TECHNICAL FIELD

This invention relates to a rotor yoke for electric machine, in particular an electric rotating machine, such as a permanent magnet synchronous motor, and to an electric machine, in particular an electric rotating machine, such as a permanent magnet synchronous motor, comprising a rotor yoke according to the invention. Further, the present invention relates to a method for manufacturing a rotor yoke according to the invention.

TECHNICAL BACKGROUND

In general, electric rotating machines, such as permanent magnet synchronous motors, with surface mounted or interior permanent magnets are usually built up similarly. Especially when it comes to the main rotational part, the rotor yoke, according to state of the art, there are predominantly three main types of rotor yokes: namely the laminated rotor yoke; the solid rotor yoke; and the non-ferromagnetic rotor yoke. With respect to the energy efficiency and the power-to-weight ratio, the laminated rotor yoke is most often the preferred solution since the effects of eddy-current losses are reduced. However, due to the fact that laminated rotor yokes comprise multiple layers of ferromagnetic material, which are glued together, the laminated rotor yoke has significant drawbacks concerning resistance in particular against axial loads. Furthermore, the manufacturing process is tedious and thus expensive. Solid rotor yokes are easier and more cost-efficient to produce, but these yokes have drawbacks concerning energy conversion efficiency, because eddy-currents can be built upon the whole yoke surface which leads to energy losses and warming of the whole propulsion system.

From US20070090708A1 an electric rotating machine comprising a radial magnetic rotor yoke having a magnet attachment surface and a plurality of permanent magnets attached on the magnet attaching surface of the rotor yoke is known. The stable rotor yoke is cup-like shaped and has a central bearing passage for supporting the rotor yoke. The attachment surface is grooved in order to interrupt the loops of the eddy-currents. The grooves extend in the circumferential direction and are formed by a regular zig-zag-structure having a triangular cross section.

While being able to increase electric resistance against the eddy-currents, it has been found out that the grooved rotor yoke structure according to US20070090708A1 comes along with issues concerning the adhesion strength of the magnet and the rotor yoke due to the fact that the magnet attachment surface is reduced. The triangular-shaped grooves form triangular-shaped teeth in the attachment surface having a sharp tip providing virtually no flat surface for reliably fixing, in particular gluing, the magnets. In order to ensure a strong attachment of the magnets, US2007009078A1 proposes the use of metal or resin to fill the grooves and gap between the permanent magnet attachment surface and the magnet. However, this is not ideal due to several reasons, in particular because the cutting away of metal during the production of the rotor yoke and filling gaps again up with another metal consumes time and money. The added material to fill the gaps introduces new failure modes related to adhesion failure and corrosion protection, that need to be managed. The triangular shaped teeth reduce the cross-section of ferromagnetic material and is subject to saturation that reduces the overall machine torque output by several % in turn increasing the stator winding losses which reduces machine efficiency and causes additional warming.

Another drawback is the cup-like shape of the rotor yoke which is made from one single piece of ferromagnetic, such as iron. Due to its design, the cup-like shaped rotor yoke is heavy and has a high moment of inertia, which is exceedingly unwanted, since the electric rotating machine is used at variable-speeds, which means the yoke is accelerated and decelerated within a short time. For fast-changing operating modes a high moment of inertia is obstructive.

SUMMARY

It is an object of the present invention to overcome the above-mentioned disadvantages of the state of the art, in particular to provide an improved rotor yoke combining the benefits of a laminated rotor yoke while keeping the advantages of a solid rotor yoke.

This objective is solved by the subject matter of the independent claims. Advantageous embodiments are subject to the dependent claims and will be set out herein below. In general, the current invention relates to a cost-efficient manufacturing of a rotor yoke with high durability and a good tangential and radial stiffness as well as better stator inductance values by means of reducing of eddy-current losses in the rotor yoke itself, thereby improving the efficiency of the electric machine. The following aspects of the present invention do similarly apply to surface-mounted permanent magnet machines (SPM) as well as interior permanent magnet machines (IPM) for both radial gap out-runner and radial gap in-runner electric machines. The rotor yoke according to the following aspects of the present invention is preferably solid rather than laminated due to a decreased danger of delamination and moderate deflections of the rotor yoke, especially when the load is acting more on one axial side than on the other.

According to a first aspect of the present invention, a rotor yoke for an electric machine, in particular an electric rotating machine, such as permanent magnet synchronous motor is provided. Such an electric machine may comprise a plurality of magnets attached to the rotor yoke and a stator having an armature core and an armature coil or coils wound around the armature core. The rotor yoke is preferably solid and ring-shaped, thereby defining a delicate ring structure with less intrinsic stiffness, being light in weight and easy and cost-efficient to manufacture.

The rotor yoke comprises an outer circumferential surface defining an outer diameter DA and an inner circumferential surface defining an inner diameter Di of at least 60 %, in particular at least 70 %, 80 % or at least 90 %, of D _A . Preferably, Di is at most 99 %, in particular at most 98 % or 97 %, of D _A . One of the circumferential surfaces is being configured for attaching a plurality of magnets cooperating with a stator for constituting magnetic field and the other is being configured for supporting the rotor yoke for example to a rotor housing structure. Depending on what type of rotor yoke respectively electric machine is concerned, either the inner or the outer circumferential surface holds the plurality of magnets.

According to the first aspect of the present invention, a plurality of grooves extending in the circumferential direction of the rotor yoke and configured to interrupt eddy-currents is formed into the magnets attachments circumferential surface, i. e. that circumferential surface on which the plurality of magnets are to be attached to. The present invention therefore provides a solution to reduce the impact of eddy-currents in the rotor yoke while keeping the advantage of the thin and light weight rotor yoke. The grooves formed into the magnets attachments circumferential surface generally function as an eddy-current break, in particular by breaking bigger eddy-currents into smaller ones and thus augmenting the stator inductance behavior of the electric machine. The described thin and light structure of the rotor yoke according to invention is in particular suitable for in- wheel rotor application requirements due to a very low inertia.

In an example embodiment of the present invention, the grooves are uniformly distributed along the magnets attachments circumferential surface. In other words, the distance between adjacent grooves in the direction of the rotational axis of the rotor yoke is constant. Preferably, the grooves are identically formed in shape. Ideally, the shape of the groove may be rectangular with a very thin width in the direction of rotational axis. It has been found out that the saturation effect on maximum current essentially depends on the ratio between the width d of a tooth tip with respect to width k of the tooth ground, respectively distance between grounds of two adjacent grooves, in the following also referred to as pitch k. By increasing the width d and conclusively the ratio d/k, the magnetic saturation of the yoke may be reduced and thus the torque output of the electric machine may be increased.

In an example embodiment of the present invention, the width d is in the range of o mm to 5 mm, in particular between 0,1 mm and 4 mm, 0,2 mm and 3 mm, 0,3 mm and 2 mm or between 0,4 mm and 1 mm. That is because a conflict of objectives has been identified regarding the design of the grooves and respectively the teeth. While increasing the depth of the grooves in the direction of the wall thickness of the rotor yoke as well as decreasing the distance between two adjusted grooves k improves the amount of losses, those measures decrease the torque constant of the electric machine.

In a further example embodiment of the present invention, the inner circumferential surface delimitates a constant open passageway. In other words, the rotor yoke comprises a strict constant ring with a constant cross section. This design assures the thin and light weight structure of the rotor yoke, thereby assuring a low inertia and good applicability in particular for in-wheel motor applications.

According to a further aspect of the present invention which may be combined with the previous aspects and preferred embodiments of the present invention a rotor yoke for an electric machine, in particular, an electric rotating machine, such as a permanent magnet synchronous motor, is provided. Such an electric machine may comprise a plurality of magnets attached to the rotor yoke and a stator having an armature core and an armature coil or coils wound around the armature core. The rotor yoke is preferably solid and ring-shaped, thereby defining a delicate ring structure with less intrinsic stiffness, being light in weight and easy and cost-efficient to manufacture. The rotor yoke is ring-shaped and comprises an outer circumferential surface and an inner circumferential surface, one of the circumferential surfaces being configured for attaching a plurality of magnets and the other being configured for supporting the rotor yoke in particular to a housing structure of the rotor of the electric machine. Depending on what type of rotor yoke respectively electric machine is concerned, either the inner or the outer circumferential surface holds the plurality of magnets. The rotor yoke is preferably solid, thereby defining a delicate ring structure with less intrinsic stiffness, being light in weight and easy and costefficient to manufacture.

Further, a plurality of grooves extending in the circumferential direction and configured to interrupt eddy-currents in the rotor yoke is formed into the inner and/or the outer circumferential surface. Preferably, the grooves are formed into the magnets attachment circumferential surface.

According to the second aspect of the present invention, teeth are formed between adjacent grooves having a flat attachment surface for the magnets extending in the direction of the rotational axis of the rotor yoke. The formation of the grooves into the circumferential surface results in the formation of teeth, wherein each tooth is delimited by a pair of adjacent grooves. One advantage of the flat attachments surface is the improved adhesion of the magnets to the magnets attachment surface due to the increased attachment area. On the contrary with regard to the prior art design of the grooves for example according to US 2007/009 0708 Al where triangular-shaped grooves are shaped into the attachment surface resulting in triangular respectively cone-shaped teeth, which lead to a sharp tip of the teeth only providing align contact with the magnets, the present invention provides a solution for a better attachment of the magnets on rotor yoke attachment surface. For example, the grooves may also be triangularly shaped, however, the teeth maybe formed to have a truncated cone-shape each having a flat attachment surface for the magnets. Further, it has been found out that increasing the width d of the flat attachment surface reduces the magnetic saturation of the rotor yoke and thus increases the torque output of the electric machine.

According to an example embodiment, a width d of the flat attachment surface defined by the teeth is calculated and according to the following equation: d = [0,7 * (k - 2 tan(^)); 1,3 *

(k - 2 tan(^))] ; wherein hi relates to a depth of the grooves in the direction of the wall thickness of the rotor yoke, k relates to a distance between grounds of two adjacent grooves, respectively the pitch of the plurality of grooves, and a relates to an opening angle of the grooves. The equation gives a range for the value d providing suitable geometric structures in order to achieve the inventive effects. Preferably, the equation only represents cut with an a>o°. To fully determine the rectangular slot embodiment, the parameters hi (slot depth), d (tooth width) and k (pitch) need to be considered. The two latter define also the 4 ^th dimension which is dependent - a slot width is calculated according to the following equation for o=o°: s=k-d. In particular, the parameters maybe chosen according to the following pattern:

Using the sharpest viable angle a which particularly is limited by manufacturing viability. Preferably, the angle a maybe greater than 50 °. Optimizing h and k based on the desired performance, wherein increasing h improves the amount of losses but decreases the torque constant of the electric machine, decreasing h converges the performance towards the full yoke, decreasing k improves the amount of losses but decreases the torque constant of the electric machine wherein a low value of k increases the duration of the additional manufacturing process proportionally, and increasing k converges the performance towards the full yoke. In a typical optimization process, to obtain the optimal values of h and k systematically the output values of eddy-current losses and torque constant may be calculated for different combinations of those parameters obtaining a pareto frontier. An optimal combination may be selected from the pareto front based on the requirements prioritizing solutions with an increased value h rather than a decreased value k.

In an another example embodiment of the present invention, a width d of the flat attachment surface defined by the teeth is bigger than 0,1 mm and less than 5 mm, in particular between 0,1 mm and 4 mm, 0,2 mm and 3 mm, 0,3 mm and 2 mm or between 0,4 mm and 1 mm. Those values for the width of the flat attachment surface have been proven to optimize the conflict of objectives regarding the reduction of eddy-current losses while maintaining a high torque constant.

According to a further development of the present invention, the teeth are formed in a way to establishing an area contact with a magnet. By providing an area contact with the magnet, rather than a line contact as established in the prior art, the adhesion of the magnets may be improved while at the same time a reduced magnetic saturation of the rotor yoke and an increased torque output may be achieved due to the effects described above.

According to a further aspect of the present invention which may be combined with the previous aspects and preferred embodiments of the present invention a rotor yoke for an electric machine, in particular, an electric rotating machine, such as a permanent magnet synchronous motor, is provided. Such an electric machine may comprise a plurality of magnets attached to the rotor yoke and a stator having an armature core and an armature coil or coils wound around the armature core. The rotor yoke is preferably solid and ring-shaped, thereby defining a delicate ring structure with less intrinsic stiffness, being light in weight and easy and cost-efficient to manufacture.

The rotor yoke is ring-shaped and comprises an outer circumferential surface and an inner circumferential surface, one of the circumferential surfaces being configured for attaching a plurality of magnets and the other being configured for supporting the rotor yoke in particular to a housing structure of the rotor of the electric machine. Depending on what type of rotor yoke respectively electric machine is concerned, either the inner or the outer circumferential surface holds the plurality of magnets. The rotor yoke is preferably solid, thereby defining a delicate ring structure with less intrinsic stiffness, being light in weight and easy and costefficient to manufacture.

Further, a plurality of grooves extending in the circumferential direction and configured to interrupt eddy-currents in the rotor yoke is formed into the inner and/or the outer circumferential surface. Preferably, the grooves are formed into the magnets attachment circumferential surface.

According to the further aspect of the present invention, the grooves are formed by machining, in particular milling, eroding, rolling or wateijet cutting, or by laser cutting. The plurality of grooves may be manufactured simultaneously or subsequently. The manufacturing technologies are configured to guaranty a thin cut, thereby minimizing the groove size and maximizing the teeth size.

In a further example embodiment of the present invention, which may be confined with the previous aspects in preferred embodiments, the grooves are spirally formed into the magnets attachment circumferential surface of the rotor yoke. The grooves may be formed by a spiral cut into the rotor yoke, very similar to cutting a thread, i.e. in the case of an in-runner like cutting a thread onto a bolt, in the case of the out-runner like cutting a thread in a nut. The grooves may also extend parallel to the circumferential extension direction.

Spiral grooves vs. parallel grooves is essentially a machining issue. Spiral shaped grooves can be manufactured in one single cut, by the method presented below, and also by other possible methods. That means only one start and one end of the cut leading to faster manufacturing, longer tool life, improving the control and quality of the cut. It also means that the part is structurally more stable in terms of sheer stress caused by forces acting radially on the yoke - unevenly in the axial direction. In another example embodiment of the present invention, the direction of extension of the grooves comprises a circumferential component and an axial component orientated in the direction of the rotational axis of the rotor yoke. In particular, the grooves are spiral shaped. Alternatively, only one groove defining a spiral shaped may be formed into the magnets attachment circumferential surface.

In another example embodiment of the present invention, the grooves comprise a preferably constant polygonal cross section, wherein particularly the grooves comprise a rectangular or truncated cone-shape cross section.

In another example embodiment of the present invention, a depth h _± of the grooves is in the range of 1 % to 40 %, in particular in the range of 2,5 % to 25 % or in the range of 5 % to 20 %, with respect to radial wall thickness h ₂ of the rotor yoke. It has been found out that the groove depth is essentially related to the inverter PWM frequency and may be chosen independently of the rotor yoke wall thickness. Preferably, the target depth for the grooves is in the range of 0,2 mm to 3 mm, in particular in the range of 0,4 mm to 2 mm or in the range of 0,6 mm to 1 mm, for 16 kHz. Further, it has been found out that the higher frequency the lower needed groove depth to achieve the desired effect on the reduction of eddy-current losses and a high torque constant of the electric machine.

In another example embodiment of the present invention, the rotor yoke is solid and particular manufactured by a single solid preferably ferromagnetic piece. In terms of stiffness and stability with regard to load effects on the rotor yoke, the solid rotor yoke has been proven better, in particular because there is no danger of delamination.

According to a further aspect of the present invention, which may be combined with the previous aspects of the preferred embodiments of the present invention, an electrical machine, in particular an electric rotating machine, such as a permanent magnets synchronous motor is provided. The electric machine comprises a magnetic motor with a rotor yoke configured according to one of the preceding aspects or preferred embodiments, a stator, in particular made of laminated material, and a plurality of permanent magnets constitutes a magnetic field between the rotor and the stator.

According to a further aspect of the present invention, method for manufacturing a rotor yoke according to one of the previous aspects or previous preferred embodiments of the rotor yoke is provided. The grooves of the plurality of grooves maybe formed simultaneously into the rotor yoke surface or subsequently. According to a further aspect of the present invention which may be combined with the previous aspects and preferred embodiments of the invention, a method for introducing a plurality of grooves configured to interrupt eddy-currents into a rotor yoke in particular according to one of the preceding aspects and/or preceding preferred embodiments is provided.

The method comprises: rotatably mounting and rotating the rotor yoke, in particular on a helical thread with respect to its rotational axis; moving the rotating rotor yoke in the direction of rotational axis for a step k per rotation; and while rotation and axially moving the rotor yoke, machining, in particular milling, eroding, rolling or waterjet cutting, or laser cutting the grooves into the rotor yoke.

In a preferred embodiment of the present method, the method further, comprises calibrating laser power and the speed of rotation to obtain a repeatable depth of cut for the grooves.

It is noted that the method according to the invention can be defined such that it realizes the rotor yoke according to the described aspects of the invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description refers to the accompanying drawings. The same reference numbers may be used in different drawings to identify the same or similar elements. In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular structures, functionality, etc. in order to provide a thorough understanding of the various aspects of the claimed invention.

However, it will be apparent to those skilled in the art having the benefit of the present disclosure that the various aspects of the invention claimed may be practiced in other examples that depart from these specific details. In certain instances, descriptions of well-known devices and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

Fig. 1 shows a schematic top view of an exemplary embodiment of a rotor yoke with magnets according to the invention; Fig. 2 shows a schematic cross section view of the rotor yoke of figure 1;

Fig. 3 shows a schematic perspective view of the rotor yoke of figures 1 and 2;

Fig. 4 shows a perspective detailed in view of the rotor yoke of figures 1 to 3;

Fig. 5 shows a further detailed in cross-section view of the rotor yoke of figures 1 to 4;

Fig. 6 shows a further example embodiment of a rotor yoke according to the invention;

Fig. 7 shows a schematic graph illustrating the saturation effect on maximum torque; and

Fig. 8 shows a schematic cross-sectional view of an example embodiment of an electric machine according to the invention.

DETAILED DESCRIPTION

In figures 1 to 3, the general design of an example embodiment of a rotor yoke according to the present invention, which in general is indicated with the reference numeral 1, is illustrated. As particularly illustrated in figure 1, the rotor yoke is formed as a ring defining one outer circumferential surface 5 and an inner circumferential surface 7 which, depending on the type of electric machine, may either be configured for attachment of a plurality of magnets 3 or for supporting the rotor yoke 1 to a housing structure 9, shown in figure 8. For illustration purposes only, the enclosed drawings exemplarily show an out-runner type rotor yoke 1 were the outer circumferential surface 5 is configured for supporting the rotor yoke 1 and the inner circumferential surface 7 configured for attachment of a plurality of magnets 3 which, as shown in particular in figure 1, are equidistantly distributed in the circumferential direction on the inner circumferential surface 7, however, could in another embodiment be distributed in a non-equi distant way.

Further in figure 1, an essential design characteristic of the rotor yoke 1 according for the present invention is shown with regard to the thin and light weight structure of the rotor yoke 1. The radial wall thickness h ₂ (figure 5) of the rotor yoke 1 is veiy thin wherein particularly the inner diameter Di of the inner circumferential surface 7 is at least 60%, in figure 1 around 90%, of the outer diameter D _A of the outer circumferential surface 7. The rotational axis is indicated with the reference sign R in the drawings. Referring to figures 2 and 3, a further design characteristic of the rotor yoke according to the present invention is illustrated with regard to the thin wall structure of the rotor yoke 1 having a width L in the direction of the rotational axis R being significantly smaller than the diameter values Di respectively DA. Due to the shown thin and light weight structure of the rotor yoke 1 it has a very low inertia and is therefore preferably applicable for in- wheel motor application requirements.

In figures 4 to 6, a further essential aspect of the rotor 1 according to the present invention is shown by means of two example embodiments. The rotor yoke 1 is provided with a plurality of grooves 11 extending in the circumferential direction and configured to interrupt eddy-currents in the rotor yoke 1 itself formed into the magnets attachment circumferential surface, i.e. in the example embodiment of the figures the inner circumferential surface 7.

In the embodiment according figures 4 and 5, the grooves 11 are triangularly shaped having an opening angle a, a radial depth h _r in the direction of the wall thickness h ₂ of the rotor yoke 1 and a step or pitch k which relates to a distance between grounds of two adjacent grooves 11. Between two adjacent grooves 11 a teeth 13 is formed which defines a flat attachment surface 15 for attaching the magnets 3 wherein the width of the flat attachment surface 15 in the direction of the rotational axis R is indicated by the reference sign d. In the embodiment according to figures 4 and 5, the teeth 13 have a truncated cone-shape resulting in a significant plane or 2-dimensional flat attachment surface 15 for improved attaching of the magnets 3 increasing the torque output of the corresponding electric machine.

The wall thickness of the rotor yoke 1 is generally indicated with the reference sign h ₂ . As may be seen in figures 4 and 5, the axial end sections of the rotor yoke 1 are free of grooves 11. In other words, the first groove 11 considered from one of the axial end surfaces 17, 19 oppositely arranged with regard to rotational axis R are arranged in a distance, preferably being bigger than the distance d of the flat attachment surface of the teeth 13, from the corresponding axial end surface 17, 19. The magnets attachment surface extends over the entire width of the rotor yoke. There is no additional grooved surface around magnets as is the case in the prior art US’708.

In figure 6, a further example embodiment of a rotor yoke 1 according to the present invention is shown which in general is identical to the rotor yoke 1 according to the previous figures, however, distinguishes with regard to the shape of the plurality of grooves 11 and teeth 13. Different than the truncated cone-shaped teeth 13 according to the embodiment of figures 4 and 5, the teeth 13 according to figure 6 are rectangular shaped which is achieved by rectangular shaped grooves 11. In this configuration, the step or pitch k is similar preferably identical to width d of the flat attachment surface 15 of the teeth 14.

In figure 7, a graph illustrating the saturation effect on the maximum torque is shown wherein the abscissa relates to a ratio d/ and the ordinate relates to maximum torque in relative units. As may be seen, with decreasing d/ ratio, i.e. a decreasing value d, the maximum torque decreases as well.

In the schematic cross sectional view of a section of an electric machine according to the invention is shown in Figure 8. The electric machine comprises a rotor 1 according to the previous embodiments with magnets 3 attached thereto and a stator 21 comprising a stator housing 23 and a stator bladestack 25. The rotor yoke 1 is supported on a rotor housing structure 9, comprising separate housing components 27, 29, which may for example be formed by means of aluminium or plastic in particular by means of injection molding. The electric machine further comprises a bearing 31 for rotationally supporting the rotor 1 and for transmitting the torque generated by the electric machine to a torque consumer, such as for example a wheel of vehicle.

The features disclosed in the above description, the figures and the claims may be significant for the realization of the invention in its different embodiments individually as in any combination.

REFERENCE SIGN LIST

1 Rotor yoke

3 Magnet

5 Outer circumferential surface

7 Inner circumferential surface

9 Housing structure n Groove

13 Tooth

15 Flat attachment surface

17, 19 Axial end surface

21 Stator

23 Stator housing

25 Stator bladestack

27, 29 Housing part

31 Bearing

R Rotational axis a Opening angle k pitch d Width of the flat attachment surface of the teeth h-t Depth of the groove h ₂ Radial wall thickness of the rotor yoke

D _A Outer diameter

Di Inner diameter

L Width of the rotor yoke