Rotor structure for a permanent magnet electrical machine

A rotor structure of a permanent magnet electrical machine and a method for manufacture of the rotor structure. The rotor structure has a shaft. An outer surface of the shaft is provided with a permanent magnet. A support collar encircles the permanent magnet. A protective piece is located between the outer surface of the shaft and an inner surface of the support collar and is arranged as an axial mechanical extension for the permanent magnet. The protective piece reduces local maximum values of forces forming between the support collar and the permanent magnet during rotation of the rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Finnish patent application 20070761 filed 9 Oct. 2007 and is the national phase under 35 U.S.C. §371 of PCT/FI2008/050447 filed 31 Jul. 2008.

TECHNICAL FIELD

The invention relates to a rotor structure for a permanent magnet electrical machine, which can operate as an electric motor or electric generator. The invention also relates to a method for the manufacture of a rotor for a permanent magnet electrical machine. The invention further relates to a permanent magnet electrical machine.

BACKGROUND OF THE INVENTION

Materials for permanent magnets, such as neodymium iron barium (NeFeB) or samarium cobolt (SmCo), with which strong magnetic flux density 1 . . . 1.2T can be achieved, are typically very brittle. Especially the ability of the permanent magnet materials to withstand tensile stress is poor. Thus, permanent magnets in the rotor of a permanent magnet electrical machine usually have to be strengthened with support structures made of more viscous material. The said strengthening is very important especially in permanently magnetised high-speed electric machines, in which the peripheral speed of the outer surface of the rotor can be even hundreds of meters in a second.

FIG. 1aillustrates the state-of-the-art rotor structure for a permanent magnet electrical machine, seen from the side.FIG. 1billustrates the cross section A-A of the said rotor structure. The rotor structure has a shaft101with permanent magnets102-113on its outer surface. The shaft is preferably made of ferromagnetic steel. The outer surface of the shaft101is provided with a sleeve115made of magnetically non-conductive material, with through-holes for the permanent magnets102-113. The arrows drawn to the permanent magnets102-113describe the magnetising direction of each permanent magnet. A support collar114has been arranged onto the sleeve115and the permanent magnets102-113. The support collar114is shown in longitudinal section inFIG. 1a. The sleeve115can be attached to the shaft101by, for example, screws116and/or a crimped joint based on thermal expansion phenomenon, i.e. thermal crimping. The sleeve115can be made, for example, of plastic, aluminium, titanium or some other suitable magnetically non-conductive material. In this document, material, which is not ferromagnetic, is called magnetically non-conductive material. In the rotor structure shown inFIGS. 1aand1b, each permanent magnet102-113is assembled of several axially consecutive pieces. In this document, axial refers to the direction of the rotor's rotation axis160.FIG. 1cillustrates a longitudinal section of area B shown inFIG. 1a. The longitudinal section plane is the x,y plane marked toFIGS. 1aand1c.FIG. 1ccorresponds to an exemplary situation, in which the rotor structure rotates around the rotation axis160. Permanent magnets are influenced by centrifugal force, which causes the permanent magnets to be pressed against the support collar114. In other words, the support collar114directs radial forces to the permanent magnets, the forces keeping the permanent magnets on the circular orbit. Forces between the permanent magnets and the support collar stretch the support collar. InFIG. 1c, the stretching of the support collar has been strongly exaggerated in order to demonstrate the phenomenon.FIG. 1cshows how the permanent magnet106is pressed against the support collar114. A strong surface pressure is applied to the area of the permanent magnet106illustrated by the arrow130. The said strong surface pressure stresses the brittle permanent magnet material106and may even cause fractures. An adversely high surface pressure may also be directed to the support collar114. The situation can be somewhat improved by rounding the edge of the permanent magnet106indicated by the arrow130and/or by selecting the length of the support collar so that the ends of the support collar and permanent magnets are aligned with each other as closely as possible in the axial direction. The realisation of these matters is complicated by the hard machineability of the permanent magnets and the possible moving of the support collar in relation to the permanent magnets.

One state-of-the-art solution is to attach the permanent magnets102-113to the shaft101so that the pressing of the permanent magnets against the support collar114would be lighter. The said attachment is generally done by gluing. However, in high-speed electric machines this solution is generally not feasible, because achieving a sufficiently strong attachment between the brittle permanent magnets102-113and the shaft101is a very challenging task. On the other hand, if the attachment between the permanent magnets and the shaft were able to endure the stress caused by the centrifugal force, i.e. the said attachment could keep the permanent magnets on the circular orbit, internal tensile stresses would be generated to the permanent magnets. This would not be advantageous, because the ability of several permanent magnet materials to tolerate tensile stress is poor.

SUMMARY OF THE INVENTION

The invention relates to the rotor structure of a permanent magnet electrical machine, which can operate as an electric motor or electric generator. The rotor structure of a permanent magnet electrical machine according to the invention has:a shaft;a permanent magnet;a support collar, which encircles the said shaft and the said permanent magnet; anda protective piece, which is located between the outer surface of the said shaft and the inner surface of the said support collar and which has been arranged as an axial mechanical extension for the said permanent magnet, and whose mass midpoint is located at a distance from the geometrical rotation axis of the rotor structure.

The invention also relates to a method for the manufacture of a permanent magnet electrical machine. In the method of the invention:a protective piece is installed so that the said protective piece will form an axial mechanical extension for the permanent magnet in the said rotor and that the mass midpoint of the said protective piece will stay at a distance from the geometrical rotation axis of the rotor; anda support collar is installed to encircle the said rotor shaft, the said permanent magnet and the said protective piece so that the said protective piece will remain between the outer surface of the said shaft and the inner surface of the said support collar.

The invention further relates to a permanent magnet electrical machine, which can operate as an electric motor or electric generator. The permanent magnet electrical machine of the invention contains a rotor, which hasa shaft fitted to the frame of the said electric machine with bearings;a permanent magnet;a support collar, which encircles the said shaft and the said permanent magnet; anda protective piece, which is located between the outer surface of the said shaft and the inner surface of the said support collar and which has been arranged as an axial mechanical extension for the said permanent magnet, and whose mass midpoint is located at a distance from the geometrical rotation axis of the rotor structure.

The considerable advantage achievable with the invention is that by means of the said protective piece it is possible to reduce the local maximum values of the forces formed between the support collar and the permanent magnet as the rotor rotates without having to shape the permanent magnet or to strive for a structure, in which the ends of the support collar and the permanent magnet are matched as closely as possible in the axial direction.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

FIGS. 1a,1band1chave been explained earlier in this document in connection of the description of the state-of-the-art technology.

FIG. 2aillustrates the rotor structure of a permanent magnet electrical machine according to an embodiment of the invention, seen from the side.FIG. 2bis a longitudinal section of a detail of the said rotor structure. The rotor structure has the shaft201, with permanent magnets202-206provided on its outer surface. The shaft is preferably made of ferromagnetic steel. On the outer surface of the shaft201there is the sleeve215made of magnetically non-conductive material, with through holes for the permanent magnets202-206. The surface collar214is located on the sleeve215and the permanent magnets202-206. InFIG. 2a, the support collar214is shown in longitudinal section. The sleeve215can be attached to the shaft201, for example, by screws and/or a crimp joint based on thermal expansion phenomenon, i.e. thermal crimping. The sleeve215can be made, for example, of plastic, aluminium, titanium, or some other suitable magnetically non-conductive material. Next to the permanent magnets202-206there are protective pieces220-229. The protective pieces are preferably located so that a protective piece operates as an axial mechanical extension for the permanent magnet. It is also possible to install protective pieces between axially consecutive permanent magnet pieces. In the structure illustrated inFIG. 2c, the protective pieces are located as axial mechanical extensions for the permanent magnets202-206towards the ends250and251of the support collar214. Each permanent magnet202-206consists of five axially consecutive pieces.

FIG. 2bis a longitudinal section of the vicinity of the protective piece220shown inFIG. 2a. The longitudinal section plane is the x,y plane marked toFIGS. 2aand2b.FIG. 2bcorresponds to an exemplary situation, in which the rotor structure rotates around the rotation axis260. The permanent magnets and protective pieces are influenced by centrifugal force, which causes the permanent magnets and protective pieces to be pressed against the support collar214. In other words, the support collar214applies radial forces to the permanent magnets and protective pieces, the forces keeping the permanent magnets and protective pieces on the circular orbit. Forces between the permanent magnets and protective pieces as well as the support collar stretch the support collar. InFIG. 2b, the stretching of the support collar has been strongly exaggerated in order to demonstrate the phenomenon.

FIG. 2bshows how the protective piece220and the permanent magnet202are pressed against the support collar214. Strong surface pressure is applied to the edge of the protective piece220indicated by the arrow230, because the sleeve215around the shaft201does not apply as high a pressure to the inner surface of the support collar as the protective pieces and permanent magnets. The protective piece220reduces the surface pressure that the support collar214applies to the edge of the permanent magnet202indicated by the arrow231. The density (kg/m3) of the protective piece material is preferably within the area of 0.3 . . . 1.3 times the density of the permanent magnet material. The viscosity of the protective piece material is preferably higher than the viscosity of the permanent magnet material. The protective pieces can be made, for example, of aluminium, titanium, stainless steel, brass, bronze, or copper.

In the rotor structure according to an embodiment of the invention, the support collar214is made of carbon fibre, the fibres of which are substantially in the tangential direction. In this document, tangential direction refers to the direction of the trajectory of the rotary movement.

In the rotor structure according to an embodiment of the invention the support collar214is made of titanium.

The rotor structure illustrated inFIG. 2ahas four poles, but it is obvious for one skilled in the art that the structural principle shown inFIG. 2acan also be applied to rotor structures, in which the number of poles is two or more than four.

FIG. 3aillustrates the rotor structure of a permanent magnet electrical machine according to an embodiment of the invention, seen from the side.FIG. 3billustrates the cross-section A-A of the rotor structure in question. The rotor structure has the shaft301, with permanent magnets302and303provided on its outer surface. The shaft is preferably made of ferromagnetic steel. The arrows drawn to the permanent magnets302and303indicate the magnetisation direction of each permanent magnet. The permanent magnets302and303are located in recesses on the outer surface of the said shaft301. There is a support collar314around the shaft301and the permanent magnets302and303. InFIG. 3a, the support collar314has been shown in longitudinal section. The rotor structure has protective pieces, which are located between the outer surface of the said shaft and the inner surface of the said support collar and which are arranged as axial mechanical extensions for the permanent magnets. The protective pieces320and321are axial mechanical extensions for the permanent magnet302.

In the rotor structure according to an embodiment of the invention, pieces305,306,307and308of magnetically non-conductive material are provided next to the surfaces of the permanent magnets302and303pointing to the tangential direction. The pieces305,306,307and308of magnetically non-conductive material can be, for example, plastic, aluminium, titanium, or some other suitable non-ferromagnetic material.

In the rotor structure according to an embodiment of the invention, each permanent magnet302and303has a radial play330and331between the support collar314and the shaft301. In the situation depicted inFIG. 3b, the permanent magnets302and303can move on the plane of the picture surface3baway from the rotation axis360. In the situation depicted inFIG. 3b, the spaces330and331between the permanent magnets and the support collar214make possible the play of the permanent magnets302and303. Depending on the position of the permanent magnets302and303in the said radial play, the spaces making possible the play can be either totally or partly also between the outer surface of the shaft301and the permanent magnets302and303. As can be seen fromFIG. 3b, the mutual fit of the permanent magnets302and303and the support collar314is not mechanically tight, so that the support collar can be pushed to place by using a movement in the direction of the rotation axis332without putting the support collar into contact with the permanent magnets of possibly brittle material during installation.

In the rotor structure according to an embodiment of the invention, the spaces330and331that enable the radial play of the permanent magnets302and303contain air. In the rotor structure according to another embodiment of the invention, the spaces330and331that enable the radial play of the permanent magnets302and303contain elastic material, which can be, for example, silicon.

The rotor structure illustrated inFIGS. 3aand3bis bipolar, but it is obvious for one skilled in the art that the structural principle shown inFIGS. 3aand3bcan also be applied to rotor structures, in which the number of poles is more than two.

FIG. 4illustrates the rotor structure of a permanent magnet electrical machine according to an embodiment of the invention, seen from the side. The rotor structure has two axially successive sections471and472, each contaning:permanent magnets (such as permanent magnet402),a support collar encircling the shaft401and the said permanent magnets, andprotective pieces (such as the protective piece420) located between the outer surface of the said shaft and the inner surface of the said support collar, and arranged as axial mechanical extensions for the permanent magnets.

The structure illustrated inFIG. 4is advantageous especially when the sleeves415and415aand/or support collars414and414ahave different thermal expansion factor from the shaft401. With the structure illustrated inFIG. 4it is possible to reduce the axial mechanical stresses caused by temperature variations. InFIG. 4, the support collars414and414ahave been shown in longitudinal section, and the permanent magnets and protective pieces have been shown as black surfaces.

In the rotor structure of a permanent magnet electrical machine according to an embodiment of the invention there are more than two axially successive sections, each containing permanent magnets, a support collar, and protective pieces.

FIG. 5illustrates a permanent magnet electrical machine according to an embodiment of the invention. The electric machine has the frame581, to which the stator core582is attached. The grooves in the stator core have the stator winding583. The rotor of the electric machine584has:a shaft501, which is fitted to the frame581of the electric machine with bearings;permanent magnets (such as permanent magnets502and503),a support collar514, which encircles the said shaft and the said permanent magnets, andprotective pieces (such as protective pieces520and521), located between the outer surface of the said shaft and the inner surface of the said support collar, and arranged as axial mechanical extensions for the permanent magnets.

The support collar514, frame581, stator core582, stator winding583, and bearings585and586have been shown in longitudinal section inFIG. 5.

In the electric machine according to an embodiment of the invention, at least one of the bearings585and586is an active magnetic bearing.

In the electric machine according to an embodiment of the invention, both bearings585and586are active magnetic bearings.

In the electric machine according to an embodiment of the invention, at least one of the bearings585and586is a gas bearing.

In the electric machine according to an embodiment of the invention, at least one of the bearings585and586is a rolling bearing, which can, for example, be a ball or roller bearing.

In the electric machine according to an embodiment of the invention, at least one of the bearings585and586is a sliding bearing, which can be, for example, a rocker segment bearing.

FIG. 6is a flow chart of the method according to an embodiment of the invention for the manufacture of a rotor for a permanent magnet electrical machine. In step601aprotective piece is installed so that the said protective piece will form an axial mechanical extension for the permanent magnet in the said rotor. In step602a support collar is installed to encircle the said rotor shaft, the said permanent magnet and the said protective piece so that the said protective piece stays between the outer surface of the said shaft and the inner surface of the said support collar. The support collar can be installed, for example, by pushing it with an axial movement to encircle the said shaft, the permanent magnet and the protective piece.

FIG. 7aillustrates the rotor structure of a permanent magnet electrical machine according to an embodiment of the invention, seen from the side.FIG. 7bis a cross-section A-A of the rotor structure in question. The rotor structure has the shaft701, which is encircled by a tubular permanent magnet702. The shaft is preferably made of ferromagnetic steel. The shaft701and the permanent magnet702is encircled by the support collar714. The rotor structure has the protective pieces720-725, which are located between the outer surface of the said shaft and the inner surface of the said support collar, and which are arranged as axial mechanical extensions for the permanent magnet. There are four protective pieces720-723shown inFIG. 7b. It is also possible to use one ring-shaped protective piece instead of the protective pieces720-723. However, it is preferable that the ring-shaped protective piece is made of more elastic material than the permanent magnet702. InFIG. 7a, the permanent magnet702, support collar714and protective pieces720,722,724and725are shown in longitudinal section. The rotor structure has the support flanges771and772, which are arranged to support the permanent magnet702, support collar714and protective pieces720-725in the axial direction. The said support flanges771and772can be attached to the shaft701, for example, by screws and/or by a crimping joint based on thermal expansion phenomenon, i.e. thermal crimping.

As is obvious for one skilled in the art, the invention and its embodiments are not restricted to the embodiment examples shown, but the invention and its embodiments can be varied within the independent patent claim. The expressions describing the existence of features contained in the claims, for example “the rotor structure has a shaft”, are open so that the presentation of the features does not exclude the existence of such other features that have not been shown in the independent patent claims.