THE SAME

FIELD OF THE INVENTION

The present invention relates to a rotor for an electric machine. The present invention also relates to an electric machine comprising said rotor.

BACKGROUND OF THE INVENTION

In an electric machine rotor provided with a plural number of magnetic poles, arranged substantially at equal intervals along the circumference of the rotor, each magnetic pole has a direct pole axis, or a direct axis. Two adjacent direct pole axes form an angle that is bisected by a quadrature axis. The reactance corresponding to the direct pole axis is called a direct-axis reactance and the reactance corresponding to the quadrature axis a quadrature- axis reactance. A reluctance torque is proportional to difference of inverse values of the quadrature-axis reactance and the direct-axis reactance, which dif- ference can be written as 1 /X _q - 1/X _d . Therefore a reluctance torque can be increased by increasing the direct-axis reactance or by reducing the quadrature-axis reactance.

A known rotor of a reluctance machine comprises a rotor core having a plurality of flux guide sections located along circumferential direction of the rotor core, each of the plurality of flux guide sections comprising a plurality of flux paths made of a material of high permeance and a plurality of flux barriers made of a material of low permeance. The flux paths and flux barriers are located alternately along radial direction of the flux guide section. For mechanical reasons there are bridges made of a material of high permeance connect- ing adjacent flux paths.

The bridges cause leak flux from one flux path to another. The leak flux decreases efficiency and maximum torque and increases no-load current thereby deteriorating electrical properties of the electric machine. Therefore minimizing leak flux in bridges improves electrical properties of the electric ma- chine.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to improve electrical properties of an electric machine whose rotor comprises bridges connecting adjacent flux paths. The objects of the invention are achieved by a rotor which is character- ized by what is stated in the independent claim 1 . The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the idea of providing a rotor core with a plurality of permanent magnets each one of which is located adjacent a corre- sponding bridge and adapted to saturate the corresponding bridge.

An advantage of the present invention is that electrical properties of an electric machine whose rotor comprises bridges connecting adjacent flux paths can be improved without need to modify the bridges in any way.

European Patent application 12197147.7 discloses an invention for improving electrical properties of an electric machine having a rotor comprising stacked rotor sheets. Present invention can be exploited together with said earlier invention, wherein permanent magnets are placed adjacent bridges of the rotor sheets disclosed in the earlier application.

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 a cross section of an electric machine according to an embodiment of the invention;

Figure 2 shows an enlarged detail of the cross section of Figure 1 ;

Figure 3 shows a rotor sheet of a rotor according to another embodiment of the invention;

Figure 4 shows four rotor sheets axially stacked, each of the rotor sheets being similar to the rotor sheet of Figure 3; and

Figure 5 shows a rotor comprising the rotor sheet stack of Figure 4.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a cross section of an electric machine according to an embodiment of the invention. The electric machine comprises a rotor and a stator. The rotor comprises a rotor core having flux guide sections FG1 , FG2, FG3 and FG4 located along circumferential direction of the rotor core. Herein a circumferential direction is a curved direction which is perpendicular to axial direction and radial direction of the rotor.

Each of the flux guide sections FG1 - FG4 comprises flux paths P1 , P2, P3 and P4 made of a material of high permeance and flux barriers B1 , B2, B3 and B4 made of a material of low permeance located alternately along ra- dial direction of the flux guide section. Each of flux barriers B1 - B4 extends from a first extremity to a second extremity, the first and the second extremities being situated adjacent surface of the rotor core and being spaced apart from one another in a circumferential direction. Each flux barrier B1 - B4 is arranged to provide a high magnetic resistance between adjacent elements made of a material of high permeance.

Each of the flux barriers B1 - B4 defines a corresponding imaginary centre line extending from a first point on a rotor surface to a second point on the rotor surface, the first point and the second point being spaced apart from one another at the rotor surface in a circumferential direction of the rotor. In Figure 1 an imaginary centre line of flux barrier B1 of flux guide sections FG1 has been illustrated by a dash line CL1 1 .

Flux paths P1 - P4 are arranged to conduct magnetic flux from a first extremity of the flux path to a second extremity thereof, both the first ex- tremity and the second extremity being situated at the rotor surface and being spaced apart from one another at the rotor surface in a circumferential direction of the rotor. Flux paths P1 - P3 are shaped in such a way that the circumferential extremities, i.e. those above referred to as the first and the second extremity, are at a substantially greater radial distance from the mid-axis of the rotor than an outer surface of a centre portion of the flux path in question.

The rotor core comprises bridges BR1 1 , BR12, BR13 and BR14 made of a material of high permeance, each of the bridges BR1 1 - BR14 extending across a corresponding flux barrier. Bridge BR1 1 extends across flux barrier B1 , bridge BR12 extends across flux barrier B2, bridge BR13 extends across flux barrier B3, and bridge BR14 extends across flux barrier B4.

The rotor of Figure 1 further comprises a centre section RCS. The centre section RCS is substantially X-shaped, with each tip of the X extending to the surface of the rotor. The direct pole axis of each magnetic pole passes through the portion of the centre section RCS that extends to the rotor surface. In the middle of the centre section RCS there is a hole RH arranged to receive a rotor shaft.

The centre section RCS is made of material of high permeance. Therefore the centre section RCS forms a centre flux path P0 for each flux guide section. Each centre flux path P0 is located adjacent a respective flux barrier B1 , and is arranged to conduct magnetic flux from a first extremity of the centre flux path to a second extremity thereof, both the first extremity and the second extremity being situated at the rotor surface.

Figure 2 shows a quarter of the cross section of Figure 1 as an enlargement. Figure 2 shows that the rotor core comprises permanent magnets M1 1 , M12 and M12P each one of which is located adjacent a corresponding bridge and adapted to saturate the corresponding bridge. A reluctance of a saturated bridge is higher than a reluctance of an unsaturated bridge. Permanent magnet M1 1 is located adjacent bridge BR1 1 , permanent magnet M12 is located adjacent bridge BR12, and permanent magnet M12P is located adjacent bridge BR12P. Magnetization direction of permanent magnet M1 1 is de- noted with arrow MD1 1 , magnetization direction of permanent magnet M12 is denoted with arrow MD12, and magnetization direction of permanent magnet M12P is denoted with arrow MD12P. A magnetization direction of each of the permanent magnets M1 1 , M12 and M12P is substantially perpendicular to a direction in which corresponding bridge extends across associated flux barrier.

Herein a magnetization direction of a magnet is a direction parallel to a line extending from a south pole of the magnet to a north pole of the magnet. Thus the magnetization direction is defined to be parallel to a general direction of a magnetic field inside the magnet. The magnetization direction defined here is rectilinear also outside the magnet despite the fact that outside a magnet a magnetic field thereof extends from a north pole to a south pole and therefore the magnetic field has a curvilinear form.

In alternative embodiments a magnetization direction of each permanent magnet diverges from a direction in which the corresponding bridge extends across the flux barrier without being substantially perpendicular to the direction in which the corresponding bridge extends across the associated flux barrier. In an embodiment a magnetization direction of each permanent magnet diverges from a direction in which the corresponding bridge extends across the flux barrier by an angle greater than 45°.

A total volume of a permanent magnet corresponding to a bridge is not more than five times a volume of the bridge. In an embodiment a total volume of each permanent magnet is one to two times a volume of the corresponding bridge. In alternative embodiments at least one permanent magnet may comprise a plurality of partial magnets wherein a total volume of such permanent magnet is the total volume of the partial magnets.

Permanent magnets are relatively small compared to associated flux barriers. Each flux barrier across which there extends at least one bridge having an associated permanent magnet has a length along the corresponding imaginary centre line which is multiple compared to a total length of said associated permanent magnet along the corresponding imaginary centre line. Length of bridges and permanent magnets is not included in a length of the flux barrier. In an embodiment each flux barrier across which there extends at least one bridge having an associated permanent magnet has a length along the corresponding centre line which is at least ten times a total length of said associated permanent magnet along the corresponding centre line. For example flux barrier B1 across which there extends bridge BR1 1 has a length along imaginary centre line CL1 1 which is tens of times a total length of permanent magnet M1 1 along the imaginary centre line CL1 1 .

Permanent magnets M1 1 , M12 and M12P are lateral magnets each one of which is located adjacent a corresponding bridge in a lateral direction perpendicular to the axial direction of the rotor. Permanent magnet M1 1 is lo- cated adjacent bridge BR1 1 , permanent magnet M12 is located adjacent bridge BR12, and permanent magnet M12P is located adjacent bridge BR12P. Permanent magnet M12P is located radially inward with respect the bridge BR12P. Each lateral magnet M1 1 , M12 and M12P is located along an imaginary centre line of corresponding flux barrier. Imaginary centre line CL1 1 of flux barrier B1 passes through permanent magnet M1 1 , and imaginary centre line of flux barrier B2 passes through permanent magnets M12 and M12P.

Permanent magnet M12 is located clockwise with relation to the adjacent bridge BR12. Also a corresponding permanent magnet in flux guide section FG3 is located clockwise with relation to the adjacent bridge. Correspond- ing permanent magnets in flux guide sections FG2 and FG4 are located anticlockwise with relation to the adjacent bridge. Herein directions clockwise and anticlockwise refer to Figures 1 and 2.

Bridge BR1 1 is a spl it bridge comprising a first bridge side wall BR1 1 A and a second bridge side wall BR1 1 B spaced apart in direction of the imaginary centre line CL1 1 of flux barrier B1 , the first bridge side wall BR1 1 A and the second bridge side wall BR1 1 B accommodating lateral magnet M1 1 between them. A volume of a split bridge is a sum of volumes of its side walls.

Bridges BR1 1 , BR12, BR13 and BR14 are intermediate bridges each one of which interrupts corresponding flux barrier. Further, bridges BR1 1 , BR12, BR13 and BR14 are quadrature axis bridges, each being positioned such that a quadrature axis of the rotor traverses the quadrature axis bridge. Quadrature axis passing through flux guide section FG1 has been denoted with reference mark q _A in Figure 1 . In the split bridge BR1 1 the quadrature axis passes between the first bridge side wall BR1 1A and the second bridge side wall BR1 1 B.

Bridge BR12P is a peripheral bridge traversing corresponding flux barrier B2 at an outer end thereof. It should be noticed that each flux barrier of the rotor core of Figure 1 comprises a peripheral bridge at both ends thereof. Each peripheral bridge is made of a material of high permeance. Due to the peripheral bridges there is an unbroken ring made of a material of high perme- ance extending around the rotor core. The unbroken ring strengthens the structure of the rotor core. Only peripheral bridge BR12P is denoted with a reference mark.

In some embodiments a rotor comprises a rotor core having a plurality of rotor sheets stacked between end plates. Figure 3 shows a rotor sheet RS1 ' of a rotor according to one such embodiment. Figure 4 shows four rotor sheets RS1 ' axially stacked. Figure 5 shows a rotor comprising the rotor sheet stack of Figure 4.

Rotor sheet RS1 ' comprises flux guide sections FG1 ', FG2', FG3' and FG4' located along circumferential direction of the rotor sheet RS1 '. Each of the flux guide sections FG1 ' - FG4' comprises flux paths P1 ', P2', P3' and P4' made of a material of high permeance and flux barriers B1 ', B2', B3' and B4' made of a material of low permeance. Flux paths and flux barriers are located alternately along radial direction of corresponding flux guide section.

In flux guide section FG1 ' none of flux barriers B1 ' - B4' has a bridge interrupting the flux barrier. Flux barrier B1 ' comprises a magnet slot MS1 1 ' adapted to accommodate a permanent magnet M1 1 ', and flux barrier B2' comprises a magnet slot MS12' adapted to accommodate a permanent magnet M12'. Each of the magnet slots is located in a centre of corresponding flux barrier such that a symmetry axis of the flux barrier bisects the magnet slot. Flux guide section FG3' is identical with flux guide section FG1 '. Flux guide section FG3' is located π radians (180°) away from flux guide section FG1 '.

In flux guide section FG2' a bridge BR21 ' interrupts the flux barrier B1 ', and a bridge BR22' interrupts the flux barrier B2'. In flux guide section FG4' a bridge BR41 ' interrupts the flux barrier B1 ', and a bridge BR42' inter- rupts the flux barrier B2'. Each of the bridges BR21 ', BR22', BR41 ' and BR42' is an intermediate bridge interrupting a corresponding flux barrier. Flux guide section FG4' is identical with flux guide section FG2'. Flux guide section FG4' is located π radians (180°) away from flux guide section FG2'.

In flux guide section FG2' magnet slot MS1 1 ' of flux guide section FG1 ' has been replaced by bridge BR21 ', and magnet slot MS12' of flux guide section FG1 ' has been replaced by bridge BR22.' Except for their middle portions flux guide sections FG1 ' and FG2' are identical with each other.

Bridges in flux guide sections FG2' and FG4' are positioned such that a quadrature axis of the rotor traverses them. Each of the bridges BR21 ', BR22', BR41 ' and BR42' is symmetrical with relation to corresponding quadra- ture axis.

The rotor sheet RS1 ' is symmetrical with relation to both a vertical line and a horizontal line, the vertical line coinciding with a quadrature axis bisecting flux guide section FG1 ', and the horizontal line coinciding with a quadrature axis bisecting flux guide section FG2'.

Rotor sheet RS1 ' comprises more bridges close to the centre axis of the rotor sheet than further from the centre axis of the rotor sheet. The innermost flux barriers B1 ' have a total of two bridges. Flux barriers B2' also have a total of two bridges. The outermost flux barriers B3' and B4' do not have any bridges.

Rotor sheet RS1 ' comprises a plurality of connection apertures.

Each flux path P1 ' comprises a connection aperture CA1 ', and each flux path P2' comprises a connection aperture CA2'. The connection apertures are adapted to receive bolts or bars for pressing rotor sheets of a stack together. Alternatively rotor sheets may be attached to each other by gluing, for exam- pie.

Figure 4 shows a stack of four rotor sheets RS1 ', each one of the rotor sheets being similar with rotor sheet RS1 ' shown in Figure 3. Successive rotor sheets are offset by ττ/2 radians (90°), wherein flux guide section FG1 ' of each inner rotor sheet is stacked between flux guide sections FG2' and FG4' of adjacent rotor sheets. Herein an inner rotor sheet is a rotor sheet which is axi- ally located between two adjacent rotor sheets.

The rotor sheet stack of Figure 4 comprises permanent magnets M1 1 ' and M12'. Each of the permanent magnets M1 1 ' and M12' is an axial magnet located axially adjacent a corresponding bridge and adapted to satu- rate the corresponding bridge. Each axial magnet M1 1 ' is located axially adjacent bridge BR41 ', and each axial magnet M12' is located axially adjacent bridge BR42'. Each of the axial magnets is adapted to be axially pressed between two elements at least one of which is a bridge corresponding to the axial magnet.

In Figure 4, a magnetization direction of one permanent magnet M1 1 ' is denoted with arrow MD1 1 ', and a magnetization direction of one permanent magnet M12' is denoted with arrow MD12'. Both magnetization direction MD1 1 ' and magnetization direction MD12' is parallel to an axial direction of the rotor sheet stack. Also, both magnetization direction MD1 1 ' and magnetization direction MD12' is substantially perpendicular to a direction in which corre- sponding bridge extends across associated flux barrier. For example, magnetization direction MD12' of axial magnet M12' is perpendicular to a direction in which bridge BR42' extends across flux barrier B2'. Permanent magnets located in the same flux barrier of the rotor sheet stack have the same magnetization direction such that a magnetic flux passes through permanent magnets located at a common axial line. Thus both permanent magnets M1 1 ' located in the flux barrier B1 ' have the same magnetization direction, and both permanent magnets M12' located in the flux barrier B2' have the same magnetization direction.

The stack of Figure 4 comprises axial magnets adjacent quadrature axis bridges. In an alternative embodiment there are axial magnets adjacent peripheral bridges too.

A rotor core comprising stacked rotor sheets may comprise lateral magnets. Successive rotor sheets may have lateral magnets at different sides of corresponding bridges. For example, a lateral magnet adjacent a bridge in a rotor sheet is located clockwise with relation to the bridge and a lateral magnet adjacent a corresponding bridge in an adjacent rotor sheet is located anticlockwise with relation to the corresponding bridge.

Figure 5 shows a rotor comprising the rotor sheet stack of Figure 4. The rotor comprises an end plate PL1 ' located at a first end of the rotor, and an end plate PL2' located at a second end of the rotor, and the rotor sheet stack of Figure 4 stacked between the end plates PL1 ' and PL2'. Flux guide sections of the rotor core are formed by the plurality of flux guide sections FG1 ', FG2', FG3' and FG4' of the stacked rotor sheets RS1 '.

Each of the axial magnets M1 1 ' and M12' in the rotor of Figure 5 is pressed between two elements at least one of which is a bridge corresponding to the axial magnet. Each axial magnet that is located adjacent an end plate is pressed between the end plate and a bridge of an adjacent rotor sheet. Elements between which an axial magnet is located exert pressing forces on the axial magnet, the pressing forces keeping the axial magnet in position during rotation of the rotor.

Rotor sheets RS1 ' shown in Figure 4 can be manufactured from a ferromagnetic plate by punching . A rotor sheet can be formed by a single punching process, wherein also the bridges of flux barriers are formed by the single punching process. A rotor core may comprise exclusively one type rotor sheets in which case the rotor sheets may be formed with one punching tool. Said rotor sheets of one type may be automatically rotated for stacking.

In alternative embodiments rotor sheets of a rotor do not have to be identical with each other. Further, in embodiments where rotor sheets are identical, the rotor sheet used does not have to be symmetrical with relation to both a vertical line and a horizontal line, as the rotor sheet RS1 '. In an embodiment a rotor core has a plurality of rotor sheets stacked in an axial direction, each of the plurality of rotor sheets comprising a plurality of flux guide sections located along circumferential direction of the rotor sheet, a plurality of flux guide sections of the rotor core being formed by the plurality of flux guide sections of the stacked rotor sheets. The plurality of flux guide sections of each of the plurality of rotor sheets comprises a first flux guide section in which a first number of the plurality of flux barriers has an intermediate bridge, and a second flux guide section in which a second number of the plurality of flux barriers has an intermediate bridge, the first number being different than the second number.

In a further embodiment a rotor core comprising lateral magnets has a plurality of rotor sheets stacked in an axial direction, the plurality of rotor sheets being arranged in a plurality of rotor sheet groups. Each rotor sheet group comprises at least one rotor sheet, rotor sheets of each group being located successively to each other in the axial direction. Rotor sheets of each rotor sheet group are identical with each other. Also positions of rotor sheets in a rotor sheet group are identical, which means that each flux guide section in a rotor sheet group comprising more than one rotor sheets is located adjacent an identical flux guide section. In other words identical flux guide sections are aligned in a rotor sheet group while corresponding flux guide sections are offset by a predetermined angle a in adjacent rotor sheet groups. The predeter- mined angle a may be defined by an equation ^N FGS where N _F GS is a number of flux guide sections in a rotor sheet. A number of flux guide sections in a rotor sheet is equal to the number of poles. The prede- termined angle a is greater than half of an angle between adjacent poles.

Although Figure 1 depicts a rotor having four poles and Figure 3 depicts a rotor sheet of a four pole rotor the present invention is usable for any number of poles. Further, although Figures depict rotors and rotor sheets of a synchronous reluctance machine the present invention can be applied to other types of machines as well.

The material of low permeance is selected on the basis of the type of the electric machine. In a reluctance motor the flux barriers may be filled with solid or powdery substances that are weakly conductive both as regards the magnetic flux and electricity. Useful substances include, depending on the embodiment, resins, plastics and carbon fibres. Naturally, in some embodiments the material of low permeance may be air.

It will be obvious to a person skilled in the art that the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.