EP4035254A1

EP4035254A1 - Hybrid transient magnetisation rotor

Info

Publication number: EP4035254A1
Application number: EP20764429.5A
Authority: EP
Inventors: Misa MILOSAVLJEVIC; Denis GROSJEAN; Fabrice LE BERR
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2019-09-23
Filing date: 2020-09-04
Publication date: 2022-08-03
Also published as: WO2021058267A1; FR3101208A1; FR3101208B1

Abstract

The invention relates to a compression device comprising an electrical machine having a hybrid magnetisation rotor with a cylindrical permanent magnet (5) and a hollow shaft (7) made of a material with temporary or modulatable magnetisation.

Description

HYBRID TRANSITIONAL MAGNET ROTOR

Technical area

The present invention relates to the field of compression devices driven by an electric machine, in particular the invention relates to a turbocharger driven by an electric machine.

It relates in particular to a device for compressing a gaseous fluid, here air, by a compressor, alone or associated with a turbine to form a turbocharger, to then send it to all devices and more particularly to the inlet of a internal combustion engine. Indeed, as is widely known, the power delivered by an internal combustion engine is dependent on the quantity of air introduced into the combustion chamber of this engine, a quantity of air which is itself proportional to the density of that tune.

Thus, it is customary to increase this amount of air by compressing the outside air before it is admitted into this combustion chamber when high power is required. This operation, called supercharging, can be carried out by any means, such as a single compressor driven electrically by an electric machine (electrified compressor), or by a compressor associated with a turbine and an electric machine to form an electrified turbocharger.

Prior art In the two aforementioned cases, the electrical machine associated with the compressor can be of different types.

One of these types is an electrical machine with a small air gap and windings close to the rotor which allows optimal guidance of the magnetic flux and optimized efficiency. This type of electric machine has the advantage of a certain compactness, which can sometimes become problematic for its cooling and which requires the use of a specific system to evacuate its losses.

In order not to be intrusive on the air inlet of the compressor, this type of electric machine is conventionally positioned on the back of the compressor in the case of an electrified compressor, or between the compressor and the turbine in the case of an electrified compressor. electrified turbocharger, and this despite the presence of an unfavorable thermal environment in the latter because it is close to the turbine. Generally, the connection between the compressor, the turbine and the electrical machine is rigid. This type of machine can also be positioned on the compressor side but at a distance relatively far from the air inlet so as not to disturb the latter. The connection between the compressor and the machine is then rigid or made using a mechanical coupling.

This type of system is better described in patents US2014373532A, US2009056332A, US2010266430A, US2010247342A, US6449950B, US7360361, EP0874953A1 or EP0912821A1.

The other of these types is an electric machine with a large air gap (called an "Air Gap" machine), the air gap of which can sometimes measure several centimeters. in order to allow the working fluid to pass through this air gap, thus allowing an integration as close as possible to the compression systems, in a more favorable thermal environment.

This arrangement of an electric machine nevertheless has the disadvantage of disturbing and limiting the passage of the magnetic flux between the rotor and the stator through the large air gap, which contributes to limiting the intrinsic efficiency of the electric machine as well as its specific performances (specific power and volume). The high losses in this type of concept also make it necessary to develop specific cooling to remove heat from the rotor and stator or to limit specific performance. Those skilled in the art will have to compensate for this drawback in particular by implementing very high control currents, which increase the cost of the inverter and cause losses in the stator.

This type of electric machine is described in particular in patents EP1995429A1, US2013169074A or US2013043745A.

A third type of electrical machine is the stator grid machine, which can be positioned upstream of the compressor, and for which the air passes through the stator.

Patent application FR3074622A1 details several structures of electric rotor and of mounting the rotor on the shaft of a turbocharger, by relating a rotor to a shaft opening on the compressor side. This rotor is particularly interesting for electric machines with stator grid. These structures are interesting for integrating rotors on already produced turbochargers.

However, most of the time, these structures are hardly compatible with an industrial mass production process, given the manufacturing tolerances and the balancing operations required to be carried out on the rotor to arrive at such a system. In addition, these solutions may require machining a bore into a magnet, which is complex and reduces the magnetic performance of the magnet. Alternatively, a person skilled in the art can also improve the assembly by simplifying it by the use of a solid magnet.

One of the problems with the electrification of compressors concerns the manufacturing costs of the electrical machine and more particularly of the rotor. Indeed, the stresses due to the very high speed of rotation of the turbocharger, of approximately 200,000 revolutions / min and more, are particularly trying for the mechanical resistance of the magnet of the rotor, which induces significant costs to manufacture a device which makes it possible to guarantee the good resistance of the magnetic material.

Another problem is the loss due to the magnetic coupling forces between the rotor and the stator (called no-load losses), more particularly during freewheeling phases, when the electric machine is not activated. Indeed, during phases of non-use of the assistance of the electric machine, the permanent magnet contained in the rotor causes resistance to the free rotation of the axis of the compressor. One solution to this problem is the use of a material with temporary or modular magnetization. In the case of the implementation of a rotor produced solely or mainly with materials with temporary or modulable magnetization, there nevertheless remains a problem to be overcome, namely the difficulty of detecting the rotational movement of the rotor and more particularly of detecting the position of the rotor because of the initial magnetization of the rotor non-existent or too weak to be detectable and usable by the inverter. The present invention overcomes this problem by proposing a hybrid rotor by the judicious introduction of a permanent magnet within a rotor made of a material with temporary or modulable magnetization. Summary of the invention

The present invention relates to a device for compressing a fluid comprising an electrical machine, said electrical machine comprising a rotor and a stator, said compression device comprising a compression shaft on which at least one compression wheel is mounted, said rotor being connected to one end of said compression shaft, said rotor comprising a cylindrical permanent magnet and a hollow shaft, said hollow shaft comprising a hoop portion radially surrounding said cylindrical magnet, characterized in that said cylindrical permanent magnet is a solid piece coaxial with said rotor, said hollow shaft is made of a material with temporary or modular magnetization.

According to one embodiment, the thickness of the hoop is greater than the radius of the cylindrical permanent magnet. According to one embodiment, said permanent magnet cylindrical is made from neodymium, such as NbFeBr, SmCo,

Ferrite or other hard magnetic material. According to one embodiment, said hollow shaft is made of a steel alloy of the iron-chromium-cobalt, aluminum-nickel-cobalt AINiCo type, or any equivalent material. According to one embodiment, the hollow shaft integrally comprises an annular cylindrical portion inserted in said at least one compression wheel, said annular cylindrical portion preferably connecting the compression shaft with play to said at least one compression wheel. .

According to one embodiment, the rotor comprises handling means. According to one embodiment, the rotor has an outside diameter less than or equal to the diameter of the nose of the compression wheel. According to one embodiment, the compression device is a turbocharger combining a turbine and a compressor, in particular for an internal combustion engine.

According to one embodiment, the electric machine is arranged at the level of the gas inlet zone of said turbocharger.

According to one embodiment, the electric machine is a stator grid machine.

According to one embodiment, the electric machine is an electric generator.

Furthermore, the present invention also relates to a method for implementing the compression device in which the magnetization of said device is carried out. hollow shaft for changing said rotor from an initial state in which the rotor is considered inactive to an operating state in which the rotor is considered active.

According to one embodiment of the method of implementing the compression device, the magnetization of said hollow shaft is reduced to change said rotor from an operating state in which the rotor is considered active to an initial state in which the rotor is considered inactive.

According to one embodiment of the method of implementing the compression device, the magnetization and demagnetization (reduction of the magnetization) of said hollow shaft are carried out by applying a current pulse by means of an inverter controlling said electrical machine. , said current pulse having an amplitude greater than the normal operating pulses of the electrical machine.

According to one embodiment of the method of implementing the compression device, the change between the active and inactive state is carried out according to the following transient modes:

1. We choose the active state when we want to transmit power by the electrical machine, said power being modulated by the inverter over a range between a minimum and a maximum, and

2. We choose the inactive state when we want to reduce the magnetic torque between the rotor and the stator. Other characteristics and advantages of the method according to the invention will become apparent on reading the following description of non-limiting examples of embodiments, with reference to the appended figures and described below. Figures

Figure 1 illustrates an electric machine rotor according to one embodiment of the invention.

Figure 2 illustrates a compression device driven by an electric machine according to one embodiment of the invention.

Description of the embodiments

The present invention relates to a device for compressing a fluid, in particular a gas, driven, in a non-exclusive manner, by an electric machine. In other words, the invention relates to the assembly formed by the electric machine and the compression device. Preferably, the compression device is provided for compressing air.

The present invention has the following functional interests: Create a magnetic rotor whose magnetization and the production of magnetic flux can be controlled, thus making it possible to have a magnetically active or passive rotor depending on the use,

Ensure the mechanical strength of the rotor assembly, in particular vis-à-vis the centrifugal forces that apply during rotation,

Guarantee limited losses at the rotor and stator level, so as to improve the efficiency of the electrical part but also to limit internal heating of the rotor and stator and simplify its cooling,

Achieve a high level of concentricity between the electric rotor and the turbocharger shaft in order to obtain a complete mechanical system (turbocharger shaft with rotor of the electric machine) which can be balanced with minimum unbalance,

Present a structure compatible with the assembly of the compressor wheel on the turbocharger shaft in the case of positioning the rotor in the air flow on the compressor side (stator grid machine for example),

Tighten the compressor wheel and preload the bearings of the turbocharger bearings or the assembly with plain bearings,

Reduce the number of system components.

FIG. 1 illustrates, schematically and in a nonlimiting manner, an embodiment of the invention. Figure 1 is a sectional view of the rotor of the electric machine. The present invention relates to a device for compressing a fluid comprising an electrical machine, said electrical machine comprising a rotor and a stator, said compression device comprising a compression shaft (3) on which at least one compression wheel is mounted. (2), said rotor being connected to one end of said compression shaft (3), said rotor comprising a cylindrical permanent magnet (5) and a hollow shaft (7), said hollow shaft (7) comprising a hoop portion (4) ) radially surrounding said cylindrical permanent magnet (5). Said cylindrical permanent magnet (5) is a solid part coaxial with said rotor, said hollow shaft (7) is made of a material with temporary or modular magnetization.

We call a permanent magnet, or simply magnet in everyday language, an object made of a hard magnetic material, that is to say one whose remanent magnetization and coercive field are large. This gives it particular properties linked to the existence of the magnetic field, such as that of exerting a force of attraction on any ferromagnetic material.

A material with temporary or modulable magnetization is called a ferromagnetic material having a weak coercive field. Such a material is referred to as a soft magnetic material. Such a material needs to be placed temporarily in a magnetic field to induce its magnetization. When the magnetic field to induce its magnetization is interrupted, the material with temporary or modulable magnetization loses its magnetization at the end of a characteristic time. The magnitude of the magnetization can be controlled by modulating the inverter over an interval between a minimum and a maximum.

In Figure 1, we can distinguish the hollow body (7) which comprises the hoop portion (4) and the annular cylindrical portion (6). In addition, the cylindrical permanent magnet (5) is shown centered axially on the hollow body (7) and substantially on the left side of the hollow shaft, which should not be limiting since the magnet can be located anywhere desirable for carrying out the invention, for example in a more centered longitudinal manner or else completely to the right. It is also possible to envisage a magnet covering the entire hollow space, depending on the implementation requirements. This figure shows a thread in the hollow body, for mounting on the compressor shaft. This mounting solution thus makes it possible to tighten the compressor wheel to preload the bearings of the turbocharger bearings. The angles, chamfers and other elements of geometry may vary depending on the embodiment of the device. Figure 2 illustrates, schematically and in a non-limiting manner, one embodiment of the invention. FIG. 2 is an isometric view of the rotor of the electric machine. We can distinguish the hollow body (7) and the cylindrical permanent magnet (5) before assembly.

FIG. 3 illustrates, schematically and in a nonlimiting manner, an embodiment of the invention. Figure 3 is a sectional view of part of the compression device. The optional turbine part of the turbocharger is not shown. A distinction is made between the compression shaft (3) and the compression wheel (2). Note the use of a tubular sleeve (9) between the compression shaft (3) and the compression wheel (2). For this embodiment, the compression wheel (2) has, over part of its bore, an internal bore of larger diameter than the compression shaft (3). This portion comprising a differential bore allows the hollow shaft (7) to have a longer centering length with the compression shaft (3). In addition, the hollow shaft (7) has blind holes (13) intended for handling the rotor for its attachment to the compression shaft (3), in particular by means of operating tools (not shown). On the one hand, the compression wheel (2) is in abutment against the hollow shaft (7). On the other side, the compression wheel (2) is in abutment against a guide system (9), for example the inner ring of a bearing, the bearing cage of which is shown. In order to facilitate the assembly and positioning of the parts, the rotor may include a flat surface which is in contact with an extremal flat surface of the compression wheel (2). In addition, this feature makes it possible to arrange the rotor as close as possible to the compression wheel (2).

Alternative embodiments of this illustrated embodiment can be envisaged; for example the annular cylindrical portion (6) may have the length of the compression wheel (2), the annular cylindrical portion (6) may have a length greater than or equal to the length of the compression wheel (2), the hollow shaft (7) may not have an annular cylindrical portion (6), etc.

According to the invention, the cylindrical permanent magnet is a solid part. In other words, the cylindrical permanent magnet is a solid cylinder with no holes, bores or holes, etc. Thus, this solution does not require any machining operation (for example boring) of the cylindrical magnet, which makes it possible to simplify the manufacture of the compression device, and to increase the magnetic performance of the magnet (the volume of l (magnet is larger compared to a situation where the magnet is drilled for mounting on the driveshaft).

The positioning of the magnet coaxially in the center of the rotor makes it possible to guarantee the mechanical strength of the magnet, which is made of a material which does not benefit from good mechanical strength, in particular vis-à-vis the centrifugal forces which s 'apply when rotating.

The role of the hoop portion (4) is to contain the permanent magnet. The name of this hoop portion alludes to a preferred method of manufacture, even though in practice the person skilled in the art could use any other desirable assembly technique such as for example screwing, gluing, welding, etc. The hooping is the assembly of two parts by means of a tight fit. The outer part (hollow shaft) is called "hoop", the inner part (permanent magnet) is called "hoop". The assembly is carried out with machining tolerances which make it difficult or impossible to assemble it by hand or even by press. More precisely, the use of a friable material or at least one which does not benefit from favorable mechanical strength characteristics, does not make force-mounting desirable. The simplest solution, when it is possible without deterioration of the material, is to heat the hoop to expand it before threading it on the element that it is must be freighted. The interior element can also be cooled with liquid nitrogen or dry ice to contract it and engage it in the hoop, even if these solutions are more expensive.

To carry out the invention, the hollow shaft is preferably a part of cylindrical revolution machined from an inexpensive, magnetically active material whose production of magnetic flux (or magnetization) to the rotor is not permanent (i.e. say temporary or modular) and which benefits from excellent mechanical strength.

Indeed, materials with temporary or modular magnetization provide a considerable price argument both in terms of the material itself and in terms of machining costs. Other embodiments of the hollow shaft can however be considered, for example forging, casting or 3D printing (additive manufacturing).

The simplification of the rotor also brings an increase in the performance of the electric machine, which can make it possible, for example, to reduce the dimensions of the electric machine, which is particularly advantageous in the case of positioning the electric machine in front of the compressor inlet. . The rotor according to the invention may preferably comprise at most the following elements in a radial section: cylindrical permanent magnet and temporary or modular magnetization hoop.

According to one embodiment, the thickness of the hoop (4) may be greater than the radius of the cylindrical permanent magnet (5). Indeed, the judicious relationship between the thickness of the hoop (4) and the radius of the cylindrical permanent magnet make it possible to produce a high-performance electrical machine. More precisely, we have already seen that it is desirable to limit the mass as much as possible (to avoid iron losses in freewheel rotation) and also the eccentricity (to ensure mechanical strength, in particular with respect to centrifugal forces that apply when rotating) of the permanent magnet while maintaining sufficient magnitude at the magnetic polarity to accurately determine the positioning of the rotating rotor. Therefore, the cylindrical permanent magnet can be reduced to its simplest shape and smallest dimensions as long as the magnetic field it maintains is sufficient for the detection of the position of the rotor by the stator coils or any other sensor. this effect. In other words, on the hybrid rotor, the hoop or the modulable / temporary magnetization part is sized to produce the specified torque. The permanent magnet is sized as described above to produce a signal which determines the inside diameter of the temporarily magnetized rotor portion.

The torque determines the diameter (also constrained by aerodynamics) and the length (constrained by the dynamics of the shaft line) of the rotor.

According to one embodiment, said cylindrical permanent magnet (5) can be made from neodymium NbFeBr, SmCo, Ferrite or any other hard magnetic material. Indeed, the magnet can be made from any magnetic material. The diameter of the cylindrical permanent magnet (5) may be within a range preferably between 4mm and 12mm. However, the person skilled in the art may limit the diameter of the cylindrical permanent magnet (5) to an interval between

2mm and 4mm in the context of miniaturization or the use of materials for the realization of the cylindrical permanent magnet (5) and respectively for the hoop with specific characteristics. According to one embodiment, said hollow shaft (7) may be made of a steel alloy of the iron-chromium-cobalt, aluminum-nickel-chromium AINiCo type, etc. Indeed, the hollow shaft can be made from any material having a temporary or modulable magnetic characteristic (low cohercivity). In this embodiment, the outer radius of the hollow shaft (7) is at least 25% greater than the radius of the cylindrical permanent magnet (5). However, depending on the characteristics of the materials used, the outer radius of the hollow shaft (7) may also reach or be at least 200% larger than the radius of the cylindrical permanent magnet (5).

According to one embodiment, the hollow shaft (7) can integrally comprise an annular cylindrical portion (6) inserted in said at least one compression wheel (2), said annular cylindrical portion (6) connecting the shaft compression (3) to said at least one compression wheel (2). This cylindrical portion surrounds the compression shaft and is inserted into the bore of the compression wheel. This cylindrical portion provides long centering, preferably with clearance of the rotor relative to the compression shaft, which makes it possible to ensure better coaxiality of the compression shaft with the rotor. The part cylindrical may have a reduced outside diameter relative to the outside diameter of the rotor.

Advantageously, the annular cylindrical portion (6) has an axial length which is greater than or equal to 1.5, for example 2 to 3 times the diameter of the compression shaft (3), in order to allow optimized long centering.

According to a first variant of this embodiment, the annular cylindrical portion (6) has an axial length which corresponds substantially to the axial length of the compression wheel, in order to allow maximum long centering and to stiffen the compression wheel, in particularly for high rotational speeds. This configuration makes it possible in particular to stiffen the portion of the shaft under the compressor wheel, which can be a critical point for certain bending modes. A high level of concentricity is thus achieved between the electric rotor and the compressor shaft in order to obtain a complete mechanical system (compressor shaft with rotor of the electric machine) which can be balanced with minimum unbalance. In addition, the compressor wheel is thus tightened in order to preload the bearings of the bearings or any other guiding system usually employed in turbochargers.

According to one embodiment, the rotor may include handling means. For this embodiment, the rotor may include gripping / handling means, for the purpose of rotating the rotor, and consequently for fixing with the compression shaft by threaded assembly or any other fixing means. These gripping / handling means can in particular be holes for inserting tools, flaps to form a handling tip, etc.

According to one embodiment of the invention, the rotor, in this case the hoop portion (4), may have an outside diameter less than or equal to the diameter of the nose of the compression wheel. In this way, the flow of gas entering the compression device is not impeded by the rotor shaft. Indeed, it is important to maintain high permeability qualities, especially when the electrical machine is located in front of the compressor inlet. A reduced rotor diameter favors the creation of large openings for the passage of fresh gases to the compressor, which ensures high performance of the compressor while ensuring sufficient cooling of the electrical machine.

According to an implementation of the invention, the compression device may be a turbocharger combining a turbine and a compressor, in particular for an internal combustion engine of a vehicle. It is then a turbocharger driven by an electric machine. In this case, the compression shaft corresponds to the turbocharger shaft that connects the turbocharger turbine to the turbocharger compressor. Thus the electric machine drives both the compressor and the turbine.

According to a variant of this implementation of the invention, the electric machine can be arranged in the gas inlet (generally air) of the turbocharger system. This solution has a twofold advantage: the electrical machine can be cooled by the inlet gas flow, and conversely, the gas. intake is heated by the electric machine, which can be beneficial in certain modes of operation of the internal combustion engine, such as cold starting and warming up. In addition, this solution avoids the location of the electric machine between the compressor and the turbine, an area where the temperatures are particularly high, which is problematic for the proper functioning of an electric machine.

Preferably, the electric machine can be an electric machine with a stator grid, that is to say an electric machine having a stator comprising stator teeth around which coils are mounted, these stator teeth being of large dimensions to allow passage. air flow which guarantees excellent permittivity for the passage of fresh gases. Such a stator grid machine is described in particular in patent applications WO17050577 A1 and FR 3048022.

According to one embodiment, the electric machine can be an electric generator and more particularly of which the operation and the calls for electric power in motor and generator modes can be very transient. Indeed, the appropriate management of the electric machine can make it possible to change its operating mode from a motor mode to a generator mode or, alternatively, to a freewheel mode, when the hollow shaft is magnetically made inactive. The present invention also relates to a method of implementing the compression device by carrying out the magnetization of said hollow shaft (that is to say of the material with temporary or modulable magnetization) to make said rotor pass from one to the other. initial state in which the rotor is considered inactive to an operating state in which the rotor is considered active. Preferably, the magnetization of said hollow shaft (7) is reduced to cause said rotor to pass from an operating state in which the rotor is considered active to an initial state in which the rotor is considered inactive. According to one embodiment of the method, the magnetization and demagnetization (decrease in magnetization) of said hollow shaft (7) are carried out by applying a current pulse by means of an inverter controlling said electrical machine, said current pulse having an amplitude greater than the normal operating pulses of the electrical machine. According to one embodiment of the method, the change between the active and inactive state is carried out according to the following transient modes: a. The active state is chosen when power is to be transmitted by the electrical machine, said power being modulated by the inverter over a range between a minimum and a maximum; b. We choose the inactive state when we want to reduce the magnetic torque between the rotor and the stator.

This temporary or modulable magnetization of the rotor, possible on a material with temporary or modulable magnetization, such as FeCrCo, thanks to a current pulse is very relevant on an electrified turbocharger, whose operation and electrical power calls in engine and generator modes can be very transient. The rotor, magnetically inactive the rest of the time, offers the advantage to limit the magnetic losses ("iron" losses in particular) of the electrical system when the latter is not in use.

Claims

1. Device for compressing a fluid comprising an electrical machine, said electrical machine comprising a rotor and a stator, said compression device comprising a compression shaft (3) on which is mounted at least one compression wheel (2), said rotor being connected to one end of said compression shaft (3), said rotor comprising a cylindrical permanent magnet (5) and a hollow shaft (7), said hollow shaft (7) comprising a hoop portion (4) radially surrounding said said rotor cylindrical permanent magnet (5), characterized in that said cylindrical permanent magnet (5) is a solid part coaxial with said rotor, said hollow shaft (7) is made of a material with temporary or modulable magnetization (with flux modulation).

2. A compression device according to claim 1, wherein the thickness of the hoop (4) is greater than the radius of the cylindrical permanent magnet (5).

3. Compression device according to one of the preceding claims, wherein said cylindrical permanent magnet (5) is made from neodymium, NbFeBr, SmCo, Ferrite or any other hard magnetic material.

4. Compression device according to one of the preceding claims, wherein said hollow shaft (7) is made of a steel type alloy. iron-chrome-cobalt, aluminum-nickel-chrome AINiCo or any other flexible magnetic material.

5. Compression device according to one of the preceding claims, wherein the hollow shaft (7) integrally comprises an annular cylindrical portion (6) inserted into said at least one compression wheel (2), said annular cylindrical portion (6) axially connecting the compression shaft (3) to said at least one compression wheel (2).

6. A compression device according to one of the preceding claims, wherein the rotor comprises handling means (13).

7. A compression device according to one of the preceding claims, wherein the rotor has an outer diameter less than or equal to the diameter of the nose of the compression wheel.

8. Compression device according to one of the preceding claims, wherein the compression device is a turbocharger combining a turbine and a compressor, in particular for an internal combustion engine.

9. A compression device according to claim 7, wherein the electric machine is arranged at the gas inlet zone of said turbocharger.

10. Compression device according to one of the preceding claims, wherein the electrical machine is a stator grid machine.

11. A compression device according to one of the preceding claims, wherein the electric machine is an electric generator.

12. A method of implementing the device according to one of claims 1 to 11 wherein the magnetization of said hollow shaft is carried out to change said rotor from an initial state in which the rotor is considered inactive to an operating state in which the rotor is considered active.

13. A method of implementing the device according to one of claims 1 to 11 wherein the magnetization of said hollow shaft (7) is reduced to change said rotor from an operating state in which the rotor is considered to be active. initial state in which the rotor is considered inactive.

14. A method of implementing the device according to claims 12 or 13 wherein the magnetization and reduction of the magnetization of said hollow shaft (7) are carried out by application of a current pulse by means of an inverter controlling said machine. electrical, said current pulse having an amplitude greater than the normal operating pulses of the electrical machine.

15. A method of implementing the device according to the preceding claim, in which the change between the active and inactive state is carried out according to the following transient modes: a) The active state is chosen when power is to be transmitted by the electrical machine, said power being modulated by the inverter over a range between a minimum and a maximum; b) The inactive state is chosen when it is desired to reduce the magnetic torque between the rotor and the stator.