Patent Application
20150091410
The embodiments described herein relate generally to axial flux electrical machines, and more particularly, to methods and systems for reducing torque ripple using a stator core in an axial flux electric machine.
Permanent magnet electrical machines are used in a wide variety of systems operating in a wide variety of industries, such as in pump systems or air handling units. As such, permanent magnet electrical machines are subject to many operating conditions. Any source of periodic divergence from ideal operating conditions in the machine typically gives rise to undesired torque pulsations, which may cause vibrations, and potentially motor noise, any amount of which may be objectionable to a user. In such a machine, pulsating torque may be caused by torque ripple, which is when the back electromotive force from a plurality of permanent magnets deviates from a purely sinusoidal waveform, causing higher frequencies above a fundamental frequency of the machine.
Audible machine noise is unacceptable in many applications. Further, the cogging and the torque pulses at the shaft of the machine may be transmitted to a fan, blower assembly or other driven equipment/end device that is attached to the shaft. In such applications these torque pulses and the effects of torque ripple may result in operational deficiencies and/or acoustical noise that can be objectionable to an end user of the machine. As such, reducing the torque ripple in the machine facilitates reducing the amount of acoustic interference generated by the device powered by the machine.
In one aspect, a stator core for an axial flux electric machine is provided. The stator core comprises an axis of rotation, a stator core base, and a plurality of circumferentially-spaced stator teeth extending from the base in a direction parallel to the axis. Each stator tooth of the plurality of stator teeth comprises a top surface, a pair of opposing circumferential sides, and a modified portion defined at an intersection of the top surface and each of the pair of opposing sides. The modified portion facilitates reducing an amount of torque ripple produced by the axial flux electric motor.
In another aspect, an axial flux electric machine is provided. The axial flux electric machine comprises a rotor assembly configured to rotate about an axis of rotation. A stator core is coupled to the rotor assembly. The stator core comprises a stator core base and a plurality of circumferentially-spaced stator teeth extending from the base in a direction parallel to the axis. Each stator tooth of the plurality of stator teeth comprises a top surface, a pair of opposing circumferential sides, and a modified portion defined at an intersection of the top surface and each of the pair of opposing sides. The modified portion facilitates reducing an amount of torque ripple produced by the axial flux electric motor.
In yet another aspect, a method of manufacturing a stator core for an axial flux electric machine is provided. The method comprises forming a plurality of circumferentially-spaced stator teeth, wherein each stator tooth of the plurality of stator teeth includes a top surface and a pair of opposing circumferential sides. The method further comprises forming a modified portion at an intersection of the top surface and each of the pair of opposing sides to facilitate reducing an amount of torque ripple produced by the axial flux electric motor.
The embodiments described herein relate generally to axial flux electrical machines, and more particularly, to methods and systems for reducing torque ripple using a stator core in an axial flux electric machine.
In the exemplary embodiment, electric machine 100 is coupled to a work component (not shown) included within a commercial and/or industrial application. The work component may include, but is not limited to, a pump system, an air handling unit, and/or manufacturing machinery (e.g., conveyors and/or presses). In an alternative embodiment, the work component may include a fan for moving air through an air handling system, for blowing air over cooling coils, and/or for driving a compressor within an air conditioning/refrigeration system. More specifically, machine 100 may be used in air moving applications used in the heating, ventilation, and air conditioning (HVAC) industry.
Electric machine 100 includes a housing 102, a stator core 104, a bobbin assembly 106, a bearing assembly 108, and a rotor assembly 110. Each of housing 102, stator core 104, bearing assembly 108, and rotor assembly 110 includes a concentric opening 112 oriented about an axis of rotation 114. Bobbin assembly 106 includes a plurality of bobbins 116 that each include an opening (not shown) that closely conforms to an external shape of one of a plurality of stator core teeth 118 such that each stator tooth 118 is configured to be positioned within a bobbin 116. Machine 100 may include one bobbin 116 per stator tooth 118 or one bobbin 116 positioned on every other tooth 118. In the exemplary embodiment, each bobbin 116 is configured to insulate a plurality of copper windings 120 such that each bobbin 116 electrically insulates one winding 120 from a respective stator tooth 118.
In the exemplary embodiment, a variable frequency drive (not shown) provides a signal, for example, a pulse width modulated (PWM) signal, to electric machine 100. In an alternative embodiment, electric machine 100 may include a controller (not shown) coupled to bobbin assembly 106 by wiring 122. The controller is configured to apply a voltage to one or more of bobbins 116 at a time for commutating bobbin assembly 106 in a preselected sequence to rotate rotor assembly 110 about axis 114.
Stator core 104 includes a plurality of circumferentially-spaced stator teeth 118 that extend in the Y-direction parallel to axis of rotation 114 from a stator core base 124. In the exemplary embodiment, stator core includes thirty-six teeth 118. Alternatively, stator core 104 may include any suitable number of teeth 118 that allow machine 100 to function as described herein. In use, stator core base 124 is positioned perpendicularly about rotational axis 114 such that plurality of teeth 118 extend in the Y-direction from stator core base 124 and form a slot 126 between each adjacent tooth 118. Referring to
In the exemplary embodiment, stator core 104 is an annular ring having an aperture 112 therethrough. Stator core 104 is coupled to housing 102 by threading a plurality of fasteners 128 through a plurality of apertures 130 in housing 102 and into corresponding apertures (not shown) in stator core 104. Housing 102 includes a secondary bearing locator 132 extending from an inner face 134 of housing 102 that facilitates retaining bearing assembly 108 in place. In the exemplary embodiment, stator core 104 is a strip wound core manufactured from a long steel ribbon wound into a toroidal shape. A number of slots are punched into a single layer of the ribbon by a punch and wind machine (not shown), and as the ribbon is wound, the slots of the single layers combine to form slots 126 and teeth 118. Alternatively, stator core 104 is a laminated core comprised of a number of laminated sheets. Copper windings 120 are then covered by a bobbin 116 and slid over a tooth 118. Alternatively, windings 120 are wound around teeth 118 and are insulated from adjacent teeth by an insulating layer (not shown).
Rotor assembly 110 includes a rotor disk 136 having at least an axially inner surface 138 and a radially inner wall 140 that at least partially defines opening 112. Rotor assembly 110 also includes a plurality of permanent magnets 142 coupled to inner surface 138 of rotor disk 136. In the exemplary embodiment, magnets 142 are coupled to rotor disk 136 using an adhesive. Alternatively, magnets 142 may be coupled to disk 136 using any retention method that facilitates operation of machine 100 as described herein. Plurality of permanent magnets 142 are symmetrical, which facilitates manufacturing a single magnet design for use with each magnet 142 within the plurality of permanent magnets 142. Alternatively, plurality of magnets 142 may be non-symmetrical. Furthermore, each magnet 142 has a substantially flat profile which minimizes waste during manufacturing, and therefore, minimizes cost. Magnets 142 are preferably composed of Neodymium Iron Boron (NdFeB) material. However, alternative materials such as Samarium Cobalt or Ferrite are suitable. Rotor disk 136 further includes mounting apertures 144 for mounting rotor disk 136 to a workpiece (not shown) and a primary bearing locator 146 extending from inner surface 138 for facilitating proper positioning of bearing assembly 108. In the exemplary embodiment, rotor disk 136 is manufactured using a sintering process from, for example, Soft Magnetic Alloy (SMA), Soft Magnetic Composite (SMC), and/or powdered ferrite materials. In an alternative embodiment, rotor disk 136 is machined and/or cast from a solid metal, for example, steel. Similarly, stator core 104 is comprised of a metal, such as a steel alloy, that provides a magnetic attraction between permanent magnets 142 and stator 104 to retain rotor disk 136, bearing assembly 108, and stator 104 in place within machine 100.
Bearing assembly 108 is secured between secondary bearing locator 132 and primary bearing locator 146. Bearing assembly 108 includes an inner race 148 and an outer race 150 with a plurality of ball bearings 152 positioned therebetween. Primary bearing locator 146 of rotor disk 136 engages and locates bearing assembly 108 by engaging outer race 150 of bearing assembly 108. In turn, housing 102 includes an inner face 134 from which extends a secondary bearing locator 132 such that it engages inner race 148 of bearing assembly 108 to further position bearing assembly 108 and to secure bearing assembly 108 between stator core 104 and rotor disk 136. The magnetic force between magnets 142 coupled to rotor assembly 110 and stator 104 acts to hold machine 100 together. Further, bearing assembly 108 provides an axially-oriented air gap 154 between rotor disk 136 and stator core 104 that facilitates rotation of rotor assembly 110 relative to stator core 104.
In operation, bobbins 116 coupled to stator core 104 are energized in a chronological sequence that provides an axial magnetic field which moves clockwise or counterclockwise around stator core 104 depending on the pre-determined sequence or order in which bobbins 116 are energized. This moving magnetic field intersects with the flux field created by the plurality of permanent magnets 142 to cause rotor assembly 110 to rotate about axis 114 relative to stator core 104 in the desired direction to develop a torque which is a direct function of the intensities or strengths of the magnetic fields.
In the exemplary embodiment, each tooth includes a modified portion 170 at the intersection of first side 162 and top surface 160 and also at the intersection of second side 164 and top surface 160. Modified portion 170 extends substantially an entire radial length of each tooth between inner surface 156 and outer surface 158. Modified portion 170 facilitates reducing torque ripple generated by machine 100, and therefore reduces the amount of noise produced by machine 100. In the exemplary embodiment, modified portion 170 includes a chamfer at the intersection of first side 162 and top surface 160 and also at the intersection of second side 164 and top surface 160. Modified portion 170 is formed during manufacture of stator core 104 such that when the punch and wind machine punches the slot from the ribbon, it also forms the chamfer on stator teeth 118. Previous attempts to chamfer edges resulted in deformed tooth shape. The punch and wind method of manufacturing produces consistent chamfer dimensions along portion 170 without deforming teeth 118. More specifically, the punch and wind machine forms the chamfer of modified portions 170 on the circumferential sides 162 and 164 or each tooth 118, while radially inner edge 166 and radially outer edge 168 remain unchanged except to account for modified portion 170.
In the exemplary embodiment, modified portion 170 has a constant chamfer width in the range of between approximately 0.5 millimeters (mm) and approximately 2.0 mm between inner surface 156 and outer surface 158. More specifically, modified portion 170 has a constant chamfer width in the range of between approximately 1.0 mm and approximately 1.5 mm. Alternatively, modified portion 170 may have any size chamfer width that facilitates operation of machine 100 as described herein. Furthermore, modified portion 170 may be tapered to have a varying width, as is shown in
Each tooth 218 includes a modified portion 270 at the intersection of first side 262 and top surface 260 and also at the intersection of second side 264 and top surface 260. Modified portion 270 includes a chamfered portion that decreases in width between radially outer surface 258 and radially inner surface 256. Similarly,
In the embodiments described above, stator teeth 118, 218, and 318 are each provided with modified portions 170, 270, and 370, respectively. A alternative embodiment is illustrated in
In the example shown in
The axial flux electric machine described herein includes a plurality of stator teeth that include modified edges that facilitate reducing torque ripple produced by the machine while maintaining a substantially constant amount of total torque production. The tooth modifications are formed at the intersection of the circumferential sides and the top surface of each tooth. The modification may include a chamfered portion having a constant width along a radial length of the tooth, or the chamfered portion may be tapered such that the width of the chamfer either increases or decreases from an radially inner surface of the tooth to a radially outer surface. The circumferential edges of each tooth may alternatively be rounded or notched to facilitate reducing torque ripple. Furthermore, forming the chamfered portions on a steel ribbon during manufacture of the stator core using a punch and wind machine facilitates forming the chamfered portions without deforming the shape of the stator teeth, therefore reducing the time and cost required for fabrication. Reducing the torque ripple caused by the machine significantly reduces the amount of noise that the machine produces. The modified stator teeth facilitate reducing the torque ripple, and therefore the noise generated, while maintaining a substantially constant amount of torque generated by the machine.
The embodiments described herein relate to axial flux permanent magnet electrical machines and methods of manufacturing the same. More specifically, the embodiments relate to a stator core that reduces torque ripple and diminishes the noise produced by the machine. More particularly, the embodiments relate to forming opposing modified portions on each stator tooth of the stator core to soften the interaction of magnetic flux from a permanent magnet on the stator tooth and therefore reduce torque ripple. The methods and apparatus are not limited to the specific embodiments described herein, but rather, components of apparatus and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with radial flux electric machines and methods, and are not limited to practice with only the axial flux electric machines and methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other electrical machine applications.
Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
