner and outer peripheries. Between the peripheries and conductor bars, a plurality of elongated and outwards-widening apertures is thus formed at even distances in the direction of the rotor radius, ferromagnetic pieces 12 of the corresponding shape with the apertures being inserted in the apertures and creating paths for the magnetic flux in axial direction through the otherwise non- ferromagnetic frame plate 8 of the rotor 2.

Figure 2 illustrates an electrical machine of the invention, in which there is a shaft 1 rotating with respect to the machine frame, and a disc-like rotor 2 supported to the shaft, which is in accordance, for instance, with Figure 1. In the direction of the shaft 1 on the first side of the rotor 2 there is a stator 4 supported to the machine frame and comprising a stator winding 3. There is a small air gap 14 between the rotor 2 and the stator 4, and after the corresponding air gap on the other side of the rotor, elements known as such in the art can be used to conduct the magnetic flux. The ferromagnetic pieces 12 are placed in the apertures extending through the rotor 2 so that together with the rotor plate they constitute a circular plate of a uniform thickness. On the side of the stator 4, surface plates 5 are attached to the surfaces of the ferromagnetic pieces, these surface plates entirely covering the ferromagnetic pieces 12 and extending somewhat over the rotor frame plate 8. The thickness of the surface plates 5 corresponds to or is larger than the air gap 14 between the stator 4 and the rotor 2, the air gap being about 1 mm and the thickness of the surface plate being about l-4mm.

Figure 3 shows an electrical machine corresponding to Figure 2, with the same reference numerals indicating the same parts as in Figure 2. The difference compared with Figure 2 is the ferromagnetic pieces 13, which have been arranged to extend outwards from the surface of the rotor plate 2 towards the stator 4 so that the surface plates 5 are not in contact with the rotor 2 but at an appropriate distance from it. This distance is for instance 3-8πun, preferably about 5mm. Yet further, the air gap 14 between the surface plates 5 and the stator 4 is about lmm and the thickness of the surface plate is larger than this, for instance about 2-4mm.

Figure 4 illustrates an embodiment largely corresponding to that of Figure 3 , in which the ferromagnetic pieces 13 are similar, that is, they extend through the rotor 2 and further to a distance towards the stator 4. In this embodiment, the surface plate 15 is comprised of a uniform ring, whereas in the embodiment of Figure 3 there is a separate surface plate on each ferromagnetic piece 13, even though the gap between these separate surface plates may be very small. In the embodiment of Figure 4, a rotor yoke 16 made of ferromagnetic material, such as construction steel, has also been attached to the outer surface of the rotor 2, that is, on the surface opposite the stator 4. This circular plate of a uniform thickness further strengthens the rotor and thus enables even higher rotation speeds. Further, it constitutes a path for the magnetic flux between the ferromagnetic pieces 13 extending through the rotor 2. In Figure 2, the corresponding rotor yoke 16 is only a ring-shaped disc, located only on the ferromagnetic pieces. Preferably the rotor yoke is not a separate part but it is comprised of the same uniform piece with the ferromagnetic pieces 13.

In the embodiments of Figures 3 and 4, a ring-shaped space is formed between the surface plates 5 and 15 and the plate surface of the rotor 2, on the outer periphery of which the ferromagnetic pieces 13 extending out from the rotor constitute radial fins. Thus, a blower is formed between the rotor and the stator, providing efficient ventilation and removing heat generated in the structures. Furthermore, the construction appropriately increases the heat transfer area of the rotor thus improving cooling of the construction .

Figure 5 shows an embodiment of the invention corresponding to Figure 3, where there is a similar stator construction 4 on both sides of the rotor 2. The ferromagnetic pieces 13 extending through the aluminium frame of the rotor 2 project to both sides of the rotor to a distance from the rotor surface. At both ends of the ferromagnetic pieces 13 there is a surface plate 5, from which at a distance of a narrow gap 14 there is the stator 4.

Further, Figure 6 is a cut-away drawing showing a detail of a rotor of the invention, where ferromagnetic pieces 18 are extending through the aluminium frame 17. The pieces 18 extend to a distance from the surface of the aluminium frame 17, surface plates 19 being attached to the outer surface of these pieces, chese surface plates extending over the edges of the pieces 18. Thus, nearly closed cavities 20 extending in the direction on the rotor radius are formed on the surface of the rotor, these cavities constituting the efficient fin structure of the centrifugal blower. It is also possible, as presented above, that the surface plates 19 form a uniform ring, in which case the cavities 20 are closed at all their sides and open only at their ends .

The invention is not restricted to the embodiments described above as examples, but many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. An axial flux induction electrical machine comprising a frame, a shaft (1) bearing-mounted to the frame, a disc-like rotor (2) supported to the shaft, a stator (4) comprising a stator winding (3) and supported by the frame on the first side of the rotor in axial direction, in which case the disc-like rotor (2) comprises a circular non-ferromagnetic rotor frame (8) of an essentially uniform thickness fabricated of a material with a high electrical conductivity and comprising a uniform inner periphery (9) and an outer periphery (10) and conductor bars (11) fabricated of the same material and galvanically connecting the peripheries; the conductor bars together with the inner and outer peripheries forming in addition to the rotor frame also the cage winding of the rotor; between the inner periphery and the outer periphery there is a plurality of ferromagnetic pieces (12) extending through the frame plate and being spaced apart from each other at an appropriate distance so that the radial conductor bars of the rotor are appropriately located between the pieces, charac t eri z ed in that the ferromagnetic piece (12,13) has a surface plate (5, 15) on the surface against the stator, the resistivity of this surface plate being essentially higher than the resistivity of the ferromagnetic piece.

2. An axial flux induction electrical machine as defined in claim 1, charac t er i z ed in that the resistivity of the surface plate is more than 0.2 μΩm.

3. An axial flux induction electrical machine as defined in claim 1 or in claim 2, charac ter i z ed in that the resistivity of the surface plate is between 0.3 and 1.5 μΩm.

4. An axial flux induction electrical machine as defined in any of claims 1-3 , charac t eri z ed in that the surface plate is of cobalt iron, martensitic steel or aluminium-iron alloy.

5. An axial flux induction electrical machine as defined in any of claims 1-4, charac t er i z ed in that the thickness of the surface plate is larger than the air gap between the rotor and the stator.

6. An axial flux induction electrical machine as defined in claim 5, charac t er i z ed in that the thickness of the surface plate is 1-4 mm.

7. An axial flux induction electrical machine as defined in any of claims 1-6, charac teri z ed in that the surface plate (5) is essentially level with the surface of the rotor frame (8) .

8. An axial flux induction electrical machine as defined in any of claims 1-6, charac t eri z ed in that the ferromagnetic pieces (13) extend from inside the rotor frame to a distance outside the surface of the rotor frame.

9. An axial flux induction electrical machine as defined in claim 8, charac teri z ed in that the surface plate (5,15) extends outside that surface of the ferromagnetic piece which is against the stator.

10. An axial flux induction electrical machine as defined in any of claims 1-9, charac t eri z ed in that the surface plates constitute a uniform ring of plates (15) that covers those surfaces of all the ferromagnetic pieces (13) which are against the stator (4) .

11. An axial flux induction electrical machine as defined in any of claims 1-10, charac t er i z ed in that the ferromagnetic pieces (12,13) together with the surface plates (5,15) constitute the fin structure of the centrifugal blower.

12. An axial flux induction electrical machine as defined in any of claims 1-11, charac t er i z ed in that the ferromagnetic pieces (12,13) are pieces extending in the direction of the radius of the rotor and essentially taking the form of a truncated narrow sector.

13. An axial flux induction electrical machine as defined in any of claims 1-12, charac ter i z ed in that the rotor frame (8) is of aluminium or copper, comprising for instance one or a plurality of aluminium plates,

14. An axial flux induction electrical machine as defined in any of claims 1-13, charac teri z ed in that the surface plate

(5,15) has an electrically highly conductive coating

(16), such as copper coating.

15. An axial flux induction electrical machine as defined in any of claims 1-14, charac teri z ed in that there is a rotor yoke (16) made of ferromagnetic material attached to the surface of the non-ferromagnetic rotor frame (8) on the opposite side of the stator (4) .