Segmented rotor for a slotless electronically commutated electric motor, and electric motor comprising such a rotor

Abstract

The invention relates to a rotor for an electric motor, comprising a slotless stator and an electronic commutator, wherein the rotor has permanent magnet segments. According to the invention, the permanent magnet segments are mutually spaced at least over an axial region of the rotor along at least one sectional plane, and the rotor has a respective electric insulation between the permanent magnet segments along the at least one sectional plane. A normal of the respective sectional plane runs perpendicularly to the axial direction of the rotor and perpendicularly to the q-­axis of the rotor if the rotor has one pole pair and perpendicularly to one of a plurality of q­-axes of the rotor if the rotor has more than one pole pair. A soft magnetic material is introduced between the permanent magnet segments along the sectional plane(s), said material being electrically insulated by means of the electric insulation of the permanent magnet segments.

Description

[0001] Segmented rotor for a slotless, electronically commutated electric motor and electric motor with such a rotor

[0002] The present invention relates to a rotor for an electric motor with a slotless stator and electronic commutation according to the preamble of independent claim 1 and to an electric motor with such a rotor according to the preamble of claim 15.

[0003] A rotor according to the preamble of independent claim 1 has permanent magnet segments.

[0004] In electronically commutated electric motors, the pulse-width modulation (PWM) of the power electronics induces additional losses in the electric motor. These losses are particularly significant in low-inductance motors, such as electrically commutated electric motors with slotless stators. The main effect is eddy currents in the rotor, with the additional losses reaching a maximum at a duty cycle of 50% for a fixed DC link voltage. There are already known solutions to this problem, but they are fraught with additional complications.

[0005] A well-known approach is the use of additional inductors in the electronics, which can reduce current ripple and thus losses. However, depending on the system, these additional inductors can become large, heavy, and expensive. Furthermore, resistive losses occur in these components, making this approach less advantageous.

[0006] It is also known to select the winding and / or DC link voltage so that the duty cycle is close to 100%. This avoids the problem of PWM-induced losses. However, without adjusting the DC link voltage, this is only possible for motors that are primarily operated at maximum speed. If operation is to take place at different speeds, a complex and expensive control system is required to adjust the DC link voltage, which also impairs the dynamics of the control system. Furthermore, this approach only partially solves the problem with sinusoidal commutations, since the voltage has a sinusoidal waveform and thus the duty cycle changes continuously from zero to the maximum.

[0007] Another well-known approach is the axial segmentation of the rotor into permanent magnet segments. The rotor is segmented along sectional planes whose normals point in the axial direction of the rotor, and the individual permanent magnet segments are electrically isolated from each other. However, effective suppression of eddy currents requires a large number of such permanent magnet segments, since the axial thickness of the permanent magnet segment discs should be smaller than their diameter. This makes this approach complex to manufacture and expensive.

[0008] The object of the present invention is therefore to provide a rotor for an electric motor with a slotless stator and electronic commutation, which reduces the additional losses induced by the pulse width modulation (PWM) of the power electronics and thereby avoids or at least mitigates the problems of the various known approaches described above.

[0009] The object is achieved by the features of independent claim 1. Accordingly, in a generic rotor, the object is achieved according to the invention in that the permanent magnet segments are spaced from one another at least over an axial region of the rotor along at least one sectional plane and the rotor has electrical insulation between the permanent magnet segments along the at least one sectional plane, wherein a normal of the respective sectional plane runs perpendicular to the axial direction of the rotor and perpendicular to a q-axis of the rotor in a case that the rotor has one pole pair, and perpendicular to one of several q-axes of the rotor in a case that the rotor has more than one pole pair.

[0010] An extension of the cutting planes in the axial direction is limited to the axial region of the rotor, wherein the axial region of the rotor preferably comprises at least 90%, particularly preferably at least 95%, of the axial length of the rotor.

[0011] The inventive solution effectively suppresses the eddy currents in the rotor induced by PWM. This requires a significantly smaller number of permanent magnet segments than the conventional approach using a cutting plane with a normal in the axial direction of the rotor.

[0012] This makes the rotor much easier and more cost-effective to manufacture.

[0013] Advantageous embodiments of the present invention are the subject of the subclaims.

[0014] In a particularly preferred embodiment of the present invention, the electrical insulation is created by means of an air gap, an insulating material, such as preferably a potting compound, plastic films or plates, paper, or a coating, such as preferably an electrically insulating adhesive. Effective electrical insulation effectively suppresses the induced eddy currents of the PWM. An electric motor can be designed as a multi-phase or single-phase electric motor and can rotate or oscillate during operation. In a further preferred embodiment of the present invention, a soft magnetic material is introduced between the permanent magnet segments along the cutting plane(s), which material is electrically insulated from the permanent magnet segments by means of the electrical insulation.This increases the inductance along the q-axis(es), which can reduce current ripple and thus also reduce induced eddy currents. Electrical insulation can be achieved with a relatively low electrical resistance on the order of a few ohms, for example, 1-10 ohms, as this already reduces eddy currents.

[0015] Preferably, the soft magnetic material is designed as a sheet or laminated core, with a lamination direction running either parallel to the normal of the respective cutting plane or in the axial direction of the rotor. This enables the formation of a high-magnitude inductance in a simple and cost-effective manner.

[0016] In a preferred embodiment, the saturation field strength of the soft magnetic material is higher than the flux density of the rotor. Preferably, the soft magnetic material consists of FeSi sheets, FeNi sheets, or a soft magnetic composite. This prevents saturation of the soft magnetic material, ensuring effectiveness in preventing losses due to PWM-induced eddy currents.

[0017] According to another particularly preferred embodiment, the distance between the permanent segments in the direction of the normal to the respective section plane over at least 50% of the extent of the section plane parallel to the q-axis within an outer circumference of the rotor is less than 8%, preferably less than 5%, of the rotor diameter. This allows the permanent segments to be sufficiently large relative to the rotor diameter.

[0018] In a further preferred embodiment, the laminated core has a lamination direction parallel to the normal of the respective cutting plane and fewer than nine, preferably fewer than five, laminations. The lamination direction is the direction in which the laminations are placed or stacked on top of one another. The lamination direction is thus perpendicular or orthogonal to the lamination plane.

[0019] In another preferred embodiment, the laminated core has its lamination direction in the axial direction of the rotor, and the extension of the laminated core in the direction of the normal to the respective section plane increases at the outer circumference of the rotor. This allows more high-frequency field to be guided into the laminated core, which has a positive effect on reducing losses due to PWM-induced eddy currents. Furthermore, the laminates are positioned in the rotor by the positive engagement with the permanent magnet segments.

[0020] According to a further preferred embodiment, the rotor has only one pole pair and more than two, preferably three, permanent magnet segments, wherein the cutting planes are preferably arranged axially symmetrically to the q-axis of the rotor. In rotors with only one pole pair, the use of more than two permanent magnet segments can further reduce losses due to PWM-induced eddy currents. Three permanent magnet segments are usually sufficient.

[0021] According to another preferred embodiment, the rotor has more than one pole pair, and the rotor's cutting planes run along the rotor's q-axes. Rotors with more than one pole pair form more than one q-axis. By slicing the cutting planes along each of these q-axes, PWM-reduced eddy currents can be effectively prevented throughout the entire rotor circumference.

[0022] In another particularly preferred embodiment of the present invention, the permanent magnet segments are bonded to each other in an electrically insulated manner. This segmentation makes the rotor structure more fragile. However, the bonding allows for the production of a sufficiently stable rotor.

[0023] According to a preferred embodiment of the present invention, shaft stubs are bonded to the end faces of the rotor. Particularly in rotor designs with only one pole pair, a continuous shaft is less practical, as eddy currents could flow through the shaft. Shaft stubs are used to prevent this.

[0024] In a further preferred embodiment of the present invention, the rotor is surrounded by an electrically non-conductive encapsulation, which is preferably a carbon fiber tube or a ceramic sleeve, or consists of a synthetic resin lamination of the outer surface. This can further improve stability. The carbon fiber tube has the additional advantage that carbon fibers have a similar coefficient of expansion to magnetic material. This makes the rotor robust against temperature changes. The carbon fiber tube can also be impregnated with epoxy resin and cured or baked. This increases the strength of the rotor, and the coating also serves as insulation.

[0025] In another preferred embodiment, the axial region in which the permanent magnet segments are spaced apart from one another constitutes only a portion of the total axial length of the rotor, and the permanent magnet segments are designed to be unsegmented at one axial end of the rotor in an end section that preferably comprises less than 10%, particularly preferably less than 5%, of the axial length of the rotor. This reduces the fragility of the rotor while still reducing the PWM-induced eddy currents. The embodiment with such an end section is particularly preferred for rotors with only one pole pair.

[0026] In another preferred embodiment, the rotor is designed as a four-pole Halbach rotor comprising eight permanent magnet segments, wherein permanent magnet segments are electrically insulated from one another at the cutting planes along the q-axes and these cutting planes are provided with soft magnetic sheets which are also electrically insulated from the permanent magnet segments.

[0027] Furthermore, the present invention relates to an electric motor with a slotless stator and electronic commutation, wherein the electric motor has a rotor according to at least one of the embodiments described above.

[0028] In the following, non-limiting embodiments of the present invention are explained in more detail with reference to exemplary drawings.

[0029] The drawings show:

[0030] Figure 1: a sectional view of an embodiment of a rotor according to the invention with a pole pair, a cutting plane and a laminated core with lamination direction parallel to the normal of the cutting plane,

[0031] Figure 2: a sectional view of a second embodiment of a rotor according to the invention with a pole pair, a cutting plane and a laminated core with lamination direction in the axial direction of the rotor,

[0032] Figure 3: a sectional view of a third embodiment of a rotor according to the invention with a pole pair, two cutting planes and two laminated cores with the lamination direction parallel to the normals of the cutting planes,

[0033] Figure 4: axial sectional views of the third embodiment shown in Figure 3,

[0034] Figure 5: a sectional view of a fourth embodiment of a rotor according to the invention with two pole pairs, two cutting planes and four laminated cores with lamination directions parallel to the normals of the cutting plane, Figure 6: a sectional view of a fifth embodiment of a rotor according to the invention with two pole pairs, two cutting planes and one laminated core with lamination direction in the axial direction of the rotor,

[0035] Figure 7: a schematic sectional view of a fifth embodiment of a rotor according to the invention, which is designed as a four-pole Halbach rotor.

[0036] In the following figures, identical parts are provided with identical reference numerals. Where a figure contains reference numerals that are not explicitly addressed in the corresponding figure description, reference is made to previous or subsequent figure descriptions. The figures merely show sectional views of exemplary embodiments of rotors according to the invention. The other components of the electric motor, which is also claimed, are not shown in the figures. For the components of the electric motor according to the invention, reference is made to the general description.

[0037] Figure 1 shows a sectional view of a first embodiment of a rotor 1 according to the invention within an axial region 8 of the rotor 1, in which the rotor 1 is divided into several permanent magnet segments 2. The rotor 1 has two permanent magnet segments 2 in the axial region 8, the magnetization directions of which are shown as thick black arrows and which form a single-pole pair magnetic field. The magnetization direction is also indicated by the d-axis 5 of the rotor 1. The two permanent magnet segments 2 are spaced from one another along a sectional plane 3. The sectional plane 3 has a normal 10 that is oriented perpendicular to the axial direction of the rotor 1 and perpendicular to the q-axis 6 of the rotor 1.Soft magnetic material in the form of a laminated core 4 is inserted between the two permanent magnet segments 2 along the cutting plane 3, wherein the laminated core 4 of this embodiment has a lamination direction parallel to the normal 10 of the cutting plane 3 and comprises eight laminations. Alternatively, fewer laminations, for example fewer than five laminations, can preferably be used. The two permanent magnet segments 2 and the laminated core 4 are electrically insulated from one another by electrical insulation. The electrical insulation between the permanent magnet segments 2 along the cutting plane 3 prevents PWM-induced eddy currents, which are particularly prevalent in electronically commutated electric motors with slotless stators and have a negative impact on the operation of such an electric motor.The laminated core 4 also increases the inductance in the q-axis 6, which reduces the current ripple and thus allows the eddy currents to be further reduced. The electric motor can be a multi-phase electric motor. Figure 2 shows a sectional view in the axial region 8 of a second exemplary embodiment of a rotor 1 according to the invention. The essential difference from the first exemplary embodiment is that the laminated core has a lamination direction in the axial direction of the rotor 1 and the extension of the laminated core 4 increases in the direction of the normal 10 on the sectional plane 3 to the outer circumferences of the rotor 1. This allows, on the one hand, more high-frequency field to be conducted into the axial laminated core 4 and, on the other hand, the laminations of the laminated core 4 are positioned in relation to the permanent magnet segments 2 by a positive fit.For this embodiment, it is also important that the laminated core 4 is electrically insulated from the permanent magnet segments 2 by electrical insulation.

[0038] Figure 3 shows a sectional view in the axial region 8 of a third embodiment of a rotor 1 according to the invention, wherein this rotor 1 has three permanent magnet segments 2, all of which have the same magnetization direction and thus form a single-pole pair magnetic field. The permanent magnet segments 2 are spaced from one another along two sectional planes 3, each of which has normals 10 oriented perpendicular to the axial direction of the rotor 1 and perpendicular to the q-axis 6 of the rotor 1. The sectional planes 3 are arranged axially symmetrically to the q-axis 6 in the rotor. Laminated cores 4 are again inserted between the permanent magnet segments 2 along the sectional planes 3, wherein the laminated cores 4 are electrically insulated from the adjacent permanent magnet segments 2 by electrical insulation.

[0039] Figure 4 shows an axial sectional view of a rotor 1, which has a sectional view in the axial region 8, like the third exemplary embodiment shown in Figure 3. The rotor 1 is segmented in the axial region 8 and has three permanent magnet segments 2 divided by two sectional planes. However, this segmentation is not present over the entire axial length of the rotor 1. Thus, the exemplary embodiment in Figure 4 has an end section 9 at one axial end, in which the permanent magnet segments 2 are not separated from one another. In the end section 9, the permanent magnet can be formed in one piece, so that the stability of the rotor 1 can be increased. Since the axial region 8, with the segmentation into several permanent magnet segments 2, encompasses a large part of the axial length, preferably at least 90% of the axial length, the PWM-induced eddy currents can nevertheless be sufficiently reduced.Shaft stubs are glued to the respective axial ends of the rotor 1 shown in Figure 4, which, together with the segmented permanent magnet, form the shaft 7 of the rotor 1. Figure 5 shows a sectional view of a further embodiment of a rotor 1 according to the invention, wherein this rotor 1 is a two-pole pair rotor. The rotor 1 has four permanent magnet segments 2, which are magnetized according to the thick black arrows. The permanent magnet segments 2 are spaced from one another along two sectional planes 3, wherein the normals 10.1 of the sectional planes 3 are each aligned perpendicular to the axial direction of the rotor 1 and perpendicular to the possible q-axes 6.1, 6.2 of the rotor 1. The laminations are arranged in the direction of the q-axis 6, 6.1, 6.2, which means that the q-axis 6, 6.1, 6.2 lies in the lamination plane. The d-axis 5 are each electrically perpendicular to the q-axis 6, 6.1, 6.2.In a motor with a single pole pair, the d-axis 5 is both electrically and mechanically or geometrically perpendicular to the q-axis 6 and thus parallel to the normal 10. In rotors with multiple pole pairs, there are multiple d-axes 5 and q-axes 6.1, 6.2 depending on the number of pole pairs. The d-axes 5 and q-axes 6.1, 6.2 are each electrically perpendicular at a 90° angle to each other. The mechanical angle between the d-axes 5 and the q-axes 6.1, 6.2, on the other hand, corresponds to half of 360° divided by the number of poles. For a rotor 1 with two pole pairs, the mechanical angle between the d-axis 5 and the q-axes 6.1, 6.2 is 360° / 2 / 4 = 45°. For a three-pole pair rotor 1, the mechanical angle between the d-axis and the q-axis is 360° / 2 / 6 = 30°.Soft magnetic material in the form of laminated cores 4 is inserted along the cutting planes 3, with a lamination direction parallel to the normal to the cutting planes. The laminated cores 4 and the permanent magnet segments 2 are electrically insulated from each other. No shaft of the rotor 1 is provided between the laminated cores 4 in the center of the rotor 1. In an alternative embodiment, a shaft made of an electrically non-conductive material can be arranged.

[0040] Figure 6 also shows a sectional view of a further embodiment of a rotor 1 according to the invention, which is designed as a two-pole pair rotor 1. In contrast to the embodiment shown in Figure 5, this embodiment has a laminated core 4 with lamination direction in the axial direction of the rotor 1, which is inserted along the sectional planes 3 between the permanent magnet segments 2. The extension of the laminated core 4 in the direction of the normal 10 of the sectional planes 3 increases at the outer circumference of the rotor, whereby on the one hand more high-frequency field can be guided into the axial laminated core 4 and the laminations of the laminated core 4 are positioned relative to one another and to the permanent magnet segments 2. In the embodiment shown in Figure 6, a shaft 7 of the rotor 1 is guided through the axial laminated core 4.In this embodiment, the shaft 7 has a diameter which is greater than the minimum distance between the permanent magnet segments 2 along the cutting planes 3. The field is guided around the shaft 7. Figure 7 shows a sectional view through a further exemplary embodiment of a rotor 1 according to the invention, wherein the rotor 1 is a two-pole-pair Halbach rotor 1 formed from eight permanent magnet segments 2 with magnetization directions marked as black arrows. These permanent magnet segments 2 are also spaced from one another along two cutting planes 3 by means of electrical insulation, wherein the normals 10.1, 10.2 of the cutting planes 3 are each aligned perpendicular to the axial direction of the rotor 1 and a respective q-axis 6.1, 6.2 of the rotor 1.Soft magnetic material in the form of laminated cores 4 can also be introduced along these cutting planes 3, wherein the laminated cores 4 are in turn insulated from the permanent magnet segments 2 by electrical insulation. For the exemplary embodiments described above, the electrical insulation can be created, among other things, by means of an air gap, an insulating material, plastic films or small plates, paper, or a coating such as an electrically insulated adhesive. The rotors shown can also be covered with a carbon fiber tube and laminated with synthetic resin, or covered with a ceramic sleeve, so that the stability of the rotor 1 can be increased. It is important that this type of encapsulation is not electrically conductive, since this would cancel out the segmentation effect.

[0041] List of reference symbols

[0042] 1 rotor

[0043] 2 permanent magnet segments

[0044] 3 Cutting plane 4 Laminated core

[0045] 5 d-axis

[0046] 6, 6.1, 6.2 q-axis

[0047] 7 Wave

[0048] 8 axial area 9 end section

[0049] 10, 10.1, 10.2 Normal

Claims

Claims Rotor (1) for an electric motor with a slotless stator and electronic commutation, wherein the rotor (1) has permanent magnet segments (2), characterized in that the permanent magnet segments (2) are spaced from one another at least over an axial region (8) of the rotor (1) along at least one sectional plane (3) and the rotor (1) has an electrical insulation between the permanent magnet segments (2) along the at least one sectional plane (3), wherein a normal (10, 10.1, 10.2) of the respective sectional plane (3) runs perpendicular to the axial direction of the rotor (1) and perpendicular to a q-axis (6) of the rotor (1), in a case that the rotor (1) has a pole pair, and perpendicular to one of several q-axes (6.1, 6.2) of the rotor (1), in a case where the rotor (1) has more than one pole pair, wherein a soft magnetic material is introduced between the permanent magnet segments (2) along the sectional plane(s) (3), which soft magnetic material is electrically insulated from the permanent magnet segments (2) by means of the electrical insulation. Rotor (1) according to claim 1, characterized in that the electrical insulation is created by means of an air gap, an insulating material, such as preferably a potting compound, plastic films or small plates, paper or a coating, such as preferably an electrically insulating adhesive. Rotor (1) according to claim 1 or 2, characterized in that the soft magnetic material is designed as a sheet or laminated core (4), wherein a lamination direction runs either parallel to the normal (10, 10.1, 10.2) of the respective sectional plane (3) or in the axial direction of the rotor (1).Rotor (1) according to claims 1 to 3, characterized in that a saturation field strength of the soft magnetic material lies above the flux density of the rotor (1), wherein the soft magnetic material preferably consists of FeSi sheets, FeNi sheets, or a soft magnetic composite. Rotor (1) according to one of claims 1 to 4, characterized in that a distance between the permanent segments (2) in the direction of the normal (10, 10.1, 10.2) of the respective sectional plane (3) over at least 50% of the extent of the sectional plane (3) parallel to the q-axis (6, 6.1, 6.2) within an outer circumference of the rotor (1) is less than 8%, preferably less than 5%, of the rotor diameter. Rotor (1) according to one of claims 1 to 5, characterized in that the laminated core (4) has a lamination direction parallel to the normal (10, 10.1, 10.2) of the respective section plane (3) and fewer than nine, preferably fewer than five, laminations. Rotor (1) according to one of claims 1 to 5, characterized in that the laminated core (4) has a lamination direction in the axial direction of the rotor (1), and an extension of the laminated core (4) in the direction of the normal (10, 10.1, 10.2) of the respective section plane (3) increases on the outer circumference of the rotor (1). Rotor (1) according to one of claims 1 to 7, characterized in that the rotor (1) has only one pole pair and more than two, preferably three, permanent magnet segments (2), wherein the section planes (3) are preferably arranged axially symmetrically to the q-axis (6) of the rotor (1).Rotor (1) according to one of claims 1 to 7, characterized in that the rotor (1) has more than one pole pair and the sectional planes (3) of the rotor (1) run along the q-axes (6) of the rotor (1). Rotor (1) according to one of claims 1 to 9, characterized in that the permanent magnet segments (2) are glued to one another in an electrically insulated manner. Rotor (1) according to one of claims 1 to 10, characterized in that shaft stubs are glued to the end faces of the rotor (1). Rotor (1) according to one of claims 1 to 11, characterized in that the rotor (1) is surrounded by an electrically non-conductive encapsulation, which is preferably a carbon fiber tube or a ceramic sleeve or consists of a synthetic resin lamination of the outer surface.Rotor (1) according to one of claims 1 to 12, characterized in that the axial region (8) in which the permanent magnet segments (2) are spaced from one another only makes up a partial region of the total axial length of the rotor (1) and the permanent magnet segments (2) at an axial end of the rotor (1) in an end section (9) which preferably has less than 10%, particularly preferably less than 5%, of the axial length of the rotor (1), are designed to be unsegmented. Electric motor with a slotless stator and electronic commutation, characterized in that the electric motor has a rotor (1) according to one of claims 1 to 13.

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