A ROTOR OF A SYNCHRONOUS RELUCTANCE MACHINE AND A METHOD FOR MANUFACTURING THE SAME

A rotor for a synchronous reluctance machine includes a first layered structure having ferromagnetic sheets stacked in a direction of a quadrature axis of the rotor and being separated from each other by layers of non-ferromagnetic material, a second layered structure similar to the first layered structure, and a ferromagnetic center part between the first and second layered structures in the direction of the quadrature axis and attached to the first and second layered structures. The ferromagnetic center part is a single piece of ferromagnetic material that is wider in a direction of the direct axis of the rotor than in the direction of the quadrature axis. The width of the ferromagnetic center part in the direction of the quadrature axis is greater than a thickness of each ferromagnetic sheet in order to improve the mechanical strength of the rotor.

FIELD OF THE TECHNOLOGY

The disclosure relates generally to rotating electric machines. More particularly, the disclosure relates to a rotor of a synchronous reluctance machine. Furthermore, the disclosure relates to a synchronous reluctance machine and to a method for manufacturing a rotor of a synchronous reluctance machine.

BACKGROUND

Rotating electric machines, such as motors and generators, generally comprise a rotor and a stator which are arranged such that a magnetic flux is developed between these two. A rotor of a synchronous reluctance machine comprises typically a ferromagnetic core structure and a shaft. The ferromagnetic core structure is arranged to have different reluctances in the direct d and quadrature q directions of the rotor. Thus, the synchronous reluctance machine has different inductances in the direct and quadrature directions and thereby the synchronous reluctance machine is capable of generating torque without a need for electric currents and/or permanent magnets in the rotor.

Different reluctances in the direct and quadrature directions can be achieved for example with salient poles so that the airgap is wider in the direction of the quadrature axis than in the direction of the direct axis. Typically, a salient pole rotor is however not suitable for high speed applications where the airgap should be smooth and where mechanical stress maxima in the rotor construction should be minimized as well as possible. Another approach to provide different reluctances in the direct and quadrature directions is based on cuttings in a rotor structure so the cuttings increase the reluctance in the direction of the quadrature axis more than in the direction of the direct axis. This approach is straightforward to use in cases where a rotor has a laminated structure comprising ferromagnetic sheets stacked in the axial direction of the rotor since the cuttings can be made on the sheets one-by-one. The approach based on the cuttings is however not free from challenges. One of the challenges is related to isthmuses formed by the cuttings because high local mechanical stresses may take place in the isthmuses and thereby the isthmuses may constitute weak points of the rotor structure. A third approach to provide different reluctances in the direct and quadrature directions is based on a stack of ferromagnetic sheets which are separated from each other with layers of non-ferromagnetic material so that the reluctance is greater in a direction perpendicular to the sheets than in a direction parallel with the sheets. This approach is typically used in synchronous reluctance machines having two or more pole-pairs and it may be challenging in conjunction with a synchronous reluctance machine having only one pole-pair.

SUMMARY

In this document, the word “geometric” when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric point, a straight or curved geometric line, a geometric plane, a non-planar geometric surface, a geometric space, or any other geometric entity that is zero, one, two, or three dimensional.

In accordance with the invention, there is provided a new rotor for a synchronous reluctance machine that has only one pole-pair. A rotor according to the invention comprises:a first layered structure comprising first ferromagnetic sheets stacked in a direction of the quadrature q axis of the rotor, the first ferromagnetic sheets being separated from each other by first layers of non-ferromagnetic material,a second layered structure comprising second ferromagnetic sheets stacked in the direction of the quadrature axis, the second ferromagnetic sheets being separated from each other by second layers of the non-ferromagnetic material, anda ferromagnetic center part located between the first and second layered structures in the direction of the quadrature axis and attached to the first and second layered structures.

The above-mentioned ferromagnetic center part is a single piece of ferromagnetic material that is wider in the direction of the direct d axis of the rotor than in the direction of the quadrature axis, and the width of the ferromagnetic center part in the direction of the quadrature axis is greater than the thickness of each of the above-mentioned ferromagnetic sheets. The ferromagnetic center part which is made of solid ferromagnetic material and which is thicker than the ferromagnetic sheets improves the mechanical strength of the rotor compared to a situation where a layered structure extends through a rotor because greatest mechanical stresses caused by the centrifugal force take typically place at the geometric axis of rotation or in its vicinity. Thus, in the above-described rotor, the solid ferromagnetic material is utilized in the area where maximal mechanical stresses may occur.

In accordance with the invention, there is provided also a new synchronous reluctance machine. A synchronous reluctance machine according to the invention comprises:a stator comprising stator windings for generating a rotating magnetic field in response to being supplied with alternating currents, anda rotor according to the invention, the rotor being rotatably supported with respect to the stator.

In accordance with the invention, there is provided also a new method for manufacturing a rotor of a synchronous reluctance machine having only one pole-pair. A method according to the invention comprises:stacking first ferromagnetic sheets and first layers of non-ferromagnetic material so as to form a first layered structure where the first layers of the non-ferromagnetic material separate the first ferromagnetic sheets from each other,stacking second ferromagnetic sheets and second layers of the non-ferromagnetic material so as to form a second layered structure where the second layers of the non-ferromagnetic material separate the second ferromagnetic sheets from each other,stacking the first layered structure, a ferromagnetic center part, and the second layered structure so that the ferromagnetic center part is, in the direction of the quadrature q axis of the rotor, between the first and second layered structures and the first and second ferromagnetic sheets are stacked in the direction of the quadrature axis, the ferromagnetic center part being a single piece of ferromagnetic material that is wider in the direction of the direct d axis of the rotor than in the direction of the quadrature axis, and the width of the ferromagnetic center part in the direction of the quadrature axis being greater than the thickness of each of the ferromagnetic sheets, andattaching the first and second ferromagnetic sheets, the first and second layers of the non-ferromagnetic material, and the ferromagnetic center part together to constitute a uniform element.

Various exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in conjunction with the accompanying drawings.

DESCRIPTION OF EXEMPLIFYING AND NON-LIMITING EMBODIMENTS

The specific examples provided in the description given below should not be construed as limiting the scope and/or the applicability of the appended claims. Furthermore, it is to be understood that lists and groups of examples provided in the description given below are not exhaustive unless otherwise explicitly stated.

FIG. 1ashows a cross-section of a rotor101according to an exemplifying and non-limiting embodiment of the invention, andFIG. 1bshows a side view of the rotor101. The cross-section shown inFIG. 1ais taken along a line A-A shown inFIG. 1bso that the geometric section plane is parallel with the xy-plane of a coordinate system199. In this exemplifying case, it is assumed that the cross-section is the same at different axial positions on the active part of the rotor, e.g. a cross-section taken along a line A′-A′ shown inFIG. 1bis the same as the cross-section shown inFIG. 1a. The rotor101comprises a first layered structure102that comprises first ferromagnetic sheets stacked in the direction of the quadrature q axis of the rotor101. The first ferromagnetic sheets are separated from each other by first layers of non-ferromagnetic material. InFIGS. 1aand 1b, two of the first ferromagnetic sheets are denoted with references104and105and two of the first layers of the non-ferromagnetic material are denoted with references106and107. The rotor101comprises a second layered structure103which is similar to the first layered structure102and which comprises second ferromagnetic sheets stacked in the direction of the q-axis. The second ferromagnetic sheets are separated from each other by second layers of the non-ferromagnetic material. The rotor101comprises a ferromagnetic center part108which is located between the first and second layered structures102and103in the direction of the q-axis and which is attached to the first and second layered structures. The ferromagnetic center part108is a single piece of ferromagnetic material that is wider in the direction of the direct d axis of the rotor than in the direction of the q-axis. The width Wq of the ferromagnetic center part108in the direction of the q-axis is greater than the thickness of each of the first and second ferromagnetic sheets. Due to the above-mentioned layers of the non-ferromagnetic material, the reluctance of the rotor101is greater in the direction of the q-axis than in the direction of the d-axis. As a skilled reader can understand based onFIG. 1a, the ferromagnetic center part108constitutes a part of a flow path for a magnetic flux when the rotor101is acting as a rotor of a synchronous reluctance machine. A shaft120can be, for example but not necessarily, the same piece of material as the ferromagnetic center part108.

In a rotor according to an exemplifying and non-limiting embodiment of the invention, the width Wq of the ferromagnetic center part108in the direction of the q-axis is at least three times the thickness of the ferromagnetic sheets. In a rotor according to an exemplifying and non-limiting embodiment of the invention, the width Wq of the ferromagnetic center part108in the direction of the q-axis is at least five times the thickness of the ferromagnetic sheets. In a rotor according to an exemplifying and non-limiting embodiment of the invention, the width Wq of the ferromagnetic center part108in the direction of the q-axis is at least ten times the thickness of the ferromagnetic sheets. The ferromagnetic center part108which is made of solid ferromagnetic material and which is thicker than the ferromagnetic sheets improves the mechanical strength of the rotor101compared to a situation where a layered structure extends through a rotor because strongest mechanical stresses caused by the centrifugal force take place typically at the geometric axis of rotation, i.e. in the ferromagnetic center part108.

In a rotor according to an exemplifying and non-limiting embodiment of the invention, the ferromagnetic sheets and the ferromagnetic center part108are made of ferromagnetic steel and the non-ferromagnetic material between adjacent ones of the ferromagnetic sheets is austenitic steel. Furthermore, there can be layers of the non-ferromagnetic material between the ferromagnetic center part108and ferromagnetic sheets closest to the ferromagnetic center part108. It is however also possible that the ferromagnetic sheets closest to the ferromagnetic center part108are directly attached to the ferromagnetic center part108. Depending on mechanical stresses, it is also possible that the non-ferromagnetic material is for example copper or brass. The ferromagnetic material and the non-ferromagnetic material are advantageously selected so that their coefficients of thermal expansion are close to each other.

A rotor according to an exemplifying and non-limiting embodiment of the invention comprises solder or brazing joints for attaching the ferromagnetic sheets, the layers of the non-ferromagnetic material, and the ferromagnetic center part108together to constitute a uniform element. A rotor according to another exemplifying and non-limiting embodiment of the invention comprises diffusion welded joints for attaching the ferromagnetic sheets, the layers of the non-ferromagnetic material, and the ferromagnetic center part108together to constitute a uniform element.

In the exemplifying rotor101illustrated inFIGS. 1aand 1b, the ferromagnetic sheets are planar and surfaces of the ferromagnetic center part108attached to the first and second layered structures102and103are planar and parallel with each other.FIG. 2ashows a cross-section of a rotor201according to another exemplifying and non-limiting embodiment of the invention, andFIG. 2bshows a side view of the rotor201. The cross-section shown inFIG. 2ais taken along a line A-A shown inFIG. 2bso that the geometric section plane is parallel with the xy-plane of a coordinate system299. In this exemplifying case, it is assumed that the cross-section is the same at different axial positions on the active part of the rotor201. The rotor201comprises a first layered structure202that comprises first ferromagnetic sheets stacked in the direction of the quadrature q axis of the rotor201. The first ferromagnetic sheets are separated from each other by first layers of non-ferromagnetic material. InFIGS. 2aand 2b, two of the first ferromagnetic sheets are denoted with references204and205and two of the first layers of the non-ferromagnetic material are denoted with references206and207. The rotor201comprises a second layered structure203which is similar to the first layered structure202and which comprises second ferromagnetic sheets stacked in the direction of the q-axis. The second ferromagnetic sheets are separated from each other by second layers of the non-ferromagnetic material. The rotor201comprises a ferromagnetic center part208which is located between the first and second layered structures202and203in the direction of the q-axis and which is attached to the first and second layered structures. The ferromagnetic center part208is a single piece of ferromagnetic material that is wider in the direction of the direct d axis of the rotor than in the direction of the q-axis. The width Wq of the ferromagnetic center part208in the direction of the q-axis is greater than the thickness of each of the first and second ferromagnetic sheets. A shaft220of the rotor201can be, for example but not necessarily, the same piece of material as the ferromagnetic center part208.

In the exemplifying rotor201illustrated inFIGS. 2aand 2b, the ferromagnetic sheets are curved having concave sides towards the ferromagnetic center part208. Correspondingly, surfaces of the ferromagnetic center part208which are attached to the first and second layered structures202and203are curved so that the width of the ferromagnetic center part208in the direction of the q-axis is tapering towards edges of the ferromagnetic center part208. The curved shapes of the ferromagnetic sheets, of the layers of the non-ferromagnetic material, and of the ferromagnetic center part208help reducing mechanical stresses between the ferromagnetic and non-ferromagnetic materials. InFIG. 2a, the width Wq means the maximum width of the ferromagnetic center part208in the direction of the q-axis. The width Wq can be for example at least 3, 5, or 10 times the thickness of each ferromagnetic sheet.

FIG. 2cshows a side view of a rotor201aaccording to an exemplifying and non-limiting embodiment of the invention.FIGS. 2d, 2e, and 2fshow cross-sections of the rotor201aso that the cross-section shown inFIG. 2dis taken along a line A1-A1shown inFIG. 2c, the cross-section shown inFIG. 2eis taken along a line A2-A2shown inFIG. 2c, and the cross-section shown inFIG. 2fis taken along a line A3-A3shown inFIG. 2c. Concerning each of the cross-sections shown inFIGS. 2d-2f, the geometric section plane is parallel with the xy-plane of the coordinate system299. The rotor201ais otherwise similar to the rotor201illustrated inFIGS. 2aand 2bbut, as shown inFIGS. 2eand 2f, the layers of the non-ferromagnetic material are shaped to form axial channels for conducting cooling fluid e.g. air. InFIG. 2f, one of the axial channels is denoted with a reference240. For example, as shown inFIG. 2f, the layer207of the non-ferromagnetic material has a center portion and side portions so that axial channels are formed between the center portion and the side portions. It is also possible to have e.g. axial grooves on the layers of the non-ferromagnetic material so as to form the axial channels. In this exemplifying case, the layers of the non-ferromagnetic material are shaped to form outlet channels from the axial channels to the airgap surface of the rotor so that the rotor constitutes a blower when the rotor is rotating. InFIGS. 2cand 2e, one of the outlet channels is denoted with a reference241. InFIG. 2c, a flow of cooling fluid is depicted with a dashed line250. In this exemplifying case, the outlet channels are located at one end of the rotor201a. In cases where a stator has a radial cooling channel in the middle of the stator, the outlet channels are advantageously located in the middle of the rotor. It is also possible that the axial channels extend through the rotor in the axial direction and there are no outlet channels of the kind discussed above.

FIG. 3illustrates a synchronous reluctance machine according to an exemplifying and non-limiting embodiment of the invention. The synchronous reluctance machine comprises a rotor301according to an embodiment of the invention and a stator309. The rotor301is rotatably supported with respect to the stator309. Arrangements for rotatably supporting the rotor301with respect to the stator309are not shown inFIG. 3. The stator309comprises stator windings310for generating a rotating magnetic field in response to being supplied with alternating currents. The stator windings310can be for example a three-phase winding. The rotor301can be for example such as illustrated inFIGS. 1aand 1b, or such as illustrated inFIGS. 2aand 2b, or such as illustrated inFIGS. 2c-2f.

FIG. 4shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for manufacturing a rotor of a synchronous reluctance machine. The method comprises the following actions:action401: stacking first ferromagnetic sheets and first layers of non-ferromagnetic material so as to form a first layered structure where the first layers of the non-ferromagnetic material separate the first ferromagnetic sheets from each other,action402: stacking second ferromagnetic sheets and second layers of the non-ferromagnetic material so as to form a second layered structure where the second layers of the non-ferromagnetic material separate the second ferromagnetic sheets from each other,action403: stacking the first layered structure, a ferromagnetic center part, and the second layered structure so that the ferromagnetic center part is, in the direction of the quadrature q axis of the rotor, between the first and second layered structures and the first and second ferromagnetic sheets are stacked in the direction of the q-axis, the ferromagnetic center part being a single piece of ferromagnetic material that is wider in the direction of the direct d axis of the rotor than in the direction of the q-axis, and the width of the ferromagnetic center part in the direction of the q-axis being greater than the thickness of the ferromagnetic sheets, andaction404: attaching the first and second ferromagnetic sheets, the first and second layers of the non-ferromagnetic material, and the ferromagnetic center part together to constitute a uniform element.

It is worth noting that the actions401-403can be carried out in an order different from the order mentioned above and presented inFIG. 4.

In a method according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned attaching is implemented by soldering or brazing.

In a method according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned attaching is implemented by diffusion welding.

In a method according to an exemplifying and non-limiting embodiment of the invention, the ferromagnetic sheets are planar and surfaces of the ferromagnetic center part attached to the first and second layered structures are planar and parallel with each other.

In a method according to an exemplifying and non-limiting embodiment of the invention, the ferromagnetic sheets are curved having concave sides towards the ferromagnetic center part, and surfaces of the ferromagnetic center part attached to the first and second layered structures are curved so that the width of the ferromagnetic center part in the direction of the quadrature axis is tapering towards edges of the ferromagnetic center part.

In a method according to an exemplifying and non-limiting embodiment of the invention, the ferromagnetic sheets and the ferromagnetic center part are made of ferromagnetic steel and the non-ferromagnetic material is austenitic steel.

FIGS. 5aand 5billustrate phases of a method according to an exemplifying and non-limiting embodiment of the invention for manufacturing a rotor of a synchronous reluctance machine. The method comprises cutting the above-mentioned ferromagnetic center part from a block521of ferromagnetic material e.g. ferromagnetic steel. InFIGS. 5aand 5b, the ferromagnetic center part is denoted with a reference508. The cutting can be for example wire cutting. Thereafter, remnant pieces522and523of the block521are used as pressing tools for pressing the ferromagnetic sheets and the layers of the non-ferromagnetic material against the ferromagnetic center part508so as to shape the ferromagnetic sheets and the layers of the non-ferromagnetic material to have the desired curved shapes. InFIG. 5b, one of the ferromagnetic sheets is denoted with a reference504and one of the layers of the non-ferromagnetic material is denoted with a reference506. The ferromagnetic sheets, the layers of the non-ferromagnetic material, and the ferromagnetic center part508are attached together e.g. by soldering, brazing, or diffusion welding. Thereafter, the resulting rotor preform is lathed according to a dashed line circle shown inFIG. 5b.

In a method according to an exemplifying and non-limiting embodiment of the invention, the ferromagnetic sheets, the ferromagnetic center part, and the layers of the non-ferromagnetic material are made using the hot isostatic pressing “HIP” which reduces porosity of metals and thus increases the mechanical strength. It is also possible that the ferromagnetic sheets and the layers of the non-ferromagnetic material are deposited on the ferromagnetic center part and on each other using the HIP. In this exemplifying case, some of the method phases shown inFIG. 4are merged and carried out simultaneously.