Axial flux rotor of an electric machine with continuous Halbach magnets

Abstract

A rotor for rotatable mounting on a rotating axis in an axial flux electric motor comprises a permanent magnet (PM) disk with a continuous Halbach magnetization profile. The axial flux electric motor also includes a non-rotatable stator with a plurality of conductive stator magnetic poles arranged radially about the rotating axis. The rotor is axially spaced from one side of the stator. A motor vehicle employing the subject axial flux electric motor is also contemplated.

Description

INTRODUCTION

[0001] The disclosure relates to a rotor with continuous Halbach magnets for an electric axial flux machine.

[0002] An electric motor is a machine that converts electrical energy into mechanical energy. The operation of an electric motor is based on the electromagnetic interaction between permanent magnets and the magnetic field generated by the machine's selectively excited coils. Electric motors are divided into two categories based on the direction of the magnetic field: axial flux motors and radial flux motors. The arrangement of the gap between the rotor and the stator of the machine, which is parallel to the axis of rotation in an axial flux motor and radial in a radial flux motor, determines the direction of the motor's magnetic flux.

[0003] Axial and radial flux motors can be designed as switched reluctance machines. Generally, a reluctance machine induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor of such a machine typically has no windings and generates torque through magnetic reluctance. Subtypes of reluctance machines include synchronous, variable-speed, switching, and variable-speed stepper motors. Reluctance motors can deliver high power density, but at high loads, significant torque ripple (the difference between maximum and minimum torque during one revolution) can occur due to reluctance when combined with the torque of magnets, which can also generate noise.

[0004] Typically, axial flux motors have a comparatively denser and shorter flux path. Furthermore, the rotor magnets can be located farther from the axial motor's central rotational axis than radial flux motors. As a result, axial flux motors generally have higher torque-to-weight and torque-to-size ratios than radial flux motors when the rotor diameter is significantly larger than its axial length. The magnets on the rotor of an axial flux motor, like all forms of AC synchronous motor, are attracted to the rotating field generated by a surrounding ring of independent electromagnets in the stator. Because the switching of the magnets that causes the field to rotate is usually not perfectly smooth, the rotor suffers from torque ripple. DESCRIPTION

[0005] An axial flux electric motor includes a non-rotatable stator defining a rotational axis and having a plurality of conductive stator magnetic poles radially disposed about the rotational axis. The axial flux electric motor also includes a first rotor axially spaced from one side of the stator and rotatably mounted coaxially with the rotational axis. The first rotor is characterized by a first rotor outer surface facing the stator and includes a first permanent magnet (PM) disk having a continuous (non-segmented) Halbach magnetization profile.

[0006] The axial flux electric motor may additionally comprise a second rotor axially spaced from another side of the stator, having a second rotor outer surface facing the stator, and rotatably mounted coaxially with the rotational axis. The second rotor may include a second PM disk having the continuous Halbach magnetization profile.

[0007] The continuous Halbach magnetization profile of both the first and second PM disks may exhibit a cyclic pattern.

[0008] The cyclic pattern of the first PM disk may be phase-aligned with the cyclic pattern of the second PM disk.

[0009] The cyclic pattern of the first PM disk may be phase-shifted from the cyclic pattern of the second PM disk.

[0010] Both the first PM disk and the second PM disk may include a plurality of concentrically arranged rings. Each of these rings may exhibit the continuous Halbach magnetization profile.

[0011] The first rotor may additionally include a first support structure, and the second rotor may also include a second support structure. In such an embodiment, each of the plurality of concentrically arranged rings of the first and second PM disks may be attached to the respective first and second support structures.

[0012] Both the first PM disk and the second PM disk may have a spiral structure with the continuous Halbach magnetization profile.

[0013] The stator may be characterized by a printed circuit board (PCB) structure that defines the magnetic poles of the stator.

[0014] The stator may be characterized by a slotless structure that defines the magnetic poles of the stator.

[0015] A motor vehicle using such an axial flux electric motor is also considered.

[0016] The above features and advantages, as well as other features and advantages of the present disclosure, will become apparent from the following detailed description of the embodiment(s) and the best mode(s) for carrying out the described disclosure, taken in conjunction with the accompanying drawings and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic diagram of a motor vehicle having a powertrain that uses an axial flux electric motor-generator for propulsion. Fig. 2 is a schematic, exploded perspective partial view of the Fig. 1, showing a stator assembly having first and second rotors, each of which includes a permanent magnet (PM) disk with a continuous Halbach magnetization profile, according to one embodiment of the disclosure. Fig. 3 is a schematic close-up of a partial side view of the Fig. 2, showing a slotless design of the stator structure and a phase angle shift between the continuous Halbach magnetization profile of the first and second PM disks, according to the disclosure. Fig. 4 is a schematic close-up of the Fig. 2, showing a printed circuit of the magnetic poles of the stator assembly, according to the disclosure. Fig. 5 is a schematic partial front view of a representative motor-generator rotor showing a concentric ring structure of the PM disk, according to one embodiment of the disclosure. Fig. 6 is a schematic partial front view of a representative motor-generator rotor showing a spiral ring structure of the PM disk, according to one embodiment of the disclosure. DETAILED DESCRIPTION

[0017] The embodiments of the present disclosure described herein are intended to be exemplary. Other embodiments may take various and alternative forms. Furthermore, the drawings are generally schematic and not necessarily to scale. Some features may be exaggerated or reduced in size to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for showing one skilled in the art how to variously employ the present disclosure.

[0018] Certain terms are used in the following description for reference purposes and are not to be considered limiting. For example, terms such as "above" and "below" refer to directions in the drawings to which reference is made. Terms such as "front," "back," "forward," "backward," "left," "right," "behind," and "side" describe the orientation and / or location of portions of components or elements within a consistent, but arbitrary, frame of reference that will become clear by reference to the text and accompanying drawings describing the components or elements under discussion. In addition, terms such as "first," "second," "third," etc., may be used to describe individual components. This terminology may include the terms mentioned above, their derivatives, and terms of similar import.

[0019] In Fig. 1, a motor vehicle 10 having a powertrain 12 is depicted. The vehicle 10 may be, but is not limited to, a commercial vehicle, an industrial vehicle, a passenger vehicle, an aircraft, a watercraft, a train, or the like. It is also contemplated that the vehicle 10 may be a mobile platform, such as an airplane, an all-terrain vehicle (ATV), a boat, a personal mobility device, a robot, and the like, to fulfill the purposes of this disclosure. The powertrain 12 includes a first power source 14, depicted as an electric motor-generator, configured to produce a first power source torque T1 (in Fig. 1) for propelling the vehicle 10 via driven wheels 16, for example, relative to a road surface. The motor-generator 14 is configured as a reluctance machine in which non-permanent magnetic poles are induced on the motor's ferromagnetic rotor(s), which are described in detail below.

[0020] As in Fig. 1, the powertrain 12 may also include a second power source 20, such as an internal combustion engine, configured to produce a second power source torque T2. The power sources 14 and 20 may cooperate to propel the vehicle 10 and are operatively connected to a transmission assembly 22. The transmission assembly 22 may be configured to transmit the first and / or second drive torques T1, T2 to an axle drive unit 24, which in turn may be connected to the driven wheels 16. The first power source 14, referred to below as a motor-generator, may be mounted, for example, on the second power source 20, on the transmission assembly 22, or on the axle drive unit 24, or may be attached to the structure of the vehicle 10 as a standalone assembly. Alternatively, a corresponding first energy source 14 can be installed in each driven wheel 16 as a wheel hub motor.As shown, the vehicle 10 additionally includes a programmable electronic controller 26 configured to control the powertrain 12 to produce a predetermined power source torque T, as well as various other vehicle systems. The vehicle 10 also includes an energy storage system 28, such as one or more batteries configured to generate and store electrical energy to power the power sources 14 and 20.

[0021] As in Fig. 2, the motor-generator 14 includes a non-rotatable stator 30. The stator 30 defines a rotational axis X and includes a stator core 30A and a plurality of magnetic poles 30B arranged radially about the rotational axis X. The stator 30 has two opposing sides—a first side 30-1 and a second side 30-2. The motor-generator 14 also includes a first rotor 32 axially spaced from the first side 30-1 of the stator 30 and forming an effective air gap therebetween. The first rotor 32 is rotatably mounted coaxially with the rotational axis X and has a first rotor outer side or surface 32-1 facing the stator 30 and an opposing rotor outer side or surface 30-2. The first rotor 32 includes a first permanent magnet disk 36.As shown, the PM disk 36 has a continuous Halbach magnetization profile 38, where the term “continuous” specifically describes an uninterrupted or non-segmented structure with magnetic field vectors that rotate in a repeating flux pattern along a circular path.

[0022] In general, a "Halbach array" is a special arrangement of discrete permanent magnets that produce a spatially rotating or alternating magnetic field vector that enhances the magnetic field on one side of the array while reducing the field to nearly zero on the other side. The net effect of the Halbach array is a one-sided magnetic flux distribution, roughly similar to that of many horseshoe magnets placed side by side with like poles touching. The most notable advantage of a Halbach array is the generation of a significantly stronger magnetic field on one side of the array with a minimized stray field on the opposite side of the array.

[0023] The continuous Halbach magnetization profile 38 is a spatially rotating or alternating magnetic field vector that has the effect of focusing and amplifying the magnetic flux on the first rotor outer surface 32-1, while canceling it out on the opposite surface 32-2. As a result, the magnetic field generated by the continuous Halbach magnetization profile 38 is very strong on the first rotor outer surface 32-1. Furthermore, the magnetic field generated in this way is significantly stronger than other rotor structures with an analogous amount of magnetic alloy, for example, with discrete, alternating south and north pole permanent magnets arranged on a ferromagnetic rotor core. The motor-generator 14 may additionally include a second rotor 40 rotatably mounted coaxially to the rotation axis X and axially spaced from the second side 30-2 of the stator 30.The second rotor 40 may be characterized by a second rotor outer surface 40-1 facing the stator 30 and an opposite rotor outer side or surface 40-2.

[0024] As in Fig. 2, the second rotor 40 includes a second PM disk 42 with an analog continuous Halbach magnetization profile 38. As shown in Fig. 3, the stator core 30A may be characterized by a slotless structure that defines the stator's magnetic poles 30B. Generally, slotted stators utilize a group of electrical steel laminations assembled into a rigid stack. The outer portion of each lamination contains a ring of material from which a pattern of teeth is radially arranged and projects toward the rotor. Electromagnetic coils are then wound around the teeth or inserted into each slot between the teeth (as in Fig. 2). The laminated stack and wound copper coil form the stator assembly. Slotless stators generally improve the smoothness of electric motors, produce predictable motor torque with minimal nonlinear effects, and reduce noise and vibration through low cogging, enabling cooler and smoother operation and higher speeds. Such slotless stators work advantageously with sine drivers, such as the PM disks 36, 42 with the continuous Halbach magnetization profile 38, to produce lower torque ripple and negligible output torque distortion. Alternatively, the magnetic poles 30B of the stator 30, as shown in Fig. 4, be defined by a printed circuit board (PCB) structure. In PCB stators, the copper windings of conventional stators are replaced by an ultra-thin printed circuit board (PCB) with the copper conductors located at key locations. The PCB stator structure results in a more efficient machine that requires only a fraction of the copper required for a conventional motor.

[0025] The continuous Halbach magnetization profile 38 may extend around the circumference of the respective rotor 32, 40 such that the first PM disk 36 has a cyclic or sinusoidal pattern 44-1 and the second PM disk 42 has a corresponding cyclic pattern 44-2. Each of the first and second rotors 32, 40 with the respective PM disks 36, 42 may be characterized by the absence of a ferromagnetic rotor core. As installed in the motor-generator 14, the cyclic pattern 44-1 of the first PM disk 36 may be phase-aligned with the cyclic pattern 44-2 of the second PM disk 42 (shown in the Fig. 2 and Fig. 4). Alternatively, the cyclic pattern 44-1 of the first PM disk 36 may be rotated or phase-shifted relative to the cyclic pattern 44-2 of the second PM disk 42, as shown in Fig. 3 shown.

[0026] For example, as in Fig. 3, the first and second rotors 32, 40 may be generally identical, with the PM disk 36 being a substantial duplicate of the PM disk 42. The phase angle shift between the respective PM disks 36, 42 would cause the cyclic pattern 44-1 to be out of alignment relative to the cyclic pattern 44-2. Such a phase angle shift of the cyclic pattern 44-1 relative to the cyclic pattern 44-2 may be used to reduce the cogging torque between the two PM disks 36, 42 and the stator 30 and to change the magnetic reluctance of the electric motor 14. Magnetic reluctance is generally defined as the ratio of magnetomotive force (mmf) to magnetic flux, which represents the resistance to magnetic flux, and generally depends on the geometry and composition of an object.One can imagine two rotor disks 36, 42 whose poles have either the same or opposite magnetic polarities. With the same polarity, two back-to-back stators 30 would be used, sharing a common back iron. With opposite polarity, a common stator disk 30 can be used for and between the two rotor disks 36, 42.

[0027] As in Fig. 5, each of the first and second PM disks 36, 42 may include a plurality of concentrically arranged rings 46. In such an embodiment, each ring 46 may have the continuous Halbach magnetization profile 38. As shown in Fig. 2, the first rotor 32 may additionally include a first support structure 48-1, while the second rotor 40 may include a second support structure 48-2. The first and second support structures 48-1, 48-2 may be used to secure concentric rings 46 of the respective PM disks 36, 42 in Fig. 5. The respective first and second support structures 48-1, 48-2 can be made of ferrous materials such as silicon steel or non-ferrous materials such as aluminum or a polymer. Alternatively, as in Fig. 6, each of the first PM disk 36 and the second PM disk 42 has a spiral, radially shaped structure 50 with the continuous Halbach magnetization profile 38.

[0028] In summary, the continuous Halbach magnetization profile 38 of the PM disks 36, 42 provides an electric motor rotor with an alternating or cyclic magnetization field pattern that rotates relative to the stator. This spatially rotating magnetic field focuses and amplifies the magnetic field on the rotor surface facing the stator. The net effect of the continuous Halbach magnetization profile of the PM disks is a stronger magnetic field and higher motor output torque compared to rotor structures with alternating south and north pole permanent magnets arranged on a ferromagnetic rotor core. The electric motor can also utilize specific stator configurations to complement the aforementioned properties of the PM disk(s) 36, 42 and further enhance the effectiveness of the continuous Halbach magnetization profile and motor operation.

[0029] The detailed description and the drawings or figures support and describe the disclosure, but the scope of the disclosure is defined solely by the claims. While some of the best modes and other embodiments for carrying out the claimed disclosure have been described in detail, various alternative designs and embodiments exist for carrying out the disclosure defined in the appended claims. Furthermore, the embodiments illustrated in the drawings or the features of various embodiments recited in this description are not necessarily intended to be independent embodiments.Rather, it is possible that each of the features described in one of the embodiments may be combined with one or more other desired features of other embodiments, resulting in other embodiments not described in words or by reference to the drawings. Accordingly, such other embodiments fall within the scope of the appended claims.

Claims

[1] Axial flux electric motor, comprising: a non-rotatable stator defining a rotational axis and having a plurality of conductive stator magnetic poles arranged radially about the rotational axis; and a first rotor axially spaced from one side of the stator and rotatably mounted coaxially to the axis of rotation, characterized by a first rotor outer surface facing the stator and comprising a first permanent magnet (PM) disk having a continuous Halbach magnetization profile. [2] An axial flux electric motor according to claim 1, further comprising a second rotor axially spaced from another side of the stator, characterized by a second rotor outer surface facing the stator and rotatably mounted coaxially to the rotation axis, and comprising a second PM disk having the continuous Halbach magnetization profile. [3] The axial flux electric motor of claim 2, wherein the continuous Halbach magnetization profile of each of the first PM disk and the second PM disk has a cyclic pattern. [4] The axial flux electric motor of claim 3, wherein the cyclic pattern of the first PM disk is phase-aligned with the cyclic pattern of the second PM disk. [5] The axial flux electric motor of claim 3, wherein the cyclic pattern of the first PM disk is phase-shifted relative to the cyclic pattern of the second PM disk. [6] The axial flux electric motor of claim 2, wherein each of the first PM disk and the second PM disk comprises a plurality of concentrically arranged rings, each ring comprising the continuous Halbach magnetization profile. [7] The axial flux electric motor of claim 6, wherein the first rotor further comprises a first support structure and the second rotor further comprises a second support structure, and wherein each of the plurality of concentrically arranged rings of the first and second PM disks is attached to the respective first and second support structures. [8] The axial flux electric motor of claim 2, wherein each of the first PM disk and the second PM disk comprises a spiral structure having a continuous Halbach magnetization profile. [9] An axial flux electric motor according to claim 1, wherein the stator is characterized by a printed circuit board (PCB) structure defining the magnetic poles of the stator. [10] An axial flux electric motor according to claim 1, wherein the stator is characterized by a slotless structure defining the stator magnetic poles.

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