FIELD OF THE INVENTION

The present invention relates to an electric machine.

BACKGROUND ART JP patent publication 10-336929 discloses an electric machine. A plurality of seat plates are arranged on the circumference of a yoke at intervals in the circumferential direction and fastened with first bolts to the yoke. Magnet pieces are placed on the seat plates and a laminate obtained by laminating thin steel plates is placed as a pressing member on the magnet pieces. The magnet pieces are clamped between the laminate and the seat plates by means of the fastening force of second bolts. The magnet pieces, the laminate, the seat plates and the second bolts comprise a magnetic block.

US patent 6,879,075 discloses a permanent magnet synchronous machine. A circumferentially oriented rotor assembly is provided with alternat- ing permanent magnets and magnetic pole pieces. In order to reduce the amount of leakage flux in the rotor and increase the effective length of the permanent magnet, a trapezoidal or otherwise tapered permanent magnet structure is used. By alternating trapezoidal permanent magnet, and magnet pole pieces, a higher intensity magnetic field is created in the air gap utilizing the same radial space in the motor without altering the weight or volume of the motor compared to conventional machines.

US patent 3,567,974 discloses a permanent magnet electric machine. Figure 4 shows permanent magnet blocks fastened between wedge- shaped pole cores and abutment members. The abutment members form con- jointly with the iron structure of the rotor a supporting star structure into which the permanent magnets and the pole cores are placed. The pole cores are fastened with non-ferromagnetic screws to the rotor.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to achieve an electric machine with an improved pole arrangement.

The electric machine according to the invention is characterized by what is stated in the independent claim 1 . Some preferred embodiments of the invention are disclosed in the dependent claims.

The electric machine according to the invention comprises a cylin- drical stator, a cylindrical, rotating rotor frame within the stator, said rotor frame having an outer surface and an axial direction, and a pole arrangement on the outer surface of the rotor frame, wherein said pole arrangement comprises a plurality of segment pieces disposed circumferentially around the outer surface of the rotor frame, a plurality of pole pieces disposed circumferentially around the outer surface of the rotor frame and disposed alternatingly with said segment pieces, and permanent magnet pieces between the segment pieces and the pole pieces, the segment pieces, the permanent magnet pieces and the pole pieces forming a continuous ring around the rotor frame, the electric ma- chine comprising a plurality of such rings after each other in the axial direction.

The electric machine according to the invention is characterised in that:

- each segment piece comprises a first inner edge seated against the outer surface of the rotor frame, a second opposite outer edge facing towards the stator, and side edges between the first inner edge and the second outer edge, said side edges comprising at the second outer edge inclined portions receiving the permanent magnet pieces, each segment piece being fastened with a first bolt to the rotor frame,

- each pole piece comprises a first inner edge facing towards the outer surface of the rotor frame, a second outer edge facing towards the stator, and inclined side edges between the first inner edge and the second outer edge, each pole piece being fastened with a second bolt to the rotor frame, whereby the magnet pieces are pressed between the inclined side edges of the pole pieces and the inclined portions of the side edges of the segment pieces when the second bolt is tightened,

- at least one axial air channel passes through the rings and at least one radial air duct passes between the rings.

The pole arrangement according to the invention results in a reliable fastening of the segment pieces, the pole pieces and the permanent magnets to the rotor frame.

The pole arrangement according to the invention also results in a minimized leakage flux at the upper and lower ends of the permanent magnets.

The pole arrangement according to the invention also results in axial air channels passing through the machine and radial air ducts enabling efficient cooling of the machine. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

Figure 1 shows an axial cross section of an electric machine.

Figure 2 shows an axonometric view of a part of a pole arrangement in an electric machine.

Figure 3 shows a radial cross section of the pole arrangement shown in figure 1 .

Figure 4 shows an axial cross section of a first cooling arrangement in an electric machine.

Figure 5 shows an axial cross section of a second cooling arrangement in an electric machine.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows an axial cross section of an electric machine. The electric machine comprises a cylindrical stator 100 and a cylindrical, rotating rotor frame 10 within the stator 100, said cylindrical rotor frame 10 having an outer surface 10a and an axial direction A-A. The electric machine comprises further a pole arrangement 200 on the outer surface 10a of the rotor frame 10. There is an air gap G between the inner surface of the stator 100 and the outer surface of the pole arrangement 200. The cylindrical rotor frame 10 comprises a radial partition wall 9 in the middle of the axial direction A-A of the cylinder. A rotor shaft 8 is fastened to the radial partition wall 9, said rotor shaft 8 being supported by bearings 7a, 7b at each end to the frame structures F of the elec- trie machine. The rotor frame 10 is of a non-magnetic material.

Figure 2 shows an axonometric view of a part of the pole arrangement in the electric machine and figure 3 shows a radial cross section of the pole arrangement shown in figure 1 .

The pole arrangement 200 comprises a plurality of segment pieces 20 disposed circumferentially around the outer surface 10a of the rotor frame 10. Each of said segment pieces 20 comprises a first inner edge 21 seated against the outer surface 10a of the rotor frame 10, a second opposite outer edge 22 facing towards the stator 100, and two side edges 23, 24 between the first inner edge 21 and the second outer edge 22. The side edges 23, 24 com- prise inclined portions 23a, 24a at the second outer edge 22. The inclined por- tions 23a, 24a of the side edges 23, 24 of the segment pieces 20 form advantageously a V so that the apex of the V faces towards the stator 100. Each of said segment pieces 20 is fastened with a first, preferably non-magnetic bolt 30 to the rotor frame 10. The first edge 21 of each segment piece 20 compris- es a dovetail joint 25. There is a corresponding dovetail locking piece 31 at the upper end of the first bolt 30. The free end of the first bolt 30 extends through a hole in the rotor frame 10 to the inside of the rotor frame 10. A nut 32 can be threaded on the free end of the first bolt 30. The segment piece 20 can be secured to the rotor frame 10 by tightening the nut 32 against the inner surface of the rotor frame 10.

The pole arrangement 200 comprises further permanent magnet pieces 41 , 42 positioned on the inclined portions 23a, 24a of the side edges 23, 24 of each segment piece 20. The permanent magnet pieces 41 , 42 have a rectangular form. The inclined portions 23a, 24a of the side edges 23, 24 of the segment pieces 20 have small protrusions engaging with the side surfaces of the permanent magnet pieces 41 , 42. The permanent magnets 41 , 42 are thereby locked against sliding movement along the surface of the inclined portions 23a, 24a of the side edges 23, 24 of the segment pieces 20. The width of the permanent magnet pieces 41 , 42 in the axial direction is thus a little bit smaller than the corresponding width of the pole pieces 50.

The pole arrangement 200 comprises further a plurality of pole pieces 50 disposed circumferentially around the outer surface 10a of the rotor frame 10 and disposed alternatingly with said segment pieces 20. Each of said pole pieces 50 comprises a first inner edge 51 facing towards the outer surface 10a of the rotor frame 10, a second outer edge 52 facing towards the stator 100, and two inclined side edges 53, 54 between the first inner edge 51 and the second outer edge 52. The first inner edge 51 is at a radial distance from the outer surface 10a of the rotor frame 10. The second outer edge 52 forms the air gap G to the stator 100. The inclined side edges 53, 54 form a V so that the apex of the V is facing towards the rotor frame 10. Each of said pole pieces 50 are fastened with a second, preferably non-magnetic bolt 60 to the rotor frame 10, whereby the magnet pieces 41 , 42 are pressed between the inclined side edges 53, 54 of the pole pieces 50 and the inclined portions 23a, 24a of the side edges 23, 24 of the segment pieces 20 when the second bolt 60 is tightened. There is a fastening piece 61 embedded into the pole piece 50. The second bolt 60 can be screwed into the treads of the fastening piece 61 in or- der to secure the second bolt 60 to the fastening piece 61 and thereby to the pole piece 50. A nut 62 can be screwed on the threads on the free end of the second bolt 60. The pole piece 50 can be secured to the rotor frame 10 by tightening the nut 62 against the inner surface of the rotor frame 10. The in- dined side edges 53, 54 of the pole piece 50 will press the permanent magnets 41 , 42 against the inclined upper portions 23a, 23b of the side edges 23, 24 of the segment piece 20 when the nut 62 is tightened.

The segment pieces 20, the permanent magnet pieces 41 , 42 and the pole pieces 50 form a continuous ring around the outer surface 10a of the rotor frame 10. The electric machine comprises a plurality of such rings after each other in the axial direction A-A. Figure 2 shows two axially adjacent rings, but the electric machine can comprise any number of rings.

The segment pieces 20 are composed of a plurality of thin electrical steel plates. Electrical steel is an iron alloy which may have from zero to 6.5% silicon. The electrical steel plates in the segment pieces 20 are pressed together by rivets 71 extending through the packet of electrical steel plates in the axial direction. There are further blind bores 72 in the axial side surfaces of the segment pieces 20. Every second segment piece 20 is turned around 180 degrees so that the blind bores 72 will receive the knobs of the rivets 71 of the axially adjacent segment piece 20. This means that the segment pieces 20 can be situated with the side surfaces attached to each other. The outermost metal plates in the package of metal plates are a little bit thicker in order to be stiff enough to form a straight side surface when the rivets 71 tighten the package. The thickness of the segment piece 20 in the axial direction is a little bit greater than the corresponding thickness of the pole piece 50.

The pole pieces 50 are also composed of a plurality of thin electrical steel plates. The electrical steel plates in the pole pieces 50 are pressed together by rivets 73 extending through the package of electrical steel plates in the axial direction. The outermost electrical steel plates in the package of elec- trical steel plates are a little bit thicker in order to be stiff enough to form a straight side surface when the rivets 73 tighten the package.

A first axial air channel A1 is formed in a space restricted by the lower portions 23b, 24b of the side edges 23, 24 of two circumferentially adjacent segment pieces 20, the outer surface 10a of the rotor frame 10 and the first edge 51 of the pole piece 50. The first axial air channel A1 extends in the axial direction A-A through the whole pole arrangement 200. A first radial air duct R1 is formed between two axially adjacent pole pieces 50. This is due to the fact that the segment pieces 20 are a little bit thicker in the axial direction A-A compared to the pole pieces 50. The first radial air duct R1 extends from the second edge 52 of the pole piece 50 into the first axial air channel A1 . The first radial air duct R1 extends also between the axially adjacent side surfaces of the permanent magnet pieces 41 , 42.

A second axial air channel A2 extends in the axial direction A-A through each axially adjacent segment piece 20. The second axial air channel A2 can be formed e.g. by punching an opening with a predetermined cross- section into the middle portion of each thin electrical metal plate forming the segment piece 20. The second axial air channel A2 will thus extend in the axial direction A-A through the whole pole arrangement 200.

A second radial air duct R2 is formed between two axially adjacent segment pieces 20. The outermost metal plates in the segment pieces 20 have been formed so that a second radial air duct R2 is formed between two axially adjacent segment pieces 20. The second radial air duct R2 extends from the second edge 22 of the segment piece 20 into the second axial air channel A2.

The area of the radial cross section of the first axial air channel A1 and the second axial channel A2 could be in the order of 20 to 35% of the area of the radial cross section of the segment piece 20. The width of the radial air ducts R1 , R2 in the axial direction A-A could be in the order of 10 to 20% of the width of the segment piece 20 in the axial direction A-A.

The rotor frame in figure 1 is a so called H-type rotor frame. The pole arrangement according to the invention can be used in any rotor frame having a cylindrical surface to which the segment pieces and the pole pieces can be fastened. The ring of segment pieces 20 and pole pieces 50 which is situated on the rotor frame 10 at the partition wall 9 cannot be secured to the rotor frame 10 with nuts 32, 62 situated inside the rotor frame 10. The segment piece 20 can be provided with a hole extending from the second surface 22 of the segment piece 20 to a seat in the interior of the segment piece and a smaller hole extending from the seat to the first surface 21 of the segment piece 20. The segment piece 20 can be fastened with a first bolt 30 extending in the smaller hole into a thread in the partition wall 9. The first bolt 30 can be tightened with a bushing extending through the bigger hole from the second surface 22 of the segment piece 20 to the seat and the nut of the first bolt 30 on the seat. The pole pieces 50 in the ring can be fastened in a corresponding way. These holes extending to the outer surface 22 of the segment piece 20 and the outer surface of the pole piece 50 will to some extent reduce the magnetic capacity of the segment piece 20 and the pole piece 50. This is the reason for not using such a fastening arrangement in the other rings.

The upper end surfaces of the permanent magnets 41 , 42 at the second edge 52 of the pole piece 50 are freely facing to air in the air gap G between the rotor and the stator 100. The lower end surfaces of the permanent magnets 41 , 42 at the first edge 51 of the pole piece 50 are freely facing to air in the first axial air channel A1 . The lower portions 23b, 24b of the side edges 23, 24 of two circumferentially adjacent segment pieces 20 have cavities below the lower ends surfaces of the permanent magnets 41 , 42 in order to increase the air space below the lower surfaces of the permanent magnets 41 , 42. This arrangement will minimize the leakage flux at said end surfaces. The fact that there is no magnetic material but instead air at the lower end surfaces of the permanent magnets 41 , 42 will reduce the leakage flux at said lower end surfaces of the permanent magnets 41 , 42 considerably. The leakage flux is that magnetic flux which does not cross the air gap and link to the stator winding, thus providing no useful magnetic field. The arrangement of the permanent magnets 41 , 42 and the magnetic pole pieces 50 is advantageous in using the space in the machine. There is a higher intensity magnetic field created in the air gap utilizing the same radial space in the machine without altering the weight or volume of the machine.

Figure 4 shows an axial cross section of a first cooling arrangement in an electric machine. Cooling air L1 is conducted from a ventilation housing 300 attached to the outer surface of the stator 100 towards both ends of the rotor 200. The cooling air L1 penetrates into the axial air channels A1 , A2 in the rotor 100 and passes towards the middle of the rotor 100. A part of the air penetrating into the first A1 and the second A2 axial air channels will also penetrate into the first R1 and the second R2 radial air ducts in the rotor 100. The air penetrating into the radial air ducts R1 , R2 in the rotor 200 will further cross the air gap G between the rotor 200 and the stator 100 and penetrate into corresponding radial air ducts R10 in the stator 100. The radial air ducts R10 in the stator open into a stator cavity 1 10 and said stator cavity 1 10 opens into the middle portion of the ventilation housing 300. Cooling pipes 1 10 pass across the ventilation housing 300. A cooling liquid or cooling air L30 is conducted through the cooling pipes 310. The return air L10 passing from the sta- tor 100 back to the middle portion of the ventilation housing 300 will pass across the cooling pipes 310 between the partition walls 320 in the ventilation housing 300 and further to the outer surface of the ventilation housing 300. The return air L10, which has been heated in the rotor 200 and the stator 100, will be cooled when passing across the cooling ducts 310. The direction of the return air L10 is changed at the outer surface of the ventilation housing 300 so that the return air L10 flows once again across the cooling pipes 310 where said air is further cooled. Said cooled air L1 is then conducted back into the motor towards the ends of the rotor 200. There is thus a closed cooling air cir- culation between the machine and the ventilation housing 300. This cooling air arrangement forms a symmetrical cooling of the machine.

The pole arrangement 200 on the rotor frame 10 produces pressure when the rotor frame 10 is rotating. The radial air ducts R1 , R2 work according to the same principal as the wings in a radial fan. The middle portion of the ventilation housing 300 or the end portions of the ventilation housing 300 could also be equipped with fans.

Figure 5 shows an axial cross section of a second cooling arrangement in an electric machine. The arrangement in the ventilation housing 300 differs from that shown in figure 4. The return air L10 passing from the stator housing 1 10 into the middle portion of the ventilation housing 300 will penetrate trough a heat converter 330a, 330b to either end of the ventilation housing 300. The return air L10 is sucked through the heat exchangers 330a, 330b with radial fans 340a, 340 situated at each end of the ventilation housing 300. The radial fans 340a, 340b blow the cooled air L1 back into the machine to- wards the ends of the rotor 200. Each radial fan 340a, 340b is driven by a motor, preferably an electric motor 350a, 350b. This cooling air arrangement forms also a symmetrical cooling of the machine.

The pole arrangement according to the invention is especially suitable for a medium sized rather slow rotating electric machine. The rotation speed could be e.g. in the order of 400 rpm. The diameter of the rotor frame could be in the order of 1 m or more. The pole arrangement according to the invention could, however, also be used in a faster rotating electric machine in which case either end or both ends of the rotor shaft 8 could be equipped with a fan. Said fan would then blow cooling air from the ventilation housing 300 to the axial air channels A1 , A2 in the rotor.

The invention is not limited to the examples described above but may vary within the scope of the claims.