Local induction heat treatment

Abstract

This disclosure provides "localized induction heat treatment." A method for manufacturing a stator is provided. The method may include stamping steel into laminations, each lamination having an inner edge region defining residual stress associated with magnetic permeability. The method may further include exposing the laminations to a varying magnetic field such that, for each lamination, the eddy current density generated near the inner edge region is maximized, thereby heating the inner edge region relative to the central region of the lamination to reduce the residual stress and core loss.

Description

Technical Field

[0001] This disclosure relates to a heat treatment process for steel used in electric motors. Background Technology

[0002] Electrical steel is a unique type of steel used to produce specific magnetic properties. It is typically manufactured from cold-rolled strip steel with a thickness of less than 2 mm. These strips are cut and shaped to form the laminated cores of transformers, as well as the stators and rotors of electric motors. Electrical steel is generally shaped using various methods, such as stamping, cutting (e.g., waterjet or laser cutting), and pressing, a process in which a flat portion of the steel is placed in a die and pressed with tools to create a desired surface. Stamping electrical steel can induce residual stresses, where the internal stress distribution is locked within the material. These stresses exist even when no external load is applied to the material.

[0003] Residual stress in electrical components can lead to core losses and reduced energy efficiency. Core losses exist in certain devices, such as transformers, inductors, AC motors, and alternators, which involve cores subjected to varying magnetic fields. Ideally, the magnetic field transmitted through these devices might be lost in the core and dissipated through heat or noise, or both. Residual stress, and consequently core losses, can be reduced through a metalworking process called annealing. Annealing is a heat treatment process that alters the physical properties of the material being treated and sometimes its chemical properties. Induction annealing is a type of annealing that involves heating electrical components by generating eddy currents within the component. Summary of the Invention

[0004] According to one embodiment of this disclosure, a method for manufacturing a stator is provided. The method may include stamping steel into laminations, each lamination having an inner edge region defining residual stress associated with magnetic permeability. The method may further include exposing the laminations to a varying magnetic field such that, for each lamination, the eddy current density generated near the inner edge region is maximized, thereby heating the inner edge region relative to the central region of the lamination to reduce the residual stress and core losses.

[0005] According to another embodiment of this disclosure, a method for manufacturing a rotor assembly is provided. The method may include stamping steel into laminations, each lamination having an outer edge region defining residual stress associated with magnetic permeability. The method may further include exposing the laminations to a varying magnetic field such that, for each lamination, the eddy current density generated near the outer edge region is maximized, thereby heating the outer edge region relative to the central region of the lamination to reduce the residual stress and the magnetic permeability.

[0006] According to yet another embodiment of this disclosure, a method for manufacturing an electric motor is provided. The method may include stamping steel into laminations, each lamination having an inner edge region and an outer edge region. The method may further include causing relative movement between the laminations and a coil to generate an alternating magnetic flux, exposing the laminations to the magnetic flux such that the eddy current density generated near the edge regions is maximized, thereby heating the edge regions relative to the central region of the laminations to reduce residual stress and increase the permeability. Attached Figure Description

[0007] Figure 1 This is a flowchart illustrating an existing technology process including conventional stress-relief annealing for electrical steel.

[0008] Figure 2 This is a flowchart illustrating a process including a local induction heat treatment process for electrical steel.

[0009] Figure 3 This is a top view of an exemplary rotor lamination, showing an exemplary local induction heat treatment area.

[0010] Figure 4 This is a perspective view of an exemplary toroidal surface or ring undergoing a localized induction heat treatment process according to an embodiment of the present disclosure.

[0011] Figure 5 This is a perspective view of a rotor assembly undergoing a local induction heat treatment process according to another embodiment of the present disclosure.

[0012] Figure 5A This is a partial top view showing the laminations, magnetic flux, and parasitic magnetic flux.

[0013] Figure 6 This is a perspective view of an example stator assembly undergoing a local induction heat treatment process according to another embodiment of the present disclosure.

[0014] Figure 7 This is a perspective view of an example stator assembly undergoing a local induction heat treatment process according to another embodiment of the present disclosure.

[0015] Figure 8 This is a perspective view of an example stator assembly undergoing a local induction heat treatment process according to another embodiment of the present disclosure. Detailed Implementation

[0016] As requested, detailed embodiments of the invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely illustrative of the invention, which may be embodied in a variety of alternative forms. The drawings are not necessarily drawn to scale; some features may be enlarged or minimized to show details of specific components. Therefore, the specific structural and functional details disclosed herein should not be construed as limiting, but merely as a representative basis for teaching those skilled in the art to employ the invention in different ways.

[0017] A permanent magnet motor is characterized by magnets mounted on or embedded in a rotor surrounded by a stator. These magnets are coupled to an internal magnetic field generated by the motor current, which is produced by the electrical input to the stator. Similar to other alternating current (AC) induction motors, power is supplied through stator windings.

[0018] Independent stator tooth groups and stator windings form multiple magnetic poles, which generate a flux flow pattern when the stator coils are excited by a multiphase sinusoidal voltage. For example, a three-phase motor has a total of 8 magnetic poles and 48 slots. A set of 6 slots is the characteristic of each magnetic pole in a specific example of a 48-slot motor disclosed herein. The magnetic flux formed by the stator windings interacts with the rotor flux formed by the permanent magnets in the rotor of the permanent magnet motor, resulting in rotor torque when the stator windings are excited by a multiphase voltage.

[0019] The permanent magnets of the rotor can be positioned or oriented in different ways to generate a desired magnetic field. Each of the magnetic poles can be formed by a single permanent magnet oriented as a single magnetic pole (i.e., north or south pole) in a radially outward direction. The rotor's magnetic poles can also be formed by multiple sets of permanent magnets arranged to collectively form a magnetic pole. One such arrangement orients the magnets in a V-shaped pattern. The inner portion of the "V" has similar magnetic poles that cooperate to form the rotor's magnetic poles. Each of the permanent magnets can be disposed in a recess or cavity to hold the permanent magnet. These recesses or cavities are typically rectangular and sized to accommodate the permanent magnet.

[0020] The torque generated by the magnetic flux field of the stator windings and rotor produces both uniform and varying torque components. The total output torque of the motor is a combination of these two components. Due to the variable torque component, torque pulsation occurs, causing oscillations in the motor's torque output speed when the motor is functioning as a motor. Torque pulsation in an electric motor is caused by the interaction between the harmonic magnetic flux generated by the permanent magnets and the current in the stator windings. Reducing the harmonic magnetic flux generated by the permanent magnets will reduce torque pulsation and iron losses.

[0021] It has been found that localized induction heat treatment of certain components can reduce iron losses. More specifically, localized induction heat treatment results in an increase in the permeability of the treated area and a reduction in core losses of 2% to 10%. The reduction in core losses is related to a reduction in the energy required for heat treatment. In another embodiment, localized induction heat treatment can be used to reduce the permeability in certain regions of the lamination to reduce parasitic magnetic flux. While the example of an electric motor has been mentioned above, this disclosure is intended to reduce iron losses caused by residual stresses in or near the stamped edges of electrical steel and is not limited to the components described above.

[0022] Electrical steel is used in electrical applications to construct the aforementioned electrical devices. Electrical steel can comprise a range of alloys possessing magnetic properties beneficial to motor structures. Ferroalloys suitable for electrical steel can comprise up to 6.5% silicon percentage. Electrical steel is typically formed into sheets, which can be cut or stamped to form laminations. During use, the periodic changes in the applied magnetic field consume energy in the electrical steel; this phenomenon is known as core loss. By reducing core loss in electrical steel, the efficiency of electrical components can be improved.

[0023] The flowchart 10 of the conventional manufacturing process includes receiving fully machined electrical steel 12 and stamping the electrical steel sheet into the final shape 14 using a punch and die. Fully machined steel refers to electrical steel with an insulating coating, fully heat-treated, and possessing specified magnetic properties. After stamping 14, the sheets can be stacked and assembled, as shown in 16. The stamping process prior to operation 16 involves strong shear forces at the cut edges of the shape, resulting in plastic deformation in these areas. Plastic deformation, or strain, produces residual stresses that affect the magnetism of the core. More specifically, residual stresses, typically located near the stamping edges, can lead to an increase in permeability in the region near the stamping edges. The decrease in permeability and the increase in hysteresis loop can increase core losses, thereby reducing the performance and efficiency of the electrical device. Core losses can be referred to as iron losses and are interchangeable.

[0024] Various techniques can be used to remove residual stress caused by the cutting process. For example, stress-relief annealing (SRA) can be used. SRA involves heat-treating a stamped electrical steel sheet or assembled iron core at high temperatures for an extended period. Previous stress-relief annealing techniques subjected the stamped electrical steel sheet or assembly to temperatures of 750°C for at least 30 minutes. In some cases, the annealing time may exceed 240 minutes. The lengthy annealing time increases costs due to the long cycle time. A vacuum or protective gas environment is required during SRA to prevent oxidation of the electrical steel. Other methods of SRA may include induction heating of the stamped electrical steel sheet or assembled iron core. SRA performed by induction heating may involve powering a coil or magnetic core, or both, such that electromagnetic flux penetrates the entire electrical steel sheet or assembled iron core.

[0025] refer to Figure 2 Flowchart 100 is shown, illustrating a process that includes localized heat treatment of the laminated sheets via induction heating. It should be noted that... Figure 2 The flowcharts in this document are for illustrative purposes only, and the methods should not be construed as limited to... Figure 2 The flowchart in Method 100. Some steps of Method 100 can be rearranged, while others can be omitted entirely.

[0026] As indicated at 102, fully processed electrical steel is received, and as indicated at 104, the electrical steel is stamped into the desired shape and size. As mentioned above, fully processed steel refers to electrical steel with an insulating coating, fully heat-treated, and possessing the specified magnetic properties. After stamping at 104, as indicated at 106, laminations are stacked and assembled. This assembly can be for a rotor, stator, or transformer, or any suitable component that, when assembled, forms a motor. Then, as indicated at 108, the lamination assembly or stack can be subjected to a localized heat treatment process via induction heating. As indicated at 110, the stack can be assembled onto other components to form a motor or transformer.

[0027] refer to Figure 3 The image shows a top view of a lamination (such as rotor lamination 120). As previously described, rotor lamination 120 is one of many rotor laminations that can be stacked to form a rotor core or lamination stack 128. Rotor lamination 120 includes an outer edge 140 and an inner edge 142, the outer edge forming the outermost portion of lamination 120, and the inner edge defined by a central opening 143. Lamination 120 may include a plurality of weight-reducing holes 141 arranged radially around the central opening 143. Lamination 120 may define a plurality of magnet openings 144. Magnet openings 144 may be arranged in a pair that are symmetrically opposite each other (e.g., mirror images). More specifically, magnet openings 144 may be arranged to form a V-shape. Typically, the outer edge 120 and the central opening are formed by a cutting or punching operation. The weight-reducing holes of the inner edge 142 and the magnet openings 144 may be formed by a punching operation. The magnet opening 144 includes an outer edge 146, which is located closest to the outer edge 140 of the lamination 120. The region between the outer edge 146 of the magnet opening 144 and the outer edge 140 of the lamination 120 may be referred to as the bridging region 148 of the lamination.

[0028] The stamping operation forming the outer edge 140 of the lamination induces plastic deformation and residual stress in the outer edge region 150 of the lamination 120. This residual stress in the outer edge region 150 reduces permeability and increases core losses in the outer edge region 150, particularly in the bridging region of the lamination 120. Heat treatment of the lamination can reduce the residual stress in the outer edge region 150. Therefore, permeability can be increased and core losses in the outer edge can be reduced. Because residual stress is more prevalent in the outer edge region 150, localized heat treatment of the outer edge region 150 may be more efficient than heat treatment of the entire lamination 120. Localized heat treatment of the lamination 120 may require less time (reduced cycle time) and less power.

[0029] As will be described in more detail below, the laminate 120 can be locally heat-treated by induction heating. Induction heating involves applying alternating current through coiled wires, either individually or coiled around a magnetic core located near the laminate 120 or the laminate stack 128. The alternating current generates an alternating magnetic field that induces eddy currents within the laminate 120 to heat the laminate and reduce residual stress present in the laminate. As the frequency of the alternating current increases, the eddy currents concentrate in the edge regions (e.g., outer edge region 150) relative to the central region of the laminate. This concentration of eddy currents in the edge regions can be termed the "skin effect." The skin effect refers to the tendency of alternating current distribution within a conductor to result in a maximum current density near the conductor surface, decreasing with increasing conductor depth. The shaded outer edge region 150 indicates the region of maximum current density in the laminate 120. The current density has a depth δ measured between the outer edge 140 and the inner edge of the outer edge region 150. The second shaded region 154 is a concentric ring set inside the outer edge region 150, and represents a current density smaller than the current density of the outer edge region 150.

[0030] Although Figure 3 The rotor laminations are shown, but the description of the heat treatment and skin effect principle described above also applies to the stator laminations, toroidal surfaces, and other components of the motor.

[0031] refer to Figures 4 to 6 This provides various illustrations of electrical components subjected to localized heat treatment via induction heating. See details. Figure 4The process may include a C-shaped induction heater 122, which includes a magnetic core 126 and a conductor 124 surrounding the core 126. The magnetic core 126 includes a pair of opposing ends 126a and 126b located on each side of a toroidal surface or ring 119. More specifically, ends 126a and 126b are positioned above and below the outer edge region of the toroidal surface or ring 119. A high-frequency alternating current is provided through the conductor 124, causing an alternating magnetic field to penetrate the portion of the toroidal surface or ring 119 located between the opposing ends 126a and 126b. The penetrating alternating magnetic field can generate eddy currents that flow through the resistance of the toroidal surface or ring, causing the portion located between the pair of ends to be heated.

[0032] refer to Figure 5 and Figure 5A A method is provided for localized induction heat treatment of the rotor core or lamination stack 128 using two C-shaped induction heaters 122a and 122b. In this embodiment, the localized induction heat treatment is used to reduce the permeability of the bridging region 148 of the rotor lamination 128. Reducing the permeability of the bridging region 148 can reduce the parasitic magnetic flux 150 of the bridging region 148. Parasitic magnetic flux can lead to increased iron losses and reduced motor efficiency. See details... Figure 5A The diagram shows a partial detail of the lamination 120 or lamination stack 128. The laminations define a pair of magnet cavities 144, each housing a magnet 160. A first bridging region (such as a central bridging region 148a) lies between the pair of magnet cavities 144. A second bridging region (such as an outer bridging region 148b) lies between the edge of the lamination 120 and the exterior of the magnet cavities 144. Magnetic flux 154 is indicated by curved arrows and indicates the flow of magnetic flux from each of the magnets 160 toward the stator (not shown). Parasitic magnetic flux, or leakage, refers to a portion of the magnetic field reaching the stator through the bridging regions rather than through the outer edges. Parasitic magnetic flux 150a is indicated by a circular dashed line near the central bridging region 148a. A second set of parasitic magnetic flux 150b is indicated by a circular dashed line 150b near the outer bridging region 148b.

[0033] Parasitic magnetic flux can be reduced by decreasing the cross-sectional area or thickness of the bridging region 148. However, if the thickness is reduced too much, the forces acting on the stack during operation may cause the stack to break or fracture. Parasitic magnetic flux within the bridging region 148 can be reduced by decreasing its permeability. The permeability of the bridging region (or other regions of the stack) can be reduced by locally induction heating the bridging region 148.

[0034] Return to reference Figure 5The diagram illustrates localized induction heating of the bridging region. A first induction heater 122a is located above and below the central bridging region 150a of the stack 128. Each of the induction heaters 122a and 122b can rotate about the stack 128 to heat-treat portions of the stack 128. In another embodiment, the induction heaters 122a and 122b can remain stationary while the stack 128 can rotate.

[0035] refer to Figure 6 This provides localized induction heat treatment for a stator assembly 132 comprising multiple stator laminations 134. Each of the stator laminations 134 includes multiple stator teeth 138 arranged circumferentially around an inner edge 136 of the stator lamination 134. In this embodiment, an inner coiled wire 125 is disposed within the inner edge 136 of the stator assembly 132, while an outer coil 124 is disposed around the outer edge 134 of the stator assembly 132. Alternating current can be supplied to the outer coil 124 and the inner coil 125 to generate an alternating magnetic field that penetrates both the inner and outer edge regions of the stator assembly 132.

[0036] refer to Figure 7 This diagram illustrates a perspective view of a process 200 for induction heating of various portions of an induction-heated motor component (such as stator assembly 136) according to an embodiment of the present disclosure. The process may include a conveyor belt 202 or other suitable arrangement that supplies the stator assembly 136 in the direction indicated by directional arrow D, such that the stator assembly passes through an induction heater 203. The induction heater 203 may include a C-shaped magnetic core 204, the core including a lower end 204a and an upper surface 204b. A coil or coiled wire 124 may be wound around a portion of the magnetic core 204 and supplied with alternating current. As the stator assembly 136 passes between the lower end 204a and the upper end 204b of the C-shaped magnetic core, an alternating magnetic field or magnetic flux 206 passes through the stator assembly 136, causing the stator assembly to be locally heat-treated by induction heating as described above. The conveyor belt 202 may pause, such that the stator assembly 136 is positioned between the lower end 204a and the upper end 204b of the C-shaped magnetic core for a sufficient time (e.g., one to two minutes) to locally heat-treat edge regions.

[0037] The time required for sufficient heat treatment of the edge regions of the motor component or stator assembly 136 can vary depending on the cross-sectional area, the thickness of individual laminations, and the thickness of the lamination stack. In another embodiment, the conveyor belt can move at a predetermined speed such that the stator assembly 136 is positioned between the lower end 204a and the upper end 204b of the C-shaped magnetic core for a sufficiently long time. After the motor component or stator assembly 136 has been sufficiently heat-treated, a treated motor component or stator assembly 136' is obtained. This component or stator assembly can then be assembled with other components to manufacture a motor.

[0038] refer to Figure 8 This illustration shows a perspective view of a process for induction heating motor components, such as stator assembly 136, according to another embodiment of the present disclosure. In this embodiment, a coil 124, sized to surround the stator assembly 136, is positioned above a conveyor belt 202, and the coil is lowered when the stator assembly 136 is positioned below the coil 124. The coil 124 is powered by an external power source (not shown) to generate an alternating magnetic field, or magnetic flux 206, through the stator assembly 136. The coil 124 may surround the stator assembly 136 for a sufficient time to locally heat-treat the edge portions of the stator assembly 136.

[0039] While exemplary embodiments have been described above, these embodiments are not intended to describe all possible forms of the invention. Rather, the terms used herein are descriptive rather than restrictive, and it should be understood that various changes may be made without departing from the spirit and scope of the invention. Furthermore, features of various implementation embodiments may be combined to form further embodiments of the invention.

[0040] According to the present invention, a method of manufacturing a stator includes: stamping steel into laminations, each lamination having an inner edge region defining residual stress associated with magnetic permeability; and exposing the laminations to a varying magnetic field such that for each lamination, the eddy current density generated near the inner edge region is maximized, thereby heating the inner edge region relative to the central region of the laminations to reduce the residual stress and core loss.

[0041] According to one embodiment, the changing electromagnetic field is generated by applying alternating current through a coil located near the laminations.

[0042] According to one embodiment, exposing the stack to the changing magnetic field causes an increase in the permeability of the inner edge region.

[0043] According to one embodiment, the frequency is selected based on the cross-sectional area of ​​the stacked wafers.

[0044] According to one embodiment, the duration of the exposure is selected based on the thickness of the stack of sheets.

[0045] According to one embodiment, stamping includes stamping the steel into a stack, each stack having an outer edge region defining a second residual stress and a second permeability, and exposing the stack to the varying magnetic field includes, for each stack, generating a maximum eddy current density near the outer edge region to heat the outer edge region relative to the central region of the stack to reduce the residual stress and the permeability.

[0046] According to one embodiment, exposing the lamination includes: positioning a metal member near the surface of the lamination and applying an alternating current at a predetermined frequency to a conductor wound around a portion of the metal member for a predetermined duration to generate eddy currents within the lamination and heat the lamination to reduce residual stress in the inner edge region.

[0047] According to the present invention, a method of manufacturing a rotor includes: stamping steel into laminations, each lamination having an outer edge region with magnetic permeability; and exposing the laminations to a varying magnetic field such that for each lamination, a first eddy current density generated near the outer edge region is maximized to heat the outer edge region relative to the central region of the lamination to reduce the magnetic permeability and mitigate magnetic flux leakage.

[0048] According to one embodiment, the changing electromagnetic field is generated by applying alternating current through a coil located near the laminations.

[0049] According to one embodiment, the coil is wound around a C-shaped magnetic core, the C-shaped magnetic core including a pair of ends spaced apart from each other such that the stack is positioned between the pairs of ends during the exposure.

[0050] According to one embodiment, the pair of ends are spaced apart above and below the bridging region of each of the laminations, and the bridging region is the area disposed between the outer edge and the edge of the magnet cavity defined by each of the laminations.

[0051] According to one embodiment, the stamping includes stamping steel into stacks, each stack having an outer edge region with residual stress and the magnetic permeability.

[0052] According to one embodiment, the invention is further characterized in that the laminates are exposed to a second varying magnetic field, such that for each laminate, a second eddy current density is generated near the outer edge region to maximize the heating of the outer edge region relative to the central region of the laminate, thereby reducing residual stress and increasing the permeability of the outer edge region.

[0053] According to one embodiment, the generated second eddy current density is less than the generated first eddy current density.

[0054] According to the present invention, a method of manufacturing an electric motor includes: stamping steel into laminations, each lamination having an inner edge region and an outer edge region; and moving the laminations relative to a coil to generate an alternating magnetic flux to expose the laminations to the magnetic flux, such that the eddy current density generated near the edge regions is maximized to heat the edge regions relative to the central region of the laminations to reduce the residual stress and increase the permeability.

[0055] According to one embodiment, inducing relative movement includes moving the laminations between a pair of stationary ends of a C-shaped magnetic core that contacts the coil and generates the alternating magnetic flux.

[0056] According to one embodiment, the invention is further characterized in that the stacked sheets are held between the pair of ends for a predetermined duration.

[0057] According to one embodiment, inducing relative movement includes causing the C-shaped magnetic core to surround the stack.

[0058] According to one embodiment, inducing relative movement includes moving the coil along the central axis of the stack such that the coil circumferentially surrounds the stack.

[0059] According to one embodiment, inducing relative movement includes moving the coil along the central axis of the stack such that the coil is circumferentially surrounded by the inner edge region of the stack.

Claims

1. A method for manufacturing a stator, comprising: Steel is stamped into laminations, each lamination having an inner edge region that defines residual stress associated with magnetic permeability; as well as The laminations are exposed to a varying magnetic field in the axial direction of the laminations, such that for each lamination, the eddy current density generated near the inner edge region is maximized, thereby heating the inner edge region relative to the central region of the laminations to reduce the residual stress and core loss.

2. The method as described in claim 1, wherein, The changing magnetic field is generated by applying alternating current through a coil located near the laminations.

3. The method of claim 2, wherein, Exposing the stacked sheets to the changing magnetic field causes an increase in the permeability of the inner edge region.

4. The method of claim 3, wherein, The frequency of the magnetic field is selected based on the cross-sectional area of ​​the laminate.

5. The method of claim 1, wherein, The duration of the exposure is selected based on the thickness of the stack of wafers.

6. The method of claim 1, wherein, The stamping process includes stamping the steel into laminations, each lamination having an outer edge region defining a second residual stress and a second permeability, and exposing the laminations to the varying magnetic field includes, for each lamination, generating a maximum eddy current density near the outer edge region to heat the outer edge region relative to the central region of the lamination to reduce the second residual stress and the second permeability.

7. The method of claim 1, wherein, Exposing the laminations includes: positioning a metal member near the surface of the laminations and applying an alternating current at a predetermined frequency to a conductor wound around a portion of the metal member for a predetermined duration to generate eddy currents within the laminations and heat the laminations to reduce residual stress in the inner edge region.

8. A method for manufacturing a rotor, comprising: Steel is stamped into laminations, each lamination having an outer edge region with magnetic permeability; as well as The laminations are exposed to a varying magnetic field in the axial direction toward the laminations, such that for each lamination, a first eddy current density is generated near the outer edge region to maximize the heating of the outer edge region relative to the central region of the laminations, thereby reducing the permeability and mitigating magnetic flux leakage.

9. The method of claim 8, wherein, The changing magnetic field is generated by applying alternating current through a coil located near the laminations.

10. The method of claim 9, wherein, The coil is wound around a C-shaped magnetic core, which includes a pair of ends spaced apart from each other such that the lamination is positioned between the pairs of ends during the exposure.

11. The method of claim 10, wherein, The pair of ends are spaced apart above and below the bridging region of each of the laminations, and the bridging region is the area between the outer edge and the edge of the magnet cavity defined by each of the laminations.

12. The method of claim 8, wherein, The stamping process includes stamping steel into stacks, each stack having an outer edge region with residual stress and the magnetic permeability.

13. The method of claim 8, further comprising: The laminations are exposed to a second, varying magnetic field, such that for each lamination, the second eddy current density generated near the outer edge region is maximized to heat the outer edge region relative to the central region of the lamination, thereby reducing residual stress and increasing the permeability of the outer edge region.

14. The method of claim 13, wherein the generated second eddy current density is less than the generated first eddy current density.

15. A method for manufacturing an electric motor, comprising: Steel is stamped into stacks, each stack having an inner edge region and an outer edge region; as well as The laminations are moved relative to the coil to generate an alternating magnetic flux in the axial direction of the laminations, exposing the laminations to the magnetic flux. This maximizes the eddy current density generated near the inner and outer edge regions, thereby heating the inner and outer edge regions relative to the center region of the laminations. This reduces the residual stress in the inner and outer edge regions and increases their permeability.

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