FIELD

The present invention relates to a synchronous reluctance electric machine and specifically to a permanent magnet assisted rotor structure. BACKGROUND

Synchronous reluctance machines typically comprise stators with poly-phase windings, which form a plurality of poles. The stator windings forms rotating magnetic field for rotating a rotor of the machine, which is spaced from the stator by an air gap.

The rotor does not typically have windings but has a number of poles in form of magnetically permeable segments. The rotor is formed as a structure where each pole has a direction of minimum reluctance (d-axis) and maximum reluctance (q-axis).

When current is applied to the windings of the stator, the rotor will attempt to align its most magnetically permeable direction (d-axis) to the peak flux formed to an air gap between the stator and the rotor. This results into rotary motion of the rotor at the same or synchronous speed as the rotation of the stator magnetic field. The rotation of the rotor can be conveyed to the exterior of the machine by a rotor shaft fixed to and extending to the exterior of the rotor.

The rotor is typically constructed of a stack of mutually similar ferromagnetic sheets or laminations or disks. The sheets are given an unisotropic structure by means of alternating permeable segments or flux paths and flux insulating flux barriers.

To provide mechanical strength to the sheet, the flux barriers may be split into two or more sections divided by bridges connecting two flux paths. This has, however, the disadvantage that the magnetic flux tends to propagate along the bridge, which is disadvantageous in view of torque provided by the rotor.

In some known solutions, permanent magnets may be set to the flux barriers to the proximity of the bridges such that the escaping of the flux to the bridges would be avoided. This is, however, not optimal in view of maximizing the torque of provided by the rotor. SUMMARY

An object of the present invention is thus to provide an apparatus and a method of constructing an apparatus so as to alleviate the above disadvantages.

The objects of the invention are achieved by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

By help of the invention, the torque provided by the machine is maximized while optimizing the use of the permanent magnetic material. 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 embodiment of a rotor sheet;

Figure 2 shows some embodiment of placing a permanent magnet to a flux path;

Figure 3 shows an embodiment of a two-pole rotor sheet provided with permanent magnet;

Figure 4 shows an embodiment of a four-pole rotor sheet with per- manent magnets;

Figure 5 shows an embodiment of a rotor sheet;

Figure 6 shows another embodiment of a pole of a rotor sheet;

Figure 7 illustrates propagation of flux paths in the embodiment of

Figure 6;

Figure 8 shows an embodiment of a four-pole rotor sheet;

Figure 9 shows another embodiment of a pole of a four-pole rotor sheet;

Figure 10 shows another embodiment of a four-pole of a rotor sheet; and

Figure 1 1 shows another embodiment of a four-pole of a rotor sheet.

DESCRIPTION OF EMBODIMENTS

The embodiments discussed in the following relate to permanent magnet assisted synchronous reluctance electric machines such as motors and generators. In the embodiments, the rotor of the electric machine is formed of a stack of laminations/sheets to reduce eddy currents. The sheet forms thus a cross section of the rotor.

Figure 1 shows, by way of an example, an embodiment of a rotor lamination/sheet 100. The circular rotor sheet 100 has a centrally positioned shaft hole 102 for receiving a cylindrical machine shaft.

To the rotor sheet there are formed four pole sectors of which the top-right sector is depicted by reference number 104. Each pole sector has a plurality of alternating flux paths 106A, 106B and flux barriers 108A, 108B. Flux paths are made of magnetically permeable material in order to provide a prop- agation path for a magnetic flux induced by a stator of the machine. The flux barriers are magnetically insulating, and may be air, for instance. In an embodiment, the rotor sheet is made of thin electric steel sheet to which the hole for machine shaft and cut-outs or openings for flux barriers have been perforated by punching or by laser cutting, for instance.

Figure 1 shows also a permanent magnet 1 10 for optimizing the flux propagation along the rotor sheet. In an embodiment, the permanent magnet is placed onto the flux path. In the embodiments, the permanent magnet is placed onto a flux path, which resides between two flux barriers. As shown in Figure 1 , there may be provided permanent magnets in at least substantially every flux path. There may also be several permanent magnets on a flux path.

Figure 1 illustrates one rotor sheet. The rotor of the electric machine is constructed by stacking a plurality of similar sheets and fixing them together in the same mutual rotary position to each other. When the rotor sheets have been attached together, the apertures in the rotor sheets form channels through the rotor. Such channels are thus formed by the flux barriers. Furthermore, the sheets may have openings for receiving the permanent magnets. When a plurality of rotor sheets are aligned together, those openings form channels for receiving permanent magnets therein. In an embodiment, the permanent magnet may have a length, which substantially equals the length of the rotor.

Figures 2a to 2d illustrate portions of flux paths and installation of permanent magnets therein. In the figures, the vertical direction is considered to be a longitudinal direction of the flux path and horizontal direction is considered to be the width of the flux path. The width of the flux path is denoted by WP in figures 2a to 2d.

In 2a, the permanent magnet 210A is arranged perpendicularly to the longitudinal direction of the flux path 206A, which is the principal direction of the magnetic flux. The magnetising direction of the permanent magnet is perpendicular to the width of the magnet. The permanent magnet may extend at least substantially across the flux path. In practical implementation, there may be formed an opening to the flux path for receiving the permanent magnet. Thus, in 2a there are small portions of permeable material in the flux path to the left and to the right of the permanent magnet 210A.

In an embodiment, the cross section of the permanent magnet is rectangular having a height and width. In the shown embodiments, the height of the permanent magnet is the dimension, which is at least substantially parallel to the longitudinal direction of the flux path, and width of the permanent magnet is the direction that is at least substantially parallel to the width of the flux path. The width of the permanent magnet is denoted by WM in figures 2a to 2d. The height is thus the smaller of the dimensions height and width of the magnet.

In 2a, the width of the permanent magnet substantially equals to the width of the flux path.

Figure 2b shows another embodiment of arranging a permanent magnet to the flux path. In this embodiment, the permanent magnet 210B is arranged to an oblique position with respect to the longitudinal direction of the flux path 206B. In this embodiment the width of the magnet (WM) exceeds the width of the flux path (WP). The width of the magnet is defined here to be the distance of the extremes of the magnet. In Figure 2b the width of the magnet equals thus the length of the top (or bottom) side of the magnet. The height is the distance between the top and bottom sides of the magnet.

In Figure 2c, the permanent magnet comprises two sections, which are arranged to angle compared to each other. The width of the permanent magnet may be calculated as the sum length of the magnet sections eg. upper edge, of the magnet. Also in this embodiment, it can be seen that the width of the permanent magnet exceeds the width of the flux path.

In Figure 2d the permanent magnet is arranged perpendicular to the longitudinal direction of the flux path. However, there is provided a widening in the flux path for receiving the permanent magnet. However, the flux path comprises narrower portions on both sides of the widening. Thus, in this embodi- ment, the width of the magnet (WM) exceeds the width of the flux path (WP) measured in its narrowest position. In Figures 2a to 2d, the cross section of permanent magnet has a cross section area, which is formed as a product of height and width of the permanent magnet. In the embodiments, the cross section area of the permanent magnet is at least equal to the cross section area of the flux path. The cross section area of the flux path is determined as a product of the width of the flux path multiplied with height of the permanent magnet. Preferably, the cross section area of the permanent magnet is greater than the cross section area of the flux path.

Figure 3 shows a two-pole rotor having a first pole sector 304A and a second pole sector 304B. The figure is simplified in that no shaft hole is illustrated. Figure 3 also shows a stator 314 surrounding the rotor. The air gap between the rotor and stator is depicted by the reference 312.

In the rotor, there are provided alternating flux paths 306A, 306B and flux barriers 308A, 308B. The flux path 306B, for instance, resides be- tween flux barriers 308A and 308B.

In a synchronous reluctance machine the direction of minimum reluctance is called as d-axis, and the direction of maximum reluctance is called as q-axis. In the two-pole construction of Figure 3, the d-axis is parallel to the flux paths 306A, 306B, and the q-axis is perpendicular to the d-axis. In this construction, the magnetizing direction of the permanent magnet(s) 310 is the d-axis direction highlighted by the bold double-headed arrow. The permanent magnet thus aims to intensify the magnetic flux induced by the stator to the flux path.

In the shown embodiment, the permanent magnet 310 extends through the rotor from the shaft to the outer perimeter of the rotor. The permanent magnet may be continuous and made of one piece. As shown, there may be bridges between the flux paths 306A, 306B, which bridges are of the same material as the flux paths. The bridges may be broader than the permanent magnet whereby the permanent magnet may be arranged between bridge sides, that is, interior of the bridge. Also in this embodiment, the permanent magnet is arranged substantially perpendicular to the longitudinal direction of the flux path and has a width greater than the flux path.

Figure 4 shows an embodiment of a four-pole rotor cross-section, that is, rotor sheet 400. Each pole thus forms about a quarter of the rotor sheet 400 cross-section. Each pole has four flux barriers and five flux paths. One flux path is denoted by reference 406 and an adjacent flux barrier by 408. It can be seen that each flux barrier and flux path have a curved form and are arranged between two points in a pole sector. The flux barrier 408 has two portions having a bridge between the portions. The bridge connects the flux path 406 to another flux path residing on the other side of the flux barrier 408.

The figure also shows the principal d-axis directions. The principal d-axes are thus substantially between the pole sectors, and the q-axis is in the middle of each pole. However, broadly defined, the d-axis refers to the direction of the flux path, and q-axis is a perpendicular direction to that. Thus, in the case of curved flux paths of Figure 4, the direction of the d-axis depends on the position on the flux path.

In Figure 4, there are permanent magnets placed onto the flux paths. In the flux paths closest to the shaft hole, the permanent magnets are substantially perpendicular to the flux path, whereby their magnetising direction is along the flux path being the d-axis direction.

In the adjacent flux paths when going towards the perimeter of the rotor sheet, the permanent magnets may be arranged slightly in an oblique position with respect to the flux path. Reference 410 shows one permanent magnet placed onto the flux path 406. The magnetising direction of permanent magnet 410 is thus only substantially parallel to the d-axis of the rotor sheet and the permanent magnet causes a slight divergence to the propagation of the magnetic flux. However, by positioning the permanent magnet in an oblique position, it is possible to a use a greater amount of magnetic material in guiding and intensifying the flux thereby providing advantages in electrical properties of the rotor.

In Figure 4 the permanent magnets are placed close to the bridges that connect flux paths over a flux barrier. The permanent magnet arranged this way prevents as much as possible the magnetic flux to escape from one flux path to another.

Figure 5 shows one embodiment of a four-pole rotor sheet. In this embodiment, there is one permanent magnet in each flux path except the one that is closest to the outer perimeter of the sheet. In this embodiment, the flux paths such as the paths 506A, 506B are not uniform parabolas as in Figure 1 but are non-uniform or irregular in their shape.

The flux paths may have alternating broader and narrower sections and there may be arranged a widening to the flux path 506A, 506B at the point where the permanent magnet 510A, 510B is placed to the flux path. The widening may comprise a receptacle/opening for receiving the permanent magnet. There may thus be small amounts of sheet material around the magnet for housing the magnet within the flux path. The receptacle may be arranged in a slightly tilted position with respect to the flux path. That is, the height being the shorter dimension of the rectangular opening is not parallel to the flux path longitudinal direction.

The permanent magnet may extend substantially across the flux path. The cross section (width) of the permanent magnet 510A, 510B is thus greater than the width of the flux path to which the permanent magnet is in- stalled, when this comparison is carried out with respect to the narrowest position of the flux path 506A, 506B in question. As can be seen in Figure 5, the flux path 506A is narrower to the right and to the left of the place where magnet 510B is installed.

Figure 5 also shows how the d-axis varies with respect to one flux path. The d-axis direction is illustrated by two-headed arrows in the flux paths on both sides of the permanent magnets 510A, 510B. It can be seen that the magnetising directions of the permanent magnets 510A, 510B are substantially parallel to the d-axis of the sheet being the longitudinal direction of the flux path. Also in this embodiment, the permanent magnet 510A is arranged onto the flux path 506A, which resides between two flux barriers 508A, 508B.

As can be seen, the flux barriers extend almost to the edge or perimeter of the sheet but, however, a small bridge of sheet material connecting neighbouring flux paths around the flux barrier remains between the end of the flux barrier and the outer edge of the sheet.

Figure 6 shows another embodiment of a quarter of a four pole rotor sheet. The permanent magnets 610A, 610B are arranged into an oblique position with respect to the respective flux paths 606A, 606B whereby the amount of magnetic material for guiding the magnetic flux can be maximized. In the embodiments, the permanent magnet may have been turned 0 upto even 90 degrees from the transverse position. Preferably the angle between the longitudinal direction of the flux path and the longitudinal direction of the magnet are about in 45 degrees angle to each other.

There may be provided a pocket or receptacle formed to the flux path for receiving the permanent magnet. In an embodiment, the receptacle is such that comprises sheet material around the magnet.

It can be seen that the flux paths closer to the sheet perimeter are shorter than the flux paths that are closer to the sheet middle such that the path 606A, for instance, is shorter than the path 606B that in interior of the path 606A. The paths may also become broader the closer the path is to the centre of the sheet. Correspondingly, the permanent magnets placed onto the rotor sheet may get smaller the closer it is on the edge of the sheet. For instance, magnet 610A is smaller than magnet 610B.

Figure 7 highlights the behaviour of the magnetic flux in the situation of Figure 6. It can be seen that the magnetic flux propagates substantially along the flux path until it reaches the permanent magnet. In the permanent magnets of Figure 6, the magnetic field is diverged about 60 degrees. However, the angle of the magnetising direction of the magnet can vary between 0 to 90 degrees, for instance, to the longitudinal direction of the flux path (d-axis). When the magnetic field has propagated through the permanent magnet, the magnetic flux returns to the d-axis direction and diverges substantially an equal angle to an opposite direction compared to the situation before the magnet.

Figure 8 show a further embodiment of how the permanent magnets can be arranged in a rotor sheet.

Figure 8 resembles the embodiment of Figure 4. However, the embodiment of Figure 8 differs from Figure 4 in that the flux barriers are continu- ous between the ends of the barrier. That is, the flux path is not divided into parts having bridges between the parts as in Figure 4. However, also in this embodiment, the permanent magnets are arranged substantially to the middle of the flux path in longitudinal direction, which corresponds to the principal q- axis of the pole.

It is also noted that flux paths closest to the shaft hole may have permanent magnets arranged such that the permanent magnet at least substantially blocks the flux path similarly as in the other flux paths. The magnetic flux is thus forced to propagate through the permanent magnet. Figure 8 shows the flux paths in the horizontal direction, where permanent magnets 810A and 810B have been arranged. The magnet 810A, for instance, is arranged such that the top left pole flux propagates from the left towards the top of the sheet, and bottom left pole flux propagates via magnet 810B from the left towards the bottom of the sheet.

Figure 9 shows another embodiment. The embodiment shows one sector of a four-pole rotor sheet. In this embodiment, there is provided a permanent magnet 910, which extends across several flux paths 906 and flux bar- riers 908. In an embodiment, the permanent magnet extends across all flux barriers. In an embodiment, the permanent magnet also extends across all flux paths that are between two flux barriers. The permanent magnet may also extend to the two flux paths that are closest to the centre of the sheet and to the flux path that is furthest away from the centre of the sheet. The permanent magnet may arranged in a non-radial direction to the rotor sheet. In this way the length of the permanent magnet can be maximised.

There is formed a corresponding recess to the rotor sheet for receiving the magnet. In an embodiment, the permanent magnet extends substantial- ly from a pole sector edge to a rotor sheet perimeter. Pole edge refers here to the line or area where the pole changes to another pole, that is, the flux paths have another polar point around which the magnetic field is formed.

Figure 10 shows still another embodiment. In this embodiment, the north-west pole has a permanent magnet 1010A that extends through several flux barriers and flux paths. The permanent magnet may extend substantially from the perimeter of the sheet to the shat opening 1002 of the sheet.

The pole to the south-west has no permanent magnets installed therein. The two poles on the right have principally mutually similar solutions where the permanent magnets form a chain of magnets 1010B, 1010C, which extend substantially from the shaft hole to the outer perimeter of the rotor. There are small spaces between the magnets. In this embodiment, each individual magnet in the chain of magnets is arranged to extend across the flux barrier but is so wide that it also extends to the neighbouring flux paths. The flux paths are wide enough to outline the permanent magnet from both sides.

Figure 1 1 shows one embodiment of utilizing, in different poles of the sheet, a combination of a chain of magnets 1010B, 1010C, a through magnet 1010A extending substantially from the sheet perimeter substantially to the shaft hole 1002, and no magnets at all. Other alternatives can be formed utilizing these three concepts. A rotor sheet may comprise one to three of these concepts. Each concept may be arranged to one or more poles. However, as the embodiments relate to permanent magnet assisted rotors, in at least one pole there is provided either a through magnet or a chain of magnets. To give some examples, there may be provided a four-pole sheet where two opposing poles have chain of magnets and the two other poles have no magnets at all. In another embodiment, two opposing poles may have through magnets and the other two opposing poles may have chains of magnets. Various alter- natives may be contemplated due to mechanical strength of the sheet. As the through magnets need corresponding large recesses for the magnets, the mechanical strength to the sheet may be required by providing chain of magnets or having magnet-free poles in some other poles.

Figure 1 1 shows also that at least some of the flux barriers may comprise flux barrier sections. The flux barrier sections 1008A, 1008B form one flux barrier. Between the flux barrier sections there is provided a bridge, which connects flux paths on both sides of the flux barrier. The bridge may be broader than the permanent magnet therein such that sufficient mechanical strength is achieved. It can be seen that there is sheet material on both sides of magnets in all the chain of magnets 1010B, 1010C and the through magnet 1010A.

Figure 1 1 shows another embodiment, where there are permanent magnets 1 1 10 which are parallel to the principal q-axis of each pole. In the fig- ure, the permanent magnets are shown to be continuous, which provides very good torque properties for the rotor but may be challenging in view of mechanical strength.

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