Brush adjuster for external excitation of a rotor, which is arranged on a rotor shaft, of an electric motor, arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuable brush adjuster, electric motor, vehicle and method for operating an electric motor

Abstract

The invention relates to a brush adjuster for external excitation of a rotor, which is arranged on a rotor shaft, of an electric motor. Here, the brush adjuster (2) has a first and a second contact brush (4, 6), each of which can be pressed against an associated slip ring (8, 10) of the rotor shaft. The contact brushes (4, 6) can be spring-pretensioned against the slip rings (8, 10) by means of an armature (18) of an electromagnet (12) and a supporting section of the electromagnet, the supporting section interacting with the armature (14) by means of a spring (20), wherein, by means of a coil former (16) of the electromagnet (12), an electromagnetic force which acts in or against a pretensioning direction of the spring (20) and can be variably adjusted depending on requirements can be impressed onto the armature (18) in order to intensify or lessen a contact pressure on the contact brushes (4, 6) by means of the spring (20) and in so doing avoid lifting off of the contact brushes (4, 6) during operation of the electric motor. The invention also relates to an arrangement, an electric motor, a vehicle and a method for operating an electric motor.

Description

[0001] Description

[0002] Brush actuator for external excitation of a rotor of an electric motor arranged on a rotor shaft, arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuated brush actuator, electric motor, vehicle and method for operating an electric motor

[0003] The invention relates to a brush actuator for the external excitation of a rotor of an electric motor arranged on a rotor shaft, an arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuated brush actuator for the external excitation of a rotor of the electric motor arranged on the rotor shaft. The invention also relates to an electric motor, in particular for driving a vehicle, having such an arrangement, as well as to a vehicle having such an electric motor or such an arrangement. The invention also relates to a method for operating an electric motor.

[0004] One object underlying the invention is to ensure reliable external excitation of a rotor of an electric motor. Another object underlying the invention is to ensure this external excitation in an energy-saving manner.

[0005] This object is achieved by a brush actuator having the features of claim 1. Claim 4 protects an arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuated brush actuator for externally exciting a rotor of the electric motor, which is arranged on the rotor shaft. Claims 8 and 9 protect an electric motor and a vehicle with such an arrangement, respectively. Claim 10 protects a method for operating an electric motor. The subclaims relate to advantageous developments. A brush actuator for externally exciting a rotor of an electric motor arranged on a rotor shaft is proposed, wherein the brush actuator has a first and a second grinding brush, each of which can be pressed or pressed against an associated slip ring of the rotor shaft.The grinding brushes can be spring-loaded against the slip rings via an armature of an electromagnet and a support section of the electromagnet that interacts with the armature via a spring, wherein an electromagnetic force acting in or against a preload direction of the spring, which can be variably adjusted as required, can be impressed on the armature via a coil body of the electromagnet in order to increase or decrease the pressure or pressing of the grinding brushes via the spring and to prevent the grinding brushes from lifting off during operation of the electric motor.

[0006] The armature can be hinged against a fork-shaped force distribution element, which can be divided into two arms and supported against the slip rings.

[0007] It is proposed that the grinding brushes be made of silver graphite brushes.

[0008] Furthermore, an arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuatable brush actuator for external excitation of a rotor of the electric motor, which is arranged on the rotor shaft, is proposed.

[0009] The brush actuator comprises a first and a second grinding brush, each pressed against an associated slip ring of the rotor shaft. The grinding brushes are spring-loaded against the slip rings via an armature of an electromagnet and a support section of the electromagnet that interacts with the armature via a spring. An electromagnetic force acting in or against a preload direction of the spring, which can be variably adjusted as needed, can be applied to the armature via a coil body of the electromagnet in order to increase or decrease the pressure applied to the grinding brushes via the spring and thereby prevent the grinding brushes from lifting off during operation of the electric motor.

[0010] The proposed arrangement allows for the grinding brushes to be pressed against the slip rings as required. Depending on the requirements, the grinding brushes can be pressed against the slip rings either passively with spring preload or actively with spring preload.

[0011] When the electromagnet is de-energized, only a passive or static spring force acts on the grinding brushes. This is referred to as passive pressure on the grinding brushes by at least one spring, or as passive spring preload on the grinding brushes.

[0012] When the electromagnet is energized, however, the contact pressure or compression via the at least one spring is electromagnetically amplified or weakened - depending on the current supply to the electromagnet - by applying an additional electromagnetic force to the armature via the coil body of the electromagnet, either in the direction of the spring preload, thereby increasing the contact pressure or compression, or applying it counter to the spring preload, thereby weakening the contact pressure or compression. This application of the electromagnetic force occurs as needed, variably as needed, or depending on the need, or variably as needed.

[0013] This active pressing or pressure of the grinding brushes is carried out in a controlled or regulated manner and depending on or taking into account various definable influencing parameters, such as a rotor (shaft) speed, a rotor (shaft) position and / or a vehicle acceleration in the direction of the grinding brushes and the like.

[0014] In one design, the armature is hinged to a fork-shaped force distribution element, which is divided into at least two arms and supported against the slip rings. This hinged connection of the armature to the fork-shaped force distribution element ensures sufficient pressure of the brushes against the slip rings, even if the brush actuator is not ideally aligned at right angles to the rotor shaft in its operating position. This is referred to as tilt compensation by the brush actuator. This hinged connection of the armature also counteracts uneven wear of the brushes.

[0015] In a further embodiment, an insulating section projecting circumferentially in the radial direction of the rotor shaft is provided on the rotor shaft between the slip rings to form a sufficient creepage and clearance distance between the current-conducting or guiding elements or the slip rings and the brushes in order to avoid short circuits.

[0016] This insulation section functions as a beneficial rotor shaft collar, which advantageously minimizes the axial distance between the slip rings and the brushes. This rotor shaft collar thus contributes to space savings.

[0017] In another version, the grinding brushes are designed as silver graphite brushes.

[0018] By using such silver graphite brushes in conjunction with slip rings made of iron or steel, for example, the aging of the gear oil that wets the brushes and slip rings can be slowed down. Unlike with copper graphite brushes and copper slip rings, sparks do not age the gear oil. In the case of copper graphite brushes and copper slip rings, the copper reacts with the sulfur content of the gear oil during such sparks, causing the gear oil to age. The silver graphite therefore improves the durability of the gear oil.

[0019] Furthermore, an electric motor, in particular for driving a vehicle, is proposed, comprising a separately excited rotor arranged on a rotor shaft. The rotor shaft is combined with a wet-running brush actuator of the type described above for separately exciting the rotor. The brush actuator is attached to a housing of the electric motor and arranged radially to the rotor shaft, thus forming an arrangement of the type described above.

[0020] Furthermore, a vehicle with an arrangement of the type described above and / or an electric motor of the type described above is proposed.

[0021] A vehicle is defined as any type of vehicle or motor vehicle powered by an electric motor, but in particular includes passenger cars and / or commercial vehicles in the form of electric or hybrid vehicles. These can be semi-autonomous or fully autonomous vehicles.

[0022] A method for operating an electric motor is also proposed, in which method at least a first and a second grinding brush are pressed or pushed against associated slip rings of the rotor shaft for the separate excitation of a rotor of the electric motor arranged on a rotor shaft. The grinding brushes are spring-loaded against the slip rings via an armature of an electromagnet and a support section of the electromagnet which interacts with the armature via a spring, wherein an electromagnetic force acting in or against a pretensioning direction of the spring and variably adjustable as required or as required is impressed on the armature via a coil body of the electromagnet in order to press or push

[0023] To increase or decrease the pressure of the grinding brushes via the spring and to prevent the grinding brushes from lifting off during operation of the electric motor.

[0024] In one embodiment, it is proposed that the grinding brushes are passively spring-loaded up to a definable limit speed of the rotor shaft and are actively spring-loaded above this limit speed by energizing the electromagnet and are pressed or pressed against the associated slip rings in a variable manner as required or as required.

[0025] In a further embodiment, it is proposed that the grinding brushes are actively spring-loaded up to a definable first limit speed and above a definable second limit speed of the rotor shaft, which is higher than the first speed, and are pressed against the associated slip rings in a variable manner as required, wherein the two limit speeds define an intermediate speed range in which the grinding brushes are passively spring-loaded and pressed against the associated slip rings.

[0026] Passive means that the electromagnet is not energized, so that only a static spring force acts on the grinding brushes. Active, on the other hand, means that this static spring force or this static contact pressure is electromagnetically amplified or weakened via the spring by energizing the electromagnet accordingly, so that an electromagnetically adjustable force acting in or against the preload direction of the spring is impressed on the armature via the coil body of the electromagnet, and this force is required or variable depending on requirements or variable according to requirements or variable according to requirements, for example taking into account at least one of the previously mentioned influencing parameters, such as a rotor (shaft) speed, a rotor (shaft) position and / or vehicle acceleration in the direction of the grinding brushes and the like.

[0027] It is proposed that the first and second limit speeds be set, and thus the speed range between them, such that the most frequently occurring speeds in a driving cycle are covered by this speed range, in which the electromagnet is not energized. A WLTC driving cycle (WLTC = Worldwide Harmonized Light Vehicles Test Cycle) can be used as the driving cycle. Further advantages and features emerge from the subclaims and the exemplary embodiments. These are shown in the following:

[0028] Fig. 1 a proposed brush actuator in a perspective view,

[0029] Fig. 2 shows the brush actuator shown in Fig. 1 in a sectional view,

[0030] Fig. 3 shows another proposed brush actuator in a perspective view and

[0031] Fig. 4 shows the brush actuator shown in Fig. 3 in a sectional view.

[0032] The bidirectional brush actuator 2 or the bidirectional brush module 2 illustrated in Fig. 1 serves to separately excite a rotor of an electric motor arranged on a rotor shaft in the form of a synchronous machine for driving a vehicle. The electric motor is combined with a reduction gear that is oil-lubricated and oil-cooled. The gear oil also lubricates and cools the electric motor and is pumped within an oil cooling circuit that encompasses the gear box and the electric motor.

[0033] This brush actuator 2 is provided or arranged in a wet-running manner in the area of ​​an oil-lubricated bearing LS of the rotor shaft, radially to the rotor shaft, and fixed to a housing of the electric motor. An oil seal is not required at this bearing LS, allowing transmission oil to escape from the housing interior through the bearing LS and reach the brush actuator 2. This arrangement of the brush actuator 2 is sealed off from the environment by a housing cover (not shown here).

[0034] The brush actuator 2 is attached or fastened to the housing of the electric motor via a first and second mounting point AP-1, AP2. This brush actuator 2 or the electromagnet 12 of the brush actuator 2 can be electrically contacted via a bushing, for example, shaped like a trunk, which is integrally formed on the housing of an electromagnet 12.

[0035] The brush actuator 2 has a first and a second grinding brush 4, 6, each of which can be variably pressed against an associated slip ring 8, 10 of the rotor shaft. These two grinding brushes 4, 6 are each arranged within an associated brush receiving and brush guide element 7 and are movable radially relative to the rotor shaft. For example, the grinding brush 4 has a negative polarity, and the grinding brush 6 has a positive polarity.

[0036] The grinding brushes 4, 6 are spring-loaded against the slip rings 8, 10 by a spring 20 in the form of a helical spring inside the electromagnet 12 - or a solenoid 12. The spring 20 is arranged between an armature 18 and a bolt or pin 14 of a support section of the electromagnet 12. This support section is fixed to the housing of the electromagnet. The armature 18 is arranged largely within a coil body 16 of the electromagnet 12 and is longitudinally movable relative to the coil body 16. The armature 18 is also joined in sections to the bolt or pin 14, which is also arranged largely within the coil body 16 and is fixed to it.

[0037] The said support section of the electromagnet 12 comprises a cylindrical metallic return element made of, for example, iron or steel, into which the pin 14 is pressed. The pin 14 itself can also be made of metal.

[0038] The armature 18 is designed as a permanent magnet and can be formed or injection-molded from a plastic with magnetic particles, such as ferromagnetic particles. Alternatively to such a magnetic plastic design, the armature 18 can be metallic, for example, from a ferromagnetic metal or from a stainless, magnetized steel. The spring 20 therefore exerts a nominal or static force of, for example, approximately 30 N on the grinding brushes 4, 6. When the electromagnet 12 is de-energized, this is the force with which the grinding brushes 4, 6 are pressed or pressed against the slip rings 8, 10.

[0039] Below a definable limit speed of the rotor shaft of, for example, approximately 800 rpm, the electromagnet 12 can remain de-energized in order to minimize the total power loss of the brush actuator 2.

[0040] This total power loss of the brush actuator 2 results essentially from the friction of the grinding brushes 4, 6 on the rotating rotor shaft and the electrical power loss of the electromagnet 12 when the contact pressure is reduced. Below this limiting speed of approximately 800 rpm, the electrical power loss when the brush actuator 2 is actuated is greater than the friction power savings on the grinding brushes 4, 6, which, as such—compared to passive operation of the brush actuator 2—results from a reduced contact pressure resulting from the actuation of the brush actuator 2. For the sake of completeness, the friction in the electromagnet 12 and the spring 20 should also be mentioned at this point, although these power loss components are negligible or play a subordinate role.

[0041] Below this limit speed, the grinding brushes 4, 6 are therefore advantageously pressed passively against the slip rings 8, 10.

[0042] Above this limit speed, however, an active pressure of the grinding brushes 4, 6 against the slip rings 8, 10 can be provided via the armature 18 and the coil body 16 acting on the armature 18, depending on various definable influencing parameters, such as a rotor (shaft) speed, a rotor (shaft) position, vehicle acceleration in the direction of the grinding brushes 4, 6 and the like, so that a rotor (shaft) out-of-roundness, floating of the grinding brushes 4, 6 on the transmission oil which wets the slip rings 8, 10 and / or vibrations and impacts which act on the brush actuator 2 can be counteracted as required. By appropriately supplying current to the electromagnet 12, an electromagnetic force as required of, for example, up to approx.50N can be impressed on the armature 18, which additionally acts on the armature 18, so that the force exerted on the grinding brushes 4, 6 can be variably increased up to approx. 80N in order to prevent the grinding brushes 4, 6 from lifting off during vehicle operation.

[0043] The armature 18 is joined to a force distribution element 22, forming a so-called ball joint. The armature 18 comprises a joint head with a spherical shape, which is enclosed by a complementarily shaped section of the force distribution element 22 or a joint socket or ball socket of the force distribution element 22. The force distribution element 22 can be designed as an injection-molded plastic element.

[0044] Thus, the armature 18 is articulated relative to the force distribution element 22, which is divided into two arms and supported against the slip rings 8, 10.

[0045] On the rotor shaft and between the slip rings 8, 10, a plastic insulating section 24 is provided, projecting circumferentially in the radial direction of the rotor shaft, to protect against short circuits. This insulating section 24 provides sufficient insulation in the form of a creepage and clearance path between the slip rings 8, 10. This insulating section 24 forms part of the rotor shaft (not shown in Figs. 1 and 2).

[0046] It is proposed that the grinding brushes 4, 6 be designed as silver graphite brushes, which improves the durability of the gear oil.

[0047] In the embodiment according to Figs. 3 and 4, the brush actuator 2 also has a printed circuit board 28 (also called PCB; PCB = Printed Circuit Board), via which the brush actuator 2 or the electromagnet 12 of the brush actuator 2 can be electrically contacted via a socket illustrated in Fig. 3. This printed circuit board 28 has a rotor shaft position sensor and / or an acceleration sensor.The circuit board 28 is accommodated by a plastic housing section 26 which is formed on a plastic housing section of the electromagnet 12 and extends transversely, preferably orthogonally, to the rotor shaft RW and into an area around an end face of one end of the rotor shaft RW, so that the circuit board 28 is arranged transversely, preferably orthogonally, to the rotor shaft RW and opposite the end of the rotor shaft RW in such a way that the circuit board 28 covers an area around the end face of the end of the rotor shaft RW in order to be able to interact with this end face, for example inductively.

[0048] Even in this version of the brush actuator 2, the arrangement proposed here must be imagined as being closed off from the environment by a housing cover (not shown here).

[0049] Fig. 4 illustrates a metallic end of the rotor shaft RW with a rotor shaft section RWA arranged thereon, made of a plastic material that can be pushed or molded onto the end of the rotor shaft RW. Embedded in this plastic material are metallic contact means in the form of two slip rings 8, 10 and a conductor track associated with the respective slip ring 8, 10. The conductor track extends from the associated slip ring 8, 10 through the plastic section toward the separately excited rotor (not shown here) arranged on the rotor shaft RW.

[0050] The slip ring 8 is pot-shaped and is formed, for example, as a so-called deep-drawn part, wherein the bottom of the pot forms a front-side metallic section 30 - which is only partially illustrated in Fig. 4 - of the end of the rotor shaft RW in order to be able to interact with, for example, an inductive signal generator of the printed circuit board 28.

[0051] This bottom of the pot or this frontal metallic section 30 of the

[0052] Slip ring 8 has to be considered with interruptions in the circumferential direction of the

[0053] Slip ring 8 can be imagined, which can be filled with the plastic as such. Between two adjacent interruptions or recesses in this pot base is a metallic web-shaped section, which is molded onto the outer, closed, circumferential section of the slip ring 8, which interacts with the grinding brush 4.

[0054] The rotor shaft section RWA according to Fig. 4 thus performs two functions. On the one hand, it serves to separately excite the rotor of the rotor shaft RW, and on the other hand, to detect the position of the separately excited rotor. The slip ring 8 combines two functions in one component. On the one hand, it acts as a contacting element with the grinding brush 4, and on the other hand, it acts as a sensor wheel with the inductive signal generator of the circuit board 28. When interacting with the signal generator, periodic or sine-cosine voltage signals are generated, from which an absolute angular position of the rotor relative to a stator of the synchronous machine can be determined in a control unit, so that the synchronous machine can be commutated efficiently.

[0055] An acceleration sensor can also be provided on the circuit board 28, which can be used to determine vehicle accelerations in the direction of the grinding brushes 4, 6 – resulting from impacts and vibrations. These vehicle accelerations can be used – along with other vehicle parameters – to adjust the contact pressure of the grinding brushes 4, 6. Additionally or alternatively, vehicle acceleration information can also be tapped, for example, from a drive axle of the vehicle and provided via a CAN bus, to which the brush actuator 2 proposed here is connected via the circuit board 28, in order to be able to counteract such vehicle accelerations with the brush actuator 2.

[0056] And since vehicle accelerations in the direction of the brushes 4, 6 – resulting from impacts and vibrations – can be compensated for by actively pressing the brushes 4, 6, the number of brushes 4, 6 required for external excitation can be reduced to a pair. The brush actuator 2 illustrated in Figs. 1 to 4 requires very little space in the axial direction of the rotor shaft RW. It can also be easily mounted on the electric motor housing and radially to the rotor shaft.

[0057] In the embodiment of the brush actuator 2 according to Figs. 3 and 4, it differs from the embodiment of the brush actuator 2 according to Figs. 1 and 2 only in the added circuit board 28 and the plastic housing section 26 receiving it, as well as the plastic receiving section, which is also formed on the plastic housing section of the electromagnet 12 and has receiving points, via which the brush actuator 2 is or can be attached to the housing of the electric motor - and is located radially to the rotor shaft RW.

[0058] The active pressing or pressing of the grinding brushes 4, 6 against the associated slip rings 8, 10 proposed within the scope of this disclosure advantageously reduces the sliding contact friction or sliding contact friction power to a necessary minimum, depending on requirements, compared to a purely passive system in which a spring force for pressing or pressing the grinding brushes is determined based on a maximum rotor shaft speed. This also advantageously results in reduced or lower brush wear. This also reduces the electric motor's power consumption, thus increasing the range of a battery supplying the electric motor.

[0059] The pressure or compression via spring 20 can be reduced up to a definable first limit speed of the rotor shaft and increased above a definable second limit speed of the rotor shaft, which is higher than the first speed. However, above the first limit speed and up to the second limit speed, which define a speed range, the electromagnet 12 can remain de-energized.

[0060] It is proposed to define or fix the first and second limit speeds, and thus to specify the speed range defined by them or lying between them, in such a way that the most frequently occurring speeds in a driving cycle—for example, in the form of a so-called WLTC (Worldwide Harmonized Light Vehicle Test Cycle)—are covered by this speed range, in which the electromagnet can remain de-energized. This results in a minimal power requirement for the active pressing or pressure of the grinding brushes 4, 6 over the lifetime of the aforementioned vehicle.

[0061] As an alternative to the electromagnet 12 shown in Figs. 1 to 4, in which the coil body 16 is designed as a stator which acts on the armature 18 located inside it or on the armature 18 which is designed as a permanent magnet and which is enclosed by it, the electromagnet can also be designed in the form of a plunger coil - not shown here - whose armature is designed as a coil body which, as such, plunges into a stator designed as a permanent magnet and is designed to be longitudinally movable relative to it.

[0062] Although exemplary embodiments are explained in the foregoing description, it should be noted that numerous modifications are possible. Furthermore, it should be noted that the exemplary embodiments are merely examples and are not intended to limit the scope of protection, applications, or structure in any way. Rather, the foregoing description provides the skilled person with a guide for implementing at least one exemplary embodiment, whereby various modifications, particularly with regard to the function and arrangement of the described components, can be made without departing from the scope of protection as it results from the claims and equivalent combinations of features.

Claims

Patent claims 1 . Brush actuator (2) for the external excitation of a rotor of an electric motor arranged on a rotor shaft, wherein the brush actuator (2) has a first and a second grinding brush (4, 6) which can each be pressed against an associated slip ring (8, 10) of the rotor shaft, wherein the grinding brushes (4, 6) can be spring-biased against the slip rings (8, 10) via an armature (18) of an electromagnet (12) and a support section of the electromagnet (12) which cooperates with the armature (18) via a spring (20), wherein an electromagnetic force acting in or against a prestressing direction of the spring (20) and variably adjustable as required can be impressed on the armature (20) via a coil body (16) of the electromagnet (12) in order to increase or decrease the pressure of the grinding brushes (4, 6) via the spring (20) and thereby prevent the grinding brushes (4, 6) from lifting off during a To avoid operation of the electric motor.

2. Brush actuator (2) according to claim 1, wherein the armature (18) is articulated against a fork-shaped force distribution element (22) which can be supported against the slip rings (8, 10) by dividing into two arms.

3. Brush actuator (2) according to one of the preceding claims, wherein the grinding brushes (4, 6) are designed in the form of silver graphite brushes.

4. Arrangement of a rotor shaft of an electric motor and a wet-running, electromagnetically actuatable brush actuator (2) for the external excitation of a rotor of the electric motor, which is arranged on the rotor shaft, wherein the brush actuator (2) has a first and a second grinding brush (4, 6) which is each pressed against an associated slip ring (8, 10) of the rotor shaft, wherein the grinding brushes (4, 6) are spring-biased against the slip rings (8, 10) via an armature (18) of an electromagnet (12) and a support section of the electromagnet (12) which cooperates with the armature (18) via a spring (20), wherein an electromagnetic force which acts in or against a prestressing direction of the spring (20) and which can be variably adjusted as required can be impressed on the armature (18) via a coil body (16) of the electromagnet (12) in order to exert a contact pressure of the grinding brushes (4, 6) via the spring (20) and to prevent the grinding brushes (4, 6) from lifting off during operation of the electric motor.

5. Arrangement according to claim 4, wherein the armature (18) is articulated against a fork-shaped force distribution element (22) which is divided into two arms and supported against the slip rings (8, 10).

6. Arrangement according to claim 4 or 5, wherein on the rotor shaft between the slip rings (8, 10) an insulating section (24) projecting circumferentially in the radial direction of the rotor shaft is provided to form a sufficient creepage and air gap between the slip rings (8, 10).

7. Arrangement according to one of the preceding claims 4 to 6, wherein the grinding brushes (4, 6) are designed in the form of silver graphite brushes.

8. Electric motor, in particular for driving a vehicle, with a separately excited rotor arranged on a rotor shaft, wherein the rotor shaft is combined with a wet-running brush actuator (2) according to one of the preceding claims 1 to 3 and thereby forms an arrangement according to one of the preceding claims 4 to 7.

9. Vehicle with an arrangement according to one of claims 4 to 7 and / or an electric motor according to claim 8.

10. A method for operating an electric motor, in which, for the external excitation of a rotor of the electric motor arranged on a rotor shaft, at least a first and second grinding brush (4, 6) are pressed against associated slip rings (8, 10) of the rotor shaft, wherein the grinding brushes (4, 6) are spring-biased against the slip rings (8, 10) via an armature (18) of an electromagnet (12) and a support section of the electromagnet (12) which cooperates with the armature (18) via a spring (20), wherein a demand-acting, in or against a pretensioning direction of the spring (20), dependently variable adjustable electromagnetic force is impressed on the armature (18) in order to increase or decrease the pressure of the grinding brushes (4, 6) via the spring (20) and to prevent the grinding brushes (4, 6) from lifting off during operation of the electric motor.

11. Method according to claim 10, wherein the grinding brushes (4, 6) are passively spring-loaded up to a definable limit speed of the rotor shaft and are actively spring-loaded above this limit speed by energizing the electromagnet and are pressed against the associated slip rings (8, 10) in a variable manner as required.

12. The method according to claim 10, wherein the grinding brushes (4, 6) are actively spring-biased up to a definable first limit speed and above a definable second limit speed of the rotor shaft which is higher than the first speed and are pressed against the associated slip rings (8, 10) in a variable manner as required, the two limit speeds defining an intermediate speed range in which the grinding brushes (4, 6) are passively spring-biased and pressed against the associated slip rings (8, 10).

13. The method according to claim 12, wherein the first and second limit speeds are determined and thus the speed range lying between them is determined such that the speeds occurring most frequently in a driving cycle are covered by this speed range in which the electromagnet is not energized.

14. The method according to claim 13, wherein a WLTC driving cycle (WLTC = Worldwide Harmonized Light Vehicles Test Cycle) is used as the driving cycle.

15. Method according to one of claims 10 to 14, wherein the pressure via the spring is adjusted taking into account a rotor (shaft) speed, a rotor (shaft) position and / or vehicle acceleration in the direction of the grinding brushes.

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