WO2022084080A1

WO2022084080A1 - Synchronous reluctance electric machine with open tangential bridges

Info

Publication number: WO2022084080A1
Application number: PCT/EP2021/078012
Authority: WO
Inventors: Baptiste CHAREYRON; Andre Nasr; Victor MEDIAVILLA SANTOS; Thomas VALIN
Original assignee: IFP Energies Nouvelles
Priority date: 2020-10-22
Filing date: 2021-10-11
Publication date: 2022-04-28
Also published as: FR3115639A1; EP4233152A1; FR3115639B1

Abstract

The aim of the invention is to reduce magnetic short circuiting by opening the tangential bridges, thereby increasing the magnetic flux in the air gap and thus improving the electromagnetic performance of the synchronous reluctance machine.

Description

SYNCHRO-RELUCTANT ELECTRIC MACHINE WITH OPEN TANGENTIAL BRIDGES

Technical area

The present invention relates to a rotating electrical machine, in particular a synchro-reluctant electrical machine (assisted by permanent magnets), and relates more particularly to the tangential bridges situated at the level of the periphery of the rotor. Generally, such an electric machine comprises a stator and a rotor arranged coaxially one inside the other.

In synchro-reluctant machines, the architecture of the rotor plays an important role in the operating principle of this type of machine. In synchro-reluctant machines with flux barriers assisted by permanent magnets, the main design parameter that controls the trade-off between electromagnetic performance and mechanical strength is the thickness of the bridges, which connect the different parts separated by the flux barriers. These bridges generate a low reluctance path that bypasses the flux barriers and loops the magnetic flux created by the permanent magnets back directly onto them without generating a magnetic effect for the operation of the electric machine. To increase the performance of the machine, it is sought to reduce the thickness of the bridges as much as possible or to eliminate them if possible.

Prior technique

The rotor of a synchro-reluctant electric machine assisted by permanent magnets is usually formed of a rotor body with a stack of laminations placed on a rotor shaft. These laminations include housings for permanent magnets and perforations to create flux barriers to radially direct the magnetic flux from the magnets to the stator and to promote the creation of a reluctant torque.

This type of rotor is generally housed inside a stator which carries electric windings making it possible to generate a magnetic field making it possible to drive the rotor in rotation.

As described in patent application WO2016188764, the rotor of a synchro-reluctant machine may comprise a plurality of axial recesses which pass right through the plates. A first series of axial recesses, arranged radially one above the other and at a distance from each other, form housings for magnetic flux generators, here permanent magnets in the form of a bar rectangular. However, it has been found that back EMF harmonics and torque ripple are important in this type of permanent magnet assisted reluctance synchronous machine. This can generate jolts and vibrations at the level of the rotor, causing discomfort when using this machine.

Patent application WO2018083639 describes a rotor of a synchronous reluctance motor made by stacking a plurality of superimposed sheets, each of which has two series of openings. In certain openings, materials such as copper are introduced in order to operate the motor in asynchronous mode. In this case, it is possible to open the flux barriers on the periphery of the rotor in order to favor the creation of the magnetic field.

A general objective targeted by the invention is to achieve better guiding of the magnetic field in a synchro-reluctant electric machine assisted by permanent magnets in order to reduce the vibrations at the level of the rotor, in particular by opening the tangential bridges, while preserving good mechanical strength.

Summary of the invention

The invention aims to reduce the magnetic short-circuit by opening the tangential bridges, which has the impact of increasing the magnetic flux in the air gap and therefore improving the electromagnetic performance of the synchro-reluctant machine.

This device has been optimized to ensure a reduction in torque ripple, an increase in torque especially at low rotational speed and a reduction in harmonic content when idle.

According to the invention, a synchro-reluctant electric machine comprises a rotor and a stator. The rotor comprising p pairs of magnetic poles, each magnetic pole having a magnetic pole axis, each magnetic pole comprising a plurality of permanent magnets spaced in a radial direction from said rotor. The rotor includes a plurality of flux barriers, a plurality of radial bridges arranged between the permanent magnets and the flux barriers and a plurality of tangential bridges arranged at the ends of the flux barriers facing the periphery of the rotor. The tangential bridges comprise a width (a) along a radial axis of said rotor and an opening (b) along a direction tangential to said radial axis.

According to one embodiment, each magnetic pole is composed of at least two magnets positioned in axial recesses. According to one embodiment, each pole comprises at least two radially spaced flux barriers, each said flux barrier comprising an axial recess and a pair of two angled recesses, said angled recesses being positioned on either side of each axial recess and said flux barriers are substantially U-shaped.

According to one embodiment, the ratio between the width (a) and the opening (b) satisfies the following expression: 1a<b<3a, preferably 2a<b<3a.

According to one embodiment, the two inclined recesses of at least one pair of two inclined recesses each comprise a permanent magnet and the ratio between the width (a) and the opening (b) satisfies the following expression: 1a<b <3y.

According to one embodiment, the opening of the tangential bridges is located substantially at the center of said tangential bridges.

According to one embodiment, the opening of at least one set of tangential bridges is located eccentrically relative to the center of said tangential bridges.

According to one embodiment, the flow barriers have a substantially constant radial section.

According to one embodiment, said openings of said tangential bridges have a substantially rectangular radial shape.

According to one embodiment, said rotor is formed by a stack of laminations arranged on a rotor shaft, said laminations being made of ferromagnetic material.

According to one embodiment, the air gap separating said rotor and said stator has a variable, continuously undulating radial thickness.

Other characteristics and advantages of the device according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the appended figures and described below.

List of Figures

FIG. 1A schematically illustrates a portion of the radial section of a rotor in order to illustrate the positioning of the permanent magnets, the flux barriers and the bridges inside the rotor according to an embodiment of the invention. FIG. 1B schematically illustrates a portion of the radial section of a rotor without the permanent magnets according to one embodiment of the invention.

Figure 2 illustrates, schematically and in a non-limiting manner, a closed tangential bridge and an open tangential bridge.

Figure 3A shows the average torque plot and Figure 3B shows the ripple as a function of normalized opening length (^).

FIG. 4 illustrates the variation of the torque as a function of the speed according to an exemplary embodiment.

Figure 5 illustrates the evolution of the average torque as a function of the electrical position according to an example embodiment.

Figure 6 illustrates the evolution of the no-load voltage at a rotation of 8000 rpm as a function of the electrical position according to an example embodiment.

Detailed description of the invention

The present invention relates to a synchro-reluctant electric machine assisted by permanent magnets. Conventionally, such an electric machine comprises a rotor and a stator.

In a synchro-reluctant electric machine assisted by permanent magnets, empty spaces of material are created in the rotor. These empty spaces are called flow barriers. In the majority of cases the flow barriers are made of air but they can also be replaced by a non-magnetic material and use a specific shape, for example dovetail in order to increase the mechanical strength.

In addition, metal bridges are provided on the rotor to the mechanical strength of the rotor. These bridges connect the different parts of the rotor separated by the flow barriers. These bridges cause leakage flux to appear in the flux barriers and result in a loss of electromagnetic performance. The thickness of the bridges depends on the speed at which the rotor will turn. When it is desired to increase this speed limit, it would be necessary to increase the thickness of the bridges to ensure good mechanical strength, which goes against good electromagnetic performance. In order to increase the saliency ratio of the motor, in one embodiment, permanent magnets may be disposed in the flux barriers. The magnets participate in guiding the circulation of the flux in the desired axis, which causes an increase in power, efficiency and torque. In synchro-reluctant machines, the architecture of the rotor therefore plays an important role in the operating principle of this type of machine. In synchro-reluctant machines with flux barriers assisted by permanent magnets, the main design parameter which controls the compromise between electromagnetic performance and mechanical resistance is therefore the thickness of the bridges which connect the different portions of the rotor separated by the barriers of flow. These bridges generate a low reluctance path which bypasses the flux barriers and loops the magnetic flux created by the permanent magnets back directly onto them without generating any magnetic effect for the operation of the electric machine. To increase the performance of the machine, the thickness of the bridges can be reduced as much as possible.

The object of the invention is aimed at an innovative design of the rotor of a synchro-reluctant machine assisted by permanent magnets, with a preferably "Swallowtail" style structure producing a high power density. However, the object of the invention is not limited to electrical machines of the "Swallowtail" type. Indeed, the object of the invention also relates to symmetrical machines. The structure of the "Swallowtail" type barrier rotor has the advantage of increasing the average torque and reducing the torque ripple for different operating points.

Generally, this type of electric machine is composed of two types of bridges, a tangential bridge which short-circuits the barrier on the peripheral side of the rotor and a radial bridge which short-circuits the barrier on the magnet side. The radial bridges are arranged between the permanent magnets and the flux barriers, and the tangential bridges are arranged at the ends of the flux barriers near the periphery of the rotor.

Figures 1 A and 1 B illustrate, schematically and in a non-limiting manner, a partial view of an electric machine according to one embodiment of the invention. The invention consists of a synchro-reluctant electric machine comprising a rotor (1) and a stator (2). The rotor (1) comprises p pairs of magnetic poles, each magnetic pole comprising a magnetic pole axis (X), each magnetic pole comprising a plurality of permanent magnets (3) spaced in a radial direction from said rotor (1), the rotor (1) comprising a plurality of flux barriers, a plurality of radial bridges (5) arranged between the permanent magnets and the flux barriers and a plurality of tangential bridges (6) arranged at the ends of the flux barriers facing the periphery of the rotor (1). The tangential bridges (6) comprise a width (a) along a radial axis of said rotor (1) and an opening (b) along a direction tangential to said radial axis. Indeed, due to the weak influence of the tangential bridge on the mechanical response, this solution proposes to open the tangential bridge. This structure makes it possible to reduce magnetic flux leakage due to short- magnetic circuits caused by tangential bridges without disturbing the mechanical strength. Reducing this magnetic flux leakage directly implies an increase in the electromagnetic performance of the motor, thus increasing the maximum torque that the motor can generate.

The aim of the invention is to reduce the magnetic short-circuit caused by the tangential bridges, the magnetic short-circuit has the impact of reducing the magnetic flux in the air gap and therefore reducing the electromagnetic performance of the machine, while holding account of the impact on mechanical strength.

According to one embodiment, each magnetic pole is composed of at least two permanent magnets (7) positioned in axial recesses, the axial recesses are spaced radially from each other. Figure 1 illustrates, by way of example, a rotor of a synchro-reluctant electrical machine with three permanent magnets per magnetic pole, but this embodiment may alternatively comprise another number of 2, 4 or more permanent magnets. In this figure 1, there are 6 flux barriers (10), 3 radial bridges (5) and 3 tangential bridges (6) on each side of the magnetic pole axis. The tangential bridges (6) consist of a set of two branches and an opening space between the two branches. This schematic representation makes it possible to locate the components of a magnetic pole and can be reproduced at different angular intervals on the rotor.

According to one embodiment, each pole comprises at least two radially spaced flux barriers, each said flux barrier comprising a pair of two angled recesses (10), said angled recesses (10) being positioned on either side of each axial recess (9) in which the permanent magnets are arranged. The assembly formed by said flux barriers and the axial recesses have substantially a U-shape. FIG. 1A illustrates, by way of example, 3 flux barriers and a configuration with 4 pairs of poles, but the invention also relates any synchro-reluctant electric machine having a different number of pairs of poles, such as for example 3, 5 or 6 pairs of poles and a different number of flux barriers, according to the needs of the implementation of the invention.

According to one embodiment, the ratio between the width (a) and the opening (b) can satisfy the following expression: 1 a<b<3a. Preferably, the ratio between the width (a) and the opening (b) can satisfy the following expression: 2a<b<3a. These ranges of values allow an effect on the torque and on the torque ripples. Indeed, if the bridge opening is too small, the gain on the torque is too small because the flow still manages to loop back. Conversely, when the opening is too large, there is no longer any interest because the magnetic field saturates elsewhere in the machine. FIG. 1B illustrates, by way of example, a portion of the radial section of a rotor without the permanent magnets. This figure highlights the radial geometry of the rotor. We can observe the notches allowing to center in position the permanent magnets. This figure can also illustrate the configuration of a machine according to one embodiment, in which there are only radial magnets (not shown). In this configuration, the lateral permanent magnets are absent.

Figure 2 illustrates, schematically and in a non-limiting manner, a closed tangential bridge and an open tangential bridge according to one embodiment of the invention. This figure also shows the dimensions: thickness (a) and opening (b), which make it possible to characterize the invention.

According to one embodiment, the opening of the tangential bridges can be located substantially in the center or, alternatively, eccentrically with respect to the center of said tangential bridges, which can have the advantage of achieving symmetry with respect to the axis (X), or to improve magnetic performance.

According to one embodiment, the flow barriers can have a substantially constant radial section. The section of the flux barriers in the radial plane is directly linked to the section of the magnetic flux conductive parts in the same plane. The magnetic flux conductive parts (see the gray parts in Figure 1) have a substantially U-shaped shape, parallel to the flux barriers. In FIG. 1, it can be observed that the horizontal and inclined conductive sections have a substantially constant section, which makes it possible to guide and maintain the density of the magnetic flux towards the periphery of the rotor.

According to one embodiment, said openings of said tangential bridges (6) may have a substantially rectangular radial shape. Indeed, the opening of the bridge in rectangular shape is visible in Figure 2. However, the radial section can also be made in other shapes, such as for example the trapezium, with the sides inclined towards the center of the rotor or, on the contrary towards the periphery of the rotor. Other geometric shapes can be chosen depending on the manufacturing techniques used.

According to one embodiment, said rotor (1) can be formed by a stack of laminations arranged on a rotor shaft, said laminations being made of ferromagnetic material. However, other embodiments of the rotor are possible, such as molding and 3d printing (also called additive manufacturing).

According to one embodiment, which the air gap separating said rotor and said stator may have a variable, continuously undulating radial thickness. This embodiment amplifies the advantages of a machine having open tangential bridges with the advantages of a variable air gap shape in order to seek both the low speed maximum torque performance of a low air gap machine and the power performance at high rotational speed of a machine with a large air gap. According to a variant of this embodiment, the air gap can have a thickness of between 0.4 mm and 1 mm and the average thickness of said air gap is preferably equal to 0.6 mm.

It goes without saying that the invention is not limited solely to the embodiments of the recesses, described above by way of example, on the contrary it embraces all variant embodiments.

Examples

Other characteristics and advantages of the device according to this embodiment will become apparent on reading non-limiting examples of embodiments below, with reference to the appended figures and described below.

According to a first example, it is possible to implement a simplified rotor of a synchro-reluctant machine assisted by permanent magnets with four pairs of poles, each pair of poles comprising three magnets arranged radially, and six flux barriers. In the remainder of the application, this type of machine is referred to as machine No. 1. It should be noted that the size of the opening of the bridge does not impact the mechanical performance of the sheet from the moment the sheet is open. For this embodiment, the opening can satisfy the following expression: 1a<b<3a.

In order to illustrate the advantages of this embodiment for a rotor of type machine n°1, by way of example, we analyze the mechanical stresses and the estimation of the advantages for the magnetic flux as follows, by comparison with the same rotor without opening tangential bridges:

The analysis of the Von Mises stress can be used as the first criterion of mechanical strength of our sheets in centrifugation. It is then observed that the stress initially in the tangential bridge is reflected on the radial bridge thus increasing the maximum stress in the radial bridge from 409 MPa to 420 MPa. A second criterion for evaluating the impact on the mechanical performance can be the deformation of the sheet. In conclusion, the tangential bridges are less stressed mechanically compared to the radial bridges in centrifugation in the case of sheets type machine n°1. Opening the tangential bridges to improve the electromagnetic performance therefore has a low impact on the degradation of the mechanical reliability. Regarding the impact at the electromagnetic level, the width of the opening has a significant impact on the electromagnetic performance. In FIG. 3A, the evolution of the average torque is plotted, and respectively in FIG. 3B the ripple as a function of the length of the normalized opening (^). On the abscissa axis of FIGS. 3A and 3B, we find the variation of the ratio (a/b) without dimension. On the ordinate axis of FIG. 3A, we find the variation of the torque at maximum current and low speed expressed in Nm, and on the ordinate axis of FIG. 3B, we find the variation of the torque ripple, expressed in %. It is then observed in FIG. 3A that the average torque increases until it reaches a plateau for an opening around 3 times the thickness of the bridge (a). In the same way, the ripple decreases then tends to increase. Thus we can define in our case an ideal for an opening around 1 to 3 times the bridge thickness (a).

Finally, the advantage of this embodiment can be expressed in the form of the graph in figure 4, which represents the variation of the torque, on the ordinate axis, expressed in Nm as a function of the speed, on the axis abscissa, expressed in revolutions/min. The two curves A and B represent a machine with the tangential bridges open (A) and respectively a machine with the tangential bridges closed (B). In this figure 4, it can be seen that the gain was still present at high speed and makes it possible to gain several kW at high speed. There is a gain over the entire speed range, making it possible to increase the maximum power of the machine as well as the power at high speed.

According to a variant of this embodiment, in which the two inclined recesses (10) of at least one pair of two inclined recesses (10) each comprise a permanent magnet (8) and the ratio between the width (a) and the opening (b) satisfies the following expression: 1a<b<3a.

According to a first example, it is possible to implement a simplified rotor of a synchro-reluctant machine assisted by permanent magnets with four pairs of poles, each pair of poles comprising three magnets arranged radially, six flux barriers and lateral permanent magnets (8) in the spaces (10). In the rest of the application, this type of machine is called machine No. 2.

Machine n°2 is a synchro-reluctant machine assisted by lateral permanent magnets (8) and, by way of example, the mechanical stresses and the estimation of the advantages for the magnetic flux are analyzed as follows, compared to the same electric machine without opening the tangential bridges:

The analysis of the Von Mises stress can be used as the first criterion of mechanical strength of our sheets in centrifugation. We then observe that the constraint initially in the tangential bridge affects the radial bridge thus increasing the maximum stress in the radial bridge from 230MPa to 314MPa. Here the impact is greater than on machine n°1 because the side magnets bring more mass in centrifugation and stress the tangential bridges more locally. In addition, the bridges are very thin, close to the minimum possible realization limit (0.4mm), this has the impact that the quantity of material removed by the removal of tangential bridges whose thickness is identical (0.4mm) on our designs is proportionally greater. In summary, the impact of removing the tangential bridges in this example is less negligible on the mechanical performance, but it remains acceptable.

Regarding the impact at the electromagnetic level, the width of the cut has a significant impact on the electromagnetic performance. In Figure 5, the evolution of the average torque is plotted, on the abscissa axis, expressed in N m as a function of the electrical position, on the abscissa axis, expressed in °e on a scale between 0 and 60 °e, which corresponds, in our case on a rotor with 4 pairs of magnetic poles, to an angular interval of 15° mechanical. In figure 5, there is a gain of 5N.m (+1.3%) between curves A and B, which represent a rotor with the tangential bridges closed and respectively with the tangential bridges open. The analysis of this figure 5 makes it possible to observe another advantage of this embodiment, which consists of a reduction in the ripple. There is thus a gain over the entire speed range allowing us to gain approximately 2kW on the maximum power and the power at high speed.

Still by way of example, the following table 1 reflects the variation in power measured on a machine of type machine type no. 2 for the maximum power and the power obtained at a rotational speed of 8000 rpm depending on whether the measurements concern a machine type machine type n°2 provided with closed tangential bridges and respectively on a machine type machine type n°2 with open tangential bridges.

[Table 1]

In Figure 6, we have plotted the evolution of the no-load voltage (BEMF, from the English "back electromotive force") on the abscissa axis, expressed in V at a rotation of 8000 rpm as a function of the electrical position, on the abscissa axis, expressed in °e on a scale between 0 and 90°e. Curves A and B represent a machine with closed and open tangential bridges respectively. This difference is significantly negligible. on the variation of the BEMF but the difference is especially visible on the harmonic content. In fact, it is possible to observe in the zones with a maximum positive and negative amplitude a reduced harmonic content, which represents a significant improvement in the implementation of this embodiment.

Claims

Claims Synchro-reluctant electric machine comprising a rotor (1) and a stator (2), the rotor (1) comprising p pairs of magnetic poles, each magnetic pole comprising a magnetic pole axis, each magnetic pole comprising a plurality of magnets permanent magnets spaced in a radial direction of said rotor (1), the rotor (1) comprising a plurality of flux barriers, a plurality of radial bridges (5) arranged between the permanent magnets and the flux barriers and a plurality of tangential bridges ( 6) arranged at the ends of the flux barriers facing the periphery of the rotor (1), characterized in that the tangential bridges (6) comprise a width (a) along a radial axis of said rotor and an opening (b) along a tangential direction to said radial axis. Machine according to claim 1, in which each magnetic pole is composed of at least two magnets (7) positioned in axial recesses. Machine according to claim 2, wherein each pole comprises at least two radially spaced flux barriers, each said flux barrier comprising an axial recess (9) and a pair of two angled recesses (10), said angled recesses (10) being positioned on either side of each axial recess (9) and said flow barriers have a substantially U-shape. Machine according to one of the preceding claims, in which the ratio between the width (a) and the opening ( b) satisfies the following expression: 1 a<b<3a, preferably 2a<b<3a. Electric machine according to one of Claims 2 or 3, in which the two inclined recesses (10) of at least one pair of two inclined recesses (10) each comprise a permanent magnet (8) and the ratio between the width (a ) and opening (b) satisfies the following expression: 1 a<b<3a. Machine according to one of the preceding claims, in which the opening of the tangential bridges is located substantially at the center of the said tangential bridges. Machine according to one of Claims 1 to 5, in which the opening of at least one set of tangential bridges is located eccentrically with respect to the center of said tangential bridges. Machine according to one of the preceding claims, in which the flow barriers have a substantially constant radial section. Electric machine according to one of the preceding claims, in which the said openings of the said tangential bridges (6) have a substantially rectangular radial shape. Machine according to one of the preceding claims, in which the said rotor (1) is formed by a stack of laminations arranged on a rotor shaft, the said laminations being of ferromagnetic material. Machine according to one of the preceding claims, in which the air gap separating the said rotor (1) and the said stator has a variable, continuously undulating radial thickness.