Electric machine comprising variable-flux and constant-flux permanent magnets

Abstract

The invention relates to an electric machine comprising a stator comprising windings (22) and a rotor comprising permanent magnets arranged in a plurality of poles, each pole of the rotor comprising two primary constant-flux permanent magnets (4) and two secondary constant-flux permanent magnets (5), characterized in that each pole of the rotor further comprises a radial-flux variable-flux permanent magnet (6), and two tangential-flux variable-flux permanent magnets (7), the variable-flux permanent magnets being magnetizable by means of one or more current pulses applied to the windings of the stator.

Description

[0001] DESCRIPTION

[0002] TITLE OF THE INVENTION: ELECTRICAL MACHINE WITH ROTOR INCLUDING PERMANENT MAGNETS WITH VARIABLE MAGNETIC FLUID

[0003]

[0001] The present invention claims priority from French application No. 2409599 filed on 10.09.2024, the content of which (text, drawings and claims) is incorporated herein by reference.

[0004]

[0002] The invention relates to an electric machine with a permanent magnet rotor. The electric machine in question may be a traction machine in an electric or hybrid vehicle.

[0005]

[0003] In particular, we are interested in types of electrical machines having on the one hand permanent magnets with constant flux and on the other hand permanent magnets with variable flux.

[0006]

[0004] Variable flux permanent magnets can be magnetically polarized as required, either to contribute to the magnetization flux in the same direction as constant flux permanent magnets, or conversely, to reduce the magnetic flux created by constant flux permanent magnets at the air gap between the rotor and stator of the machine. In the trade, variable flux permanent magnets are also called 'memory permanent magnets'.

[0007]

[0005] This capability allows, on the one hand, a rotor generating a significant magnetic flux for phases where a significant torque at zero or moderate speed is required, and on the other hand, for phases with high rotational speed, a rotor generating a lower magnetic flux in order to optimize efficiency at high rotational speeds.

[0008]

[0006] An example of such a configuration is given by document US2023108575.

[0009]

[0007] However, the use of variable flux permanent magnets must be carried out with caution, as they are immersed in a magnetic field created on the one hand by the constant flux permanent magnets located in their vicinity and on the other hand, when the machine is operating, by a field created by the excited windings in the machine's stator.

[0008] Thus, there is a risk of unintentional demagnetization of the variable flux permanent magnets during machine operation. The coercivity of variable flux permanent magnets characterizes the difficulty in changing their magnetic flux density. High coercivity reduces the risk of demagnetization, but increases the current pulse amplitudes required by the stator to change their magnetization, which leads to oversizing the inverter driving the electric machine, consequently increasing the machine's cost.Put another way, coercive force represents resistance to demagnetization, and it is difficult in practice to find a compromise between a coercive force that is too weak and a coercive force that is too strong.

[0010]

[0009] The inventors therefore sought to propose new solutions for designing and implementing an electrical machine comprising a rotor which cleverly combines the use of constant flux permanent magnets and variable flux permanent magnets.

[0011]

[0010] To this end, the present invention proposes an electrical machine comprising a stator including windings and a rotor including permanent magnets arranged according to a plurality of poles, each pole of the rotor including two main constant flux permanent magnets and two secondary constant flux permanent magnets, characterized in that each pole of the rotor further includes a radial flux variable permanent magnet, and two tangential flux variable permanent magnets, the variable flux permanent magnets being able to be magnetized by means of one or more current pulses driven in the stator windings, so as either on the one hand to contribute to establishing flux lines in addition to the flux lines produced by the constant flux permanent magnets in order to maximize the magnetic flux produced in the air gap between the rotor and the stator,either, on the other hand, in such a way as to establish flux lines in antagonism with the flux lines produced by the constant flux permanent magnets or by deflecting them, in order to minimize the magnetic flux produced in the air gap between the rotor and the stator.

[0011] Thanks to these arrangements, an optimized solution is proposed which makes it possible to obtain a maximum magnetic flux in the air gap for circumstances of high torque demands at low speed and, on the other hand, to obtain a greatly reduced magnetic flux for operation at high rotational speeds.

[0012]

[0012] The proposed combination of radial and tangential flux-changing permanent magnets makes it possible to combine a moderate coercive force necessary for the reversal of the polarization of the variable flux permanent magnets, and makes it possible to avoid unintentional and undesirable demagnetization.

[0013]

[0013] Advantageously, the size, shape factor and grade of ferromagnetic material can be chosen differently on the one hand for radial flux permanent magnets and on the other hand for tangential flux permanent magnets.

[0014]

[0014] It is noted that there are at least six magnets per pole, given that, as will be seen later, permanent magnets with variable flux and tangential flux are shared between two adjacent poles. Thus, a given pole can be referred to as a half-magnet with regard to permanent magnets with variable flux and tangential flux.

[0015]

[0015] It should be noted that permanent magnets with constant flux exhibit a very high coercive field, out of reach in practice, and consequently their residual induction remains unchanged during their use in the electrical machine of interest.

[0016]

[0016] The proposed configuration allows for a flow intensity value ratio between the maximum flow configuration and the reduced flow configuration of the order of 2.

[0017]

[0017] According to one embodiment, the radial flux variable permanent magnet is composed of a first type of AlNiCo, and the two tangential flux variable permanent magnets are composed of a second type of AlNiCo, AlNiCo generally designating an alloy composed of aluminium, nickel and cobalt.

[0018]

[0018] Wherefore, we have a degree of freedom in the choice of Alnico material shades and in particular concerning their respective coercive field.

[0019] According to one embodiment, the respective materials are chosen so that the coercive field of the first type of AlNiCo is less high than the coercive field of the second type of AlNiCo.

[0019]

[0020] Thus, since tangential flux permanent magnets are closer to the stator than radial flux permanent magnets, for a given pulse amplitude at the stator winding, the magnetization of the flux permanent magnets can be changed homogeneously for both types of material and therefore for both types of flux permanent magnets simultaneously.

[0020]

[0021] According to one embodiment, the radial flux variable permanent magnet is composed of AlNiCo 9. This grade of AINiCo material exhibits a coercive force (coercive field) in the vicinity of 1300 to 1400 Oersteds.

[0021]

[0022] According to one embodiment, the tangential flux variable permanent magnet is composed of AlNiCo 8H. This grade of AlNiCo material exhibits a coercive force in the vicinity of 1900 to 2000 Oersteds.

[0022]

[0023] According to one embodiment, the radial flux variable permanent magnet forms a first hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets, and the two tangential flux variable permanent magnets form a second hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets.

[0023]

[0024] This has advantageously introduced a new hybridization method combining the two types of permanent magnets.

[0024]

[0025] In one embodiment, the tangential flux-changing permanent magnets are shared between two adjacent poles. This allows for resource sharing between two adjacent poles.

[0025]

[0026] In one embodiment, the machine comprises N poles, and the rotor carries N radial flux-changing permanent magnets and N tangential flux-changing permanent magnets. Thanks to the sharing of the tangential flux permanent magnets, there are as many tangential flux permanent magnets as there are poles on the entire machine, whereas from the perspective of a particular pole, it benefits from the effect of two tangential flux permanent magnets, or more precisely, two half-tangential flux permanent magnets.

[0027] According to one embodiment, constant flux permanent magnets are composed primarily of an NdFeB alloy, NdFeB generally referring to an alloy composed of neodymium, iron, and boron. This material exhibits a coercive force (coercive field) exceeding 10,000 Oersteds, surpassing the field values ​​encountered inside the machine's rotor.

[0026]

[0028] According to one embodiment, considering the cross-section to the axis, the area covered by permanent magnets with variable flux is greater than the area covered by permanent magnets with constant flux.

[0027]

[0029] Consequently, more AlNiCo and less NdFeB are used compared to known solutions. Therefore, the proposed solution reduces the use of rare earth elements, such as neodymium.

[0028]

[0030] The invention further relates to an electric or hybrid vehicle comprising an electric drivetrain including an electric machine as described above.

[0029]

[0031] The invention will be further detailed by describing non-limiting embodiments, and based on the accompanying figures illustrating variants of the invention, in which:

[0030] [Fig.1] schematically represents a cross-section of an electrical machine transverse to its axis;

[0031] [Fig.2] represents a magnetization graph relating to the behavior of permanent magnets with variable flux;

[0032] [Fig.3] illustrates a section view of a rotor pole, in an example of a machine where the present invention is implemented, in a maximum air gap flux configuration;

[0033] [Fig.4] represents the magnetic field lines produced by permanent magnets according to the configuration of figure 3;

[0034] [Fig.5] is analogous to figure 3 and illustrates a section view of one pole of the stator, in a reduced air gap flux configuration;

[0035] [Fig.6] represents the magnetic field lines produced by permanent magnets according to the configuration of figure 5;

[0036] [Fig.7] shows a cross-sectional view of two rotor poles, illustrating the pooling of tangential flux-variable permanent magnets, in an example of a machine where the present invention is implemented, with a maximum air gap flux configuration shown in solid line and in a reduced air gap flux configuration shown in dashed lines.

[0037] [Fig.8] illustrates the field lines produced during the magnetization pulses of the variable flux magnets, generated by the stator windings.

[0038]

[0032] In the various figures, the same reference numerals designate identical or similar elements. For the sake of clarity, some elements are not necessarily shown to scale. This applies in particular to the air gap, the thickness of which may have been exaggerated to facilitate understanding.

[0039]

[0033] We are interested here in an electric or hybrid motor vehicle, namely a vehicle equipped with an electric drivetrain.

[0040]

[0034] In practice, the electric or hybrid vehicle includes an electromotor unit. The electromotor unit includes an electric machine, used as a motor or as an alternator depending on the driving circumstances.

[0041]

[0035] Generally in the context of the present invention, the electric machine 1 is considered to comprise two sub-assemblies: a first static sub-assembly called stator 2 and a second rotating sub-assembly called rotor 3. The machine has an axis denoted X. The rotor is mounted on a shaft 32 received on roller bearings for rotational mounting relative to the stator or the machine housing.

[0042]

[0036] The stator 2 comprises a stack, along X, of a plurality of ferromagnetic laminations 20, with notches 21 housing windings 22. This is assumed to be known in itself and therefore not described in detail here.

[0043]

[0037] The rotor 3 comprises a stack, along X, of a plurality of ferromagnetic sheets 30. The rotor is separated from the stator by an empty cylindrical gap called the air gap EF.

[0044]

[0038] The rotor 3 comprises permanent magnets arranged in a plurality of poles. In the illustrated example, the machine has 8 poles, i.e., 4 pairs of poles.

[0045]

[0039] The ferromagnetic laminations of the rotor have recesses for receiving the permanent magnets. As shown in Figure 3, after the magnets are mounted, a gap 34 remains at the short end of each magnet, preventing the magnetic field lines of a magnet from closing along a short circuit. The gap 34 is filled with air; it can be formed as an internal orifice within the pole, or as an edge orifice separating the pole shown from the adjacent pole.

[0046]

[0040] Each rotor pole comprises four constant flux permanent magnets. More precisely, each rotor pole comprises two main constant flux permanent magnets, designated 4, and two secondary constant flux permanent magnets, designated 5.

[0047]

[0041] Each constant flux permanent magnet is an NdFeB type magnet. This type of magnet has a high energy density and is widely used in permanent magnet rotating machines.

[0048]

[0042] The pole is organized around a pole axis denoted d.

[0049]

[0043] The permanent magnets are arranged symmetrically on either side of the pole axis d.

[0050]

[0044] Each of the two main permanent magnets 4 has a rectangular cross-section (transverse to the X-axis) with two long sides and two short sides. According to one embodiment, the long side is at least five times longer than the short side.

[0051]

[0045] The large sides of the two main permanent magnets are inclined with respect to the axis of pole d, at an angle between 30° and 45°, they are arranged in a V open radially outwards.

[0052]

[0046] Each of the two secondary permanent magnets 5 is also rectangular in section, but smaller in size, with two long sides and two short sides.

[0053]

[0047] According to one embodiment, the longer side is slightly larger than the shorter side.

[0054]

[0048] The secondary permanent magnets 5 are also inclined with respect to the pole axis d.

[0055]

[0049] The position and inclination of the primary permanent magnets 4 and secondary permanent magnets 5 is determined to maximize the radial flux exiting the rotor and traveling through the air gap EF towards the stator.

[0056]

[0050] The total cross-sectional area occupied by the cross-sections of the four permanent magnets with constant flux is denoted S45.

[0057]

[0051] Each pole of the rotor includes a radial flux variable permanent magnet, denoted 6.

[0052] This variable flux permanent magnet has a rectangular cross-section with two long sides and two short sides.

[0058]

[0053] The long sides are perpendicular to the axis of pole d. The induction flux at the center of this permanent magnet is directed along the radial direction, hence its name of radial flux permanent magnet.

[0059]

[0054] In addition, each pole of the rotor includes a first variable flux permanent magnet 7a arranged on a first edge of the pole which forms the boundary with the adjacent pole, positioned and oriented in such a way that the induction flux at the center of this permanent magnet is directed in the tangential or circumferential direction, hence its name of tangential flux permanent magnet.

[0060]

[0055] In addition, symmetrically with respect to the axis of pole d, the pole in question comprises a second variable flux permanent magnet 7b arranged on a second border of the pole which forms the boundary with the other adjacent pole, positioned and oriented in such a way that the induction flux at the center of this permanent magnet is directed in the tangential direction, hence its name of tangential flux permanent magnet.

[0061]

[0056] The induction fluxes produced by the permanent magnets are represented by arrows in figures 3, 5, and 7.

[0062]

[0057] The first and second radial flux permanent magnets 7a, 7b are generically identified together by the reference 7.

[0063]

[0058] The radial flux permanent magnet 6 is composed of a first type of AlNiCo. The acronym AINiCo generally designates an alloy composed of aluminium, nickel and cobalt.

[0064]

[0059] According to one example, the radial flux variable permanent magnet 6 is composed mainly of AlNiCo 9. AlNiCo 9 exhibits a coercive force (coercive field) in the vicinity of 1300 to 1400 Oersteds.

[0065]

[0060] The two tangential flux variable permanent magnets 7 are composed of a second type of AlNiCo.

[0066]

[0061] According to one example, two tangential flux-changing permanent magnets 7 are composed primarily of AlNiCo 8H. AlNiCo 8H exhibits a coercive force (coercive field) in the vicinity of 1900 to 2000 Oersteds.

[0062] Generally, the radial flux-changing permanent magnet is composed of a first type of AlNiCo, and the two tangential flux-changing permanent magnets are composed of a second type of AlNiCo.

[0067]

[0063] Generally, the respective materials are chosen so that the coercive field of the first type of AlNiCo is lower than the coercive field of the second type of AlNiCo.

[0068]

[0064] It is noted that the energy density of permanent magnets with variable flux 6.7 is much lower than the energy density of permanent magnets with constant flux 4.5.

[0069]

[0065] In practice, the energy density of permanent magnets with constant flux 4.5 is four or even five times greater than the energy density of permanent magnets with variable flux 6.7.

[0070]

[0066] The radial flux variable permanent magnet 6 forms a first hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets.

[0071]

[0067] More specifically, the radial flux permanent magnet 6 is magnetically located in series with the main permanent magnets 4 and is magnetically located in parallel with the secondary permanent magnets 5.

[0072]

[0068] Similarly, the two tangential flux permanent magnets 7 form a second hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets.

[0073]

[0069] More specifically, the first tangential flux permanent magnet 7a is magnetically located in series with the secondary permanent magnet 5 which is closest to it, and is magnetically located in parallel with the main permanent magnet 4 which is closest to it.

[0074]

[0070] Similarly, the second variable flux permanent magnet 7b is magnetically located in series with the secondary permanent magnet 5 which is closest to it, and is magnetically located in parallel with the main permanent magnet 4 which is closest to it.

[0075]

[0071] The total cross-sectional area occupied by the cross-sections of the four variable flux permanent magnets is denoted S67.

[0076]

[0072] It is noted that the value of S67 is greater than the value of S45. In other words, the area S67 covered by all the permanent magnets with variable flux of the pole is greater than the area S45 covered by all the permanent magnets with constant flux of the pole.

[0077]

[0073] As can be seen in Figure 7, the tangential flux-changing permanent magnets 7a, 7b are shared between two adjacent poles.

[0078]

[0074] Thus for the illustrated machine comprising 8 poles, and the rotor carries 8 permanent magnets with radial flux 6, and 8 permanent magnets with tangential flux 7. Of course, the same logic applies regardless of the number N of poles, whether N=8 or N is different from 8.

[0079]

[0075] By turning to figures 3 and 4, it can be seen that the magnetic flux lines imparted by the variable flux permanent magnets contribute positively to the magnetic flux lines generated by the constant flux permanent magnets.

[0080]

[0076] After reversing the magnetization polarities of the variable-flux permanent magnets, the rotor is in the situation illustrated in Figures 5 and 6. In this configuration, the magnetic flux lines emitted by the variable-flux permanent magnets deflect the magnetic flux lines generated by the constant-flux permanent magnets, causing the forces to loop back on themselves within the rotor. The result is a substantially reduced magnetic flux in the air gap EF between the rotor and the stator.

[0081]

[0077] Regarding the rate of reduction of magnetic flux brought about by the reversal of polarity of the variable flux magnets, the inventors have shown that it is possible to reduce by at least 40%, or even by at least 50%, the induction crossing the air gap EF compared to a maximum magnetic flux configuration shown in Figure 4.

[0082]

[0078] In other words, we have a ratio of between 1.8 and 2, or even more, between the maximum induction flux and the reduced induction flux, which represents a very relevant ratio compared to the ratios known in art.

[0083]

[0079] Figure 8 illustrates the circumstance in which the rotor windings 22 induce a magnetic field H in the variable flux permanent magnets to modify their magnetization.

[0084]

[0080] A computer for managing the electric machine controls this sequence of high magnetic fields by controlling the current and phase on the 3 winding phases.

[0081] Referring to Figure 2, this corresponds to the transition from the positive region of the ordinate to a negative region of the ordinate, or vice versa. When the permanent magnets with variable flux are magnetized in the direction of matching the flux generated by the magnets with constant flux, their operating points are located at points 81, 82, and 83, more generally in the upper region of the curve; the field in which they are immersed is not, of course, constant everywhere.

[0085]

[0082] After polarity reversal, their operating points are located at the points marked 85 86 87, more generally in the lower area of ​​the curve.

[0086]

[0083] Curve C1 is representative of the behavior of the second type of Alnico material, composing the tangential flux permanent magnets.

[0087]

[0084] Curve C2, in dotted lines, is representative of the behavior of the first type of Alnico material, composing radial flux permanent magnets, with a lower coercive field, but the induction is higher.

[0088]

[0085] The horizontal line B=0 corresponds to the change in direction of the residual induction. Advantageously, the coercive field values ​​chosen here make it possible to avoid unintentionally passing through point 84 or remaining in areas where B is low.

[0089]

[0086] In practice, when the machine's rotational speed increases, the electric machine control unit sends a strong pulse to the stator windings to reverse the magnetization direction of the variable-flux magnets (this is done on the fly at a predetermined speed, for example). The required pulse duration is very short, in practice a few milliseconds.

[0090]

[0087] Later, after a phase of high-speed operation, if the rotational speed decreases and falls below a predefined threshold, then the electric machine management computer drives another strong pulse (with opposite effect) on the stator windings to reverse the direction of magnetization of the variable flux magnets again and return to the maximum rotor magnetic flux configuration.

[0091]

[0088] Figure 8 illustrates, in dashed lines, the field lines 8 produced by the strong pulses in the stator winding. It can be seen that these lines 8 pass through the long sides of the radial flux permanent magnet, and the same is true for tangential flux permanent magnets.

[0092]

[0089] Returning to figure 7, it shows, for two adjacent poles, on the one hand in solid line a configuration of maximum air gap flux, and on the other hand a configuration of reduced air gap flux illustrated in dotted lines.

[0093]

[0090] The north-south alternation between two adjacent poles is observed, which cleverly allows a single tangential flux magnet 7 to be used as two half-tangential flux magnets.

[0091] Regarding the mounting of the magnets, the magnets are inserted along the X direction into the housings formed by the series of holes in the indexed ferromagnetic sheets 30, aligned with each other along the X axis. It is noted that the magnets, once inserted, are immobilized in both the radial and tangential directions by the shape of the housings that receive them.

Claims

DEMANDS 1. An electrical machine (1) comprising a stator (2) comprising windings (22) and a rotor (3) comprising permanent magnets arranged in a plurality of poles, each pole of the rotor comprising two main constant-flux permanent magnets (4) and two secondary constant-flux permanent magnets (5), characterized in that each pole of the rotor further comprises a radial variable-flux permanent magnet (6), and two tangential variable-flux permanent magnets (7), the variable-flux permanent magnets being able to be magnetized by means of one or more current pulses driven into the stator windings, so as either on the one hand to contribute to establishing flux lines in addition to the flux lines produced by the constant-flux permanent magnets in order to maximize the magnetic flux produced in the air gap between the rotor and the stator,either, on the other hand, in such a way as to establish flux lines in antagonism with the flux lines produced by the constant flux permanent magnets, or by deflecting them, in order to minimize the magnetic flux produced in the air gap between the rotor and the stator.

2. Electric machine according to claim 1, characterized in that the radial flux variable permanent magnet (6) is composed of a first type of AlNiCo, and the two tangential flux variable permanent magnets (7) are composed of a second type of AlNiCo, AlNiCo generally designating an alloy composed of aluminium (Al), nickel (Ni) and cobalt (Co).

3. Electric machine according to claim 2, in which the coercive field of the first type of AlNiCo is lower than the coercive field of the second type of AlNiCo.

4. Electric machine according to claim 2, characterized in that the radial flux variable permanent magnet (6) is composed of AlNiCo 9.

5. Electric machine according to claim 2, characterized in that the tangential flux variable permanent magnet (7) is composed of AlNiCo 8H.

6. Electric machine according to any one of claims 1 to 5, characterized in that the radial flux-changing permanent magnet (6) forms a first hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets, and the two tangential flux-changing permanent magnets (7) form a second hybrid magnetic circuit, i.e. series and parallel, with the constant flux permanent magnets.

7. Electric machine according to any one of claims 1 to 6, characterized in that the tangential flux-changing permanent magnets (7) are shared between two adjacent poles.

8. Electric machine according to claim 7, comprising N poles, the rotor carrying N radial flux-changing permanent magnets (6), and N tangential flux-changing permanent magnets (7).

9. Electric machine according to any one of claims 1 to 8, characterized in that, considering the cross-section to the axis, the area covered by the permanent magnets with variable flux is greater than the area covered by the permanent magnets with constant flux.

10. Electric or hybrid motor vehicle comprising an electric drivetrain including an electric machine according to any one of claims 1 to 9.

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