Method for balancing cogging torque of electric machine having permanent magnet rotor and slotted stator, and associated electric machine having balanced cogging torque

Abstract

The invention provides a method for balancing the cogging torque of an electric machine with a permanent magnet rotor and a slotted stator, and a corresponding electric machine with balanced cogging torque. According to the application, the actual angular width of the magnetic pole shoe of the rotor of the motor can be obtained by the formula theta m = k * theta me, k is a correction coefficient considering the edge effect and has a value of 0.8 to 1.1, theta me is the effective angular width of the magnetic pole shoe of the rotor and is determined by the equation theta me = (Q-1 / 2) theta Sp, theta Sp is the stator pitch angle, theta Sp = 360 degrees / S, and theta m = (Q-1 / 2) * theta Sp = (Q-1 / 2) * theta Sp = (Q-1 / 2) * theta Sp = (Q-1 / 2) * theta Sp = (Q-1 / 2) * theta Sp = (Q-1 / 2) * theta Sp. Q is the ratio of the slot number S to the magnetic pole number P and is an integer, and Q = S / P. The value of the correction coefficient k of the edge effect is considered, and modeling is carried out on the magnetic circuit structure of the motor by using a finite element method to determine.

Description

Technical Field

[0001] This application relates to the field of motors. More specifically, it relates to a motor with a rotor using permanent magnets or DC electromagnets, which includes a stator made of ferromagnetic material, and stator windings are installed in the stator slots. The purpose of this application is to provide a method for balancing and eliminating the total cogging torque of the motor without affecting the electromagnetic performance of the motor, especially applicable to motors where the ratio Q of the number of stator slots S to the number of rotor poles P is an integer. Background Art

[0002] The main drawback of motors with permanent magnet rotors (including brushless DC motors (BLDC) or permanent magnet synchronous motors / generators (PMSM / G) with slotted ferromagnetic stators) is that these motors generate cogging torque. The cogging torque is generated by the interaction between the permanent magnets in the rotor and stator slots due to the change in magnetic reluctance above the slot openings. The permanent magnet rotor is located at the position where the mutual interaction energy of the magnetic reluctance between the rotor and stator is minimized, thus forming a stable equilibrium state of the system.

[0003] At the stable equilibrium position, the cogging torque is zero. Near this position, in addition to the external torque that causes the rotor to deviate from this position, there is also an additional reverse torque acting on the rotor towards the stable equilibrium position. Between the two stable equilibrium positions, there is also an unstable equilibrium point where the cogging torque is also zero. A disturbance in the torque applied to the rotor at the unstable equilibrium point will result in the following situation: the rotor will turn back to the stable equilibrium position due to the generated cogging torque, which may be before or after the unstable equilibrium position.

[0004] Except for stepper motors, cogging torque is an undesirable phenomenon during the operation of most motors because it can cause harmful vibrations, noise, higher starting torque, uneven motion, and torque ripple, etc. Therefore, various methods have been developed to reduce the cogging torque.

[0005] However, most of the known methods for reducing cogging torque in the prior art will lead to a decline in the electromagnetic performance of the motor, and usually also complicate the structure, resulting in higher production costs and greater complexity. Therefore, most of the known methods can only reduce the cogging torque of slotted motors to a certain extent and cannot eliminate the cogging torque.

[0006] Various patent applications also describe a method or device by which the cogging torque generated on a certain component of the motor can be eliminated by the cogging torque of the same magnitude but opposite direction generated on another component of the motor on the same rotating shaft. Such solutions can be learned from the following documents.

[0007] CA2711543A1 describes a known solution for reducing the cogging torque of a permanent magnet motor. This solution adds a structural component based on permanent magnets, a cogging torque compensator, to the motor, which is not necessary for operation in the electromagnetic sense. The rotor part of the compensator contains an inertial mass in addition to the rotor of the motor, which increases the inertia of the rotor and thus also the starting torque. Compared with an integrated motor, this solution is more costly and less reasonable in design. The structural components of the integrated motor achieve efficient electromagnetic operation while compensating for the cogging torque (i.e., generating a driving torque during motor operation, or generating an electromotive force and current during generator operation). This solution can reduce the total cogging torque but cannot eliminate it.

[0008] The main drawback is that the described method does not provide a clear, definite, and repeatable solution for designing the shape of the cogging torque function of the compensator according to the rotation angle in order to reverse (compensate) the cogging torque of the motor.

[0009] The solution described in TW201234739A only reduces the cogging torque but does not eliminate it. This is due to the lack of a solution for the effective angle of the rotor pole shoe. The necessary conditions for compensating the cogging torque have been met, but this is not enough - the mechanical offset of the two halves (stator or rotor) of the motor is half the period of the uncompensated cogging torque function. In addition, the described generator structure actually includes two electromagnetically independent generators, that is, it is actually two motors located on the same axis, rather than an integrated motor where the two halves complement each other during operation.

[0010] The solution described in CN212258740U does not provide any description or rule regarding the relationship between the rotor pole shoe angle and the stator tooth pitch angle (number of slots). Similar to the description in TW201234739A, this solution meets the necessary conditions for compensating the cogging torque, but this is not enough - the mechanical offset of the two halves (stator or rotor) of the motor is half the period of the uncompensated cogging torque function. Another drawback is that the reduction of the cogging torque only applies to a specific motor structure, and the description does not provide a solution applicable to different types of permanent magnet rotor motors.

[0011] The solution described in US20200153367A1 reduces the cogging torque but does not fully compensate. In this solution, an even number of motors are connected to a common shaft. This is not a single integrated motor solution where different halves of the motor participate in the operation of the motor simultaneously and compensate for the cogging torque.

[0012] Permanent magnet rotor motors in contemporary industrial manufacturing typically employ various methods to reduce cogging torque. The most commonly used method is to use a fractional-slot motor (the ratio of the number of slots to the number of magnetic poles is not an integer). Without using methods to reduce cogging torque or a combination thereof, the motor will effectively be inoperable because the cogging torque will be extremely high, even comparable to the maximum driving torque, and thus the vibration of the motor will be intolerable in virtually every aspect. Therefore, the above methods can only serve as supplementary methods to reduce the already tolerable cogging torque in existing motors.

[0013] All of the above documents describe the phase shift required to compensate for and eliminate cogging torque by means of the half-period of the uncompensated cogging torque function between the components (stator or rotor) of two motors operating on a common axis or between the rotor or stator components of a motor.

[0014] Therefore, none of these documents mention or provide a clear and comprehensive solution to the problem that the phase shift between motor components alone is insufficient to eliminate and / or compensate for the cogging torque of a motor. All of the documents mentioned describe the necessary (required) conditions for eliminating (balancing) the total cogging torque of a motor, but not the sufficient conditions. This application provides a new method for manufacturing a motor having a permanent magnet rotor and a slotted stator with balanced cogging torque (zero cogging torque). Summary of the Invention

[0015] When the stator slots and rotor magnetic poles are evenly distributed, the cogging torque is a periodic function of the rotor rotation angle. Within one period, the cogging torque is equal to zero twice, each time after half a period. This occurs at alternating stable equilibrium points and unstable equilibrium points. Compared with near the unstable equilibrium point, the increment (d∣Tc∣) / dΘ of the absolute value of the cogging torque with respect to the rotation angle is usually lower near the stable equilibrium point, and vice versa. However, the extreme value (maximum absolute value) of the cogging torque function generally does not fall on an odd quarter period.

[0016] It can be said that the cogging torque function is generally asymmetric with respect to the rotor rotation angle, and its extreme value is not on an odd quarter period. A phase shift of half a period of this function does not achieve inversion (antiphase), and thus cannot fully compensate for the initial function. Therefore, in order to fully compensate for the cogging torque with an equal but opposite cogging torque, conditions must be found such that after satisfying these conditions, the cogging torque function is symmetric with respect to the rotor rotation angle and its extreme value is also symmetric, or in other words, the cogging torque function can be inverted by a phase shift of half a period.

[0017] Cogging torque is caused by the variation of reluctance above the rotor pole shoes and stator slots. Generally, the relationship between the cogging torque function and the rotation angle depends on the ratio of the stator slot opening and stator tooth angular width to the rotor pole shoe angular width. Due to the rules for creating an electromagnetic efficient magnetic path between the rotor and stator, it is almost impossible for the electromagnetic efficient structure of a motor with a permanent magnet rotor and a slotted stator to deviate from the so-called classical stator structure.

[0018] Among the most important ones are: minimizing the reluctance (the reluctance between the rotor pole shoes and the stator) and the leakage flux (the part of the flux that does not cross the current loop) simultaneously. In addition to the above rules, the size of the stator slot opening also depends on the minimum technical size of the slot opening required for installing the winding (i.e., winding or inserting the winding into the slot). This is usually the most important factor. Therefore, the angular width Θ t of the stator teeth and the angular width Θ o of the slot opening are determined according to the electromagnetic characteristics of the motor and the production process conditions.

[0019] If Θ t and Θ o and their sum Θ Sp (Θ Sp is the stator tooth pitch angle) are predetermined, the only possibility is to control the dependence of the cogging torque function on the rotation angle through the angular width of the rotor pole shoes. According to the idealized or simplified method, the flux direction between the rotor pole shoes and the stator can be considered to be radial (or the direction of the rotation axis in a machine with axial flux), regardless of the actual existing edge effects. This method can determine the effective angular width Θ me of the rotor pole shoes, at which the cogging torque function can be reversed, with a phase shift of half a period 1 / 2Θ c , where Θ c is the period of the cogging torque function, and Θ c = 360° / S, and S is the number of slots in the stator.

[0020] Due to the actual existing edge effects, the actual angular width Θ m of the rotor pole shoes is slightly different from the effective angular width. That is, a part of the magnetic flux will pass through the surfaces of the pole shoe edges and sides, so the angular width (effective angular width) of the pole shoes interacting with the stator is different from the actual angular width. The ratio of the actual angular width of the pole shoes to the effective angular width can be expressed by a correction factor k considering the edge effects, i.e., k = Θ m / Θ me . In an effective structural scheme, the value range of k is from 0.8 to 1.1.

[0021] The present application provides a method for balancing the cogging torque of an electric machine having a permanent magnet rotor and a slotted stator, where the stator has S slots, the rotor has P poles, and the ratio of the number of slots S to the number of poles P is an integer Q. The rotor and the stator include two coaxially arranged components with equal magnetism, where the above-mentioned components of the rotor or the stator are offset from each other by an angle Θ about the axis of rotation. s , where Θ s = 180° / S, and this angle corresponds to a half cycle of the cogging torque function 1 / 2Θ c .

[0022] According to the method of the present application, the actual angular width of the rotor pole shoe can be obtained by the formula Θ m = k·Θ me , where k is a correction coefficient considering the edge effect, and its value is from 0.8 to 1.1, and Θ me is the effective angular width of the rotor pole shoe, which is determined by the formula Θ me = (Q - 1 / 2)Θ Sp , where Θ Sp is the stator tooth pitch angle, and Θ Sp = 360° / S.

[0023] The present application also provides an electric machine having a permanent magnet rotor and a slotted stator with balanced cogging torque, where the stator has S slots, the rotor has P poles, and the ratio of the number of slots S to the number of poles P is an integer Q. The rotor and the stator include two coaxially arranged components with equal magnetism, where the above-mentioned components of the rotor or the stator are offset from each other by an angle Θ about the axis of rotation. s , where Θ s = 180° / S, and this angle corresponds to a half cycle of the cogging torque function 1 / 2Θ c .

[0024] In the above electric machine, the actual angular width Θ m of the rotor pole shoe corresponds to the formula Θ m = k·Θ me , where k is a correction coefficient considering the edge effect, and its value is from 0.8 to 1.1, and Θ me is the effective angular width of the rotor pole shoe, and the corresponding formula is Θ me = (Q - 1 / 2)Θ Sp , where Θ Sp is the stator tooth pitch angle, and Θ Sp = 360° / S.

[0025] By relatively rotating (but not simultaneously rotating) the two components of the rotor or the stator about the axis of rotation by the angle Θ s , an offset of the angle Θ s can occur, and this offset corresponds to a half cycle 1 / 2Θ c of the cogging torque function.

[0026] In the context of the present application, the electric machine is an electric motor or a generator.

[0027] Magnetic pole shoe angular width Θ m Ensure that the cogging torque function is inverted and phase-shifted by half a period, which is usually slightly different from the effective angular width. This difference is due to the fact that a small portion of the magnetic flux reaches the air gap not through the rotating surface of the magnetic pole shoe but from the edges and sides. Therefore, due to the above-mentioned edge effect, the effective angular width of the magnetic interaction is slightly different from the actual magnetic pole angular width. The adjustment of the actual angular width of the magnetic pole shoe corresponding to the effective angular width and the rotor magnetic circuit structure is preferably carried out by gradually approaching the optimal result (i.e., the total cogging torque is as close as possible to zero) through simulating the magnetic circuit structure of the electric machine using the finite element method (FEM). This is represented in the above-mentioned correlation by considering the correction factor k for the edge effect, whose value ranges from 0.8 to 1.1.

[0028] Other schemes for adjusting the actual angular width of the magnetic pole shoe or determining the correction coefficient k can obtain the optimal result, i.e., the total cogging torque is as close as possible to zero, through analytical solutions or trial-and-error methods.

[0029] It should be noted here that due to the inevitable errors in the manufacturing and assembly processes of the electric machine magnetic circuit structure, a small portion of the cogging torque cannot be eliminated. However, if the manufacturing and assembly precision is good, this portion can be ignored.

[0030] The value of the correction factor k considering the edge effect is preferably determined by modeling the magnetic circuit structure of the electric machine using the finite element method.

[0031] The back electromotive force shape of an electric machine with a permanent magnet rotor and a slotted stator having balanced cogging torque (zero cogging torque) enables it to achieve a trapezoidal back electromotive force as well as a sinusoidal back electromotive force. Therefore, the present application can achieve a brushless DC motor with a permanent magnet rotor or a permanent magnet electromagnet, as well as a synchronous motor with a permanent magnet rotor or a permanent magnet electromagnet rotor. It is obvious to those skilled in the art that a DC electromagnet can be used instead of a permanent magnet.

[0032] In terms of structural topology, an electric machine with a permanent magnet rotor and a slotted stator having balanced cogging torque (zero cogging torque) can adopt either an inner rotor fixed on the rotating shaft or an outer rotor. The present application can construct an electric machine having balanced cogging torque or zero total cogging torque and having axial and radial magnetic flux paths. The magnetic poles of the electric machine rotor can be composed of surface-mounted permanent magnets (SMPM) or interior permanent magnets (IPM), or can be composed of electromagnets within the rotor structure.

[0033] In terms of structure, a motor with a permanent magnet rotor and a slotted stator having balanced cogging torque (zero cogging torque) is preferably of a two-phase structure. Among other options, if the number of slots is appropriate, both parts of the stator can be equipped with windings of three or more phases. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present application will now be described with reference to the accompanying drawings, in which:

[0035] Figure 1A A graphical representation of an example of the possible shape of the cogging torque function is shown, in which the stable and unstable equilibrium points and the period Θ of the cogging torque function are marked c as well as the quarter and half periods of the cogging torque function;

[0036] Figure 1B An example of a phase-shifted cogging torque function for an irreversible half period 1 / 2Θc is shown;

[0037] Figure 1C An example of a phase-shifted cogging torque function for a reversible half period 1 / 2Θc is shown;

[0038] Figures 2A to 2F A basic schematic diagram of the motor topology of the present application is shown, in which the ratio Q of the number of slots to the number of magnetic poles is 1 and 2 respectively, and there is an offset between the motor components between the stator and the rotor. The basic schematic diagram of the topology schematically shows the cross-sections of the stator and the rotor. The stator in all the schematic diagrams is of a two-part structure, and the slots, slot openings and stator teeth are schematically shown respectively, and the angular widths Θ of the slot openings and teeth are also marked, o and Θ t . In addition, the offset angle Θ s and the stator tooth pitch angle Θ Sp are also marked. According to the topology, the rotor can be of a single-part or two-part structure and is schematically shown between the stator components. For the magnetic pole shoes, only the effective angular width Θ me is shown. The actual angular width Θ m of the magnetic pole shoes depends on the structural details and is therefore not marked in the basic schematic diagram of the topology ( Figures 2A to 2F ).

[0039] Figure 2A A basic scheme of the motor magnetic circuit topology is shown, in which the offset between the stator parts in the case of a two-part rotor when Q = 1 is shown;

[0040] Figure 2B A basic scheme of the motor magnetic circuit topology is shown, in which the offset between the stator parts in the case of a two-part rotor when Q = 2 is shown;

[0041] Figure 2CThe basic scheme of the motor magnetic circuit topology is shown, where the offset between the stator parts is shown for the single-part rotor case when Q=1;

[0042] Figure 2D The basic scheme of the motor magnetic circuit topology is shown, where the offset between the stator parts is shown for the single-part rotor case when Q=2;

[0043] Figure 2E The basic scheme of the motor magnetic circuit topology is shown, where the offset between the rotor components is shown when Q=1;

[0044] Figure 2F The basic scheme of the motor magnetic circuit topology is shown, where the offset between the rotor components is shown when Q=2;

[0045] Figure 3A , Figure 3B and Figure 3C The exemplary embodiments of the two-phase brushless DC motor produced by the method of the present application at the corresponding sections AA, BB and CC are respectively shown;

[0046] Figure 4A , Figure 4B , Figure 4C and Figure 4D An exemplary embodiment of a two-phase brushless DC motor with axial magnetic flux produced by the method of the present application at corresponding sections AA, BB, CC and DD is shown, wherein sections AA and BB are obtained at the air gap of the motor, and section DD is obtained from the central part of the rotor;

[0047] Figure 5A , Figure 5B and Figure 5C A possible exemplary embodiment of a rotor is shown;

[0048] Figure 6 This is a back electromotive force diagram of a two-phase brushless DC motor including a permanent magnet rotor with balanced cogging torque according to the first embodiment of the present application. DETAILED DESCRIPTION

[0049] For purposes of clarity, similar details and elements are indicated by the same reference numerals in different figures.

[0050] According to the method of the present application, a brushless DC motor is constructed, which has a permanent magnet rotor and a slotted stator and has radial flux, wherein the stator of the motor has eight slots and eight magnetic poles (S=8, P=8). The basic topology of the motor is as follows Figure 2A That is, the ratio of the number of slots and the number of poles of the motor is 1 (Q = 1), and the cogging torque function angle θ is staggered by half a cycle between the components of the motor and between the stators. s =1 / 2θc = 180° / S = 22.5°, and the rotor consists of two parts, but there is no offset between these two parts.

[0051] Figures 3A to 3C The structure of the above-mentioned motor is shown in simplified views of three cross-sections. For clarity, the windings of the motor, some fastening parts, bolts, hole diameters, and possible position sensors of the rotor are omitted. The stator components 1a and 1b are fixed on their respective separate housing components 5c, offset by 22.5° relative to each other. The "mushroom-shaped" magnetic pole shoes of the rotors 2a and 2b are fixed on the permanent magnets 3a and 3b of the two halves of the rotor respectively.

[0052] The effective angular width of the magnetic pole shoes is 22.5°, and the actual angular width of the curved surface of the rotating surface of the pole shoes is 21.875°, thus ensuring the above-mentioned effective angular width. The flux conductor of the rotor 4 is shared by the two halves of the rotor, and the permanent magnet groups 3a and 3b of the two halves of the rotor are both fixed thereon. The longitudinal distance between the permanent magnet groups 3a and 3b is the same as the distance between the stator components 1a and 1b. The flux conductor of the rotor 4 is firmly fixed on the shaft 6, and the shaft 6 can rotate freely on the ball bearings 7a and 7b, while the ball bearings 7a and 7b are fixed in the bearing seats of the housing components 5a and 5b (end covers).

[0053] Figures 3A to 3C A brushless DC (BLDC) motor with radial flux, a constant air-gap size of 0.5 mm, and a rotor structure using permanent magnets is shown. It has P = 8 magnetic poles on the two halves of the rotor and the same number of stator teeth on the two halves of the stator. The tooth width of each stator tooth is 43° (the number of slots S = 8, the stator tooth pitch angle Θ Sp = 45°, and the ratio of the number of slots to the number of magnetic poles Q = S / P = 1). The period of the uncompensated cogging torque function generated on the two halves of the motor is Θ c = 360° / 8 = 45°.

[0054] To ensure that when the rotor rotates 45° / 4 = 11.25° from the stable equilibrium position, the uncompensated cogging torque reaches the maximum value and the cogging torque function of the rotation angle is symmetric about the extreme value, the effective angle of the magnetic pole shoes of the rotor must be Θ me = (Q - 1 / 2)Θ Sp = 1 / 2×45° = 22.5°. The two halves of the stator are fixed to each other, and the offset angle Θ s = 45° / 2 = 22.5°, corresponding to half of the period of the uncompensated cogging torque, thus ensuring the effect of mutual compensation of the cogging torque applied to the rotor components on the same shaft. The above-mentioned offset can also be carried out between the two halves of the rotor, but in this example, due to structural reasons, the offset is carried out between the stator components.

[0055] Therefore, the shape of the pole shoes of the rotor should ensure that almost all magnetic fluxes flow radially through the rotating surface of the pole shoes, thereby minimizing the mutual influence between the pole shoes and the stator teeth through the side surfaces of the pole shoes. To this end, the cross-section of the pole shoes should be in the shape of a "mushroom". In addition, the actual angular width of the pole shoes of the rotor should be slightly smaller than the effective angular width.

[0056] This magnetic circuit structure has been gradually approximated by computer simulation using the finite element method, achieving the best results. It has been found that under the selected magnetic circuit geometry, the actually adjusted angular width Θ of the pole shoes m should be 21.875°, that is, the correction factor k = 0.97(2): Θ m = Θ me ·k = 22.5°·0.97(2) = 21.875°.

[0057] To achieve this result, first, the uncompensated cogging torque function must be found and the actual value of 22.5° of the pole shoes is used in the calculation model (using FEM software). On this basis, by performing a half-period phase shift on the initial cogging torque function and then adding the initial function and the phase-shifted function, the compensated (balanced) cogging torque function is obtained. It has been found that when the pole shoe angle is the given value (22.5°), the deviation of the compensated (balanced) cogging torque function from zero is greater than the simulation accuracy. In each iteration, the above process must be continued with two decreasing step sizes, where the actual pole shoe angles of the rotor are: 22.0°; 21.75°; 21.875°. For the last actual angle (21.875°), the difference between the compensated (balanced) cogging torque function and zero does not exceed the simulation accuracy. Therefore, this angular width can be regarded as the actual angular width of the pole shoes during motor manufacturing, and the correction factor k considering the edge effect is determined, and its value is 0.97(2).

[0058] It should be noted here that the rotor structure described above is only an example and not the only one. For example, surface-mounted permanent magnets (SMPMs) can also be used, or permanent magnets or permanent magnet electromagnets located in the rotor structure, which are different from the described solution. Figures 5A to 5C Some non-exhaustive examples are provided, which show the radial flux, the inner rotor, and different numbers of pole pairs.

[0059] Therefore, it is only necessary to ensure that in the selected magnetic circuit structure, the actual angle of the pole shoes corresponds to the correct effective angle, which can be achieved by adjusting the actual dimensions and geometries of the pole shoes by trial and error, or preferably by computer simulation using the finite element method to gradually approximate the best results.

[0060] In a preferred embodiment of the motor, both halves of the stator are equipped with single-phase windings, which can be wound into concentrated windings or waveform windings.Figure 6 The shape of the back electromotive force in the two-phase structure of the motor is given. In the preferred operation of the motor, current commutation occurs in each phase winding through two H-bridges, one corresponding to each phase. In the simplest case, direct commutation can be used to control the H-bridge transistors, which are controlled by a rotor position sensor, such as a Hall effect sensor or any other sensor that transmits the position of the magnetic pole shoes. The commutation of the phase windings can also be controlled without a rotor position sensor (sensorless control).

[0061] To minimize commutation torque ripple, a microprocessor can be used for advance angle control.

[0062] The main advantage of the motor described in this application is the absence of cogging torque, that is, the total cogging torque generated is zero, which makes the motor operate smoothly and quietly and enables it to be applied in situations where it cannot be applied due to the presence or existence of cogging torque. For example, the motor can be used as a generator in a wind turbine. Due to the absence of cogging torque, the minimum wind speed of the wind turbine can be significantly reduced, thereby increasing the capacity factor. In addition to the absence of cogging torque, the motor constructed according to this application also has more excellent electromagnetic characteristics than other known zero-cogging torque motors (slotless and / or coreless or non-core motors), and thus has a higher specific power.

[0063] The additional advantages of the brushless DC motor with balanced cogging torque (zero cogging torque) are as follows:

[0064] – High specific power;

[0065] – Due to the relatively large distance (gap) between the magnetic pole shoes, the machine has good resistance to ferromagnetic dust, and thus can utilize the ventilation air to cool through the inner surface of the machine structure;

[0066] – The commutation torque ripple is negligible or non-existent (when equipped with a relevant control system).

[0067] Based on the method of this application, another example of a brushless DC motor is constructed, which has a permanent magnet rotor and a slotted stator and has axial (along the direction of the rotation axis) magnetic flux. The motor has twelve magnetic poles and twelve stator slots (P = 12, S = 12). The basic topology of the motor is as Figure 2C shown. That is, Q = S / P = 1, and the cogging torque function or angle Θ between the stator components is offset by half a cycle s = 180° / S = 15°. The rotor is a single component, and the surface of the permanent magnet facing the stator component acts as the magnetic pole shoe. The effective angular width Θ me of the latter is 15°.

[0068] For clarity, Figures 4A to 4DThe above motor is simplified. In the above structural diagram, the windings of the motor, some fastening details, bolts, hole diameters, and possible position sensors of the rotor are omitted. The stator components 1a and 1b are respectively fastened to the housing components 5a and 5b, staggered by 15° from each other. A set of permanent magnets 3 (consisting of twelve ring-segment magnets) is fastened in the structure of the rotor 8, and the rotor 8 is rigidly fixed to the shaft 6. The shaft 6 can rotate freely on the ball bearings 7a and 7b, and the ball bearings 7a and 7b are installed in the bearing seats of the housing components 5a and 5b.

[0069] The axial-flux motor has similar technical advantages and characteristics to the above radial-flux motor. Due to the shorter axial-flux path compared to the radial-flux path, the specific power (power density) and driving torque inherent in the axial-flux motor are slightly higher. The number of stator cores of this motor is relatively small, and the rotor magnetic circuit consists only of permanent magnets.

[0070] List of reference numbers

[0071] 1a, 1b Stator components

[0072] 2a, 2b Pole shoes

[0073] 3 Set of permanent magnets

[0074] 3a, 3b Set of permanent magnets

[0075] 4 Magnetic flux conductors

[0076] 5a, 5b, 5c Housing parts

[0077] 6 Shaft

[0078] 7a, 7b Ball bearings

[0079] 8 Rotor

Claims

1. A method for balancing the cogging torque of an electric machine, the electric machine having a permanent magnet rotor and a slotted stator, the stator having S slots, the rotor having P magnetic poles, the ratio of the number of slots S to the number of magnetic poles P being an integer Q, the rotor and the stator comprising two coaxially aligned components of equal magnetism, the coaxially aligned components of the rotor or the stator being offset from each other by an angle Θ about the axis of rotation s , where Θ s = 180° / S, characterized in that The actual angular width of the pole shoes of the rotor corresponds to the equation Θ m = k·Θ me , where k is a correction factor taking into account the edge effect, and its value is from 0.8 to 1.1, and Θ me is the effective angular width of the pole shoes of the rotor and corresponds to the equation Θ me = (Q - 1 / 2)Θ Sp , where Θ Sp is the stator tooth pitch angle, and Θ Sp = 360° / S.

2. The method according to claim 1, characterized in that, The rotor includes two parts, and the two parts are offset from each other by an angle Θ on the rotation axis s .

3. The method according to claim 1, wherein The stator includes two parts, and the two parts are offset from each other by an angle Θ on the rotation axis s .

4. The method according to any one of the preceding claims, characterized in that, The value of the correction coefficient k considering the edge effect is determined by modeling the magnetic circuit structure of the motor using the finite element method.

5. A motor, the motor having a balanced cogging torque, the motor having a permanent magnet rotor and a slotted stator, the stator having S slots, the rotor having P poles, the ratio of the number of slots S to the number of poles P being an integer Q, the rotor and the stator including two coaxially aligned components of equal magnetism, the coaxially aligned components of the rotor or the stator being offset from each other by an angle Θ about the axis of rotation s , where Θ s = 180° / S, characterized in that The actual angular width Θ of the pole shoes of the rotor m corresponds to the equation Θ m = k·Θ me , where k is a correction factor taking into account the edge effect, and its value is from 0.8 to 1.1, and Θ me is the effective angular width of the pole shoes of the rotor and corresponds to the equation Θ me = (Q - 1 / 2)Θ Sp , where Θ Sp is the stator tooth pitch angle, and Θ Sp = 360° / S.

6. The motor according to claim 5, characterized in that, The rotor includes two parts, and the two parts are offset from each other by an angle Θ on the rotating shaft s .

7. The motor according to claim 5, characterized in that, The stator includes two parts, and the two parts are offset from each other by an angle Θ about the rotation axis s .

8. The electric machine according to any one of claims 5 to 7, characterized in that The motor is an electric motor.

9. The electric machine according to any one of claims 5 to 7, characterized in that The motor is a generator.

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