WO2011120564A1

WO2011120564A1 - Rotor disc, rotor assembly, synchronous machine, and method of producing thereof

Info

Publication number: WO2011120564A1
Application number: PCT/EP2010/054238
Authority: WO
Inventors: Freddy Magnussen; Heinz Lendenmann; Juan Soler Abello; Jorge Sanz Remon; Reza Rajabi Moghaddam
Original assignee: Abb Research Ltd
Priority date: 2010-03-30
Filing date: 2010-03-30
Publication date: 2011-10-06

Abstract

The present invention relates to rotor assemblies (1) of synchronous machines (100) and manufacturing method for the rotor assemblies, in particular to rotor assemblies with press-fitting elements which provide improved bonding of rotor discs. Some of the press-fitting elements (10a-10d; 12a-12d; 52a-52d) are positioned close to the outer circumference of the rotor discs, i.e. at positions between 3/4 the radius and the radius, and some (11a-11d; 55a-55d) are positioned closer to the axis of rotation, i.e., at positions smaller than 3/4 of the radius.

Description

ROTOR DISC, ROTOR ASSEMBLY, SYNCHRONOUS MACHINE, AND METHOD

OF PRODUCING THEREOF

The present disclosure relates to a rotor disc, in particular, to a rotor disc with press- fitting elements which provide improved bonding of a plurality of rotor discs. The present d isclosu re relates to rotor assemblies of synchronous machines, synchronous machines and a manufacturing method for a rotor assembly. The present invention particularly relates to synchronous reluctance machines.

BACKGROUND OF THE INVENTION

Generally, synchronous reluctance machines include a stator with poly-phase windings forming a plurality of poles in a manner resembling the stator of an induction motor. The rotor assembly of the synchronous reluctance machine does normally not include electrical windings but has a number of poles in the form of portions with a higher magnetic permeability. The rotor assembly is formed as an anisotropic structure where each pole of the reluctance machine has a direction of minimum reluctance, a so-called „direct axis" or "d-axis", and a d irection of maxim um reluctance, a so-called "quadrature axis" or "q-axis".

When sinusoidal currents are applied to the poly-phase windings in the stator, an approximately sinusoidal magnetic flux waveform is produced in an air gap formed between the stator poles and an outer contour of the rotor assembly. The rotor will attempt to align its most magnetically permeable direction, the d-axis, to the direction of the peak flux by displacing its d-axis of minimum reluctance until alignment of the magnetic fields in the stator poles and rotor poles is obtained. The alignment process results in rotary motion of the rotor assembly at the same speed as the rotating magnetic field of the stator, i.e. at synchronous speed. The air gap flux generates a torque which can be conveyed to the external of the reluctance machine for example by a rotor shaft bonded to the rotor assembly and extending through a central axis thereof. The rotor assembly includes a stack of transversely oriented rotor discs. The rotor discs include alternating layers of magnetically permeable and magnetically non- permeable and/or electrically insulating layers, such as varnish, to prevent generation of an axially-oriented flow of eddy-currents between neighbouring rotor discs that would lower the efficiency of the synchronous reluctance machine.

The rotor assembly must, in many cases, exhibit high structural strength to allow rotation at h igh speeds and withstand elevated operation temperatures over extended periods of time. At the same time, the rotor assembly should exhibit low mag netic fl ux lea kage to improve power efficiency and power factor of the synchronous reluctance machine. The rotor assembly should furthermore be designed for cost-effective and flexible manufacturing processes.

US 6,675,460 discloses a method of making composite powder metal disks for a rotor assembly of a synchronous reluctance machine. The powder metal disks are aligned axially along a central shaft and attached to the central shaft by mating key and keyways structures. A keyway is arranged on an interior surface of each powder metal disc and the keyway is pressed against the key structure on the central shaft to lock the metal disc and the central shaft to each other.

US 5,296,773 discloses a rotor assembly with a plurality of axially-extending stator laminations that are bonded to each other and to a star-shaped core by high temperature cure epoxy. Metallic end caps are secured to the axially extending stator laminations to increase centrifugal strength of the rotor.

WO 2006/121225 discloses a rotor assembly of a synchronous reluctance motor. The rotor assembly includes a plurality of transversely oriented silicon steel sheets or laminations arranged in-between upper and lower ends plates. Each rotor lamination includes fixing points and rivet holes. A plurality of fixing points are inserted through the silicon steel laminations and fixing point fixing grooves formed on inwardly facing surfaces of the upper and lower end plates. The rivets extend through rivet holes in the silicon steel sheets and the upper and lower ends plates to couple the laminated core and the end plates to each other. Generally, high speed tests show that the laminations tend to bend outwards in the outer parts of the lamination, i.e. in regions further away from the centre of rotation, due to the centrifugal force so that, due to the fixing points of WO 2006/121225, the laminations are bent axially in operation of the motor. This may result in banana- shaped laminations in operation. In order to prevent this situation, it is necessary to provide strong end plates of the rotor disc stack that press the stack of laminations together.

There is a need for a rotor disc, the rotor assembly of which does not experience axial bending or the like in high speed operation. There is further a continued need in the art to provide alternative and/or improved methods of bonding rotor laminations in rotor assemblies. Adhesive agents such as epoxy resins are often highly toxic and require extensive precautionary measures in the manufacturing process to protect workers and the environment. These factors lead to increasing manufacturing time and costs. Furthermore, adhesively bonded rotor laminations may have difficulties in withstanding high temperature levels and mechanical stress for example imparted by centrifugal forces during high speed and/or high power operation. The fabrication and mounting of additional and complex mechanical structures such as cores, guide pins and rivets, end plates etc to bond rotor laminations together tend to increase manufacturing time and costs.

SUMMARY OF THE INVENTION

In view of the above, a rotor disc adapted for use in a rotor assembly for a synchronous machine is provided. The rotor disc has a centre and a radius, and includes at least one portion with a first magnetic permeability and at least one barrier with a second magnetic permeability. The at least one portion with the first magnetic permeability and the at least one barrier with the second magnetic permeability are positioned and shaped as to form at least one axis of minimum reluctance (d-axis) and at least one axis of maximum reluctance (q-axis). The rotor disc further includes at least one outer press-fitting element positioned off the axis of maximum reluctance of the rotor disc and at a radial distance to the centre of the rotor disc of at least 3/4 times the radius of the rotor disc. It further includes at least one inner press-fitting element positioned at a radial distance to the centre of the rotor disc of less than 3/4 times the radius of the rotor disc.

The positioning "off the at least one axis of maximum reluctance" is to be understood as including a distance to the at least one axis of maximum reluctance of at least 1 mm, more typically at least 5 mm, even more typically at least 15 mm. The term "radial" or "radial distance" as used herein shall refer to the radial direction of the rotor disc once it is in operation.

The magnetic permeability is the measure of the ability of a material to resist the formation of a magnetic field within itself. The reference to the magnetic permeability is to be understood as a reference to the magnetic permeability at a magnetic polarisation of 1 .5 Tesla.

The term "a portion with a first permeability" shall be understood such that this portion is made of a material that has substantially a first permeability. For instance, this portion could be a portion of steel, in particular of rolled steel. To those skilled in the art it is known that a small variation of the permeability might occur within the portion. This variation could be caused by the material itself. For instance, steel has a permeability that might vary in the range of a few percents within the steel portion. Further, the rotor disc production method might include a process step such as rolling which could also lead to a slightly varying permeability. For instance, the resulting permeability in the rolling direction could differ from the permeability perpendicular to the rolling direction at some percent.

The condition that the first magnetic permeability of the portion is larger than the second magnetic permeability of the barrier shall be understood within the present disclosure such that the average magnetic permeability of the portion is larger than the average magnetic permeability of the barrier.

The axes of maximum reluctance (q-axes) and minimum reluctance (d-axes) as used herein refer to the geometrical axes of the rotor disc that correspond to the magnetic property of the rotor disc once a magnetic flux is produced between the stator poles and the rotor assembly. In this situation, the rotor assembly will attempt to align its most magnetically permeable direction to the direction of the peak flux. This direction is called "axis of minimum reluctance (d-axis)" herein. In case of a rotor disc having portions with a higher magnetic permeability and barriers with a lower magnetic permeability thereinbetween, the axes can be geometrically assigned to the rotor disc layout by a person skilled in the art easily.

In embodiments, a rotor disc is provided with several axes of minimum reluctance such as two. The number of axes of minimum reluctance corresponds typically equal to the number of poles. Within the following description, the "at least one axis of minimum/maximum reluctance" is referred to as the "axis of minimum/maximum reluctance".

Typically, the axes of maximum reluctance are positioned in the middle between the axes of minimum reluctance. Their number is typically equal to the number of poles, such as four. Typically, rotor discs have an even number of q-axes and d-axes, for example 2, 4, 6, etc. q-axes and respectively 2, 4, 6, etc. d-axes.

Accord ing to aspects, at least one of the at least one press-fitting elements is positioned at a radial distance to the centre of the rotor disc of at least 3/4 times the radius of the rotor disc on or close to the at least one axis of minimum reluctance of the rotor disc.

The term "close to" in this context is to be understood as including slight deviations. In particular, when talking about the d-axis, it is typical that a central flux path of magnetically permeable material is positioned along the d-axis. For instance, the width of th is central flux path may be up to 20 mm, typically up to 1 5 mm at its smallest position. The path typically extends from the central opening of the rotor disc to the rotor disc circumference in a radial direction. Typically, the central flux path is positioned symmetrical to the d-axis with the d-axis correlating to the middle of the path. The term "close to the d-axis" is to be understood as including press-fitting elements positioned on the central flux path. Optionally, the term "close to the d-axis" is to be understood as including press-fitting elements that are between 0 mm and 10 mm, typically between 0 mm and 5 mm away from the d-axis. According to a further aspect, a rotor assembly for use in a synchronous machine is provided. The rotor assembly includes a plurality of rotor discs according to the present d isclosure . In particular, the rotor assembly may be end plate free. Alternatively or additionally, the rotor assembly may be free of any support structure such as guide pins, rivets, bolts or the like. Generally, the rotor shaft shall not be understood as support structure herein.

According to a further aspect, a rotor assembly for use in a synchronous machine is provided. The rotor assembly comprises a plurality of rotor discs with each rotor disc including at least one portion having a first magnetic permeability, at least one barrier having a second magnetic permeability, and at least one press-fitting element for bonding the rotor discs together. The rotor assembly is end plate free.

According to a further aspect, a synchronous machine is provided. The synchronous machine includes a stator adapted to be connected to a power supply; and a rotor assembly according to the present disclosure.

The synchronous mach ine may be a synchronous reluctance mach ine. The synchronous machine may be adapted to be connected to or may even be provided with a drive of the machine.

Typically, the second magnetic permeabil ity is lower than the first magnetic permeability. The at least one portion having the first magnetic permeability optionally includes highly magnetically permeable material such as a ferromagnetic alloy or ferromagnetic metal for example silicon iron motor steel.

The at least one barrier having the lower magnetic permeability may include magnetically non-permeable materials such as diamagnetic, paramagnetic materials, or combinations thereof. In particular, the magnetically non-permeable material may be air. The barriers having a lower magnetic permeability are optionally formed as apertures or cut-outs in the rotor disc material. This allows a pattern of the portions with a h igher magnetic permeabil ity and barriers having a lower magnetic permeability to be formed on each rotor disc. Forming can be done, e.g., by punching or stamping operations on a metallic carrier comprising the magnetically permeable material.

Hence, according to the present disclosure, it is possible to create a rotor assembly even in the absence of rotor shafts, bolts, guide pins or similar support structures. Since the press-fitting elements may include, or be completely formed in, rotor disc material , such as the material with a higher magnetic permeability of a rotor lamination or disc, costly and time-consuming support elements or structures can be avoided in the manufacturing process. This simplifies the manufacturing process of the rotor assembly and makes it considerably more flexible.

Further, in view of the resulting mechanical strength of the rotor assembly, it is possible to provide an endplate-free rotor assembly. In other words, the rotor assembly consists of the rotor discs and the shaft only when assembled. According to the present disclosure, the term "endplate" shall refer to one or more extra discs which are mounted to the axial ends of the rotor assembly in order to stick the stack of rotor discs together. "Endplates" as understood herein differ in shape from the discs positioned between the endplates, such as the rotor discs described herein. Typically, the thickness of the endplates is larger than the thickness of the rotor discs. Endplates do typically not contribute to the electromagnetic properties of the rotor assembly. Endplates normally do not have designated axes of maximum and minimum reluctance.

According to embodiments, the rotor disc is formed as a single unitary element fabricated by punching or stamping a metallic carrier. The metallic carrier may include a strip of ferromagnetic material or alloy with relative permeability, μτ, larger than 550, according to embodiments larger than 2000 or even larger than 5000, such as about 7000 or more.

Typically, the rotor disc is provided with a central opening. Once the rotor discs are assembled to form a rotor assembly, in most embodiments, a rotor shaft is inserted through the central opening. The rotor shaft may be magnetically non-permeable or insulating.

The rotor disc includes at least one press-fitting element positioned off the axes of maximum reluctance, i.e., off the quadrature axes ("q-axes"). The press-fitting element is positioned with a radial distance to the centre of the rotor disc of at least 3/4 times the radius of the rotor disc. The rotor disc additionally includes at least one press-fitting element positioned at a radial distance to the centre of the rotor disc of less than 3/4 times the radius of the rotor disc. Typically the additional at least one press-fitting element is positioned at a radial distance to the centre of the rotor disc of less than 1/2 times the radius of the rotor disc.

The radius of the rotor disc shall be understood as the distance extending from the centre of the disc to the circumference of the disc. The centre of the disc is the rotational axis once the rotor disc is assembled in a rotor assembly.

In case the disc has cut-outs on the circumference or is not completely circular, the rad ius is considered as the distance between the centre of the disc to the circumferential point of the disc which is positioned furthest away from the centre. Hence, in case of rotor discs that do not have a perfectly circular shape, the "radius" as used here in is to be u nderstood as the maximum radius. According to embodiments, the radius of the rotor disc is at least 20 mm. In most embodiments, the radius is smaller than 300 mm.

According to embodiments, the press-fitting elements positioned at a distance of larger than 3/4 of the radius (the outer press-fitting elements) and the press-fitting elements positioned at a distance of smaller than 3/4 of the radius (the inner press- fitting elements) have a distance to each other that is at least 1 /5 of the radius, optionally at least 1/3 of the radius. In absolute terms, the distance between the inner press-fitting elements and the outer press-fitting elements is larger than 1 0 mm according to embodiments.

Generally, a "press-fitting element" as used herein is understood as a limited area on the rotor d isc that has been treated such as to bu ild a deformation . In most embodiments, the press-fitting element includes a protrusion out of the rotor disc and an indentation into the rotor disc. Typically, the protrusion and the indentation are located at opposite surfaces, i.e., sides of the rotor disc. For instance, the protrusion and indentation may be generated by a punch mark. The press-fitting element as understood herein is typically fully embedded in the rotor disc. In most embodiments, the press-fitting elements are made of the rotor disc material. They typically represent a deformation of a magnetically permeable surface portion of the rotor disc. Within the present disclosure, such a combination of a protrusion and an oppositely located indentation may also be called "knob".

The press-fitting elements act as attachment elements that bond the radially aligned rotor discs to each other once the rotor assembly is produced. The press-fitting elements are normally configured to mate to corresponding press-fitting elements arranged on neighbouring rotor discs. According to embodiments, all rotor discs used in a rotor assembly are identical.

According to typical embodiments, the rotor disc includes a multitude of press-fitting elements such as at least two, at least four, at least 8, or at least 12. At least one of the press-fitting elements, typically as many as the number of poles present in the rotor disc, is positioned with a distance to the centre of the rotor disc of at least 3/4 of the radius of the rotor disc.

According to embodiments, at least four, optionally precisely four of the press-fitting elements have a distance to the centre of the rotor disc of at least 3/4 of the radius of the rotor disc and are positioned on the axis of minimum reluctance. According to embodiments that may be combined therewith, at least four, optionally precisely four of the press-fitting elements have a distance to the centre of the rotor disc of less than 3/4 of the radius of the rotor d isc. They may be positioned on the axis of minimum reluctance.

The press-fitting element is placed at aligned locations on opposing or facing plane surfaces of the rotor d isc. This configuration of press-fitting element allows neighbouring rotor discs to be bonded to each other by application of pressure once the respective press-fitting elements of the neighbouring rotor discs have been appropriately oriented and aligned along a rotational axis of the rotor assembly. In th is manner, it is possible to fabricate a coherent and/or self-supporting rotor assembly. In particular, it is possible to produce a coherent and/or self-supporting rotor assembly that can be manipulated without cores or support structures such as, guide pins, rivets, nuts and bolts, epoxy glue bolts, guide pins etc. In particular, it is possible to produce a rotor assembly without endplates, i.e., an endplate-free rotor assembly.

In the art it is assumed that the positioning of press-fitting elements is advantageous where there exists some portions with a higher magnetic permeability of the rotor disc that conduct magnetic flux in an unwanted direction such as along the q-axis. Hence, it is assumed that the knobs should be arranged to increase magnetic reluctance of the rotor d isc along the q-axis thereof. Therefore, according to rotor assemblies known to the applicant, the press-fitting elements are positioned along the q-axis.

In particular, the conduction of magnetic flux in an unwanted direction may take place where the rotor disc includes one or more radial bridge portions. Generally, a bridge portion is understood herein as a magnetically conductive path interconnecting portions with a higher magnetic permeability thereby crossing barriers having a lower magnetic permeability. The effect of the typically radially oriented bridge portion is detrimental to the magnetic properties of the rotor disc because it tends to conduct magnetic flux along a quadrature axis (q-axis) of the rotor disc. On the other hand, the presence of the one or more radial bridge portions increases the mechanical strength of the rotor disc structure.

According to embodiments known to the applicant, the magnetic properties of the rotor disc have been improved by punching the radial bridge portions along the q-axis of the rotor disc so as to form press-fitting elements. These punch marks formed by the punching process create localized mechanical stress in deformed areas of the radial bridge portion or portions in question to deteriorate their magnetically conductive properties, i.e. increasing the reluctance of the radial bridge portion or portions.

According to the present disclosure, however, press-fitting elements are provided off the axis of maximum reluctance, such as on or close to the axis of m in imum reluctance of the rotor disc. In other words, at least one press-fitting element is provided at a position that has been thought to be detrimental to the property of the rotor d isc. However, long-term experiments have shown that the benefits in mechanical strength outweigh the disadvantages in the electromagnetic property of the disc. In particular, providing the press-fitting elements on or close to the d-axis means that the mechanical stress put upon the rotor disc is kept small.

Further, as stated previously, is has been found out through experimentation that the rotor discs tend to bend outwards in the outer parts of the rotor discs due to the centrifugal force if the press-fitting elements are positioned only close to the centre, such as within three quarters of the radius of the rotor disc.

However, by positioning at least one of the press-fitting elements off the q-axis at a distance larger than 3/4 of the radius of the rotor disc and at least one of the press- fitting elements at a distance smaller than 3/4 of the radius of the rotor disc as described herein, the rotor discs do not become banana-shaped at high rotational speeds. According to typical embodiments, the press-fitting elements located at a distance to the centre of smaller than 3/4 of the radius, are positioned off the q-axis as well, optionally on or close to the d-axis.

This allows the controlled operation of the rotor assembly. According to embodiments of a rotor assembly, it is even possible to dispense with an endplate. This, in turn, simplifies the production process, the material needed , and the overall rotor assembly production costs.

According to the present disclosure, the synchronous machine may particularly be a synchronous reluctance machine. It is further possible that the synchronous machine as described herein is a synchronous permanent magnet machine.

Further, the term "machine" typically embraces both motor and generator. The machine as described may operate converting electrical energy into kinetic energy, i .e. , as a motor. The machine as described may also operate converting kinetic energy into electrical energy, i.e., as generator. According to typical embodiments, the machine as described is suitable and/or adapted for operating both as motor and generator. According to other embodiments, the machine is suitable and/or adapted to work either as motor or generator. According to a further aspect, a method of manufacturing a rotor assembly for a synchronous reluctance machine is provided. The manufacturing method includes producing a metallic carrier to produce a plurality of rotor discs according to the present disclosure. Producing particularly includes forming press-fitting elements on each rotor disc by punching a portion with a higher magnetic permeability of the rotor disc to simultaneously create an indentation and a projection on respective opposing plane surfaces of the rotor disc. The method further includes radially aligning and stacking the plurality of rotor discs, and applying a predetermined compressive force to end portions of the plurality of rotor discs to bring the plurality of rotor discs into mutual abutment, thereby bonding the plurality of rotor discs to each other.

The rotor assembly manufacturing may include forming press-fitting elements on each rotor disc by punching a portion with a higher magnetic permeability of the rotor disc. Thereby, an indentation and a projection on respective opposing surfaces of the rotor disc are provided.

According to embodiments, the rotor assembly manufacture includes aligning the plurality of rotor discs along a rotational axis to align respective indentations and projections of neighbouring rotor discs and pressing the indentations and the projections of the neighbouring rotor discs. Thereby, the plurality of rotor discs are bonded to each other.

Further aspects, advantages and features of the present invention are apparent from the dependent claims, the description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures wherein:

Fig. 1 is a surface view of a rotor disc according to embodiments described herein. Fig. 2 is a surface view of a rotor disc according to embodiments described herein. Fig. 3 is a surface view of a rotor disc according to embodiments described herein. Fig. 4 is a surface view of a rotor disc according to embodiments described herein. Fig. 5 is a surface view of a rotor disc according to embodiments described herein. Fig. 6 is a surface view of a rotor disc according to embodiments described herein. Fig. 7 is a surface view of a rotor disc according to embodiments described herein. Fig. 8 is a surface view of a rotor disc according to embodiments described herein. Fig . 9 is a three dimensional perspective view of a rotor assembly according to embodiments described herein.

Fig. 10 is a cross-sectional view of a synchronous reluctance motor perpendicular to the axis of rotation according to embodiments herein.

Notably, the surface views of the rotor disc embodiments can also be understood as cross-sectional views of embodiments of the rotor assembly.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. The figures are schematical. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. It is intended that the present disclosure includes such modifications and variations.

Fig . 1 shows a rotor disc 2 adapted to be part of a rotor assembly. As will be discussed in more detail below, the rotor assembly includes a plurality of rotor discs, which are similar or identical to the rotor disc 2, that are stacked and attached to each other.

The rotor disc 2 includes four substantially identical poles distributed evenly around a rotor disc surface. Each pole covers a portion of 90° of the rotor disc. A first pole extends across a first quarter of the rotor disc 2 and is limited by the dashed line "P1 " and "P2". Each pole includes five portions with a higher magnetic permeability 3a to 3e separated by four equally spaced and intermittently positioned barriers having a lower magnetic permeability 4a to 4d. Each of the five portions with the higher magnetic permeability 3a - 3e has an arm-shaped geometry with each arm extending between the first and second predetermined angular sections or coordinates along a circumferential edge 5 of the rotor disc 2. The circumferential edge portion 5 of the rotor disc 2 generally serves to increase a saliency ratio and power factor of the rotor disc 2.

The rotor disc is adapted to pivot around its centre A once assembled with further discs to form a rotor assembly. For illustration, the rotational direction of the rotor disc in operation of a rotor assembly is indicated by arrow RA in Fig. 1 . Generally, there is no preferred direction, i.e. the rotor disc is shaped and formed symmetrical in the sense that there is no predetermined rotational direction (i.e., clock-wise or counterclockwise). The plane of the rotor disc, i.e. the plane in which the disc is adapted to rotate, is perpendicular to the central axis and shall be called "rotation plane" herein.

Figs. 1 -8 further illustrate the d-axes lying on the dashed symmetry lines P1 and P2, and the q-axes lying on the dashed symmetry lines S3 and S4. For example, in Figs. 1 to 8, two d-axes are lying on each dashed symmetry line P1 and P2 extending respectively in opposite directions from the centre A, and two q-axes are lying on dashed symmetry lines S3 and S4 extending respectively in opposite directions from the centre A. In case of additional permanent magnets, the magnetic d-axes and q- axes might switch. However, within the present disclosure it is referred to the "d-axis" and "q-axis" from a geometrical viewpoint. In other words, the "d-axis" and "q-axis" as understood herein shall be understood as the axis of the minimum reluctance (d-axis) and the axis of maximum reluctance (q-axis) of the rotor disc without consideration of the permanent magnets.

Typically, in embodiments disclosed herein, the directions of the d-axes and q-axes towards the centre or from the centre away are not considered. For example, a first d-axis of two adjacent d-axes has typically a direction towards the centre A whereas the second d-axis has a direction radially from the centre A away.

While the rotor disc 2 includes a total of four poles in the present embodiment illustrated in Fig. 1 (and thus four d-axes and four q-axes), other embodiments may include a smaller or larger number of poles such as 2 poles or 6 poles. It is also possible to have more than 6 poles, for instance up to 20 without departing from the scope of the present disclosure.

The number of poles generally refers to the number of substantially identical patterns of portions with a higher magnetic permeability and intermittent barriers having a lower magnetic permeability. These identical patterns are typically positioned axially symmetrical along the rotational axis of the rotor disc. "Substantially" within this context is to be understood as comprising deviations due to production tolerances.

In most embodiments, the number of poles, which is typically equally to the number of d-axes and q-axes, respectively, refers to the number of possible axes of symmetry in the rotation plane that can be thoug ht of. For instance, in the embodiments shown in Figs. 1 -8, the two dashed lines referred to as P1 and P2 form two axes of symmetry. Two further axes of symmetry are illustrated by the dashed lines S3 and S4.

The rotor disc 2 includes a central opening 7 adapted for receiving a rotor shaft. According to embodiments described herein, the shape of the central opening is generally circular and may have one or more additional recesses, such as key-holes, or salients. In many cases, the number of additional recesses or salients is equal to the number of poles of the rotor disc.

The rotor d isc typically includes a highly magnetically permeable material, for example, ferromagnetic metals or alloys. According to embodiments, the relative permeability, μτ, is larger than 550, or even more preferably larger than 800, such as larger than 1000. Normally, the relative permeability is below 2000.

The rotor disc material may particularly be provided in form of a metallic carrier of silicon iron motor steel. This material is relatively inexpensive and can be provided as carrier sheets or elongated strips of suitable dimensions for punching or stamping operations. Suitable rotor disc dimensions typically have a diameter of larger than 5 cm and/or smaller than 30cm or 50cm. The thickness of the rotor disc is normally between 0.1 mm and 5 mm. In typical embodiments, the thickness is between 0.27mm and 0.65 mm such as 0.50mm. The thickness is a compromise between a rotor disc as thin as possible in order to reduce the eddy-current and the production costs. The use of thinner rotor plates involves higher costs per weight of the material, a higher number of shaping and punching processes, a higher number of assembling steps, etc. Further, in view of the press-fitting elements of the present disclosure, arbitrary small th icknesses are not possible due to the mechanical strength requirements.

Depending on the needed performance, the given size and shape requirements for the rotor may vary depending on the requirements imposed by a particular synchronous reluctance machine.

According to typical embodiments, the entire rotor disc is formed as a single unitary element. It may be fabricated in a rapid cost-effective manner by punching or stamping strips of silicon iron motor steel with, for instance, relative permeability, μτ, of below 7000 to form a plurality of substantially identical rotor discs with highly portions with a higher magnetic permeability.

According to the embodiments shown in the Figures, the rotor disc includes four bridge portions 6a - 6d. A radially extending bridge portion 6a, or radial bridge portion, is interconnecting the fourth and fifth portions with the higher magnetic permeability 3d and 3e. Without departing from the present disclosure, the bridge segments can be omitted, or further bridge segments, such as between the further portions with the higher magnetic permeability (such as between reference numbers 3a, 3b, 3c, and 3d) may be added.

Fig. 1 shows four press-fitting elements 10a-10d positioned off the q-axes, in more detail on the four d-axes distant to the rotor disc centre and close to the rotor disc's edge. The press-fitting elements 10a-10d are formed as punch marks arranged on the portions with a higher magnetic permeability of each rotor disc. The press-fitting elements are shown as having a rectangular shape.

According to embodiments, the press-fitting elements described herein may be of any elongated shape such as rectangular or elliptic. The term "rectangular" particularly includes a non-quadratic shape with one side being larger than the other side of the press-fitting element. In the event of a rectangular shape, it is typical that the corners are rounded.

In other embodiments, the press-fitting elements described herein may have an elongated shape. For example, in some embodiments at least one, typically at least the half, of the press-fitting elements may have a shape formed like a tilde (e.g. "~").

Fig. 1 illustrates an embodiment having all together eight press-fitting elements with all the press-fitting elements being positioned on the d-axis. Fig. 3 shows the press- fitting elements 10a-10d, and the press-fitting elements 1 1 a-1 1 d . Some of the press- fitting elements 10a-10d are positioned at a distance of larger than 3/4 of the rotor disc's radius, some of the press-fitting elements 1 1 a-1 1 d are positioned at a distance of smaller than 3/4 of the rotor disc's radius. In the embodiment illustrated with respect to Fig. 1 , half of the press-fitting elements are positioned at a distance of about half the radius of the rotor disc.

The larger number of press-fitting elements increases the mechanical stability of the rotor assembly without at the same time deteriorating the electromagnetic properties of the disc unacceptably. It has been found out that the existence of the press-fitting elements at a distance of more than three quarters away from the disc's centre and the additional positioning of press-fitting elements closer to the disc's centre than 3/4 of the rad ius, avoids axial bend ing of the rotor discs in operation of the rotor assembly. The embodiments illustrated guarantee that the rotor discs are not bent in operation but remain straight. This, in turn, allows an endplate-free rotor assembly with these discs.

The press-fitting elements 10a-10d positioned at a radial distance to the centre of more than 3/4 of the radius shall also be called "outer press-fitting elements" herein. The press-fitting elements 1 1 a-1 1 d positioned at a radial distance to the centre of less than 3/4 of the radius shall also be called "inner press-fitting elements" herein. The distance between the inner press-fitting elements and the centre of the rotor disc may be at least 20 mm. The distance between the outer press-fitting elements and the inner press-fitting elements is typically at least half the radius.

According to embodiments, the distance between the inner press-fitting elements to the central opening is at least 1/8 of the radius of the rotor disc, typically at least 1/6 of the radius of the rotor disc. For instance, if the rotor discs are shrink-fitted on a rotor shaft to form the rotor assembly, the shaft axis has a similar effect on the rotor discs as the press-fitting elements. That is, the rotor discs are fixedly joined together at the rotor shaft by shrink-fitting them to the rotor shaft. It is sufficient that the inner press-fitting elements have a certain distance to the disc's central opening in such a case.

During assembling, the press-fitting elements described, e.g., punch marks, are brought into frictional engagement with a neighbouring rotor disc.

According to embodiments, the punch marks described herein include an upwardly projecting protrusion on a first surface of the rotor disc and an oppositely-located indentation on an opposing surface of the rotor disc. For rotor assembly, this allows aligned pairs of protrusions and indentations on neighbouring rotor discs to be forced into frictional engagement and bond these to each other by press-fitting. This press- fitting attachment techn ique of neighbouring rotor d iscs of the coherent rotor assembly may accordingly rely on the press-fitting elements that are formed entirely within the rotor disc material.

In particular, it is possible to form a rotor assembly by rotor discs as described herein only. In other words, according to embodiments, the rotor assembly does not have end plates located at the opposite ends of the rotor. Fu rther, accord ing to embodiments, the rotor assembly is additionally or alternatively free of any support structure such as guide pins, rivets, nuts, bolts, epoxy glue bolts, or the like that extend through the plurality of rotor discs.

Accord ing to typical embod iments, the sizes of the press-fitting elements as described herein are in the range of between 0.1 mm times 2.5 mm and 2.0 mm times 6.0 mm. Even more typically, the size of the press-fitting elements is in the range of between 0.5 mm times 2.0 mm and 2.5 mm times 4.5 mm such as 1 .0 mm times 4.0 mm. These sizes have proved to be a fair compromise between increasing the mechanical stability of the rotor assembly and not deteriorating the magnetic properties in an unacceptable manner. According to embodiments, which can be combined with other embodiments, all or part of the press-fitting elements as used herein are of elliptic or circular shape. Typical diameters or circularly shaped press-fitting elements range between 0.2 mm and 3 mm, more typically between 0.5 mm and 1 .5 mm. Generally, the circular press- fitting elements allow reduced rigidity per press-fitting element for assembly of the rotor assembly. However, their stability impact on the rotor disc is smaller than in case of rectangular or elliptic press-fitting elements so that more press-fitting elements can be provided without deteriorating the mechanical properties of the rotor disc.

According to embodiments, the inner press-fitting elements have an elongated shape, and the outer press-fitting elements are of circular shape. The elongated press-fitting elements allow for a high stability in the assembly of the rotor assembly. However, their impact on the electromagnetic properties is rather high. If they are positioned within a specific radial distance to the centre, such as within 3/4 of the radius, they are typically embedded in a sufficiently large amount of material with the higher permeability which reduces this effect.

According to embodiments combinable therewith, the outer press-fitting elements are of circular shape. The contribution to the mechanical strength of the rotor assembly is smaller in comparison to e.g. rectangular or elliptic shapes, but the impact on the electromagnetic properties is smaller as well. In those regions where the press-fitting elements are positioned in regions close to barriers having a small permeability, circular press-fitting elements may be chosen.

Such an arrangement is exemplarily shown in Fig. 2. Therein, the outer press-fitting elements 10a-10d positioned on the d-axis are of circular shape. The inner press- fitting elements 1 1 a-1 1 d positioned on the d-axis are of rectangular shape.

Although Figs. 1 and 2 show only two press-fitting elements 10a-10d per direct axis, it is possible that one, three or even more press-fitting elements are positioned on the direct axis at a distance of at least three quarters of the radius of the disc. Generally, and not limited to the embodiments depicted in the Figures, a multitude of press- fitting elements may be positioned adjacent to each other, such as with a distance to each other of below 5 mm, optionally even below 2 mm.

The number of press-fitting elements is typically a multiple of the number of poles of the rotor disc. According to the embodiments shown in the figures, the number can be eight or more. Further embodiments may have twelve or even more press-fitting elements. In other embodiments, such as those with only two poles, the number of press-fitting elements may be at least four.

Although circular press-fitting elements are illustrated only in Fig . 2, it shal l be emphasized that the elongated press-fitting elements shown in other embodiments of the present disclosure can generally be replaced by other shapes such as the circular press-fitting elements or by elliptic press-fitting elements. The present disclosure particularly includes the combination of roundly shaped press-fitting elements and press-fitting elements with an elongated shape such as rectangular press-fitting elements.

The depth of the press-fitting elements, i.e. their dimension perpendicular to the rotary plane of the rotor disc, is typically in the range of between 0.1 mm and 1 mm, more typically between 0.4 mm and 0.8 mm. The depth of the press-fitting elements is to be understood as the depth of the indentation formed in the press-fitting element. According to typical embodiments, this indentation depth refers also to the height of the protrusion of the press-fitting element.

More generally, the depth may depend on the thickness of the rotor disc. According to embodiments, the depth is at least 30% or even 40% of the rotor disc thickness. The depth is optionally even larger than 100%, 130% and may reach values of up to 200% of the rotor disc thickness. The larger the depth, the better the rotor discs are fixed together by the press-fitting elements. However, at the same time, larger depths decrease the mechanical stability of the rotor disc, and, in the event that the press- fitting elements are positioned on the d-axis, deteriorate the electromagnetic properties of the rotor disc.

Typically, non-quadratic elongated press-fitting elements such as rectangular or elliptic press-fitting elements are oriented towards the flux direction within the rotor disc at the press-fitting element position. That is, the longer side of the non-quadratic elongated press-fitting elements is oriented in parallel to the flux direction of the rotor disc once a magnetic field is applied from the outside of the rotor disc. Examples for the orientation of the press-fitting elements are given in the following.

The orientation of the press-fitting elements positioned on or close to the d-axis is typically so that the larger extension is oriented rad ial ly. In other words, for electromagnetic reasons, the orientation of the larger side of the elongated press- fitting elements is typically parallel to the d-axis flux in order to interfere as little as possible. For instance, if the press-fitting elements are of rectangular or elliptic shape, the longer side, which is the so-called "major axis" in the case of the ellipse, is oriented in a radial direction of the disc.

In addition or alternatively to the press-fitting elements positioned on or close to the d-axis, it is possible to provide press-fitting elements on or close to the axis of maximum reluctance, i.e., the q-axis. The position on the q-axis further enhances the magnetically insulating property of the q-axis. At the same time, the rotor assembly benefits from the increased stability in the assembled rotor assembly due to the press-fitting elements.

The term "close to the q-axis" is to be understood as including press-fitting elements that are positioned between 0 mm and 10 mm, typically between 0 mm and 5 mm away from the q-axis.

Figs. 3 to 5 illustrate q-axis press-fitting elements.

According to embodiments, the rotor disc has four poles and twelve press-fitting elements. Eight of the press-fitting elements are positioned on or close to the d-axis, four of the press-fitting elements are positioned on or close to the q-axis. Generally, four of the press-fitting elements may have a distance to the centre of the rotor disc of at least 3/4 of the radius to the centre of the rotor disc and are positioned on the axis of minimum reluctance. Further four of the press-fitting elements may also be positioned on the axis of minimum reluctance with a distance of smaller than 3/4 of the radius to the centre of the rotor disc. Further four of the press-fitting elements may be positioned on the axis of maximum reluctance. Their distance to the centre of the rotor disc may be larger than 3/4 of the radius of the rotor disc.

Such an arrangement is shown in Fig. 3, additionally to the four outer press-fitting elements 10a-10d and the four inner press-fitting elements 1 1 a-1 1 d positioned on the d-axis, four press-fitting elements 12a-12d are provided that are of rectangular shape and positioned on the q-axis.

The orientation of non-circular press-fitting elements positioned on or close to the q- axis is typically so that the larger extension is oriented perpendicularly to the radial direction of the rotor disc, which is, in many embodiments, perpendicular to the q- axis. For instance, if the elongated press-fitting elements are shaped in a non- quadratic manner, the longer side, which is also called the major axis in the case of the elliptical shape, is oriented perpendicular to the radial direction of the rotor disc. At this position, the direction perpendicular to the radial direction corresponds to the flux direction.

According to embodiments, the central opening may include a keyhole structure at the central opening of the rotor disc. The keyhole 7 structure as shown in Figs. 2-7 includes an evenly distributed keyhole structure 8 in the form of four rectangular cutouts. Typically, the cut-outs are adapted to mate to keyways formed in the rotor shaft (not shown).

In Fig. 4, additionally to the four press-fitting elements 10a-10d positioned on the d- axis, each two press-fitting elements 13a and 13a', 13b and 13b', 13c and 13c', 13d and 13d' are positioned close to the q-axis.

The arrangement shown in Fig. 4 is representative for typical embodiments of the present disclosure. Accordingly, it is possible to arrange a multitude of two or more press-fitting elements per axis, such as the d-axis or the q-axis, with the press-fitting elements being positioned symmetrically to the respective axis. For instance, two press-fitting elements may be positioned adjacent to each other close to, but not on, one of the d-axes or one of the q-axes of the rotor disc. The shape of these press- fitting elements may optionally be circular, rectangular, or elliptic. The term "adjacent" is understood as typically embracing a distance of maximally 5 mm, optionally maximally 2 mm between the press-fitting elements' edges.

According to embodiments, the rotor disc includes one or more bridge portions. Within the present disclosure, "bridge portion" and "bridge" are used as synonyms. In typical embodiments, the bridges are made of the same material as the portions with the higher magnetic permeability. Typically, the number of bridges is a multiple of the number of poles. For instance, in the case of a four pole disc, the number of bridges is typically four, eight, twelve or sixteen.

The bridge portions can be radially extending, such as e.g. the bridge portions 6a-6d in the Figs. 1 -10. The bridge portions can be circumferentially extending, such as the bridges shown in Figs. 1 -10 on the circumference of the rotor disc radially outside the barriers with the lower magnetic permeability 4a-4d . The circumferential bridges interconnect the portions with a higher magnetic permeability 3a-3e. In Fig. 6, some of the circumferential bridges are exemplarily referred to by numbers 51 a-51 d. Note that the size of the circumferential bridges is exaggerated in the figures for illustrative purposes.

The rotor disc according to the present disclosure may include one or more circumferentially extending bridge portions interconnecting the portions with a higher magnetic permeability of the rotor disc. These circumferentially extending bridge portions may be located in proximity of the circumferential edge of the rotor disc to mechanically support and strengthen the rotor disc. The one or more radial bridge portions and/or circumferentially extending bridge portions are optionally formed in rotor disc material by a punching or stamping process.

A magnetically conductive or permeable path formed by the radial bridge portions 6a-6d is detrimental to the d-axis magnetic properties of the rotor disc 2 because the radial bridge portions conduct magnetic flux along a q-axis. Further, the magnetically conductive or permeable path formed by the circumferential bridge portions 51 a - 51 d is also detrimental to the d-axis magnetic properties of the rotor disc. However, the bridge portions provide mechanical strength to the rotor disc structure. In particular in the event that radially oriented bridge portions are provided along the q-axis, but also in the case of circumferentially oriented bridge portions, the provision of press-fitting elements on or close to the q-axis helps eliminate, or at least lessen, the detrimental effect caused by a q-axis magnetically conductive or permeable path formed by the bridge portions.

It is possible to provide press-fitting elements close to or on some or all of the radial and/or circumferential bridge portions. Generally, the press-fitting elements close to or on bridge portions are off the direct axis according to the present disclosure. Some or all of them may be positioned on the quadrature axis. The term "close to" is to be understood as embracing press-fitting elements that are positioned adjacent to a bridge portion. Optionally, the term "close to" in this context is to be understood as including press-fitting elements that are positioned less than 7 mm, typically less than 3 mm away from the respective bridge portion.

Fig. 5 shows embodiments of further press-fitting elements 52a-52d and 55a-55d that are positioned off the d-axis. The press-fitting elements are positioned on or close to the bridges 6a-6d and 51 a-51 d.

In addition, further press-fitting elements are positioned on the rotor disc, such as in the example of Fig. 5 wherein further press-fitting elements are positioned on the d- axis and q-axis. Generally, it is possible to combine the press-fitting elements on or close to bridges with all the embodiments and the press-fitting element configurations described herein, in particular with those shown in Figs. 1 -5. With respect to the press-fitting elements on or close to bridges, it does not matter whether the other press-fitting elements are of elongated shape.

Typically, the diameter of each press-fitting element on or close to the bridge portion may lie between 0.5 mm and 2.0 mm depending on dimensions of the radial bridge portion in question. The pairs of press-fitting elements create localized mechanical stress in each of the respective punched areas of the radial bridge portions so as to deteriorate their magnetic conductive properties. The pairs of punch marks are therefore operative to increase magnetic reluctance of the rotor disc in the q-axis of each of the poles while retaining a desirable mechanical strengthening effect on the rotor disc structure provided by the radial bridge portions.

In embodiments, one or more of the circumferentially extending bridge portions may additionally include press-fitting elements 52a-52d to create localized deformations and further increase d-axis magnetic reluctance of the poles. This is also illustrated in the fourth pole in Fig. 5. These press-fitting elements are also provided on the other poles in the same manner but are not depicted for sake of a better overview. Further, the embodiment depicted can be combined with all other embodiments described herein.

Generally, the rotor disc may include radially extending cut-outs. Typically the cutouts are positioned on the q-axis. Even more typically, the cut-outs are positioned symmetrically around the q-axis. According to embodiments, the radially extending cut-outs are of shallow shape. Normally, the rotor d isc has as many rad ial ly extending cut-outs as poles. In most embodiments, the radially extending cut-outs are positioned on the circumference of the rotor disc.

For instance, Fig . 6 shows the shallow radially extending cut-outs 9a-9d . In the assembled rotor assembly, the cut-outs 9a-9d form grooves which are visible in the rotor assembly 1 of Fig. 8. The cut-outs improve the electromagnetic property of the rotor disc. However, at the same time, they reduce the mechanical stability of the rotor disc in a region that experiences the largest stress at high speeds in operation of the rotor assembly.

As previously-mentioned, the plurality of rotor discs may be bonded to each other by aligned pairs of press-fitting elements formed as mating protrusions and indentations on facing surfaces of neighbouring rotor discs. A centrally located opening 7 extends through the rotor assembly 1 and is shaped and dimensioned to mate to a rotor shaft (not shown).

For rotor assembly, a pair of neighbouring rotor discs are typically aligned in radial d irection and along a rotational axis so that the opposing protrusions and indentations of punch marks on respective facing rotor disc surfaces can be brought into contact. An appropriate pressure is thereafter applied to the end discs of the rotor assembly whereby the opposing pairs of protrusions and indentations are brought into frictional engagement and press-fitted together to form the coherent rotor assembly of mutually bonded rotor discs.

According to embodiments, the rotor disc may additionally comprise permanent magnets. For instance, some or all of the barriers with the second permeability may be equipped with one or more permanently magnetic material such as Neodymium Iron Boron (Nd-Fe-B) and/or ferrites.

Such an embodiment is illustrated in Fig. 7 where the three permanent magnets 20a, 20b, and 20c are positioned embedded within the portions with the first permeability 3a, 3b, 3c, and 3d.

In those cases where the permanent magnets change the magnetic properties of the rotor disc in such a manner that the direct and quadrature axes are interchanged by the magnet's existence, the present disclosure, in particular the description of the positioning of the press-fitting elements, shall be understood as referring to the rotor disc without the permanent magnets. That is, even if the permanent magnets are added to the rotor disc and the q-axis of the disc without the magnets becomes the d- axis of the disc with the magnets, the "q-axis" as understood in this description shall still be understood as the original q-axis referring to the permanent magnet free rotor disc. The same is true for the d-axis, that is, the "d-axis" as understood in this description shall be understood as the d-axis referring to a permanent magnet free rotor disc.

Fig. 8 shows a further embodiment of a rotor disc. In Fig. 8, the circumferential bridges 51 a - 51 d are thinner than in the embodiment shown in Figs. 1 to 7. For example the bridges 51 a - 51 d may have a radial thickness of about 0,5 to 2 mm, for example about 1 mm. For example, with respect to the diameter of the rotor disc, the bridges have a radial thickness of about 0,5 to 2 %, for example about 1 %, of the diameter of the rotor disc.

Further, the radial outer end of the outer press-fitting elements 10a - 10d is placed rad ial inward of the rad ial inward end of the adjacent bridges 51 d . Thus, the elongated press-fitting elements are positioned at a portion of the portion with higher magnetic permeability 3e such that the outer press-fitting elements disturb less the magnetic flux.

For example, in an embodiment, the radial outward end of the press-fitting elements 10a-10d has about the double distance than the inward end of the adjacent bridges 51 d to the circumferential edge portion 5. For example, in an embodiment, the distance between the radial outward end of the outer press-fitting element and the circumferential edge portion is between 2 and 5 mm of a rotor disc having a diameter of about 1 0 cm . Thus, the distance between the radial outward end of the outer press-fitting and the circumferential portion is between 2 and 5% of the diameter of the rotor disc.

In a typical embodiment, the dimensions of the press-fitting elements has a size of about 1 x 4 mm.

Fig. 9 is a three dimensional perspective view of the rotor assembly according to embodiments. The rotor assembly includes a multitude of rotor discs, e.g. the rotor discs shown in Figs. 1 - 8. Generally, the rotor assembly is shaped as a stack of radially and rotationally aligned substantially identical individual rotor discs. The number of rotor discs is typically at least 100, more typically at least 150. The rotor may include up to 300 or even 400 rotor discs depending on the thickness of the rotor discs.

The rotor assembly 1 has a substantially cylindrically circumferential shape with a cylindrical surface area 45 formed by the plurality of circumferential edges 5 (refer to Fig. 1 ) of abutted individual rotor discs. Generally, the circumferential shape of the rotor assembly is cylindrical. In those cases where the rotor discs include cut-outs on their circumferences, these cut-outs 9a-9d form grooves on the rotor assembly with the grooves being typically parallel to the central axis.

In the embodiment of Fig. 9, the plurality of discs 2 form the rotor assembly 1 having the central opening 7 for receiving a shaft. The discs used to form this rotor assembly could be those as shown in the Figures. The discs of the rotor assembly have the press-fitting elements 10a-10d positioned on the d-axis close to the edge of the rotor assembly. In contrast to the rotor discs shown in the Figures, radially extending bridges are provided between all the portions with a higher magnetic permeability. However, as stated several times, the embodiments described herein can be combined without departing from the scope of the present disclosure.

The rotor assembly shown is endplate-free. That is, the rotor assembly does not have an endplate positioned on either axial end of the rotor assembly. Given the described shaping of the rotor discs, the rotor assembly is stable nonetheless.

The plurality of rotor discs are brought into mutual abutment in a manner where neighbouring rotor d iscs are separated only by an intermittent thin electrically insulating layer, for instance, applied as varnish. The thin electrically insulating layer between neighbouring rotor discs prevents generation of an axially-oriented flow of eddy-currents between neighbouring discs or laminations. The typical thickness of the electrically insulating layer is smaller than 0.01 mm. According to embodiments, the electrically insulating layer is applied to the disc prior to stamping or punching. In some embodiments, the stamping or punching benefits from the electrically insulating layer because the insulating material smoothens the stamping or punching step.

The coherent rotor assembly is typically manufactured by punching or stamping a strip of silicon iron motor steel with an appropriately patterned tool to produce a plurality of individual rotor discs as unitary elements. Each rotor disc is stamped to produce a circumferential edge and a central opening for mating to a rotor shaft and one or more portions with a higher magnetic permeability with adjacently positioned barriers having a higher magnetic permeability in the shape of apertures. The plurality of individual rotor discs are subsequently radially and axially aligned and stacked by a suitable alignment tool or fixture that may project through the central tunnel 7 and mate to one or more of the keyhole structures 9.

The described method of production and the described design of the rotor discs/rotor assembly allow a streamlined production. Fig. 10 illustrates a cross-sectional view of a synchronous reluctance machine 100 according to e m bod i m ents d escri bed h ere i n . Th e cross-section is ta ken perpendicular to the rotational axis of the rotor assembly. In other words, the cross- section is taken somewhere along the central opening 7 provided to receive a shaft. The rotor disc 2 has four outer press-fitting elements 10a-10d and four inner press- fitting elements 1 1 a-1 1 d . Further, each pole of the rotor disc has four portions with a h ig her magnetic permeabil ity and th ree barriers having a lower magnetic permeabil ity. The three portions with a higher magnetic permeability are interconnected by bridges.

Fig. 10 further shows the stator of the reluctance machine. Typically, the stator is positioned outside of the rotor. The stator typically has a plurality of stator slots, such as at least 3, more typically at least 18, even more typically at least 24 such as 36. In Fig. 10, the stator slots are illustrated and referred to with reference number 1 15. Each of the stator slots typically comprises a coil of wire, such as copper wire.

According to embodiments, the synchronous reluctance machine can be connected to a drive unit. The drive unit is adapted for controlling the operation of the machine. In particular, the drive unit may be connected to each of the face windings made up of coils of wire extending through the stator slots in order to apply a corresponding voltage.

Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. While various specific embodiments have been disclosed in the foregoing, those skilled in the art will recognize that the scope of the claims allows for equally effective mod ifications. Especially, mutually non-exclusive features of the embodiments described above may be combined with each other. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they incl ude equ ivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1 . A rotor disc (2) adapted for use in a rotor assembly (1 ) of a synchronous machine (100), the rotor d isc having a centre (A) and a rad ius, wherein the rotor disc comprises:

at least one portion having a first magnetic permeability (3a-3e);

at least one barrier having a second magnetic permeability (4a-4d), wherein the at least one portion having a first magnetic permeability and the at least one barrier having a second magnetic permeability are positioned and shaped as to form at least one axis of minimum reluctance (d-axis) and at least one axis of maximum reluctance (q-axis),

at least one outer press-fitting element (10a-10d; 12a-12d; 52a-52d) positioned off the at least one axis of maximum reluctance of the rotor disc and at a radial distance to the centre of the rotor disc of at least 3/4 times the radius of the rotor disc; and

at least one inner press-fitting element (1 1 a-1 1 d; 55a-55d) positioned at a radial distance to the centre of the rotor disc of less than 3/4 times the radius of the rotor disc.

2. The rotor disc according to claim 1 , wherein at least one of the inner press fitting elements (1 1 a-1 1 d; 55a-55d) is positioned off the at least one axis of maximum reluctance and/or at a radial distance to the centre of the rotor disc of less than 2/3 times the radius of the rotor disc.

3. The rotor disc according to claim 1 , wherein at least one of the inner and outer press fitting elements are positioned on or close to the at least one axis of minimum reluctance of the rotor disc (2).

4. The rotor disc according to any of the preceding claims, wherein at least one of the inner and outer press-fitting elements (10a-10d;1 1 a-1 1 d; 12a-12d; 52a-52d; 55a- 55d) is of elongated shape with the larger extension of the press-fitting element typically being oriented in the flux direction of the rotor disc.

5. The rotor disc according to any of the preceding claims, wherein at least one of the inner and outer press-fitting elements is of a circular shape.

6. The rotor disc according to any of the preceding claims, wherein at least one of the inner and outer press-fitting elements (12a-12d; 55a-55d) is positioned on or close to the at least one axis of maximum reluctance.

7. The rotor disc according to any of the preceding claims, wherein the overall number of inner and outer press-fitting elements is at least 8 and optionally at least 12.

8. The rotor disc according to any of the proceeding claims, further comprising bridge portions (6a-6d; 51 a-51 d) having the first permeability and positioned between at least two portions with the second magnetic permeability (3a-3e), wherein at least one of the inner and outer press-fitting elements (52a-52d; 55a-55d) is positioned on or close to the bridge portions.

9. The rotor disc according to any of the proceeding claims, wherein the inner and/or outer press-fitting elements comprise a deformation of a portion with a first magnetic permeability of the rotor disc made of rotor disc material with the deformation typically including a protrusion and an oppositely located indentation.

10. A rotor assembly (1 ) for use in a synchronous machine (100), wherein the rotor assembly comprises a plurality of rotor discs (2) according to any of the preceding claims.

1 1 . The rotor assembly (1 ) according to claim 10, wherein the rotor assembly is end plate free.

12. A rotor assembly for use in a synchronous machine (100), wherein the rotor assembly comprises a plurality of rotor discs (2), wherein each rotor disc comprises

- at least one portion having a first magnetic permeability (3a-3e);

- at least one barrier having a second magnetic permeability (4a-4d), and

- at least one press-fitting element (10a-10d; 1 1 a-1 1d; 12a-12d; 52a-52d; 55a-55d) for bonding the rotor discs together;

wherein the rotor assembly is end plate free.

13. A synchronous machine (100), comprising:

- a stator (1 15) adapted to be connected to a power supply;

- a rotor assembly (1 ) according to any of the claims 10-12.

14. A method of manufacturing a rotor assembly for a synchronous machine, the method comprising:

- producing a plurality of rotor discs according to claims 1 -10, wherein producing includes forming at least one press-fitting element on each rotor disc by punching a portion with a first magnetic permeability of the rotor disc to simultaneously create an indentation and a projection on respective opposing plane surfaces of the rotor disc;

- radially aligning and stacking the plurality of rotor discs,

- applying a predetermined compressive force to the plurality of rotor discs to bring the plurality of rotor discs into mutual abutment, thereby bonding the plurality of rotor discs to each other.

1 5. The method of manufacturing a rotor assembly according to claim 14, further comprising:

- aligning the plurality of rotor discs along a rotational axis to align respective indentations and projections of neighbouring rotor discs,

- pressing the press-fitting elements of the neighbouring rotor discs together to bond the plurality of rotor discs to each other.