The present disclosure concerns a rotor disc adapted for use in a rotor of a synchronous reluctance machine. More

specifically, the present disclosure relates in modifying the rotor disc dimensions in order to achieve a more

reliable synchronous reluctance machine.

BACKGROUND ART

Generally, synchronous reluctance machines include a stator with poly-phase windings forming a plurality of poles in a manner resembling a stator of an induction motor. A rotor of a synchronous reluctance machine does not normally include electrical windings but has a number of poles in the form of portions with a higher magnetic permeability. The rotor is formed as an anisotropic structure wherein each pole has a direction of minimum reluctance, a so-called "direct axis" or "d-axis", and a direction of maximum 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. 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 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 and extending through a central axis thereof.

The rotor includes a stack of transversally oriented rotor discs. The rotor discs may be coated with magnetically non- permeable material, such as varnish, to prevent generation of an axially-oriented flow of eddy-currents between

neighbouring rotor discs.

The rotor must, in many cases, exhibit high structural strength to allow rotation at high speeds and withstand elevated operation temperatures over extended periods of time. At the same time, the rotor should exhibit low

magnetic flux leakage to improve power efficiency and power factor of the synchronous reluctance machine.

EP 2 169 805 relates to a rotor for a synchronous reluctance machine. The rotor includes rotor discs in a cut-out design. Thus, the outer contours of the rotor discs are provided with magnetically insulating flux barriers at the q-axis. Such a design provokes to a torque ripple and friction losses at high rotational speeds. A similar reluctance machine is disclosed in US 5,418,415, which relates to a reluctance motor having a rotor core with salient poles. Each salient pole of the rotor core has a shape such that the gap permeance varies in proportion to where θ ₂ represents an angular position with respect to the origin set to the center of a salient pole.

EP 1 111 755 Al relates to a synchronous reluctance machine having a rotor disc with an outer peripheral contour being circular. The rotor disc comprises a strengthened stress concentration portion. US 2007/0126304 Al concerns a rotating machine with

permanent magnets in the rotor. The rotor has its minimum radial dimension at a d-axis, and the maximum radial dimension at a q-axis, using the definitions of d- and q- axes of the present disclosure. It is to be noted that due to the permanent magnets the definitions of d- and q-axes in US 2007/0126304 Al are opposite to the definitions of the present disclosure. According to the present disclosure a d- axis shall be understood as the axis of the minimum

reluctance of the rotor disc, and a q-axis as the axis of maximum reluctance of the same, while in US 2007/0126304 Al an axis coinciding with a direction of a magnetic flux passing a magnet is called a "d-axis".

Generally, high speed tests show that the rotor discs tend to expand in radial direction due to the centrifugal force, electromagnetic forces, and thermal expansion. These forces may therefore reduce the air gap size to zero during

operation of the machine so that the rotor touches the stator. Hence the rotor may be destroyed.

SUMMARY OF THE INVENTION

One object of the invention is to provide a reliable rotor disc for a synchronous reluctance machine such that a reliable operation of the machine is ensured even at high speeds and over long operating periods. A further object of the invention is to provide a reliable rotor for a

synchronous reluctance machine and a reliable synchronous reluctance machine. These objects are achieved by the devices according to appended claims 1, 11 and 13.

The invention is based on the realization that by designing a rotor disc such that the rotor has an optimal shape in operation and not in standstill, a more reliable synchronous reluctance machine is achieved.

According to a first aspect of the invention, there is provided a rotor disc adapted for use in a rotor of a synchronous reluctance machine, wherein the rotor disc comprises: a disc body having a first magnetic permeability and a circumferential surface; at least one flux barrier having a second magnetic permeability lower than the first magnetic permeability, wherein the at least one flux barrier is shaped and positioned in the disc body such that at least one axis of minimum reluctance, a d-axis, and at least one axis of maximum reluctance, a q-axis, are formed; a first radial dimension of the rotor disc at a q-axis, and a second radial dimension of the rotor disc at a d-axis, wherein the first radial dimension is smaller than the second radial dimension, and wherein a difference between the first radial dimension and the second radial dimension is between 0,05% and 3% of the maximal diameter of the rotor disc at a substantially uniform temperature of the disc body at standstill .

With the rotor disc according to the invention disclosed herein, a certain deformation of the rotor disc is allowed with reduced risk of the circumferential surface of the rotor disk coming into contact with the stator of the machine .

According to one embodiment of the invention, the difference between the first radial dimension and the second radial dimension is between 0,1% and 2%, in particular between 0,2% and 0,9%, of the maximal diameter of the rotor disc at substantially uniform temperature of the disc body at standstill .

According to one embodiment of the invention, the difference between the first radial dimension and the second radial dimension is reduced in operation with respect to

standstill. When the rotor disc is designed such that it fulfils this condition, a rotor that has its optimal shape in operation and not in standstill can be achieved. According to one embodiment of the invention, the difference between the first radial dimension and the second radial dimension at a peripheral speed of 50 m/s is less than 70%, such as less than 50% or less than 40%, of the difference between the first radial dimension and the second radial dimension at a substantial uniform temperature of the disc body at standstill.

According to one embodiment of the invention, for each pair of circumferentially adjacent q-axis and d-axis the radial dimension of the rotor disc gradually changes from the first radial dimension at the q-axis to the second radial

dimension at the d-axes. When the change of the radial dimension is gradual, air resistance and noise generation during a high speed operation of the machine are maintained low.

According to one embodiment of the invention, for each pair of circumferentially adjacent q-axis and d-axis the radial dimension of the rotor disc continuously, such as

constantly, increases from the first radial dimension at the q-axis to the second radial dimension at the d-axis. By this measure the noise generation is further reduced.

According to one embodiment of the invention, the at least one flux barrier is disposed radial inward of the

circumferential surface of the disc body. According to one embodiment of the invention, the rotor disc includes at least three flux barriers, such as four flux barriers, between the circumferential surface and a rotor disc centre at a q-axis, wherein in particular the flux barriers are spaced radial apart from each other. According to one embodiment of the invention, the rotor body is free of flux barriers at a d-axis. Thus, the magnetic flux at the d-axis is not reduced by a flux barrier, which enables a high ratio of the reluctance between the at least one d-axis and the at least one q-axis.

According to one embodiment of the invention, at least one permanent magnet element is disposed in a flux barrier. A machine using a rotor disc with permanent magnet elements may use both the magnetic torque and the reluctance torque inherent for the rotor disc.

According to a second aspect of the invention, there is provided a rotor for a synchronous reluctance machine, the rotor comprising at least one rotor disc according to an embodiment disclosed herein.

According to one embodiment of the invention, the rotor comprises at least 100 rotor discs, such as at least 150 rotor discs. The number of rotor discs may depend on the torque to be generated by the machine. Further, the number of rotor discs may depend on the thickness of the rotor discs .

According to a third aspect of the invention, there is provided a synchronous reluctance machine comprising a stator and a rotor according to an embodiment disclosed herein .

According to one embodiment of the invention, the machine comprises an air gap between the circumferential surface of a rotor disc and an inner surface of the stator, the air gap at a q-axis being wider than the air gap at a d-axis.

According to one embodiment of the invention, the air gap at a q-axis is between 1,5 and 3 times, such as between 1,7 and 2,5 times, the air gap at a d-axis.

A radial dimension shall be understood as the distance extending from the rotor disc centre to the circumferential surface of the rotor disc. The rotor disc centre is the rotational axis once the rotor disc is assembled onto a rotor shaft. When measuring the radial dimension at a certain point of the circumferential surface of the rotor disc, notches or small depressions having a circumferential extension of less than 1%, in particular less than 0,5 % of the total periphery of the rotor disc are typically not taken into account, but merely a virtual circumferential surface following the overall form of the rotor disc and extending over such notches or small depressions should be considered . 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 a generator. According to typical embodiments of the invention, the machine as described is suitable for operating both as a motor and a generator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail with reference to the accompanying drawings, wherein figure 1 shows a rotor according to one embodiment of the invention, and figure 2 shows a rotor disc and a stator according to one embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to figure 1, a rotor 1 according to the invention consists of a stack of rotor discs 100. The number of rotor discs 100 is typically at least 100, more typically at least 150. The rotor may include up to 500 or even 1000 rotor discs 100 depending on their thickness. Further, the number of rotor discs 100 may depend on the torque needed for a specific appliance. The rotor 1 has a substantially smooth envelope surface. Especially, the rotor discs 100 do not have salient poles at the outer periphery.

In a typical embodiment, the plurality of rotor discs 100 are brought into mutual abutment in a manner where

neighbouring rotor discs 100 are separated only by an intermittent thin electrically insulating layer applied as varnish. The varnish layer between neighbouring rotor discs 100 prevents generation of an axial flow of eddy-currents between the rotor discs 100. The typical thickness of the electrically insulating layer is smaller than 0,01 mm. The varnish layer is applied to the rotor discs 100 prior to stamping or punching, and the varnish thereby functions as lubrication in the punching process and results in less wear in the punching tools. The varnish is, however, not

absolutely needed either for insulating purposes or

lubrication purposes.

Figure 2 shows an embodiment of a rotor disc 100. The rotor disc 100 includes a disc body 104 in which a central opening 102 is disposed for a rotor shaft. The shape of the central opening 102 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 100. Further, the rotor disc 100 is given anisotropic structure by alternating layers of magnetically permeable segments 106 having a high magnetic permeability, and magnetically insulating flux barriers 108 having a low magnetic

permeability. In a typical embodiment, the flux barriers 108 are created by cutting material of the disc body 104 in the shape of longitudinal slots. It is thereby the air inside the cut-outs that functions as the flux barrier 108. The rotor disc 100 is typically designed to be mechanically self-sustained. Typically the magnetically permeable

segments 106 are connected by narrow tangential bridges 110 at the periphery of the rotor disc 100. The tangential bridges 110 are typically formed of rotor disc material in a punching or stamping process. Typically, the circumferential surface 116 includes the radial outward edge of the

tangential bridges 110.

Further, in an embodiment of the invention, radial bridges 112 are arranged to connect two radial adjacent magnetically permeable segments 106 in order to improve the mechanical strength of the rotor disc 100. Without departing from the present disclosure, the radial bridges 112 may be omitted, or further radial bridges 112 may be added. In typical embodiments, the radial bridges 112 are made of the same material as the segments 106 with the high relative magnetic permeability .

A magnetically permeable path formed by the radial bridges 112 is detrimental to the magnetic properties of the rotor disc 100 because the radial bridges 112 conduct magnetic flux along a q-axis. Further, the magnetically permeable path formed by the tangential bridges 110 is also

detrimental to the magnetic properties of the rotor disc 100. However, the bridge portions provide mechanical

strength to the rotor disc structure. Further, they provide a path for a heat transfer from the periphery of the rotor disc 100 to the shaft which is typically cooler than the periphery of the rotor disc 100 during operation of the machine . The rotor disc 100 shown in figure 2 includes four pole sectors 114. The pole sectors 114 are distributed evenly about a rotor disc centre X. Each pole covers a 90° sector of the rotor disc 100 in a four pole machine. Other

embodiments may include a smaller or larger number of poles such as two poles or six poles. In a six pole machine each pole covers a 60° sector. It is also possible to have more than six poles, for instance up to 20 poles.

Each pole sector 114 includes five segments 106 made of a material with a high relative magnetic permeability. Each of the five magnetically permeable segments 106 has an arm- shaped form extending between two predetermined angular positions at a circumferential surface 116 of the rotor disc 100. Four flux barriers 108 are intermittently disposed between the permeable segments 106 in a manner where an alternating pattern of magnetically permeable segments 106 and flux barriers 108 is formed along the q-axes of the rotor disc 100 from the central opening 102 towards the circumferential surface 116. The flux barriers 108 are separated from an air gap 200 between the rotor 1 and a stator 300 by tangential bridges 110.

In a typical embodiment of the invention, the rotor disc 100 is formed as a single unitary element fabricated by punching or stamping a metallic carrier. Each rotor disc 100 is stamped to produce a circumferential surface 116, a central opening 102 for mating to a rotor shaft, and flux barriers 108 in the form of apertures. The metallic carrier may include ferromagnetic metal or alloy with a high relative magnetic permeability. In an embodiment of the invention the rotor disc material is provided in the form of a metallic carrier of silicon iron motor steel. This material is relatively inexpensive and can be provided as carrier sheets or elongate strips with suitable dimensions for punching or stamping operations. In typical embodiments of the invention the rotor disc thickness is between 0,27 mm and 0,65 mm such as 0,50 mm, but this dimension may of course be larger or smaller depending on performance and other requirements for the rotor 1. The thickness is a compromise between a thin rotor disc 100 resulting in reduced eddy-currents, and a thick rotor disc 100 resulting in reduced production costs. The use of thinner rotor discs 100 involves higher costs per weight of the material, a higher number of shaping and punching processes, a higher number of assembling steps, etc. The maximal diameter d of the rotor disc 100 may be between 5 cm and 50 cm. For example, the maximal diameter d may be between 12 and 40 cm.

The rotor disc 100 is adapted to rotate around the rotor disc centre X once assembled with further rotor discs 100 to form a rotor 1. Generally, there is no preferred rotational direction, i.e. the rotor disc 100 is shaped and formed symmetrically in the sense that there is no predetermined rotational direction (i.e., clock-wise or counter ^¬ clockwise) .

Figure 2 further shows inner surface 304 of the stator 300 with a diameter D. Typically, the air gap 200 between the rotor disc 100 and the inner surface 304 of the stator 300 is designed to be as narrow as possible to provide a high torque. In other words, the circumferential surface 116 of the rotor disc 100 is typically arranged as close as

possible to the inner surface 304 of the stator 300.

At high rotational speeds the centrifugal force affecting on the rotor disc 100 increases. Since the radial outermost segments 106 are connected to the central portion of the rotor disc 100 only via tangential bridges 110, the

centrifugal forces may cause the radial dimension of the rotor disc 100 at the q-axes to increase due to the weaker structure compared to the d-axes. Further, iron losses cause the temperature of the rotor disc 100 to rise. Typically, the rotor shaft onto which the rotor discs 100 are arranged is cooler than the periphery of the rotor discs 100. Thus, the heat is conducted from the periphery of the rotor discs 100 towards the shaft. However, as the heat is mainly conducted through the disc body 104, the segments 106 separated from the shaft by the tangential or radial bridges 110, 112 are less cooled than the portions close to the d- axes. Temperature differences within the rotor discs 100 lead to thermal stress which together with the mechanical stress caused by the centrifugal force may lead to a

deformation of rotor discs 100 such that the circumferential surface 116 at q-axes comes into contact with the stator 300.

A first radial dimension Rl of the rotor disc 100 at the q- axes is smaller than a second radial dimension R2 of the rotor disc 100 at the d-axes. In other words, the radial dimension of the rotor disc 100 is alternating in

circumferential direction between the first radial dimension Rl and the second radial dimension R2. In the typical case where the inner surface 304 of the stator 300 is circular, the air gap 200 between the circumferential surface 116 of the rotor 1 and the inner surface 304 of the stator 300 is increasing when going from each of the d-axes towards respective circumferential adjacent q-axes. For example, the air gap 200 at each q-axis may be 1,5 to about 3 times the air gap 200 at the d-axes, such as about 2 times the air gap 200 at the d-axes.

In a typical example for small rotors 1 where the rotor discs 100 have a diameter of about 100 mm, the air gap 200 between the inner surface 304 of the stator 300 and the circumferential surface 116 of the rotor 1 at the d-axes is between 0,2 mm and 0,5 mm, and the air gap 200 at the q-axes is between about 0,4 mm to 1 mm. In other embodiments of the invention, for example where the rotor discs 100 have a diameter of about 160 mm, the air gap 200 at the d-axes is between about 0,3 mm to about 0,8 mm, and the air gap 200 at the q-axes is between about 0,7 mm to about 1,5 mm.

In further embodiments of the invention, the difference between the radial dimensions of the rotor disc 100 at the d-axes and at the q-axes is between about 0,1% and 2%, in particular between 0,2% and 0,9% of the maximal diameter d of the rotor disc 100.

Typically, the radial dimension of the rotor disc 100 gradually changes from the second radial dimension R2 at the d-axis to the first radial dimension Rl at the q-axis. The radial dimension may decrease continuously from the d-axes to the q-axes such that a tangent at any point of the circumferential surface 116 touches the rotor disc 100 only at a single point. The continuous decrease of the radial dimension may be constant.

A rotor 1 including a stack of rotor discs 100 may

additionally include permanent magnets. For instance, some or all of the flux barriers 108 may be equipped with one or more magnets made of permanently magnetic material such as Neodymium Iron Boron (Nd-Fe-B) or a ferrite. If, after the introduction of permanent magnets, the main magnetic flux of the machine passes through the magnets, a person skilled in the art would probably define a d-axis direction of the resulting rotor to be the direction of the main magnetic flux passing the magnets. However, the introduction of permanent magnets does not affect the directions of minimum and maximum reluctance of a rotor disc 100, and therefore the directions of the d-axes and q-axes according to the definition of the present disclosure remain the same despite of the presence of permanent magnets.

The invention is not limited to the embodiments shown above, but the person skilled in the art may, of course, modify them in a plurality of ways within the scope of the

invention as defined by the claims.