WO2014001355A2

WO2014001355A2 - Electric machine having integrated bearing arrangement

Info

Publication number: WO2014001355A2
Application number: PCT/EP2013/063312
Authority: WO
Inventors: Isak LÖFGREN
Original assignee: Pandion Haliaetus Ab
Priority date: 2012-06-27
Filing date: 2013-06-25
Publication date: 2014-01-03
Also published as: WO2014001355A3

Abstract

The present invention relates to an electric machine, comprising: a stator; a rotor for rotation around a geometrical axis of rotation, wherein said stator and said rotor in conjunction with each other generate an electromagnetic field for operating said electric machine; a bearing arrangement interposed between said stator and said rotor for allowing a relative rotational motion there between; and an electrical insulation arranged between the rotor and the stator for preventing electrical current to flow between the rotor and the stator.

Description

ELECTRIC MACHINE HAVING INTEGRATED BEARING ARRANGEMENT

Field of the Invention

The present invention relates to the field of electric machines, and more specifically to an electric machine having an integrated bearing arrangement.

Background of the Invention

Electric machines are used in a wide variety of applications due to, for example, their capability of being provided in different sizes and also to the fact that they are environmentally friendly which has been an increasing demand lately. Typical applications are, for example, industrial fans, machine tools, household appliances, blowers and pumps, etc. Moreover, in the automotive field, electric machines can also be used, for example, as starter motors for the internal combustion engine of the vehicles.

Typically, the electric machines comprise a moving part, the rotor, and a stationary part, the stator. Electromagnetic fields are formed in the rotor as well as in the stator, and the product between these electromagnetic fields give rise to a force enabling for a relative rotational motion between the stator and the rotor. The rotor or the stator of the electric machine is in most applications arranged onto a centrally positioned shaft in order to transmit the rotational motion to, for example, the internal combustion engine or other rotationally driven components. To enable the rotational motion from the electric machine to be transmitted to other components, bearings are often used. The bearings are arranged to connect the shaft to, for example, a housing or the like on a respective side of the electric machine. Bearings are also important in regards to the aspect of preventing the shaft from being moved in the translational degrees of freedom while allowing motion in the desired rotational degree of freedom.

A problem of using bearings in close proximity to the electric machine is, however, that electrical current that may be encountered in the electric machine may cause different electric potential between the rotor and the stator. This may in turn increase the risk of electrical current to flow through the bearing, thereby short-circuiting the electric machine and also damaging the bearing arrangement. US 7,317,270 presents an approach for preventing equalization between the rotor and the stator of an electric motor by using electrically insulated bearings. Hereby, a transducer system of the electric motor is electrically decoupled from the rotor and the stator.

Although the electric motor disclosed in US 7,317,270 reduces the risk of short-circuiting the electric motor and prevents the bearing arrangement to be damaged, a problem with US 7 317 270 is that the bearing arrangements are exposed to large bearing loads in terms of e.g. bending moments and shear forces from the electrical motor. As a consequence, the bearing arrangements are relatively large in size in order to sustain these bending moments and shear forces, thus making the entire electric machine large in size, expensive, etc. Hence, it would be desirable to provide an electric machine which is reduced in size and which is less expensive for

manufacturing.

Summary of the Invention

In electric machines, it is well known that an air gap between the stator and the rotor is necessary for preventing electrical current to flow between the stator and the rotor, which electrical current otherwise may short-circuit the electric machine. The present invention, however, is based on the insight that the air gap between the stator and the rotor may be replaced by an insulated bearing arrangement, contrary to prior art solutions in which bearing arrangements are provided on each side of the electric machine. Hereby, the electric machine may be reduced in size, minimizing shear forces on the bearings, as well as reducing the number of components for the electric machine.

According to an aspect of the present invention, an electric machine is provided. The electric machine comprises a stator, a rotor for rotation around a geometrical axis of rotation, wherein the stator and the rotor in conjunction with each other generate an electromagnetic field for operating the electric machine; a bearing arrangement interposed between the stator and the rotor for allowing a relative rotational motion there between; and an electrical insulation arranged between the rotor and the stator for preventing electrical current to flow between the stator and the rotor.

An advantage of having a bearing arrangement interposed between the stator and the rotor is, for example, that the electric machine may be reduced in size since the need of supporting the electric machine with bearings on each side of the electric machine is reduced. Also, in electric machines according to prior art solutions, the rotor or the stator is most often connected to a shaft for transferring the rotational motion that arises when propelling the electric machine. The present invention, however, allows for the stator or the rotor to act as rotational transferring shaft which thereby reduces the number of articles needed for providing the electrical machine. It should however be understood that the present invention is also applicable with the use of an external shaft onto which the stator or the rotor is positioned, which will be described further below. Moreover, when having bearing arrangements on each side of the electric machine, these bearing arrangements are exposed to bending moments and shear forces from e.g. the weight of the rotor and the stator, which arise due to the lever between the electric machine and the bearing arrangements. By interposing the bearing arrangement between the stator and the rotor according to the present invention such lever may be minimized and the bearing arrangement may hence be relatively free from such bending moments and shear forces, thus allowing for the use of e.g. smaller bearing arrangements with less strength in comparison to prior art solutions. Accordingly, the flexibility of choosing bearing arrangement for the electric machine is increased.

Furthermore, by providing an electrical insulation between the rotor and the stator, electrical currents are prevented from flowing between the rotor and the stator, which otherwise may short-circuit the electric machine and also damaging the bearing arrangement. The electrical insulation may be provided to the electric machine in a number of ways. For example, the rolling elements of the bearing arrangement may be constituted by a material having insulating properties, or a circumferential positioned electrical insulating layer may be provided to the stator and/or the rotor. A further description of various types of electrical insulations will be described further below.

It should be readily understood that the present invention is not limited to any specific type of electric machine for its function. The electric machine may, for example, be a permanent-magnet DC motor or a brushed DC motor. These two kinds of electric machines will be described further below, but other alternatives are of course conceivable, such as e.g. an AC motor, brushless DC motor, induction motor, etc. Furthermore, the invention should also not be construed as limited to the use of a specific type of bearing arrangement. For example, a ball bearing arrangement, needle bearing arrangement, conical bearing arrangement, sliding bearing arrangement, etc. may be used. These types of bearing arrangements, and their advantages will be described in yet more detail below. Moreover, the bearing arrangements may be provided with or without inner and outer bearing rings. In the case of not having separate inner and outer bearing rings, a surface of the stator and a surface of the rotor may act as raceway for e.g. the rolling elements of the bearing arrangement. An advantage of utilizing the stator and the rotor as raceways for the rolling elements is that the electric machine may be even further reduced in size.

According to an example embodiment, the electric machine may be an electric motor. An electric motor is configured to convert electrical energy to mechanical energy, i.e. to a rotational motion of the stator or the rotor.

However, the electric machine may also be a generator which is configured to convert mechanical energy to electrical energy.

According to an example embodiment, the bearing arrangement, stator and rotor may be coaxially aligned with each other. This may, for instance, be in the form of a radial motor or an axial motor.

Hereby, a compact electric machine may be provided where the stator, rotor and bearing arrangement are provided on the same geometric axis.

According to an example embodiment, the bearing arrangement may encircle one of the stator and the rotor, and be encircled by the other one of the stator and the rotor.

By arranging the bearing arrangement to encircle one of the stator and the rotor, and to be encircled by the other one of the stator and the rotor, an axial motor may be provided.

According to an example embodiment, the electric machine may further comprise a shaft aligned with the geometrical axis of rotation, wherein the shaft and one of the rotor and the stator are configured to rotate relative to each other.

Hereby, the shaft may transfer the rotational mechanical motion formed when converting electrical energy to mechanical energy, or vice versa. The shaft may in turn be connected to a component which is to be exposed to a rotational motion or a torque load. The shaft may either be a moving part or a non-moving part. In the case where the shaft and the rotor are configured to rotate relative to each other the shaft is a non-moving part and in the case where the shaft is a rotating part the shaft and the stator are configured rotate relative to each other. Furthermore, the shaft may in turn be connected to other components of a structure, such as, for example, another shaft coupled to an engine of a vehicle or to a gear for meshing with a corresponding gear in a gear box arrangement, etc.

According to an example embodiment, one of the stator and the rotor may be mounted onto the shaft.

By arranging the electric machine such that the stator or the rotor is configured to be mounted onto the shaft, the electric machine may be exchangeable between different shafts. Accordingly, the electric machine may be recycled if, for example, components connected to the shaft is exposed to some kind of failure. Furthermore, if the rotor is connected to the shaft; the shaft may in turn be exposed to a rotating motion when the electric machine is propelled. If, on the other hand, the stator is connected to the shaft, the shaft may be kept in a non-rotating motion such that the rotor is rotating around the shaft, which is also described above. Furthermore, according to an example embodiment, the bearing arrangement may be positioned centrally on the shaft in relation to an axial extension of the shaft.

According to an example embodiment, the shaft is formed by one of the stator and the rotor.

Hereby, the need of an externally provided shaft may be reduced. Accordingly, the electric machine may be provided with fewer components making the electric machine, for example, more compact and less expensive. The shaft formed by the rotor or the stator may, however, in turn be connected to an externally arranged shaft for transferring the rotational motion to other components. The rotor or the stator may be connected to such externally arranged shaft in a plurality of fashions, such as by means of a splined coupling, meshed gears, a universal joint, etc.

According to an example embodiment, the bearing arrangement is interposed between the stator and the rotor in a direction along the

geometrical axis of rotation.

Hereby, a radial electric machine may be arranged.

According to an example embodiment, the electric machine may be a permanent-magnet DC motor.

A permanent-magnet DC motor comprises a stator and a rotor, where one of the stator and the rotor may comprise windings of electrical wire, while the other one of the stator and the rotor comprises permanent magnets to provide the magnetic field against which the field from the windings of electrical wire interacts to produce a torque load. The permanent-magnet DC motor may use different types of permanent magnet which are considered to be known by the skilled person.

According to an example embodiment, the stator may comprise windings of electrical wire.

Hereby, if the electric machine is a permanent-magnet DC motor, the rotor may, as described above, comprise permanent magnets. If, on the other hand, the electric machine is e.g. a brushed DC motor, the rotor may also comprise windings of electrical wire. The brushed DC motor will be described further below.

According to an example embodiment, the rotor may comprise windings of electrical wire.

For the same reasons as described above, if the rotor comprises windings of electrical wire, the stator may also comprise windings of electrical wire or comprise permanent magnets depending on the specific type of electric machine chosen.

According to an example embodiment, the electric machine may be a brushed DC motor.

In a brushed DC motor, the rotor and the stator comprise windings of electrical wire. More specifically, the windings of electrical wire provided in the rotor and the stator are arranged perpendicular to each other in order to create perpendicular electromagnetic fields that interact with each other for propulsion of the electric machine.

Although different kinds of electric machines have been described above, other alternatives are also conceivable and are known to the person skilled in the art.

According to an example embodiment, the bearing arrangement may comprise an inner raceway, an outer raceway and rolling elements between the raceways, wherein at least one of the inner raceway, the outer raceway and the rolling elements is provided with insulating characteristics for preventing electrical current to flow between the rotor and the stator.

An inner- and an outer raceway are needed in order to guide the rolling elements of the bearing arrangement. The inner raceway may be constituted by an inner ring having a circumferential recess in which the rolling elements are situated. In a similar manner, the outer raceway may be constituted by an outer ring which also comprises a circumferential recess. Accordingly, the rolling elements of the bearing arrangement are situated between the inner ring and the outer ring, and more precisely located within the described recesses of the respective inner ring and outer ring. The invention is, however, not limited to the use of separate inner and outer rings for providing the needed raceways. For example, one of the inner ring and the outer ring may be constituted by the stator or the rotor. In other embodiments, the inner and outer raceways may instead be constituted by the stator and the rotor, respectively, or vice versa. If the stator and/or the rotor comprise windings of electrical wire as described above, a circumferentially arranged surface may be provided onto the rotor and stator comprising the windings, such that a well defined raceway is provided for the rolling elements. If the rotor or the stator comprises permanent magnets, these magnets may be arranged in recesses of, for example, a shaft, such that a well defined raceway for the rolling elements is provided.

Moreover, according to an example embodiment, the bearing arrangement may be constituted by a plurality of rolling elements arranged next to each other. More specifically, in an axial motor as described above, a plurality of rolling elements may be provided in an axial direction and in a radial motor as described above, a plurality of rolling elements may be provided in a radial direction.

Furthermore, in order to prevent electrical current from flowing through the bearing arrangement, at least one of the inner raceway, the outer raceway or the rolling elements may be provided with insulating characteristics.

Accordingly, the insulating characteristics should throughout the entire description be interpreted as an electrical insulation. The insulation may hence be arranged on at least one of the raceways in the form of an insulating layer, or the rolling elements may be electrically insulated, for example by being made of an electrically insulated material, such as e.g. ceramics. These different options are reflected in the example embodiments presented below.

According to an example embodiment, the rolling elements are formed by a ceramic material.

A ceramic material has, as described above, electrically insulated characteristics, thereby preventing electrical currents to flow between the stator and the rotor, i.e. through the bearing arrangement. A ceramic material is also well known and relatively easy to use when forming the rolling elements. Other electrically insulated materials are of course conceivable and well known by the person skilled in the art of materials. According to an example embodiment, at least one of the inner and the outer raceways is coated with an electrically insulating material.

As described above, an electrically insulating layer may be beneficial for preventing electrical current to flow between the stator and the rotor. By providing an electrically insulating layer on at least one of the inner- and outer raceways, there may not be a need of forming the rolling elements from an electrically insulated material as described above. Accordingly, the flexibility of choosing rolling element for the bearing arrangement is increased. The insulating materials for the raceways may, for example, be ceramics, material comprising the trademark Teflon, different kinds of polymer materials and combinations thereof, etc. It should however be understood that the bearing arrangement may be provided with both insulating layers as well as insulated rolling elements. Hereby, the electrical insulation of the bearing arrangement may be further secured.

According to an example embodiment, the inner raceway may be constituted by one of the stator and the rotor.

Hereby, a surface constituted by the stator or the rotor may act as inner raceway. Accordingly, as described above, the need of an external inner ring is reduced. An advantage is that a smaller electric machine may be provided in comparison to using a separate inner ring.

According to an example embodiment, the outer raceway may be constituted by one of the stator and the rotor.

Similarly to the above description of the stator or the rotor acting as inner ring, the stator or the rotor may also act as outer ring, depending on which one of the stator and the rotor being positioned circumferentially of the bearing arrangement. The bearing arrangement may also be provided without an inner- and an outer ring. Hereby, the size of the electric machine may be even further reduced.

According to an example embodiment, the bearing arrangement may be one of a ball bearing arrangement, needle bearing arrangement or tapered bearing arrangement.

These types of bearing arrangements are well known and may be relatively easy to handle when, for example, assembling the electric machine. Also, these kinds of bearing arrangement may be provided from suppliers with or without separate bearing rings. Brief Description of the Drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Fig. 1 is a partial cut-out perspective view of an example embodiment of an axial electric machine according to of the present invention;

Fig. 2 is a partial cut-out perspective view of a further example embodiment of an axial electric machine according to the present invention;

Fig. 3 is a partial cut-out perspective view of a still further example embodiment of an axial electric machine according to the present invention;

Fig. 4 is a partial cut-out perspective view of an example embodiment of a radial electric machine according to the present invention; and

Fig. 5 is a partial cut-out perspective view of a further example embodiment of a radial electric machine according to the present invention.

Detailed Description of Exemplary Embodiments of the Invention

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description.

Referring now to the drawings and to Fig. 1 in particular, there is depicted a partial cutout perspective view of an axial electric machine 100 comprising a rotor 102 and a stator 104. The rotor 102 comprises a plurality of rotor cores 105 provided with windings of electrical wire 106. The rotor cores 105 are in turn coupled to a shaft 108 of the electric machine 100. The shaft 108 may either be a part of the electric machine 100 or it may be arranged as a separate component which can be connected to the electric machine 100 by means of a suitable connecting element, such as e.g. a spline connection, which will be further described below in relation to the description of Fig. 2. Furthermore, in the example embodiment of Fig. 1 the stator 104 comprises a plurality of permanent magnets 1 1 1 . The rotor 102 and the stator 104 each forms electromagnetic fields and the product of these electromagnetic field give rise to a force enabling a relative rotational motion between the stator 104 and the rotor 102, i.e. electrical energy is converted to mechanical energy, in the embodiment depicted in Fig. 1 as a torque provided to the shaft 108 since the rotor 102 is connected thereto. Although the electric machine in Fig. 1 is depicted as having a relatively coarse distribution of rotor and stator magnets, the number of rotor cores as well as permanent magnets may of course be increased such that a denser distribution of permanent magnets and rotor cores are provided. The relatively coarse distribution is merely for simplified illustrative purposes.

Moreover, the electric machine 100 further comprises a bearing arrangement 1 10 which is radially interposed between the rotor 102 and the stator 104 in relation to an axial direction of the electric machine 100. The bearing arrangement 1 10 is, in the illustrated embodiment depicted in Fig. 1 , a needle roller bearing arrangement comprising a plurality of cylindrical rolling elements 1 12 allowing a relative rotational motion between the rotor 102 and the stator 104. However, other types of bearing arrangement may be used, such as a ball bearing arrangement, tapered bearing arrangement, conical bearing arrangement, etc. Also, instead of having a single row of cylindrical rolling elements 1 12, a plurality of cylindrical rolling elements may be provided in the axial direction, i.e. side by side to each other.

Furthermore, the bearing arrangement 1 10 comprises an inner bearing ring 1 14 and an outer bearing ring 1 16, wherein the inner bearing ring 1 14 is coupled to the rotor cores 105 and the outer bearing ring 1 16 is coupled to the stator 104. The inner bearing ring 1 14 and the outer bearing ring 1 16 each comprises edge portions 122, 124 for preventing the rolling elements 1 12 of the bearing arrangement from being moved axially. Moreover, the coupling of the inner bearing ring 1 14 to the rotor cores 105 and the outer bearing ring 1 16 to the stator 104 may be accomplished in a number of fashions, such as by press-fitting, shrinkage-fitting, welding, gluing, etc.

Accordingly, the inner bearing ring 1 14 and the outer bearing ring 1 16 form an inner raceway 1 18 and an outer raceway 120, respectively, for guiding the cylindrical rolling elements 1 12 inside the bearing arrangement 1 10. Although the bearing arrangement 1 10 is described as comprising a separate inner bearing ring 1 14 and outer bearing ring 1 16, the present invention is equally applicable by arranging the rotor 102 and the stator 104 such that a surface thereof is forming the respective raceways, which will be further described below. As described above, in use, the rotor 102 and the stator 104 forms electromagnetic fields which in conjunction transforms electrical energy to mechanical energy which may be output as a rotational motion of the shaft 108. However, the electromagnetic fields may give rise to electrical currents which could flow between the rotor 102 and the stator 104 which could, for example, short-circuit the electric machine 100. In order to prevent such flow of electrical current between the rotor 102 and the stator 104, the electric machine 100 further comprises an electrical insulation between the rotor 102 and the stator 104. A configuration of the electric machine 100 illustrated in Fig. 1 may provide such insulation between the rotor 102 and the stator 104 by having cylindrical rolling elements 1 12 made of an electrically insulating material, such as e.g. ceramics. The electrical insulation may instead, or additionally, be provided by having the inner bearing ring 1 14 and/or the outer bearing ring 1 16 made of an electrically insulating material. Furthermore, the electrical insulation may also be provided by an electrically insulating layer provided on the inner 1 18 and/or the outer 120 raceway of the bearing arrangement 1 10.

Now, reference is made to Fig. 2, illustrating a partial cutout

perspective view of a further example embodiment of an axial electric machine 200 according to the present invention. Many of the features in Fig. 2, as well as Fig.3 described further below, are similar to those described above in relation to Fig. 1 and may hence not be described in further detail.

The electric machine 100 in Fig. 2 comprises a rotor 202 having a plurality of permanent magnets 1 1 1 and a stator 204 comprising a plurality of stator cores 205 having windings of electrical wire 106. The rotor 202 further comprises an opening 206, here illustrated as a through-hole. The opening 206 comprises a plurality of circumferentially arranged cut-outs 208 extending in the axial direction of the electric machine 200. The cut-outs are configured to fit with corresponding protrusions of a shaft or the like, thereby forming a spline coupling between the shaft and the electric machine. Accordingly, the electric machine 200 depicted in Fig. 2 is configured to be connected to the shaft, in comparison to the electric machine in Fig. 1 which comprises an integrated shaft 108 on the rotor 102.

Furthermore, the electric machine 200 comprises a bearing

arrangement 210 which is interposed between the rotor 202 and the stator 204. A difference between the bearing arrangement 1 10 depicted in Fig. 1 and the bearing arrangement 210 in Fig. 2 is that the bearing arrangement in Fig. 2 does not have separate inner and outer bearing rings. As can be seen in Fig. 2, the rotor 202 comprises a circumferentially arranged recess 212 which hence acts as an inner raceway for the rolling elements 1 12. Also, a thin sheet 214 of, for example, an electrically insulating material is connected to the stator 204 interconnecting the plurality of stator cores 205 having windings of electrical wire 106 with each other to form an outer raceway for the rolling elements 1 12. Hereby, the rolling elements 1 12 are allowed to be larger compared to the rolling elements in the configuration depicted in Fig. 1 , since there are no separate inner and outer bearing rings taking up the space between the rotor and the stator.

The electrical insulation in the embodiment depicted in Fig. 2 may be provided by having roller bearing elements 1 12 from an electrically insulating material, such as e.g. ceramics. As an alternative or complement, the rotor 202 and/or the stator 204 may be provided with an electrical insulating layer in proximity to the bearing arrangement 210, thereby either increasing the electrical insulation or providing for the use of non-electrically insulated rolling elements.

Turning to Fig. 3, there is depicted a partial cutout perspective view of a still further example embodiment of an axial electric machine 300 according to the present invention. A difference between the electric machine 300 in Fig. 3 and electric machines 100, 200 in Figs. 1 and 2, respectively, is that neither the rotor nor the stator in Fig. 3 comprises permanent magnets. Instead, both the rotor 302 and the stator 304 comprise windings of electrical wire 306, 308. The windings of electrical wire 306 arranged on the rotor 302 are positioned circumferentially on the rotor 302 in relation to the axial direction of the electric machine 300 and the stator 304 comprises a plurality of stator cores 305 having windings of electrical wire 308 arranged substantially

perpendicularly to the windings of electrical wire 306 in the rotor 302. By arranging the windings of electrical wire 306, 308 substantially

perpendicularly to each other enable the rotor 302 of the electric machine 300 to rotate in relation to the stator 304, which is well known by a person skilled in the field of electric machines. Accordingly, the electric machine 300 depicted in Fig. 3 is a so called brushed electric machine.

Furthermore, since the rotor 302 comprises circumferentially arranged windings of electrical wire 306, the rotor 302 itself may not constitute a well defined surface for guiding the rolling elements of the bearing arrangement 310. Therefore, an inner bearing ring 312 is positioned onto the windings of electrical wire 306. Also, a thin sheet or outer bearing ring 314 is provided to the stator coils 305 in order to provide a well defined outer raceway for the rolling elements of the bearing arrangement. The electrical insulation between the rotor 302 and the stator 304 in the embodiment illustrated in Fig. 3 may hence be provided by having the rolling elements made from a material having electrically insulating properties and/or the inner and outer raceways may be provided with electrically insulating characteristics, such as e.g. an electrically insulating material and/or an insulating layer provided on the stator and/or the rotor.

Reference is now made to Fig. 4 illustrating an example embodiment of a partial cut-out perspective view of a radial electric machine 400 according to the present invention.

The radial electric machine 400 comprises a rotor 402 and two stators 404 which axially enclose the rotor 402. The rotor 402 is in the embodiment depicted in Fig. 4 coupled to a shaft 406 of the electric machine 400, while the stators 404 are coupled to axial sealing packages 408. The rotor 402 further comprises a plurality of permanent magnets 410 arranged on opposites sides of the rotor 402 in relation to the axial direction of the electric machine 400. Moreover, the stators 404 comprise a plurality of stator cores 410 having windings of electrical wire 412. Furthermore, the plurality of stator cores 410 is axially connected to a surface 414 of the stator 404.

Moreover, two bearing arrangements 416 are axially interposed between the rotor 402 and the stator 404, respectively on each side of the rotor 402. The bearing arrangements 416 are configured to allow for a relative rotational motion between the rotor 402 and the stator 404. In the example embodiment illustrated in Fig. 4, the bearing arrangements 416 are ball bearing arrangements having spherical rolling elements. Other bearing arrangements are of course also conceivable, such as tapered bearing arrangements, sliding bearing arrangements, needle bearing arrangements, etc. Furthermore, circumferentially arranged recesses are provided in the stator 404 and in the rotor 402, respectively. Hereby, well defined raceways 418, 420 are provided for the rolling elements of the bearing arrangements 416. Accordingly, the inner raceway 418 is constituted by the rotor 402 and the outer raceway 420 is constituted by the stator 404 for each of the two bearing arrangements. Hereby, no separate inner and outer bearing rings need to be provided to the bearing arrangement. The electrical insulation between the rotor 402 and the stator 404 may be provided in a similar manner as for the electric machines described above in relation to Figs. 1 - 3, i.e. having electrically insulated rolling elements, a layer on the rotor 402 and/or the stator 404, etc.

Finally, attention is now drawn to Fig. 5, illustrating a partial cut-out perspective view of a further example embodiment of a radial electric machine 500 according to the present invention. The difference between the radial electric machine 400 described in relation to Fig. 4 and the radial electric machine 500 depicted in Fig. 5 is mainly that the rotor 502 comprises a plurality of rotor cores 506 having windings of electrical wire 508, while the stator 504 comprises a plurality of permanent magnets 510. Accordingly, the plurality of rotor cores 506 are coupled to the shaft 512. As described above, the shaft 512 may be a part of the rotor 502 or it may also be a separate component detachable to the rotor 502. Furthermore, in order to provide a continuous raceway for the rolling elements of the bearing arrangement, a sheet 514 of, for example, an insulating material is connected to the rotor cores 506 on each side thereof.

Although the description in relation to Figs. 4 and 5 describes a radial electric machine 400 and 500 having a plurality of permanent magnets in the rotor or the stator, a radial electric machine may also, according to at least one example embodiment of the present invention, use a plurality of cores having windings of electrical wire for the stator as well as the rotor, making it a brushed radial electric machine. In such a case, sheets 514 connecting the rotor cores 506 in Fig. 5 may be provided to the stator cores as well.

Furthermore, although the description above describes a rotor being connected or being connectable to a shaft, the present invention is equally applicable coupling the stator to the shaft. In such a case, the shaft is a non- rotating component and the rotor may hence be coupled to another component for which it is desired to provide a rotational motion. Still further, the electric machine described above should be interpreted as to function both as an electric motor as well as a generator, and although not described above, the present invention may be equally applicable for three-phase electrical power as well as two-phase electrical power.

Thus, the above description of the example embodiments of the present invention and the accompanying drawings are to be regarded as a non-limiting example of the invention and the scope of protection is defined by the appended claims. Any reference sign in the claims should not be construed as limiting the scope.

Claims

1 . An electric machine, comprising:

- a stator;

- a rotor for rotation around a geometrical axis of rotation, wherein said stator and said rotor in conjunction with each other generate an

electromagnetic field for operating said electric machine;

- a bearing arrangement interposed between said stator and said rotor for allowing a relative rotational motion there between; and

- an electrical insulation arranged between the rotor and the stator for preventing electrical current to flow between the rotor and the stator.

2. The electric machine according to claim 1 , wherein the electric machine is an electric motor.

3. The electric machine according to any one of the preceding claims, wherein said bearing arrangement, stator and rotor are coaxially aligned with each other.

4. The electric machine according to any one of the preceding claims, wherein said bearing arrangement encircles one of said stator and said rotor, and is encircled by the other one of said stator and said rotor.

5. The electric machine according to any one of the preceding claims, further comprising a shaft aligned with said geometrical axis of rotation, wherein said shaft and one of said rotor and said stator are configured to rotate relative to each other.

6. The electric machine according to claim 5, wherein one of said stator and said rotor is mounted onto said shaft.

7. The electric machine according to claim 5, wherein said shaft is formed by one of said stator and said rotor.

8. The electric machine according to any one of claims 1 - 3, wherein said bearing arrangement is interposed between said stator and said rotor in a direction along said geometrical axis of rotation.

9. The electric machine according to claim 8, wherein said bearing arrangement is a first bearing arrangement, the electric machine further comprising a second bearing arrangement, wherein the first and second bearing arrangements enclose one of said stator and said rotor and are enclosed by the other one of the stator and the rotor.

10. The electric machine according to any one of the preceding claims, wherein said electric machine is a permanent-magnet DC motor.

1 1 . The electric machine according to any one of claims 1 - 10, wherein said stator comprises windings of electrical wire.

12. The electric machine according to any one of claims 1 - 10, wherein said rotor comprises windings of electrical wire.

13. The electric machine according to any one of claims 1 - 9, wherein said electric machine is a brushed DC motor.

14. The electric machine according to any one of the preceding claims, wherein said bearing arrangement comprises an inner raceway, an outer raceway and rolling elements between said raceways, wherein at least one of said inner raceway, said outer raceway and said rolling elements is provided with electrical insulating characteristics for preventing electrical current to flow between the rotor and the stator.

15. The electric machine according to claim 14, wherein said rolling elements are formed by a ceramic material.

16. The electric machine according to claim 14 or 15, wherein at least one of said inner and said outer raceways is coated with an electrically insulating material.

17. The electric machine according to any one of claims 13 - 16, wherein said inner raceway is constituted by one of said stator and said rotor.

18. The electric machine according to any one of claims 13 - 16, wherein said outer raceway is constituted by one of said stator and said rotor.

19. The electric machine according to any one of the preceding claims, wherein said bearing arrangement is one of a ball bearing arrangement, needle bearing arrangement, tapered bearing arrangement or roller bearing arrangement.