Method and device for controlling the magnetic field of a rotor of an electric machine for a vehicle that is at least partially electrically powered

Abstract

Electric machine for a vehicle that is at least partially electrically powered, comprising a rotor (1) arranged on a shaft (10), wherein the rotor (1) comprises at least one permanent magnet (3) and a magnetic flux barrier (2) in the form of a cavity, where the cavity has a receptacle (7) wherein the receptacle (7) is designed to reversibly store and remove a liquid, wherein - the receiving (7) is designed in the form of a container, and the container is connected via a line (9) to a liquid reservoir (8) for the dispensing and receiving of the liquid, or - the intake (7) is designed in the form of a reversibly expandable, elastic bag, wherein the bag is connected via a line (9) to a liquid reservoir (8) for the delivery and intake of the liquid, and the liquid reservoir (8) is arranged on the shaft circumference and is in mechanical contact with a piston (11) which at least partially encompasses the shaft (10) and is axially displaceable, wherein the piston (11) and the liquid reservoir (8) are designed to control the discharge and intake of the liquid from the liquid reservoir (8) via the axial displacement of the piston (11).

Description

[0001] The present invention relates to a method and a device for controlling the magnetic field of a rotor of an electric machine for a vehicle that is at least partially electrically powered. The present invention relates in particular to a rotor of an electric machine which has a variable and adaptable magnetic field.

[0002] The selection of magnets for electric machines, such as permanent magnet synchronous machines (PMSMs), is primarily based on the machine's required power output. For machine design, "strong" magnets are typically preferred to achieve the highest possible power output and motor torque, especially in the base speed range. However, at higher speeds, these magnets induce a high, speed-dependent voltage in the windings (back EMF). This induced voltage is undesirable and must be actively limited or reduced by the inverter control using countercurrent. Similarly, strong magnets generate a high active short-circuit current (ACS), which is used for safe operation. A high ACS causes unwanted temperature increases in the machine and the inverter, and both components must be specifically designed to handle these high currents in the safety case.Therefore, a speed-dependent adjustment of the machine would be desirable to overcome these disadvantages.

[0003] In JP 2005 - 269 839 A a rotating electric machine is shown, wherein a magnetic fluid is arranged in the rotor lamination stack in an opening different from the magnet holder, wherein the magnetic fluid can flow into an escape volume.

[0004] From JP H04 - 251 534 A a rotating electric machine is known in which the magnets of a rotor are designed to be displaceable in the radial direction of the rotor.

[0005] CN 1 09 450 137 A teaches a person skilled in the art a rotor with magnets arranged in magnetic pockets. Adjacent to the magnets is an air pocket which is connected to a reservoir, wherein a magnetic powder arranged in the reservoir can be displaced into the air pocket.

[0006] In WO 2014 / 043 350 A1 a thermal management system for a permanent magnet rotor is shown, wherein a liquid reservoir is arranged in the air pockets of the magnet pockets, into which a cooling liquid can be introduced.

[0007] JP 2015 - 133 832 A describes the fastening of a magnet to a magnetic pocket using an expandable potting compound.

[0008] Known electrical machines therefore still have potential for improvement.

[0009] The object of the present invention is to overcome at least one disadvantage of the prior art, at least partially. In particular, it is an object of the present invention to provide a solution that enables an improved design of an electric machine. This design allows for both high power output and low electrical and thermal stress on the other components in both high and low speed ranges.

[0010] The solution of the present invention is achieved by an electric machine having the features of claim 1, a method for controlling the magnetic field of a rotor according to claim 6, and a traction drive according to claim 8. Preferred embodiments of the invention are described in the dependent claims, in the description, or in the figures, wherein further features described or shown in the dependent claims, in the description, or in the figures may, individually or in any combination, constitute an object of the invention unless the context clearly indicates otherwise.

[0011] The present invention relates to an electric machine for an at least partially electrically powered vehicle, comprising a rotor arranged on a shaft, wherein the rotor comprises at least one permanent magnet and a magnetic flux barrier in the form of a cavity, wherein the cavity has a receptacle, wherein the receptacle is configured to reversibly insert and remove a liquid.

[0012] Such an electric machine offers significant advantages over prior art solutions.

[0013] This describes an electric machine for a vehicle that is at least partially electrically powered. The electric machine is thus suitable for propelling a vehicle, although in addition to the electric machine, other power sources, such as internal combustion engines, may also be present. Possible vehicles include cars, motorcycles, rail vehicles, and aircraft. The electric machine essentially converts electricity into mechanical energy or mechanical energy into electrical energy.

[0014] The electric machine comprises a rotor mounted on a shaft. The rotor is located on a shaft that may, for example, be situated inside a stator. The rotor can move synchronously or asynchronously with the rotating magnetic field of the stator. A fundamental requirement for the rotor's movement within the stator's alternating magnetic field is the presence of permanent magnets on or within the rotor. Depending on the machine's power output, permanent magnets of varying strengths and material compositions can be used. In addition to the magnet strength, multiple magnets can also be present. The number of magnets can be, for example, greater than four, more preferably greater than six, and even more preferably greater than twelve.

[0015] In addition to the permanent magnet, the rotor contains at least one magnetic flux barrier in the form of a cavity. This magnetic flux barrier is located within the rotor surface and is typically arranged around the magnets. Its purpose is to minimize magnetic short-circuiting within the rotor. This barrier allows the magnetic flux through the rotor air gap to be used in the stator for torque generation. Multiple magnetic flux barriers may be present, with the number depending on the number of permanent magnets.

[0016] The magnetic flux barrier has a receptacle designed to reversibly store and release a liquid. Within the volume of the magnetic flux barrier, a separate container is arranged, capable of receiving and releasing liquids. The liquid receptacle can contain varying amounts of liquid, and its volume can change with the fill level or remain constant. This can be achieved using either flexible or rigid walls. Since liquids have different electromagnetic properties compared to air, the controllable amount of liquid in the receptacle of the magnetic flux barrier can be used to introduce other electromagnetic properties that influence the overall electromagnetic properties of the rotor. For example, magnetic field lines can be short-circuited.These short-circuited field lines are then no longer available to interact with the magnetic field lines of the stator. Suitable liquids can be those with appropriate magnetic susceptibilities. It is also possible for the liquid to contain further solids, such as iron or other metal particles, which can further modify the magnetic properties of the liquid. Dispersions also fall under the term "liquid" according to the invention. The term "liquid" is used because of the desirable pumpability or flowability with low viscosity, which is particularly advantageous for reversible insertion and removal. The receptacle has a constant volume or occupies a different volume as a function of the amount of liquid inserted into the magnetic flux barrier.Only one, for example half, of the total magnetic flux barriers in the rotor, or all magnetic flux barriers, can be equipped with a receptacle. A receptacle is suitable for holding liquid if it is liquid-tight and allows for the reversible storage of varying amounts of liquid. In this context, "reversible" means that the storage and removal processes can be repeated as often as desired without significantly altering the properties of the receptacle. The receptacles can also include pipes or conduits for storing and removing liquid.

[0017] According to the invention, the electromagnetic properties of the rotor can be selectively modified by means of a reversible receptacle in a magnetic flux barrier, designed for the insertion and removal of a liquid. In particular, the electromagnetic flux lines can be selectively altered by the insertion of the liquid, in contrast to an air-filled situation. When the receptacles are filled with liquid, some of the magnetic field lines of the permanent magnets short-circuit, and a weaker magnetic field is available in the rotor-air gap compared to purely air-filled magnetic flux barriers. This effect can be controlled by the number of receptacles, the amount of liquid inserted, and the electromagnetic properties of the liquids themselves. If required, a portion of the magnetic flux from the rotor to the stator can thus be controlled to be "magnetically short-circuited."Due to the fluid's reversible storage capacity, the rotor's original electromagnetic properties can be restored by removing it from the reservoir using "conventional" magnetic flux barriers. This process can be controlled, for example, as a function of the applied torque or as a function of the rotational speed.

[0018] These advantages are achieved by incorporating a reversible fluid storage chamber into one or more of the magnetic flux barriers. Particularly at higher rotational speeds, the rotor's magnetic field can be selectively weakened by fluid storage, thus reducing the amount of externally generated field-weakening current required. This, in turn, reduces the energy / current needed for field weakening. The energy or current now released can be used for further torque generation, especially at higher rotational speeds.

[0019] The aforementioned design therefore enables a particularly advantageous configuration of an electric machine, for example, a permanent magnet synchronous motor, and especially its rotor. As described above, the rotor magnetic field, and thus the power transmission between rotor and stator, can be actively modified and adapted to the specific machine situation. The very high magnetic interaction between rotor and stator, which is preferred at low speeds, can be weakened at higher speeds by short-circuiting rotor magnetic field lines, so that other compensation mechanisms, such as the use of a field-weakening current, are reduced or completely avoided. By alternatively using the field-weakening current to generate torque, the electrical efficiency of the machine can be improved.

[0020] According to the invention, a first alternative provides that the receiving element is designed in the form of a container, the container being connected via a line to a liquid reservoir for receiving and discharging the liquid. The container can, for example, adopt the contour of the magnetic flux barriers and be partially or completely filled and emptied via the line. In this case, the container has non-elastic walls, and the liquid can be drawn off, for example, by the applied centrifugal forces or by a pumping action. Unintentional withdrawal can be prevented by the applied liquid pressure or, for example, by a valve. The intake and discharge of the liquid can, for example, be a function of the rotational speed or torque of the machine. The line can extend from the magnetic flux barrier across the rotor surface or...extend from the front face or inside the rotor towards the shaft.

[0021] According to the invention, a second alternative provides that the receptacle is designed in the form of a reversibly expandable, elastic bag, the bag being connected via a line or tube to a liquid reservoir for dispensing and receiving the liquid. To increase the forces for dispensing the liquid, it can be advantageous to use an elastic bag, for example made of an elastic plastic material, as the receptacle within the magnetic flux barrier. In the unexpanded state, without liquid, the volume of the bag is smaller than the volume of the magnetic flux barrier. Only when liquid enters the bag does it expand and more or less completely fill the volume of the magnetic flux barrier. The rotor magnetic field is then thus co-defined by a mixture of air-filled and liquid-filled magnetic flux barriers.In this configuration with a bag, valves can be omitted, as the fill level can be regulated solely by the applied pressure of the filling line or the liquid reservoir. The line can extend from the magnetic flux barrier across the rotor surface or end face, or inside the rotor towards the shaft.

[0022] According to the invention, the liquid reservoir is arranged on the shaft circumference and is in mechanical contact with a piston that at least partially encompasses the shaft and is axially displaceable. The piston and the liquid reservoir are configured to control the dispensing and intake of liquid from the reservoir via the axial displacement of the piston. The force acting on the liquid reservoir can thus be varied by the relative position of the piston along the shaft and with respect to the distance to the liquid reservoir. The force exerted by the piston can, for example, be controlled by a sleeve attached to the piston but not fixed to the shaft, which displaces the piston axially along the shaft. Mechanical forces are exerted on the liquid reservoir, and as a function of the applied force, the receptacles in the magnetic flux barriers are then further filled or emptied.Depending on the fill level of the recording, the magnetic field lines of the permanent magnets on the rotor are completely or partially short-circuited to varying degrees.

[0023] It may be further preferred that the liquid reservoir mechanically contacts a centrifugal governor, wherein the liquid reservoir and the centrifugal governor are configured to control the discharge and intake of the amount of liquid from the liquid reservoir as a function of the rotor speed. The filling of the receptacle located in the magnetic flux barrier can be indirectly controlled via a centrifugal governor, which exerts a force on the liquid reservoir proportional to the applied rotor speed. The force can be applied directly to the liquid reservoir. However, it is also possible for the centrifugal governor to act only indirectly on the reservoir. For this purpose, for example, a piston encompassing the shaft and axially displaceable can be mechanically interposed. Depending on the force exerted, the receptacle in the magnetic flux barrier is filled with or emptied of liquid.The mechanical contact between the centrifugal governor and the reservoir can be achieved, for example, via a mechanical lever.

[0024] It may be preferable to have multiple inlets, with the inlets, when filled, occupying greater than or equal to 15% and less than or equal to 80% of the total volume of the magnetic flux barriers on the rotor. To attenuate the rotor magnetic field at high speeds, it has proven advantageous to modify the magnetic properties of the magnetic flux barriers by incorporating fluid within the volume range specified above. Smaller volume changes to the magnetic flux barriers can be detrimental, as the magnetic field may be insufficiently modified or even short-circuited. Larger changes to the volume of the magnetic flux barriers can be detrimental, as the rotor's magnetic field interaction with the stator is then excessively disrupted.Preferably, the recesses in the filled state can occupy greater than or equal to 25% and less than or equal to 70%, and more preferably greater than or equal to 30% and less than or equal to 65% of the volume of the total magnetic flux barriers on the rotor. These recesses in the filled state occur when the electric machine reaches its maximum rotational speed. Preferably, every second magnetic flux barrier on the rotor can have a recess, and more preferably, every magnetic flux barrier on the rotor can have one. If not every magnetic flux barrier has a recess, the recessed magnetic flux barriers can be distributed symmetrically around the rotor axis.

[0025] It may also be preferable for the filling chamber to extend through the rotor when filled. For particularly efficient and homogeneous weakening of the magnetic field, it has proven especially suitable to fill the magnetic flux barrier as homogeneously as possible through a thickness of the rotor. This type of filling is significantly more efficient compared to an asymmetrical filling of the rotor volume.

[0026] It can also be advantageous for the rotor to have an embedded magnet. An embedded magnet can also be understood to be a buried magnet. Buried magnets are necessary to achieve reluctance, particularly when the rotor is arranged in a traction drive and experiences rotational speeds greater than 12,000 rpm, especially greater than 15,000 rpm. The weakening of the rotor magnetic field according to the invention by filling the magnetic flux barriers can be particularly efficient in cases where the rotor has at least one embedded magnet. Several embedded magnets may also be present. Embedded magnets are not only located on the rotor surface but extend at least partially in the axial direction through the rotor.At this point and at the following points mentioned in the disclosure, a rotor surface can be understood to be an end face formed in the axial direction of the rotor. In particular, the combination of a uniform filling of the magnetic flux barrier throughout the entire rotor, i.e., from rotor surface to rotor surface or from end face to end face, can result in a particularly symmetrical and thus efficient weakening of the rotor magnetic field.

[0027] Furthermore, according to the invention, a method for controlling the magnetic field of a rotor of an electric machine is used, wherein the rotor has at least one magnetic flux barrier, the magnetic flux barrier having receptacles, the receptacle being reversibly fillable with a liquid comprising electromagnetically active particles. The method according to the invention allows the magnetic field of the rotor to be influenced in a particularly efficient manner, so that, for example, a reversible change in the magnetic rotor-stator coupling can be achieved as a function of the rotational speed or the applied torque. This allows the advantages described for the rotor according to the invention to be achieved. For example, the rotor magnetic field can be selectively weakened, so that less field-weakening current needs to be generated externally and introduced into the system at high rotational speeds. This reduces the energy / current required for field weakening.The energy or current released can now be used to further increase torque, especially at higher speeds. Electromagnetically active particles can be present in the liquid, for example, in the form of a dispersion. These particles are capable of effectively short-circuiting the magnetic field generated by the permanent magnet. Examples of electromagnetically active particles include metal powders, such as iron powder.

[0028] It may be preferable to control the reversible filling of the reservoir as a function of the rotor speed, with the reservoir being filled as the rotor speed increases and emptied as the rotor speed decreases. This control of the amount of fluid stored effectively weakens the rotor's magnetic field at high speeds, thus reducing the amount of field-weakening current that needs to be actively introduced into the system. When the rotor speed is reduced again, the fluid is removed from the magnetic field barrier, restoring the original magnetic properties. The speed-dependent control in the process can be achieved via centrifugal force control or a piston acting on a reservoir as a function of the rotational speed.

[0029] Furthermore, according to the invention, a traction drive for a motor vehicle that is at least partially electrically powered is provided, wherein the electrically powered vehicle has at least one electric machine according to the invention. The use of the electric machine according to the invention can contribute in particular to efficient electrical control of a traction drive, whereby, for example, higher torques can be achieved with lower energy consumption, since less energy has to be supplied for a field-weakened current at high speeds. The saved current or energy can be used to generate torque, which can improve the efficiency of the entire electric machine, for example in the form of a permanent magnet synchronous motor.

[0030] The invention is further explained below with reference to the figures, whereby one or more features of the figures, individually or in combination, can constitute a feature of the invention. Furthermore, the figures are to be considered merely exemplary and in no way limiting. Fig. Figure 1 schematically shows a partial section of a rotor according to the state of the art; Fig. Figure 2 schematically shows a possible structure of a rotor according to the invention;

[0031] In the Fig. Figure 1 schematically depicts a partial section of a rotor for a permanent magnet synchronous motor according to the prior art, shown in an oblique top view. This section of the rotor shows the permanent magnets 3 and, at the ends of the permanent magnets 3, magnetic flux barriers 2. The permanent magnets 3 can be embedded in the surface or end face of the rotor. The magnetic flux barriers 2 are in the form of air-filled cavities within the rotor and serve the purpose of ensuring that the magnetic flux lines of the permanent magnets 3 interact predominantly with the stator magnetic field (not shown). The rotor interior 6 is located near the shaft passage 5. The rotor is bounded towards the stator by the rotor circumference 4.

[0032] In the Fig.Figure 2 schematically shows a possible structure of a rotor 1 according to the invention. The rotor 1 has permanent magnets 3, which can be buried permanent magnets 3. Magnetic flux barriers 2 are also shown, with receptacles 7 arranged in the air-filled cavities of some of the magnetic flux barriers 2. All magnetic flux barriers 2 can be provided with receptacles 7. However, it is also possible that only some of the magnetic flux barriers 2 are provided with receptacles 7. The receptacles 7 can extend through the entire rotor 1 and are connected to a liquid reservoir 8 via lines 9. Liquid can flow from the liquid reservoir 8 into the receptacles 7 through the lines 9. The receptacles 7 can be in the form of rigid containers or in the form of bags with flexible walls within the air-filled cavities of the magnetic flux barriers 2.The liquid can be a pure liquid or a dispersion of a dispersing agent, such as water, and metallic particles, such as iron particles. By introducing the liquid from the liquid reservoir 8 into the receptacles 7, the magnetic field of the permanent magnets 3 is selectively weakened, thus reducing the magnetic interaction between the rotor and stator. The degree of weakening can be controlled, for example, by the amount of liquid introduced into the receptacles 7. This amount of liquid and the flow direction can be determined, for example, by an axially displaceable piston attached to the shaft 10. This axial displacement allows liquid to be introduced from the liquid reservoir 8 into the receptacles 7 via the lines 9 and then removed again.The axially displaceable piston 11 can, for example, be moved axially via a sleeve (not shown) not attached to the shaft 10. Alternatively, the axially displaceable piston 11 can also be moved speed-dependently via a centrifugal governor (not shown). This allows larger quantities of fluid to be introduced into the receptacles 7 at high speeds. The weakening of the magnetic field by the fluid in the receptacles 7 reduces the magnetic interaction between the rotor 1 and the stator, thus reducing the active field-weakening current required. This saved current can be used to generate additional torque. The latter can improve the efficiency of the rotor 1 and the synchronous machine. Reference symbol list 1 Rotor 2 Magnetic flux barrier 3 permanent magnets 4 Rotor circumference 5 Shaft feedthrough 6 Rotor interior 7th recording 8 Liquid reservoir 9 Management 10 wave 11 Axially movable pistons

Claims

[1] Electric machine for a vehicle that is at least partially electrically powered, comprising a rotor (1) arranged on a shaft (10), wherein the rotor (1) comprises at least one permanent magnet (3) and a magnetic flux barrier (2) in the form of a cavity, where the cavity has a receptacle (7) wherein the receptacle (7) is designed to reversibly store and remove a liquid, wherein - the receiving (7) is designed in the form of a container, and the container is connected via a line (9) to a liquid reservoir (8) for the dispensing and receiving of the liquid, or - the intake (7) is designed in the form of a reversibly expandable, elastic bag, wherein the bag is connected via a line (9) to a liquid reservoir (8) for the delivery and intake of the liquid, and the liquid reservoir (8) is arranged on the shaft circumference and is in mechanical contact with a piston (11) which at least partially encompasses the shaft (10) and is axially displaceable, wherein the piston (11) and the liquid reservoir (8) are designed to control the discharge and intake of the liquid from the liquid reservoir (8) via the axial displacement of the piston (11). [2] Electric machine according to claim 1, characterized by , that the liquid reservoir (8) mechanically contacts a centrifugal governor, wherein the liquid reservoir (8) and the centrifugal governor are configured to control the discharge and intake of the quantity of liquid from the liquid reservoir (8) as a function of the rotational speed of the rotor (1). [3] Electric machine according to any one of the preceding claims, characterized by, that several inlets (7) are present, wherein the inlets (7) in the filled state occupy greater than or equal to 15% and less than or equal to 80% of the volume of the total magnetic flux barriers (2) on the rotor. [4] Electric machine according to any one of the preceding claims, characterized by , that the intake (7) extends through the rotor (1) when filled. [5] Electric machine according to any one of the preceding claims, characterized by , that the rotor (1) has an embedded magnet. [6] Method for controlling a magnetic field of a rotor (1) of an electric machine according to one of the preceding claims, wherein the rotor (1) has at least one magnetic flux barrier (2), characterized by , that the magnetic flux barrier (2) has a receptacle (7) wherein the receptacle (7) can be reversibly filled with a liquid comprising electromagnetically active particles. [7] Method according to claim 6, characterized by , that the reversible filling of the intake (7) is controlled depending on the rotational speed of the rotor (1), wherein with increasing rotational speeds of the rotor (1) the intake (7) is filled and with decreasing rotational speeds of the rotor (1) the intake (7) is emptied. [8] Traction drive for a motor vehicle that is at least partially electrically powered, comprising an electric machine according to any one of claims 1-5.

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