A high torque density interior permanent magnet motor

Abstract

The application relates to the technical field of middle-embedded motors, and discloses a high-torque-density middle-embedded motor which comprises a rotor assembly and a stator assembly, the stator assembly comprises a stator core and a stator winding, the stator core is provided with wire-winding columns which are distributed at intervals in the circumferential direction, and the stator winding is arranged on the wire-winding columns at intervals, the rotor assembly comprises a rotor core and permanent magnets, the rotor core is provided with a shaft hole located at the center, and the rotor core is provided with rotor ribs which are arranged at intervals in the circumferential direction and extend radially outward from the periphery of the shaft hole, the permanent magnets are embedded between the rotor ribs, the rotor core further comprises an annular magnetic isolation ring which is arranged concentrically with the shaft hole and is located on the radially inner side of all the rotor ribs, the inner and outer circumferential walls of the magnetic isolation ring are integrally connected with each rotor rib, and the inner circumferential wall of the magnetic isolation ring, the outer circumferential wall of the shaft hole and the rotor ribs form a magnetic isolation cavity which surrounds the shaft hole.

Description

Technical Field

[0001] This invention relates to the field of mid-drive motor technology, specifically a high torque density mid-drive motor. Background Technology

[0002] As a green mode of transportation, electric-assist bicycles place demands on their core power component—the mid-drive motor. The mid-drive motor is typically mounted in the middle of the bicycle frame, connecting the pedal cranks and rear wheel via a transmission mechanism to provide auxiliary power to the rider. Given limited space and lightweight requirements, the mid-drive motor must possess high torque density and high efficiency. To meet this need, existing mid-drive motors generally employ permanent magnet synchronous motors, with the rotor often using a spoke-type structure. This structure embeds permanent magnets radially between adjacent ribs of the rotor core. Leveraging its magnetic focusing effect, it allows for the placement of longer permanent magnets within a given rotor outer diameter, thereby achieving higher air gap flux density and torque output capability. Regarding the stator, to facilitate winding and improve slot fill factor, a modular core structure has gradually emerged. This involves dividing the stator core into multiple independent I-shaped or I-shaped units, with windings concentrated on the winding columns of each unit before being assembled into a ring. Rectangular cross-section flat enameled wire is often used for winding to fully utilize winding space and reduce copper losses.

[0003] However, the aforementioned existing technologies still have several technical problems in practical applications. On the rotor side, a low-resistivity leakage magnetic path naturally exists within the conventional spoke structure: in addition to the main magnetic circuit that closes radially outward through the stator core to generate effective torque, a portion of the magnetic flux generated by the permanent magnet leaks towards the central shaft through the inner ends of the rotor ribs, forming an ineffective closed loop inside the rotor. This internal leakage magnetic field not only directly diverts the main magnetic flux that contributes to the motor torque, reducing the motor's torque density and efficiency, but also the stray magnetic field leaking to the shaft area severely interferes with the magnetoelectric encoder installed at the shaft end, causing distortion of the rotor position detection signal and affecting the smoothness of the assist output and control accuracy. On the stator side, although the modular core combined with flat wire windings can improve the winding fill factor to some extent, insulating paper is still required between the winding post surface and the winding, as well as between winding layers, to achieve electrical isolation. Insulating paper of conventional thickness inevitably occupies limited winding window space, restricting the effective conductor cross-sectional area that can be accommodated, thus limiting further improvements in slot fill factor and consequently limiting the increase in motor power density. These issues create a bottleneck in the further miniaturization, weight reduction, and high-power development of existing mid-drive motors for electric bicycles. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a high torque density mid-drive motor, which has the advantages of improving motor torque density and winding slot fill factor, while effectively suppressing interference from magnetoelectric encoders. It solves the problems of torque limitation and encoder interference caused by internal leakage flux in existing spoke-type rotor mid-drive motors, as well as the space occupied by stator insulation structure restricting the improvement of slot fill factor.

[0005] (II) Technical Solution To achieve the aforementioned goals of increasing motor torque density and winding slot fill factor while effectively suppressing interference from magnetoelectric encoders, this invention provides the following technical solution: a high torque density mid-drive motor, comprising a rotor assembly and a stator assembly, wherein the stator assembly comprises a stator core and stator windings, the stator core having winding posts spaced apart along the circumference, and the stator windings being concentrated on the winding posts; the rotor assembly comprises a rotor core and permanent magnets, the rotor core having a centrally located shaft hole, and rotor ribs extending radially outward from the periphery of the shaft hole and spaced apart in the circumference, the permanent magnets being embedded between the rotor ribs; The rotor core also includes an annular magnetic shielding ring arranged concentrically with the shaft hole. The magnetic shielding ring is located radially inside all the rotor ribs. The inner and outer peripheral walls of the magnetic shielding ring are integrally connected to each rotor rib. The inner peripheral wall of the magnetic shielding ring, the outer peripheral wall of the shaft hole, and the rotor ribs form a magnetic shielding cavity surrounding the shaft hole array.

[0006] The stator core is formed by stacking stator laminations along the axial direction, and the rotor core is formed by stacking rotor laminations along the axial direction.

[0007] The stator core is formed by splicing multiple independent stator core units circumferentially to form a ring. Adjacent stator core units are fitted together. Each stator core unit includes a winding post and an inner end plate and an outer end plate connected to the winding post. The inner end plate is closer to the shaft hole than the outer end plate. The circumferential width of the inner end plate and the outer end plate is greater than the circumferential width of the winding post, so that the stator core unit as a whole is I-shaped. Adjacent inner end plates and outer end plates are fitted together to form a ring-shaped stator core.

[0008] An insulating shell is provided on the stator core unit. The insulating shell covers the inner end plate, the axial side of the outer end plate, and the surface of the winding column to insulate and isolate the stator winding from the inner end plate, the outer end plate, and the winding column.

[0009] The stator winding is made of flat enameled wire, the outer circumferential surface of the winding post is a flat surface, and the cross-section of the flat enameled wire is rectangular.

[0010] The stator assembly also includes insulating paper, which is laid on the surface of the winding post and the wall surface of the outer end plate facing the winding post, and is disposed between the insulating shell and the stator core unit to form a continuous insulating layer.

[0011] The permanent magnet is installed between adjacent rotor ribs to form a radially extending permanent magnet mounting groove. The two side walls of the permanent magnet mounting groove are respectively formed by the opposing sides of the adjacent rotor ribs. The side walls are parallel planes. The permanent magnet is embedded in the permanent magnet mounting groove, and the two sides of the permanent magnet are respectively in contact with the sides of the rotor ribs that form the two side walls of the permanent magnet mounting groove.

[0012] The magnetic shielding ring is a closed circular ring structure. All rotor ribs pass through the intersection of the magnetic shielding ring and are connected to the outer peripheral wall of the shaft hole. The magnetic shielding cavity is an annular cavity separated by the rotor ribs, and the cavity is filled with air.

[0013] The shaft hole is used to fix the motor shaft, and the end of the motor shaft is provided with a magneto-electric encoder. The magnetic isolation ring and the magnetic isolation cavity are located radially between the permanent magnet and the motor shaft.

[0014] The rotor assembly and stator assembly are mounted in the middle of the frame of the electric-assisted bicycle and are connected to the bicycle pedal crank and the rear wheel drive structure through a transmission mechanism.

[0015] (III) Beneficial Effects Compared with the prior art, the present invention provides a high torque density mid-drive motor, which has the following beneficial effects: 1. This high torque density mid-mounted motor, on the rotor side, utilizes an auxiliary magnetic isolation structure formed by an annular magnetic isolation ring and a magnetic isolation cavity to fundamentally optimize the internal magnetic circuit of the spoke-type rotor. The magnetic isolation cavity, leveraging the high magnetic reluctance of air, cuts off the direct leakage magnetic path from the inner end of the rotor ribs towards the shaft. The annular magnetic isolation ring forms a circumferential magnetic short-circuit ring, confining residual ineffective magnetic flux within the closed loop. This forces most of the magnetic flux generated by the permanent magnet to radially outward through the stator core to form the main magnetic circuit, significantly increasing the effective magnetic flux ratio. This allows the motor to output higher torque within the same volume, significantly improving torque density and operating efficiency. Simultaneously, this magnetic isolation structure attenuates stray magnetic fields reaching the motor shaft area to an extremely low level, effectively protecting the shaft-end magneto-electric encoder from leakage magnetic interference and ensuring the accuracy of the rotor position detection signal and the smoothness of the assist response. Furthermore, the annular magnetic shielding ring firmly connects all rotor ribs into a whole, greatly enhancing the mechanical rigidity of the rotor core. This overcomes the deformation problem that easily occurs when the inner ends of the ribs are designed too thin to increase the magnetic resistance of the leakage magnetic circuit, thus improving the structural reliability of the rotor under high-speed rotation. Multiple fan-shaped magnetic shielding chambers running axially also function as ventilation channels, facilitating air convection and heat dissipation inside the rotor and reducing the risk of high-temperature demagnetization of the permanent magnets.

[0016] 2. This high torque density mid-drive motor employs a modular I-beam core unit on the stator side. The winding posts have flat surfaces and are wound with rectangular cross-section flat enameled wire, achieving a seamless fit between the conductor and the core. This geometrically eliminates the gaps inherent in round wire winding, laying the foundation for a high slot fill factor. In particular, the use of a thin layer of insulating and thermally conductive paper on the surface of the winding posts and between the flat wire layers minimizes the space occupied by the insulation layer while meeting electrical insulation requirements. This frees up more effective volume for increasing the number of winding turns or the conductor cross-sectional area, further increasing the slot fill factor. This improvement directly leads to a decrease in winding resistance and copper losses, significantly improving the motor's power density and continuous torque output capability. This allows the motor to achieve a lighter weight and more compact size while meeting the power assist requirements, perfectly meeting the stringent lightweight and space constraints of mid-drive motors in electric bicycles. The large-area planar contact between the flat wire winding layers, combined with the thermal conductivity of the thin insulating paper, enhances the heat transfer capacity from the winding to the core, improves the thermal field distribution, reduces hot spot temperatures, and improves the motor's thermal stability and overload capacity. The insulating shell reliably isolates the core end face from the winding, eliminating the risk of end electrical breakdown. Its snap-fit ​​or adhesive fixing method facilitates automated assembly. The modular structure allows each unit to be wound independently, facilitating the use of automated winding equipment, reducing process complexity, and improving production efficiency and consistency. The synergy between rotor magnetic circuit optimization and high stator slot fill factor enables the motor to deliver strong, precise, and reliable power output within a limited space.

[0017] 3. This high torque density mid-drive motor achieves significant performance improvements and technological advantages through the synergistic combination of modular I-shaped core units, insulating shells, flat wire windings, and thin-layer insulating paper. Individual stator core units are circumferentially assembled into a ring, and the winding posts have flat surfaces, allowing for neat, layer-by-layer winding of rectangular cross-section flat enameled wire along the axial direction. This results in a large-area, tight fit between conductor layers and between the conductor and the core, completely eliminating the inherent gaps in traditional round wire winding and laying the foundation for high slot fill factor from a geometrical perspective. Thin-layer insulating thermally conductive paper is fully laid on the surface of the winding posts and extends to the end plate walls. Combined with a reduced-thickness insulating shell, this ensures reliable electrical isolation while minimizing the impact of its own thickness on the winding window space. This frees up more effective volume to increase the number of winding turns or use flat wire conductors with larger cross-sectional areas, further increasing the winding fill factor. This structural change directly reduces the DC resistance and copper loss of the windings, significantly improving the motor's operating efficiency and continuous torque output capability. This allows the motor to achieve a lighter weight and more compact size while meeting the requirements for power assist performance, precisely addressing the stringent lightweight and space constraints of electric-assist bicycles for power components. The large-area contact between the flat wires, combined with a high thermal conductivity insulating paper layer, effectively reduces the thermal resistance from the winding interior to the core, creating an efficient radial heat transfer path. This allows heat to be quickly conducted to the stator core and dissipated outwards through the outer casing, improving the motor's thermal field distribution, reducing hot spot temperatures, and enhancing thermal stability and overload capacity. The upper and lower insulating shells reliably cover the axial end faces of the core unit, eliminating the potential risk of electrical breakdown between the winding ends and the core, and providing end barriers for the winding process, which is beneficial for the orderly shaping and positioning of the coil during automated winding. The modular structure allows each core unit to independently complete lamination stacking, winding, and insulation treatment, and then be assembled by clamping with the stator shell or fastening rings. This greatly reduces the difficulty of winding the overall stator core, facilitates the use of multi-station parallel winding equipment, improves production efficiency and product consistency, and makes it easy to replace defective single parts without scrapping the entire stator.

[0018] 4. In terms of its cooperation with the rotor side, this high torque density mid-drive motor features a high slot fill factor stator and rotor magnetic isolation ring and magnetic isolation cavity structure working synergistically to enhance the magnetic circuit optimization on the rotor side, which maximizes the effective main magnetic flux entering the stator teeth. Meanwhile, the high slot fill factor on the stator side receives this magnetic flux with extremely high copper conductor filling efficiency, generating greater electromagnetic torque under the same operating current. The combined effect of these two factors enables the motor to reach new levels of power density and torque density, driving breakthroughs in the miniaturization and high performance of mid-drive motors. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the present invention; Figure 2 This is a schematic diagram of the stator assembly and rotor assembly structure of the present invention; Figure 3 This is a schematic diagram of the cross-sectional structure of the present invention; Figure 4 This is a partial cross-sectional view of the stator core of the present invention; Figure 5 This is a schematic diagram of the stator assembly structure of the present invention; Figure 6 This is a detailed schematic diagram of the stator assembly of the present invention; Figure 7 This is a schematic diagram of the magnetic circuit of the present invention.

[0020] In the diagram: 1. Rotor assembly; 2. Stator assembly; 11. Rotor core; 12. Permanent magnet; 13. Magnetic isolation ring; 14. Shaft hole; 15. Rotor rib; 21. Stator core; 22. Stator winding; 23. Insulating paper; 24. Insulating shell; 121. Permanent magnet mounting slot; 131. Magnetic isolation cavity; 211. Winding post; 212. Inner end plate; 213. Outer end plate. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Please see Figures 1-3 A high torque density mid-drive motor includes a rotor assembly 1 and a stator assembly 2. The stator assembly 2 includes a stator core 21 and a stator winding 22. The stator core 21 has winding posts 211 spaced out in the circumferential direction. The stator winding 22 is concentrated and wound on the winding posts 211. The rotor assembly 1 includes a rotor core 11 and a permanent magnet 12. The rotor core 11 has a central shaft hole 14 and rotor ribs 15 extending radially outward from the periphery of the shaft hole 14 and spaced out in the circumferential direction. The permanent magnet 12 is embedded between the rotor ribs 15. The rotor assembly 1 is rotatably coaxially disposed inside the stator assembly 2. The rotor ribs 15 are evenly distributed in the circumferential direction. The slots formed between adjacent rotor ribs 15 face the winding posts 211 of the stator core 21. The magnetic field generated by the permanent magnet 12 links with the stator winding 22 through the winding posts 211 of the stator core 21.

[0023] The rotor core 11 also includes an annular magnetic shielding ring 13 arranged concentrically with the shaft hole 14. The magnetic shielding ring 13 is located radially inside all the rotor ribs 15. The inner and outer peripheral walls of the magnetic shielding ring 13 are integrally connected to each rotor rib 15. The inner peripheral wall of the magnetic shielding ring 13, the outer peripheral wall of the shaft hole 14, and the rotor ribs 15 form a magnetic shielding cavity 131 surrounding the shaft hole 14 array. The stator core 21 is formed by stacking stator laminations along the axial direction, and the rotor core 11 is formed by stacking rotor laminations along the axial direction. The annular magnetic shielding ring 13 is made of the same magnetically conductive material as the rotor rib 15. It is integrally formed with the rotor rib 15 and the outer peripheral wall of the shaft hole 14 through a lamination stacking process. That is, the rotor laminations constituting the rotor core 11 are formed into a sheet-like structure including the annular magnetic shielding ring 13, the rotor rib 15 and the outer contour of the shaft hole 14 during the stamping process. The stacked stator laminations are fixed into an integral stator core 21 by a snap-fit ​​structure or a welding structure. The stacked rotor laminations are fixed into an integral rotor core 11 by a snap-fit ​​structure or a welding structure. The magnetic shielding cavity 131 is formed by the inner peripheral wall of the annular magnetic shielding ring 13, the outer peripheral wall of the shaft hole 14 and the side of the adjacent rotor rib 15. The magnetic shielding cavity 131 is axially connected along the rotor core 11 and is filled with air. The high magnetic reluctance of air is used to cut off the leakage magnetic path from the inner end of the rotor rib 15 to the shaft hole 14, forcing the magnetic flux generated by the permanent magnet 12 to mainly close radially outward through the stator core 21. Electric-assist bicycles have extremely high requirements for the space utilization and weight of the mid-drive motor. The mid-drive motor must output the required torque within a limited space and weight. The spoke structure, or spoked rotor structure, has a magnetic focusing effect and allows for the insertion of longer permanent magnets, enabling it to generate greater torque with a fixed rotor outer diameter. Therefore, spoked rotors are commonly used in mid-drive motors of electric-assist bicycles. However, in a conventional spoked rotor, the magnetic flux generated by the permanent magnet exists in two magnetic circuits inside the rotor. One of these is the main magnetic circuit closed by the stator core (i.e.,...). Figure 7 C1) is the effective magnetic circuit for generating electromagnetic torque. The other leakage magnetic circuit, which closes from the inner end of the rotor ribs towards the shaft, is an ineffective magnetic circuit. This ineffective magnetic circuit not only diverts the magnetic flux of the main magnetic flux, causing a decrease in motor torque, but also interferes with the magneto-electric encoder mounted on the shaft due to leakage magnetic flux passing through the shaft. This application constructs an auxiliary magnetic isolation structure inside the rotor by setting an annular magnetic isolation ring 13 and a magnetic isolation cavity 131, wherein the annular magnetic isolation ring 13 is located radially inside the rotor ribs 15. Between the end and the shaft hole 14, the magnetic isolation cavity 131 further forms a high magnetic resistance air layer between the annular magnetic isolation ring 13 and the shaft hole 14. At the same time, the annular magnetic isolation ring 13 connects all the rotor ribs 15 into one piece, which on the one hand enhances the mechanical rigidity of the rotor core 11 and avoids the deformation problem caused by the inner end of the rotor ribs 15 being designed too thin to block leakage magnetic flux. On the other hand, the magnetic flux is guided by the magnetic resistance difference configuration of the annular magnetic isolation ring 13 and the magnetic isolation cavity 131 for different radial paths.

[0024] See Figures 1-6 The stator core 21 is formed by splicing multiple independent stator core units circumferentially to form a ring. Adjacent stator core units are fitted together. Each stator core unit includes a winding post 211 and an inner end plate 212 and an outer end plate 213 connected to the winding post 211. The inner end plate 212 is closer to the shaft hole 14 than the outer end plate 213. The circumferential width of the inner end plate 212 and the outer end plate 213 is greater than the circumferential width of the winding post 211, so that the stator core unit is I-shaped as a whole. Adjacent inner end plates 212 and outer end plates 213 are fitted together to form a ring-shaped stator core 21. An insulating shell 24 is provided on the stator core unit. The insulating shell 24 covers the axial sides of the inner end plate 212 and the outer end plate 213 and the surface of the winding post 211 to insulate and isolate the stator winding 22 from the inner end plate 212, the outer end plate 213 and the winding post 211. The insulating shell 24 is a one-piece molded part made of plastic or resin. Its outline is adapted to the shape of the I-shaped stator core unit. It is fixed to the core unit by axial insertion or bonding. The inner end plate covering part of the insulating shell 24 completely covers the axial end face of the inner end plate 212 and extends radially in part. The outer end plate covering part completely covers the axial end face of the outer end plate 213 and folds inward radially to cover the axial side walls of the winding post 211. This forms an uninterrupted insulation barrier along the entire circumference and axial end face of the winding post 211, replacing the traditional split upper and lower end insulating shell structure, reducing the number of parts, and improving assembly efficiency and insulation reliability.

[0025] The stator winding 22 is made of flat enameled wire. The outer peripheral surface of the winding post 211 is a flat surface, and the cross-section of the flat enameled wire is rectangular. The stator assembly 2 also includes insulating paper 23, which is laid on the surface of the winding post 211 and the wall surface of the outer end plate 213 facing the winding post 211, and is disposed between the insulating shell 24 and the stator core unit to form a continuous insulating layer. The insulating paper 23 is made of thin insulating and thermally conductive material, pre-bent into a U-shape or frame shape consistent with the cross-sectional shape of the winding post 211, tightly laid on the flat outer peripheral surface of the winding post 211, and extends outward to the wall surface of the outer end plate 213 facing the winding post 211. The insulating shell 24 is fitted on the outside of the insulating paper 23, tightly pressing the insulating paper 23 onto the surface of the stator core unit, eliminating the air gap between the insulating paper 23 and the core. This sandwich structure allows the insulating paper 23 to provide slot insulation while being mechanically protected by the insulating shell 24, making it less prone to displacement or damage during winding. Furthermore, the insulating paper 23 is in direct contact with the iron core, and its thermal conductivity allows it to efficiently conduct the heat from the winding to the iron core for dissipation. Meanwhile, the insulating shell 24 provides additional outer insulation creepage distance and mechanical strength. Together, they form a composite insulation system with higher safety margin and a balance between thermal conductivity and insulation, providing reliable protection for the tight winding and high slot fill factor of flat enameled wire. Meanwhile, the insulating paper 23 reduces the requirements for the insulating shell 24. The insulating paper 23 is made of thin insulating and thermally conductive material with small thickness and high insulation strength. After being bent, it is attached to the surface of the winding post 211 and the wall surface of the outer end plate 213 facing the winding post, and extends to the area of ​​the inner end plate 212. This forms continuous slot insulation and interlayer insulation between the stator winding 22 and the stator core 21, minimizing the space occupied by the insulating shell 24, which is beneficial to increasing the number of winding turns or conductor cross-sectional area and improving the power density of the motor.

[0026] See Figures 2-7The permanent magnet 12 is installed between adjacent rotor ribs 15 to form a radially extending permanent magnet mounting groove 121. The two side walls of the permanent magnet mounting groove 121 are respectively formed by the opposing side surfaces of adjacent rotor ribs 15. The side surfaces are mutually parallel planes. The permanent magnet 12 is embedded in the permanent magnet mounting groove 121, and the two side surfaces of the permanent magnet 12 are respectively in contact with the side surfaces of the rotor ribs 15 that form the two side walls of the permanent magnet mounting groove 121. The magnetic shielding ring 13 is a closed circular ring structure. All rotor ribs 15 pass through the intersection of the magnetic shielding ring 13 and are connected to the outer peripheral wall of the shaft hole 14. The magnetic shielding cavity 131 is an annular cavity separated by rotor ribs 15, and the cavity is filled with air. The permanent magnet 12 is made of sintered NdFeB permanent magnet material, and the magnetization direction is either parallel magnetization or radial magnetization. The permanent magnet 12 is bonded and fixed to the two side walls of the permanent magnet mounting groove 121 with anaerobic adhesive or epoxy adhesive to fill the tiny gaps and enhance force transmission. In the connection relationship between the annular magnetic shielding ring 13 and the rotor rib 15, the rotor rib 15 extends radially from the inner part of the annular magnetic shielding ring 13 to the outer part. That is, the annular magnetic shielding ring 13 is located in the middle region of the length of the rotor rib 15 in the radial direction, so that the inner section of the rotor rib 15 extends into the shaft hole area inside the annular magnetic shielding ring 13, and the outer section extends to the outer circle of the rotor. This through-type integrated structure enhances the overall mechanical strength of the rotor. The magnetic isolation cavity 131 is divided into multiple fan-shaped chambers by the inner space of the annular magnetic isolation ring 13, the outer wall of the shaft hole 14, and the side wall of the inner section of the rotor rib 15. Each chamber runs through the center axially, forming a ring-shaped array of magnetic isolation channels, which not only provides magnetic isolation but also facilitates axial ventilation and heat dissipation inside the rotor. Further explanation of the magnetic circuit control principle of the auxiliary ring magnetic isolation structure: The radial thickness of the inner end of the rotor rib 15, i.e., the portion extending into the inner circumferential wall of the annular magnetic isolation ring 13, is... Figure 7 The value marked L1 is controlled to be relatively small, so that the leakage magnetic path from the permanent magnet 12 through the inner end of the rotor rib 15 directly closes towards the shaft hole 14. Figure 7 The component labeled C2 has a large magnetic reluctance, which significantly weakens the leakage flux; after the introduction of the annular magnetic isolation ring 13, the annular magnetic isolation ring 13 forms a circumferentially continuous short-circuit magnetic circuit inside the rotor. Figure 7 (Created as C3) This C3 can attract part of the leakage magnetic flux that would originally flow to the shaft hole 14 to the annular magnetic shielding ring 13, where it will meander circumferentially and return to the other end of the permanent magnet 12. However, C3 itself is an ineffective magnetic circuit. If the annular magnetic shielding ring 13 is too close to the permanent magnet 12, i.e., the radius is too small, the magnetic resistance of C3 will be too small, resulting in an increase in the ineffective magnetic flux passing through C3. Therefore, the radial distance between the annular magnetic shielding ring 13 and the bottom of the permanent magnet mounting groove 121 ( Figure 7 (labeled L3) is kept at a large value so that the magnetic reluctance of C3 is large enough to reduce the ineffective magnetic flux; at the same time, the radial distance from the annular magnetic shielding ring 13 to the shaft hole 14 is kept at a large value. Figure 7The distance (marked as L2) is designed to be greater than that of L3, so that the magnetic reluctance of C2 is greater than that of C3. In this way, the magnetic flux leaking from the inner end of the rotor rib 15 will preferentially choose the C3 with lower magnetic reluctance to close around the annular magnetic isolation ring 13 and bypass the area of ​​the shaft hole 14, thereby significantly reducing the leakage magnetic flux reaching the shaft hole 14 and the motor shaft, and protecting the magneto-electric encoder from magnetic field interference.

[0027] The shaft hole 14 is used to fix the motor shaft, and a magneto-electric encoder is provided at the end of the motor shaft. The magnetic isolation ring 13 and the magnetic isolation cavity 131 are located radially between the permanent magnet 12 and the motor shaft. The rotor assembly 1 and the stator assembly 2 are installed in the middle of the frame of the electric bicycle and are connected to the bicycle pedal crank and the rear wheel drive structure through a transmission mechanism. The motor shaft is pressed into the shaft hole 14 by a flat key or interference fit and is circumferentially fixed to the rotor core 11. The magneto-electric encoder is installed at one end of the motor shaft and realizes motor commutation control by sensing the rotor position angle. Because the annular magnetic shielding ring 13 and the magnetic shielding cavity 131 are radially positioned between the permanent magnet 12 and the motor shaft, the magnetic flux generated by the permanent magnet 12, when propagating inward, is forced to flow radially outward into the stator core 21 to form the main magnetic circuit. Only a very small portion of leakage magnetic energy reaches the motor shaft, thus significantly reducing magnetic field interference to the encoder, ensuring the accuracy of the encoder signal, and effectively improving the motor's torque density. This mid-drive motor is installed at the bottom bracket position of the bicycle frame, and transmits the output power of the motor shaft to the bottom bracket or rear wheel through a single or multi-stage gear reduction mechanism to assist riding.

[0028] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0029] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high torque density mid-drive motor, comprising a rotor assembly (1) and a stator assembly (2), wherein the stator assembly (2) comprises a stator core (21) and stator windings (22), the stator core (21) having winding posts (211) spaced apart circumferentially, and the stator windings (22) being concentratedly wound on the winding posts (211), and the rotor assembly (1) comprising a rotor core (11) and permanent magnets (12), characterized in that: The rotor core (11) has a central shaft hole (14) and rotor ribs (15) extending radially outward from the periphery of the shaft hole (14) and spaced apart in the circumferential direction, with the permanent magnet (12) embedded between the rotor ribs (15). The rotor core (11) also includes an annular magnetic shielding ring (13) arranged concentrically with the shaft hole (14). The magnetic shielding ring (13) is located radially inside all the rotor ribs (15). The inner and outer peripheral walls of the magnetic shielding ring (13) are integrally connected with each rotor rib (15). The inner peripheral wall of the magnetic shielding ring (13) and the outer peripheral wall of the shaft hole (14) and the rotor ribs (15) form a magnetic shielding cavity (131) surrounding the shaft hole (14) array.

2. The high torque density mid-drive motor according to claim 1, characterized in that: The stator core (21) is formed by stacking stator laminations along the axial direction, and the rotor core (11) is formed by stacking rotor laminations along the axial direction.

3. A high torque density mid-drive motor according to claim 2, characterized in that: The stator core (21) is formed by splicing multiple independent stator core units circumferentially to form a ring. Adjacent stator core units are attached to each other. Each stator core unit includes a winding post (211) and an inner end plate (212) and an outer end plate (213) connected to the winding post (211). The inner end plate (212) is closer to the shaft hole (14) than the outer end plate (213). The circumferential width of the inner end plate (212) and the outer end plate (213) is greater than the circumferential width of the winding post (211) so that the stator core unit is I-shaped as a whole. Adjacent inner end plates (212) and outer end plates (213) are attached to each other to form a ring stator core (21).

4. A high torque density mid-drive motor according to claim 3, characterized in that: An insulating shell (24) is provided on the stator core unit. The insulating shell (24) covers the axial side of the inner end plate (212), the outer end plate (213) and the surface of the winding post (211) to insulate and isolate the stator winding (22) from the inner end plate (212), the outer end plate (213) and the winding post (211).

5. A high torque density mid-drive motor according to claim 4, characterized in that: The stator winding (22) is made of flat enameled wire, the outer surface of the winding post (211) is a flat surface, and the cross-section of the flat enameled wire is rectangular.

6. A high torque density mid-drive motor according to claim 5, characterized in that: The stator assembly (2) also includes insulating paper (23), which is laid on the surface of the winding post (211) and the wall surface of the outer end plate (213) facing the winding post (211), and is disposed between the insulating shell (24) and the stator core unit to form a continuous insulating layer.

7. A high torque density mid-drive motor according to claim 1, characterized in that: The permanent magnet (12) is installed between adjacent rotor ribs (15) to form a radially extending permanent magnet mounting groove (121). The two sides of the permanent magnet mounting groove (121) are respectively formed by the opposing sides of the adjacent rotor ribs (15). The sides are parallel planes. The permanent magnet (12) is embedded in the permanent magnet mounting groove (121), and the two sides of the permanent magnet (12) are respectively attached to the sides of the rotor ribs (15) that form the two sides of the permanent magnet mounting groove (121).

8. A high torque density mid-drive motor according to claim 7, characterized in that: The magnetic shielding ring (13) is a closed circular ring structure. All rotor ribs (15) pass through the intersection of the magnetic shielding ring (13) and the outer peripheral wall of the shaft hole (14). The magnetic shielding cavity (131) is an annular cavity separated by rotor ribs (15), and the cavity is filled with air medium.

9. A high torque density mid-drive motor according to any one of claims 1-8, characterized in that: The shaft hole (14) is used to fix the motor shaft, and the end of the motor shaft is provided with a magneto-electric encoder. The magnetic isolation ring (13) and the magnetic isolation cavity (131) are located radially between the permanent magnet (12) and the motor shaft.

10. A high torque density mid-drive motor according to any one of claims 1-8, characterized in that: The rotor assembly (1) and stator assembly (2) are mounted in the middle of the frame of the electric-assisted bicycle and are connected to the bicycle pedal crank and the rear wheel drive structure through a transmission mechanism.

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