Electric machine and method of operating an electric machine

Abstract

A motor (20) is disclosed, comprising a stator (21) and a rotor (22) rotatably supported relative to the stator (21). The rotor (22) comprises two first recesses (23) and at least one second recess (24), wherein an air gap (25) is provided between the stator (21) and the rotor (22), the two first recesses (23) are provided in the rotor (22) and extend completely through the rotor (22) from the air gap (25) to a shaft (26) on which the rotor (22) is provided, the two first recesses (23) are provided in a manner that they are offset relative to each other by less than 180° along a circumference of the rotor (22), the at least one second recess (24) is provided in a manner that it is offset relative to the first recesses (23) by at least 90° along the circumference of the rotor (22), the at least one second recess (24) does not extend through the rotor (22) to the shaft (26), and a first permanent magnet (27) is provided in each first recess (23) and / or a second permanent magnet (28) is provided in the at least one second recess (24). Furthermore, a method of operating the motor (20) is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to an electric machine and a method of operating an electric machine. BACKGROUND

[0002] Generally, an electric machine comprises a stator and a rotor which is movable relative to the stator. The electric machine can operate as an electric motor or as a generator, thereby converting electrical energy into kinetic energy and vice versa. In operation, a rotor magnetic field interacts with a stator magnetic field.

[0003] In order to generate the rotor magnetic field, the rotor can have permanent magnets. The use of permanent magnets has the advantage that there are many different ways of providing permanent magnets in the rotor, which is why there are many ways of optimizing the efficiency of the electric machine.

[0004] However, permanent magnets generally account for a large part of the total cost of the electric machine.

[0005] It is an object to provide an electric machine which can be operated efficiently. It is a further object to provide an efficient method of operating an electric machine.

[0006] The objects are achieved by the subject matter of the independent claims. Advantageous embodiments and further improvements are provided in the dependent claims. SUMMARY

[0007] According to at least one embodiment of the electric machine, the electric machine comprises a stator and a rotor which is rotatably mounted relative to the stator. The stator can comprise a stator winding. To this end, the stator can have slots in which the stator winding is provided. The stator winding can be connectable to a power electronics element and designed to generate a rotating magnetic field. The rotor can be an internal rotor or an external rotor. If the rotor is an internal rotor, an outer surface of the rotor faces the stator. The rotor can be provided on a shaft. In addition, the rotor has an axis of rotation.

[0008] The rotor comprises two first recesses. The first recesses can be cavities in the rotor. The rotor can comprise a rotor core comprising a core material. The two first recesses are free of any core material. The core material can be iron. The first recesses can extend completely through the rotor along the axis of rotation of the rotor. The rotor can have exactly two first recesses.

[0009] The rotor further comprises at least one second recess. The second recess can be a cavity in the rotor. The second recess is free of any core material of the rotor. The second recess can extend completely through the rotor along the axis of rotation of the rotor. The rotor can have exactly one second recess. Alternatively, the rotor can have exactly two second recesses.

[0010] An air gap is provided between the stator and the rotor. The air gap can extend between the stator and the rotor in a direction extending parallel to the axis of rotation of the rotor.

[0011] Two first recesses are provided in the rotor and extend through the rotor from the air gap to the shaft on which the rotor is provided. This means that the first recesses are at least partially surrounded by the rotor. The first recesses can be at least partially surrounded by the material of the rotor. Each first recess adjoins the air gap. Each first recess extends from the air gap along a radial direction in a cross section through the rotor to the shaft. The cross section through the rotor extends in a plane perpendicular to the rotation axis of the rotor. The first recesses directly adjoin the shaft. The first recesses can have a straight shape. This means that the first recesses can extend along a straight line from the air gap to the shaft.

[0012] The two first recesses are provided with an offset of less than 180° relative to each other along the circumference of the rotor. This means that in a cross section through the rotor, the two first recesses are provided with an offset of less than 180° relative to each other along the circumference of the rotor. Thus, in a cross section through the rotor, the angle between the main extension directions of the first recesses is less than 180°.

[0013] The at least one second recess is provided with an offset of at least 90° relative to the first recesses along the circumference of the rotor. This means that in a cross section through the rotor, the at least one second recess is provided with an offset of at least 90° relative to the first recesses along the circumference of the rotor. Thus, the at least one second recess is provided spaced apart from the first recesses. Thus, the position at which the second recess is provided is provided with an offset of at least 90° from the position of the first recesses along the circumference of the rotor.

[0014] The at least one second recess does not extend through the rotor to the shaft. This means that the second recess partially extends through the rotor but not all the way to the shaft. The second recess can be completely provided within the rotor. This means that in a cross section through the rotor, the second recess is surrounded by the rotor on all sides. The second recess can have a different geometry than the first recesses.

[0015] A first permanent magnet is provided in each first recess and / or a second permanent magnet is provided in the at least one second recess. The first permanent magnets can have the same size and the same shape. The second permanent magnets can have a different size or a different shape than the first permanent magnets. Each first permanent magnet does not completely fill the first recess. The second permanent magnet does not completely fill the second recess. The first permanent magnets and the second permanent magnets each have a magnetic axis. The magnetic axis extends parallel to the magnetization direction of the respective magnet. This means that the magnetic axis connects two magnetic poles of each permanent magnet.

[0016] Thus, there are three different possibilities to design the rotor: a first permanent magnet is provided in each first recess and no permanent magnet in the second recess, a second permanent magnet is provided in the second recess and no permanent magnet in the first recess, or a first permanent magnet is provided in each first recess and a second permanent magnet is provided in the second recess.

[0017] For a rotor with a first permanent magnet arranged in each first recess and a second permanent magnet arranged in the second recess, the rotor magnetic field excited by the permanent magnets has four magnetic poles. In addition to this, the properties of the rotor magnetic field depend on the geometrical extension of the permanent magnets in the rotor and the arrangement of the non-magnetic material in the rotor. Thus, even if the number of permanent magnets in the rotor is less than four, the rotor magnetic field has four magnetic poles. Thus, the number of permanent magnets in the rotor can be less than the number of magnetic poles of the rotor magnetic field.

[0018] An advantage of this is that the number of permanent magnets can thus be reduced. Generally, permanent magnets greatly increase the cost of a rotor. In the case of the rotor described herein, a rotor magnetic field with four magnetic poles requires less than four permanent magnets. The cost of the rotor can be greatly reduced by reducing the number of permanent magnets required or by reducing the size of the permanent magnets of the rotor.

[0019] Furthermore, with the electric machine described herein, rotor magnetomotive force components of an order greater than one can be used to generate torque. The rotor magnetomotive force can have components of different harmonic orders. Another expression for the magnetomotive force is the field excitation curve. For example, if a rotor magnetomotive force component of the third order has a non-zero magnetic flux density, the component can be used to generate torque. The design of the rotor described herein is optimized for operation of the electric machine using rotor magnetomotive force components of an order greater than one to generate torque. Thus, the electric machine can be efficiently operated.

[0020] According to at least one embodiment of the electric machine, the rotor magnetic field excited by the first permanent magnets and the at least one second permanent magnet has four magnetic poles. This means that the rotor magnetic field has two pairs of magnetic poles. Thus, a rotor with only three permanent magnets has a rotor magnetic field with four magnetic poles. Thus, the number of permanent magnets required can be kept low and the manufacturing costs of the electric machine can be reduced.

[0021] According to at least one embodiment of the electric machine, the magnetic axes of the first permanent magnets point in opposite directions along the circumference of the rotor. This means that the magnetic axes of the first permanent magnets point in opposite directions to each other along the circumference of the rotor. In a cross-section through the rotor, the magnetic axes of the first permanent magnets can point in opposite directions along the circumference of the rotor. In a cross-section through the rotor, the angle between the magnetic axes of the first permanent magnets can be greater than 180°. Advantageously, in this arrangement, one or more magnetomotive force components of the rotor for generating torque can be amplified or one or more magnetomotive force components not used for generating torque can be suppressed. Thus, the electric machine can be efficiently operated.

[0022] According to at least one embodiment of the electric machine, in a cross section through the rotor, the magnetic axis of each first permanent magnet extends perpendicular to a radial direction. The radial direction extends in the cross section through the rotor from the shaft towards the air gap. The magnetic axis of the first permanent magnet extends perpendicular to the respective radial direction through the respective first permanent magnet. Thus, advantageously, one or more components of the magnetic motive force of the rotor for generating torque can be amplified and one or more components of the magnetic motive force of the rotor not used for generating torque can be suppressed. Thus, the electric machine can be operated efficiently.

[0023] According to at least one embodiment of the electric machine, in some places in the first recess, at least an electrically insulating material or a gas is provided. The electrically insulating material is non-magnetic. The electrically insulating material or the gas can each completely fill the first recess. Alternatively, the electrically insulating material or the gas can fill regions of the first recess in which no first permanent magnet is provided. Thus, the first permanent magnets and the electrically insulating material or the gas can each completely fill the first recess. In this respect, the electrically insulating material or the gas can be provided adjacent to the air gap and adjacent to the shaft. Thus, the first permanent magnets in the first recess can be surrounded on both sides by the electrically insulating material or the gas. The gas can be air. In those regions in which the electrically insulating material or the gas is provided, the impedance of the magnetic flux density is increased compared to the surrounding rotor material. The provision of the electrically insulating material or the gas adjacent to the air gap and adjacent to the shaft allows there to be as few closed field lines in the rotor as possible. This means that losses in the rotor are minimized. Thus, the electric machine can be operated efficiently.

[0024] According to at least one embodiment of the electric machine, each first recess is larger than the first permanent magnet provided therein. This can mean that each first recess has a larger geometrical extension than the first permanent magnet provided therein. In particular, in a cross section through the rotor, each first recess is larger than the first permanent magnet provided therein. This allows the electrically insulating material or the gas to be provided in the respective first recess together with the first permanent magnet. This allows efficient operation of the electric machine.

[0025] According to at least one embodiment of the electric machine, the at least one second recess is larger than the second permanent magnet. This can mean that the second recess has a larger geometrical extension than the second permanent magnet provided therein. In particular, in a cross section through the rotor, the second recess is larger than the second permanent magnet provided therein. For example, the second recess extends further along the circumference of the rotor than the second permanent magnet. This allows the electrically insulating material or the gas to be provided in the second recess together with the second permanent magnet. This allows efficient operation of the electric machine.

[0026] According to at least one embodiment of the electric machine, an electrically insulating material or a gas is provided at least in some places in the at least one second recess. The electrically insulating material is non-magnetic. The electrically insulating material or the gas can completely fill the second recess. Alternatively, the electrically insulating material or the gas can fill regions of the second recess in which no second permanent magnet is provided. Thus, the second permanent magnet and the electrically insulating material or the gas can completely fill the second recess. In this regard, the electrically insulating material or the gas can be provided along the circumference of the rotor at both ends of the second permanent magnet. This means that the second recess is able to extend further along the circumference of the rotor than the second permanent magnet provided therein. The second permanent magnet can be centered within the second recess. The electrically insulating material or the gas can be provided in a peripheral region of the second recess which is not filled by the second permanent magnet. Thus, the first permanent magnet in the first recess can be surrounded on both sides by the electrically insulating material or the gas. The second recess can directly adjoin the air gap. The gas can be air. Thus, air from the air gap is able to be provided in the peripheral region of the second recess. The provision of the electrically insulating material or the gas in the regions of the second recess which are not filled by the second permanent magnet allows there to be as few closed field lines in the rotor as possible. This means that losses in the rotor are minimized. Thus, the electric machine is able to be operated efficiently.

[0027] According to at least one embodiment of the electric machine, the at least one second permanent magnet has a curved shape. The second permanent magnet can extend along the circumference of the rotor in some places. Thus, the second permanent magnet can have the shape of a segment of a circle. This shape of the second permanent magnet allows the rotor, which likewise comprises two first permanent magnets, to have a magnetic field with four magnetic poles. Overall, the number of permanent magnets required and the manufacturing costs of the electric machine can thus be reduced.

[0028] According to at least one embodiment of the electric machine, the at least one second recess directly adjoins the air gap. The second recess can extend directly from the air gap into the rotor. In a cross section through the rotor, the second recess can extend in a radial direction from the air gap towards the shaft. In this regard, the second recess extends only partially and not through the rotor to the shaft. This allows the second recess to be easily manufactured and the second permanent magnet to be easily inserted into the second recess.

[0029] According to at least one embodiment of the electric machine, the at least one second recess extends along a portion of the circumference of the rotor. This means that the second recess has the shape of a segment of a circle. This allows the second permanent magnet to also have the shape of a segment of a circle. Overall, the number of permanent magnets required and the manufacturing costs of the electric machine can thus be reduced.

[0030] According to at least one embodiment of the electric machine, the at least one second recess has a main extension direction perpendicular to a radial direction in a cross section through the rotor. The second recess can have a main extension direction perpendicular to a radial direction in a cross section through the rotor. In this regard, the radial direction can extend through a center of the second recess. The second recess extends further along its main extension direction than along other directions in a cross section through the rotor. The second recess can be completely arranged within the rotor. This means that the second recess does not adjoin the air gap. This shape of the second recess having a rectangular shape in a cross section allows for the second permanent magnet to be arranged in the second recess. Thus, the second permanent magnet can be manufactured in a simple and economic way.

[0031] According to at least one embodiment of the electric machine, the electric machine comprises two second recesses completely arranged within the rotor. A second permanent magnet can be arranged in each of the second recesses. The second recesses do not adjoin the air gap. The second recesses are not connected to each other. This means that the second recesses are spaced apart from each other in the rotor. The second recesses can have a substantially rectangular shape in a cross section through the rotor. This allows for each second permanent magnet to have a rectangular shape in a cross section through the rotor. Thus, the second permanent magnet can be manufactured in a simple and economic way.

[0032] Further, a method of operating the electric machine described herein is disclosed. Thus, all features of the electric machine described herein are also disclosed for the method of operating the electric machine and vice versa.

[0033] According to at least one embodiment of the method of operating the electric machine, a magnetic motive force component of the rotor of an order greater than 1 is used for generating a torque. This means that the magnetic motive force component of the rotor of the order 1, which is called the fundamental wave, is not used for generating a torque. Instead, higher harmonics of the magnetic motive force of the rotor are used for generating a torque. This means that in the operation of the electric machine, a magnetic motive force component of the rotor of an order greater than 1 can be used for generating a torque. The magnetic motive force of the rotor can have components of different harmonic orders. Another expression for the magnetic motive force is the field excitation curve. For example, if a magnetic motive force component of the rotor of the order 3 has a non-zero magnetic flux density, said component can be used for generating a torque. The design of the rotor described herein is optimized for the operation of the electric machine using magnetic motive force components of an order greater than 1 for generating a torque. Thus, the electric machine can be efficiently operated. The magnetic motive force component of the rotor of an order greater than 1 interacts with the magnetic field of the stator for generating a torque. For this purpose, the permanent magnets can be arranged in the rotor such that the magnetic flux density is maximum for the magnetic motive force component of the rotor used for generating a torque. Advantageously, the magnetic flux density of the magnetic motive force components of the rotor not used for generating a torque can also be minimum. Both of the above are achieved by the arrangement of the first recess and the at least one second recess described herein. This allows for an efficient operation of the electric machine. BRIEF DESCRIPTION OF DRAWINGS

[0034] In the following, the electric machine and the method of operating the electric machine described herein will be explained in more detail in connection with embodiments and the drawings

[0035] Figure 1 A schematic cross-section through the electric machine according to an embodiment is shown.

[0036] Figure 2 A schematic cross-section through the rotor according to an embodiment is shown.

[0037] Figure 3 A schematic cross-section through the rotor according to another embodiment is shown.

[0038] Figure 4 A schematic cross-section through the electric machine according to an embodiment is shown. Figure 1 The distribution of the simulated magnetic flux density in the electric machine shown.

[0039] Figure 5 A schematic cross-section through the electric machine according to another embodiment is shown.

[0040] Figure 6 A schematic cross-section through the electric machine according to another embodiment is shown.

[0041] Figure 7 A schematic cross-section through the electric machine according to another embodiment is shown. DETAILED DESCRIPTION

[0042] Figure 1 A schematic cross-section through the electric machine 20 according to an embodiment is shown. The electric machine 20 comprises a stator 21 and a rotor 22 rotatably mounted with respect to the stator 21. The rotor 22 is an inner rotor provided in the stator 21. That is, in the shown cross-section, the stator 21 completely surrounds the rotor 22. An air gap 25 is provided between the stator 21 and the rotor 22.

[0043] The rotor 22 comprises two first recesses 23. The two first recesses 23 are provided in the rotor 22 and extend completely through the rotor 22 from the air gap 25 to a shaft 26 on which the rotor 22 is provided. The rotor 22 comprises a rotor core 30. The first recesses 23 extend completely through the rotor core 30 in the shown cross-section through the rotor 22. Each of the first recesses 23 extends through the rotor 22 along a radial direction r. Each radial direction r extends from a center of the rotor 22 towards the air gap 25. The two first recesses 23 extend through the rotor 22 along two different radial directions r. This means that one of the first recesses 23 extends along a different radial direction r than the other one of the first recesses 23.

[0044] Two first recesses 23 are arranged in such a way that they are offset from each other along the circumference of the rotor 22 by less than 180°. In particular, the first recesses 23 are arranged in such a way that they are offset from each other along the circumference of the rotor 22 by less than 120°. In each first recess 23, a first permanent magnet 27 is arranged. Each first recess 23 is larger than the first permanent magnet 27 arranged therein. This means that each first permanent magnet 27 does not completely fill the first recess 23. In addition to the first permanent magnet 27, in each first recess 23, an electrically insulating material 29 is arranged. In the present example, the electrically insulating material 29 is air. Air is arranged in each of the first recesses 23 between the first permanent magnet 27 and the air gap 25. Furthermore, air is arranged in each of the first recesses 23 between the first permanent magnet 27 and the shaft 26.

[0045] In a cross section through the rotor 22, each first permanent magnet 27 has a rectangular shape. The magnetic axes of the first permanent magnets 27 point in opposite directions along the circumference of the rotor 22. In the present example, this means that the magnetic axes of the first permanent magnets 27 comprise an angle of approximately 90° with respect to each other. In a cross section through the rotor 22, the magnetic axis of each first permanent magnet 27 extends perpendicular to the radial direction r.

[0046] The rotor 22 further comprises second recesses 24. The second recesses 24 are arranged along the circumference of the rotor 22 in such a way that they are offset by at least 90° with respect to the first recesses 23. In the present example, the second recesses 24 are arranged along the circumference of the rotor 22 in such a way that they are offset by more than 90° with respect to the first recesses 23. This means that each region of the second recesses 24 is arranged along the circumference of the rotor 22 in such a way that it is offset by more than 90° with respect to both first recesses 23. In a cross section through the rotor 22, the rotor 22 can be divided into two halves, the two first recesses 23 being arranged in one of the two halves and the second recess 24 being arranged in the other half.

[0047] The second recess 24 does not extend through the rotor 22 all the way to the shaft 26. The second recess 24 extends from the air gap 25 towards the shaft 26. Thus, the second recess 24 directly adjoins the air gap 25. Furthermore, the second recess 24 extends along a portion of the circumference of the rotor 22. This means that the second recess 24 has a curved shape. Furthermore, the second recess 24 has the shape of a segment of a circle.

[0048] A second permanent magnet 28 is arranged in the second recess 24. The second recess 24 is larger than the second permanent magnet 28. Thus, the second permanent magnet 28 does not completely fill the second recess 24. In addition to the second permanent magnet 28, an electrically insulating material 29 is arranged in the second recess 24. In the present example, the electrically insulating material 29 is air. Like the second recess 24, the second permanent magnet 28 has a curved shape and extends along a portion of the circumference of the rotor 22. The second recess 24 extends further along the circumference of the rotor 22 than the second permanent magnet 28. The electrically insulating material 29 is arranged in the region of the second recess 24 that extends further along the circumference of the rotor 22 than the second permanent magnet 28. The second permanent magnet 28 is centered in the second recess 24, such that the electrically insulating material 29, i.e. the air from the air gap 25, is arranged on both sides of the second permanent magnet 28.

[0049] The magnetic axis of the second permanent magnet 28 points from the air gap 25 towards the shaft 26. That is, for each position along the second permanent magnet 28, the magnetic axis extends at the respective position parallel to the radial direction r. Thus, the magnetic axis of the second permanent magnet 28 points towards the center of the rotor 22.

[0050] The rotor magnetic field excited by the first permanent magnet 27 and the second permanent magnet 28 has four magnetic poles. Figure 1 The illustrated electric machine 20 comprises exactly two first permanent magnets 27 and exactly one second permanent magnet 28.

[0051] Figure 1 The illustrated electric machine 20 can be operated by a method in which components of the magnetic motive force of the rotor 22 of an order greater than one are used to generate torque.

[0052] Figure 2 is a cross section through a rotor according to an embodiment. Figure 2 The rotor 22 of Figure 1 is used in Figure 1 the electric machine 20 illustrated. Figure 2 The rotor 22 illustrated differs from Figure 1 the rotor 22 illustrated in the shape and arrangement of the second recess 24 and the second permanent magnet 28. In a cross section through the rotor 22, the second recess 24 has a main extension direction perpendicular to the radial direction r. The main extension direction of the second recess 24 is perpendicular to the radial direction r through the center of the second recess 24. The second recess 24 is completely arranged within the rotor 22. This means that, in a cross section through the rotor 22, the second recess 24 is surrounded on all sides by the material of the rotor 22.

[0053] In a cross section through the rotor 22, the second permanent magnet 28 in the second recess 24 has a rectangular shape. The magnetic axis of the second permanent magnet 28 points from the air gap 25 to the shaft 26. Thus, the magnetic axis of the second permanent magnet 28 extends parallel to the radial direction r through the center of the second permanent magnet 28. This means that the magnetic axis of the second permanent magnet 28 extends parallel to the radial direction r through the center of the second permanent magnet 28 at every position of the second permanent magnet 28.

[0054] An electrically insulating material 29 is provided at the relatively short side edges of the second permanent magnet 28. The second recess 24 in the area of the electrically insulating material 29 can be of any shape.

[0055] The rotor magnetic field excited by the first permanent magnets 27 and the second permanent magnets 28 has four magnetic poles. Figure 2 The illustrated rotor 22 comprises exactly two first permanent magnets 27 and exactly one second permanent magnet 28.

[0056] Figure 3 A cross section through a rotor 22 according to another embodiment is shown. Figure 3 The rotor 22 of Figure 1 The illustrated rotor 22 is used in Figure 1 The illustrated motor 20. Figure 3 The illustrated rotor 22 differs from the Figure 1 The illustrated rotor 22 differs from the rotor 22 shown in that the rotor 22 comprises a total of two second recesses 24. A second permanent magnet 28 is provided in each of the second recesses 24. The second recesses 24 are completely provided within the rotor 22.

[0057] In a cross section through the rotor 22, each second permanent magnet 28 has a rectangular shape. Each second permanent magnet 28 does not completely fill the second recess 24. In each of the second recesses 24, an electrically insulating material 29 is provided adjacent to the second permanent magnet 28. The area in which the electrically insulating material 29 is provided can be of any shape. In each case, the electrically insulating material 29 is provided at the side of the second permanent magnet 28 which is closest to the air gap 25.

[0058] Two second permanent magnets 28 are provided in the rotor 22 such that they together approximately form a V shape. The second recesses 24 are spaced apart from one another. The magnetic axes of the second permanent magnets 28 point in different directions. For each of the second permanent magnets 28, the magnetic axis extends parallel with respect to the shorter side edge of the second permanent magnet 28. Furthermore, the magnetic axes of the second permanent magnets 28 approximately point toward the first permanent magnet 27.

[0059] The rotor magnetic field excited by the first permanent magnets 27 and the second permanent magnets 28 has four magnetic poles. Figure 3 The illustrated rotor 22 comprises exactly two first permanent magnets 27 and exactly two second permanent magnets 28.

[0060] Figure 4 The distribution of the simulated magnetic flux density through Figure 1 the motor 20 is shown. The rotor magnetic field 22 has four magnetic poles.

[0061] Figure 5 A schematic cross-section through a motor 20 according to another embodiment is shown. The motor 20 has the same design as the motor 20 shown in Figure 1 , the only difference being that no first permanent magnet 27 is provided in the first recess 23. Thus, the first recess 23 is free of permanent magnets. Thus, the electrically insulating material 29 completely fills the first recess 23. In the present example, the electrically insulating material 29 is air. Thus, the first recess 23 acts as a magnetic flux barrier in the rotor 22. The motor 20 has exactly two first recesses 23 completely filled with air, and exactly one second recess 24 provided with a second permanent magnet 28. Figure 5 The motor 20 shown operates in the same way as the motor 20 shown in Figure 1 .

[0062] Figure 6 A schematic cross-section through a motor 20 according to another embodiment is shown. The motor 20 has the same design as the motor 20 shown in Figure 1 , the only difference being that no second permanent magnet 28 is provided in the second recess 24. Thus, the second recess 24 is free of permanent magnets. Thus, the electrically insulating material 29 completely fills the second recess 24. In the present example, the electrically insulating material 29 is air. Thus, the second recess 24 acts as a magnetic flux barrier in the rotor 22. The motor 20 has exactly two first recesses 23 each provided with a first permanent magnet 27, and exactly one second recess 24 completely filled with air. Figure 6 The motor 20 shown operates in the same way as the motor 20 shown in Figure 1 .

[0063] Figure 7 A schematic cross-section through a motor 20 according to another embodiment is shown. The motor 20 has the same design as the motor 20 shown in Figure 1 , the only difference being that the magnetic axis of the second permanent magnet 28 extends differently than shown in Figure 1 . In the rotor 22 in Figure 7 , the magnetic axis of the second permanent magnet 28 is parallel to the radial direction r through the center of the second permanent magnet 28. This means that the magnetic axis of the second permanent magnet 28 extends parallel to the radial direction r through the center of the second permanent magnet 28 at any position of the second permanent magnet 28.

[0064] Legend of the figures

[0065] 20: motor

[0066] 21 : stator

[0067] 22: rotor

[0068] 23: first recess

[0069] 24: second recess

[0070] 25: air gap

[0071] 26: shaft

[0072] 27: first permanent magnet

[0073] 28: second permanent magnet

[0074] 29: electrically insulating material

[0075] 30: rotor core

[0076] r: radial direction

Claims

1. An electric motor (20) comprising a stator (21) and a rotor (22) rotatably mounted relative to said stator (21), said rotor (22) comprising: - Two first recesses (23), and -At least one second recess (24), in, - An air gap (25) is provided between the stator (21) and the rotor (22). - The two first recesses (23) are disposed in the rotor (22) and extend from the air gap (25) completely through the rotor (22) to the shaft (26) on which the rotor (22) is disposed. - The two first recesses (23) are configured to be offset from each other by less than 180° along the circumference of the rotor (22). - The at least one second recess (24) is configured to be offset by at least 90° relative to the first recess (23) along the circumference of the rotor (22). -The at least one second recess (24) does not extend through the rotor (22) to the shaft (26), - A first permanent magnet (27) is provided in each of the first recesses (23), and / or a second permanent magnet (28) is provided in the at least one second recess (24). - The rotor magnetic field excited by the first permanent magnet (27) and by at least one second permanent magnet (28) comprises four magnetic poles, and - The motor (20) includes exactly two first permanent magnets (27) and exactly one second permanent magnet (28).

2. The motor (20) according to claim 1, wherein, The magnetic axis of the first permanent magnet (27) points in the opposite direction along the circumference of the rotor (22).

3. The motor (20) according to claim 1, wherein, In the cross-section passing through the rotor (22), the magnetic axis of each first permanent magnet (27) is perpendicular to the radial direction (r).

4. The motor (20) according to claim 1, wherein, Electrically insulating material (29) or gas is provided in at least some places in the first recess (23).

5. The motor (20) according to claim 1, wherein, Each first recess (23) is larger than the first permanent magnet (27) disposed therein.

6. The motor (20) according to claim 1, wherein, The at least one second recess (24) is larger than the second permanent magnet (28).

7. The motor (20) according to claim 1, wherein, Electrically insulating material (29) or gas is provided in at least some locations in the at least one second recess (24).

8. The motor (20) according to claim 1, wherein, The at least one second permanent magnet (28) has a curved shape.

9. The motor (20) according to claim 1, wherein, The at least one second recess (24) is directly adjacent to the air gap (25).

10. The motor (20) according to claim 1, wherein, The at least one second recess (24) extends along a portion of the circumference of the rotor (22).

11. The motor (20) according to claim 1, wherein, In the cross section passing through the rotor (22), the at least one second recess (24) has a main extending direction perpendicular to the radial direction (r).

12. The motor (20) according to claim 1, wherein, The motor (20) includes two second recesses (24) that are fully disposed in the rotor (22).

13. A method of operating the motor (20) according to any one of the preceding claims, wherein, The magnetomotive force component of the rotor (22) of order greater than 1 is used to generate torque.

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