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, and to a method for manufacturing an electric machine.

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 val- ues of the quadrature-axis reactance and the direct-axis reactance, which difference 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 hav- ing a plurality of identical rotor sheets stacked in an axial direction. Each of the plurality of rotor sheets comprises a plurality of flux guide sections located along circumferential direction of the rotor sheet, 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 perme- ance. 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 connecting adjacent flux paths. Each flux barrier located closest to the centre axis of the rotor sheet in corresponding flux guide section has a bridge that crosses the flux barrier substantially at the quadrature axis of the flux guide section. Further, each flux barrier located second closest to the centre axis of the rotor sheet in corresponding flux guide section has a bridge that crosses the flux barrier substantially at the quadrature axis of the flux guide section.

The bridges should be as narrow as possible in order to avoid un- desirable leak flux from one flux path to another. The leak flux decreases effi- ciency and maximum torque and increases no-load current thereby deteriorating electrical properties of the electric machine. Therefore the narrower the bridges are the better electrical properties of the electric machine.

Conventionally a rotor of a reluctance machine has been manufac- tured by punching flux barriers and other required openings into a ferromagnetic sheets and stacking these sheets to form the rotor core. A plurality of rotor sheets manufactured by punching are stacked between end plates, the plurality of rotor sheets being attached to the end plates by bolts extending through the rotor sheets.

A problem associated with punching is that narrow bridges cannot be formed with it.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to improve electrical properties of an electric machine whose rotor sheets comprise bridges while being ob- tainable by punching. The objects of the invention are achieved by a rotor which is characterized 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 sheet which comprises at least two types of flux guide sections, each said type of flux guide section having a different number of flux barriers with bridges compared with the rest of the types. In a rotor the rotor sheets are arranged such that adjacent a first type flux guide section in the axial direction there is a second type flux guide section. Since rotor sheets are axially pressed against each other, a bridge in a flux barrier of a first flux guide section is also capable of supporting a flux barrier of a second flux guide section located adjacent the first flux guide section in the axial direction. In an axial series of flux barriers some flux barriers have a bridge and the others do not have a bridge. Therefore an average bridge width in the axial series of flux barriers is less than an average bridge width of actual bridges in the axial series of flux barriers.

An advantage of the invention is that electrical properties of an electric machine having rotor sheets comprising bridges crossing flux barriers can be improved without need to reduce width of the bridges. In a way the present invention reduces effective width of bridges without need to reduce actual width thereof. 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 four successive rotor sheets of a rotor according to an embodiment of the invention;

Figure 2 shows a rotor sheet of a rotor according to another embodiment of the invention;

Figure 3 shows four rotor sheets of Figure 2 axially stacked; and Figure 4 shows a rotor comprising the rotor sheet stack of Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows rotor sheets RS1 , RS2, RS3 and RS4 which are successive rotor sheets of a rotor core of a four pole rotor according to an embodiment of the invention. The rotor sheets RS1 - RS4 are adapted for a syn- chronous reluctance machine. In the rotor core the rotor sheets RS1 - RS4 are axially stacked such that rotor sheet RS2 is stacked between rotor sheets RS1 and RS3, and rotor sheet RS3 is stacked between rotor sheets RS2 and RS4.

Each of the rotor sheets RS1 - RS4 comprises flux guide sections FG1 , FG2, FG3 and FG4 located along circumferential direction of the rotor sheet. 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.

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 extremity and the second extremity ending at the rotor surface and being spaced apart from one another at the rotor surface in the direction of its circumference. 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. Flux paths P1 - P3 are inwards curved.

Each flux barrier B1 - B4 is arranged to increase the ratio of direct- axis reactance to quadrature-axis reactance. In other words, 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 rotor sheets RS1 - RS4 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 d _A 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 ending at the rotor surface.

The rotor sheets RS1 - RS4 are identical with each other. In a rotor core successive rotor sheets are offset by ττ/4 radians (90°) wherein flux guide section FG4 of rotor sheet RS2 is stacked between flux guide section FG1 of rotor sheet RS1 and flux guide section FG3 of rotor sheet RS3, and flux guide section FG3 of rotor sheet RS3 is stacked between flux guide section FG4 of rotor sheet RS2 and flux guide section FG2 of rotor sheet RS4.

In flux guide section FG1 none of flux barriers B1 - B4 has a bridge interrupting the flux barrier. In flux guide section FG2 a bridge BR21 interrupts the flux barrier B1 , a bridge BR22 interrupts the flux barrier B2, and a bridge BR23 interrupts the flux barrier B3. In flux guide section FG3 a bridge BR31 interrupts the flux barrier B1 . In flux guide section FG4 a bridge BR41 inter- rupts the flux barrier B1 , and a bridge BR42 interrupts the flux barrier B2. Except for number of bridges the flux guide sections FG1 , FG2, FG3 and FG4 are identical with each other.

In the embodiment of Figure 1 , each of the bridges is positioned such that a quadrature axis of the rotor traverses the bridge. Only a quadrature axis q _A traversing flux guide section FG1 of rotor sheet RS1 is depicted in Figure 1 , along with two direct pole axes d _A which form an angle that is bisected by the quadrature axis q _A . Each of the bridges is symmetrical with relation to corresponding quadrature axis. Further, a width of each bridge is substantially the same.

Each of the rotor sheets RS1 - RS4 comprises more bridges close to the centre axis of the rotor sheet than further from the centre axis of the ro- tor sheet. In each of the rotor sheets RS1 - RS4 the innermost flux barriers B1 have a total of three bridges while flux barriers B2 have a total of two bridges, and flux barriers B3 have a total of one bridge.

Each of the rotor sheets RS1 - RS4 comprises a plurality of connec- tion apertures. Each flux path P1 comprises a connection aperture CA1 , each flux path P2 comprises a connection aperture CA2, and each flux path P3 comprises a connection aperture CA3. 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 example.

Figure 2 shows a rotor sheet RS1 ' of a rotor according to another embodiment of the invention. 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 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' interrupts the flux barrier B2'. 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'. Except for the bridges the 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 quadrature axis. A width of each bridge is substantially the same.

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 quad- rature 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 CAT, and each flux path P2' comprises a connection aperture CA2'. The connection apertures are adapted to receive bolts for pressing rotor sheets of a stack together.

Figure 3 shows a stack of four rotor sheets, each one of the rotor sheets being identical with rotor sheet RS1 ' shown in Figure 2. Successive rotor sheets are offset by ττ/4 radians (90°).

In a further embodiment a rotor core 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 predetermined angle a may be defined by an equation

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.

In some embodiments a predetermined offset angle CH may be dif- ferent from the angle between adjacent poles. However, the predetermined angle αι is always greater than half of the angle between adjacent poles.

Figures 1 and 2 depict rotor sheets of a four pole rotor. However, the present invention is usable for any number of poles.

Each rotor sheet depicted in Figures 1 and 2 comprise a peripheral bridge assembly. The peripheral bridge assembly comprises a plurality of peripheral bridges made of a material of high permeance. The peripheral bridges form, together with outer portions of flux paths, an unbroken ring made of a material of high permeance, the unbroken ring defining an outer circumference of corresponding rotor sheet. The unbroken ring strengthens the structure of the rotor sheet.

Bridges shown in Figures 1 and 2 are radial bridges located at quadrature axis of corresponding flux guide section. Alternative embodiments may comprise bridges of different types, such as tangential bridges. Also, location of bridges may be different than what is shown in Figures 1 and 2.

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.

The invention is not limited to reluctance machines. For example, it is possible to use a rotor according to present invention in a permanent magnet machine. Basically it is possible to use present invention in any stacked rotor having bridges.

Figure 4 shows a rotor comprising the rotor sheet stack of Figure 3. The rotor further comprises end plates PL1 ' and PL2'. End plate PL1 ' is located at a first end of the rotor, and end plate PL2' is located at a second end of the rotor. The four rotor sheets are pressed between the end plates PL1 ' and PL'2 by bolts BT extending through the connection apertures CAT and CA2'.

Rotor sheets RS1 - RS4 shown in Figure 1 and rotor sheet RS1 ' shown in Figure 2 can be manufactured from 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 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 auto- matically rotated for stacking.

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.