Method for cancelling cogging torque in an electric machine with a permanent magnet rotor and a slotted stator, and associated electric machine in which cogging torque is cancelled

Abstract

The present invention provides a method for canceling cogging torque in an electric machine having a permanent magnet rotor and a slotted stator, and a corresponding electric machine in which the cogging torque is canceled. According to the present invention, the actual angular width of the pole pieces of the rotor of the electric machine is calculated using the equation Θ m =k Θ me where k is a correction factor taking into account edge effects, with a value of 0.8...1.1, and Θ me is the effective angular width of the rotor pole piece, and the equation Θ me =(Q-1 / 2)Θ Sp where Θ Sp is the stator pitch angle, Θ Sp = 360° / S, and Q is the ratio of the number of slots S to the number of magnetic poles P, which is an integer, so Q = S / P. The value of the correction coefficient k that takes into account the edge effect can be found by modeling the structure of the magnetic circuit of the electric machine using the finite element method.

Description

[Technical Field]

[0001] The present invention relates to the field of electric machines, more particularly to electric machines in which permanent magnets or DC electromagnets are used in the rotor and which have a stator made of ferromagnetic material and in whose slots stator windings are mounted.The object of the present invention is to provide a method for canceling and eliminating the total cogging torque of an electric machine without impairing its electromagnetic properties, in particular in such an electric machine in which the ratio Q between the number of slots S of the stator and the number of magnetic poles P of the rotor is an integer. [Background technology]

[0002] A major drawback of electric machines with permanent magnet rotors, which can be brushless direct current (BLDC) electric machines or permanent magnet synchronous motors / generators (PMSM / G) with slotted ferromagnetic stators, is the generation of cogging torque in these machines. Cogging torque results from the interaction of the rotor's permanent magnets with the stator slots by changing the reluctance of the electric machine above the slot openings. The permanent magnet rotor is located at a position where the reluctance and interaction energy of the system between the rotor and stator are minimum, thus creating a stable equilibrium state for the system.

[0003] At the stable equilibrium position, the cogging torque is zero, and around this position, any external torque that moves the rotor away from its position is counteracted, acting on the rotor toward the stable equilibrium position. Between two stable equilibrium positions, there are unstable equilibrium points that also have zero cogging torque. A torque disturbance applied to the rotor at an unstable equilibrium point will cause the rotor to move back toward the stable equilibrium position due to the generated cogging torque, which may precede or follow the unstable equilibrium position.

[0004] Cogging torque is an undesirable phenomenon in the operation of most electric machines, except for stepper motors, as it causes harmful vibrations, noise, higher starting torque, uneven motion, torque ripple, etc. Therefore, several methods have been developed to reduce cogging torque.

[0005] However, most known methods of reducing cogging torque in the prior art cause a degradation of the electromagnetic properties of the electric machine and often also cause structural complexity, resulting in more expensive and complex manufacturing, so that most known methods only reduce the cogging torque of slotted electric machines to a certain extent, but do not eliminate it.

[0006] Various patent applications also describe methods or installations in which a cogging torque generated in one part of an electric machine is counteracted by a cogging torque of the same magnitude generated in another part of the electric machine about a common axis of rotation but in the opposite direction. Such solutions are known from the documents listed below.

[0007] Canadian Patent Application Publication No. 2711543 (CA2711543A1) describes a known solution for reducing cogging torque in electric machines with permanent magnets. In this solution, a permanent magnet-based component, i.e., a cogging torque compensator, is added to the electric machine, which is electromagnetically unnecessary for operation. The rotor components of the compensator, in addition to the rotor of the electric machine, have inertial mass, which increases the rotor's inertia and therefore the starting torque. This solution is more expensive and less practical than an integrated electric machine, in which the structural components that compensate for the cogging torque also operate electromagnetically efficiently (i.e., generate a driving torque in motor operation or an electromotive force and current in generator operation). This solution allows for a reduction in the total cogging torque, but does not eliminate it.

[0008] The main disadvantage of the described method is that it does not offer a clear, unambiguous and reproducible solution for designing the shape of the compensator's cogging torque function as a function of the rotation angle to reverse (compensate) the cogging torque of the electric machine.

[0009] The solution described in Taiwan Patent Application Publication No. TW201234739 (TW201234739A) only reduces cogging torque, but does not eliminate it. This is due to the lack of a solution for the effective angle of the rotor pole pieces. While the necessary condition for compensating for cogging torque—a mechanical shift of the uncompensated cogging torque function by a half-period in the two halves of the electric machine (in the stator or rotor)—is met, it is not sufficient. Furthermore, the described generator structure includes essentially two electromagnetically independent generators, i.e., essentially two electric machines with respect to a common shaft, rather than an integrated electric machine, whose two halves complement each other in operation.

[0010] The solution described in Chinese Utility Model No. 212258740 (CN212258740U) does not provide any explanation or specification for the angle of the rotor pole pieces (number of slots) according to the stator pitch angle. Similar to the description in Taiwan Patent Application Publication No. TW201234739 (TW201234739A), the necessary condition for cogging torque compensation, i.e., a mechanical shift of the uncompensated cogging torque function by half a period in the two halves of the electric machine (in the stator or rotor), is met, but this alone is not sufficient. A further disadvantage is that cogging torque reduction is only possible for certain motor configurations, while the description does not provide a solution for various types of electric machines with permanent magnet rotors.

[0011] The solution described in U.S. Patent Application Publication No. 2020 / 0153367 (2020 / 0153367A1) reduces cogging torque but does not fully compensate for it. In this solution, an even number of electric motors are connected to a common shaft. This is not a single, integrated electric machine solution, where the various halves of the electric machine simultaneously participate in the operation of the electric machine and thereby mutually compensate for the cogging torque.

[0012] Modern, industrially manufactured electric machines with permanent magnet rotors always have one or another method for reducing the applied cogging torque. The most common method is the use of machines with fractional slots (i.e., the ratio of slots to magnetic poles is not an integer). Without the application of a method or combination of methods for reducing the cogging torque, the electric machine would practically be impossible to operate, i.e., the cogging torque would be excessively high, comparable to the maximum drive torque, and therefore the machine vibrations would practically be unacceptable in all respects. Therefore, the above methods can only be treated as additional methods for reducing the cogging torque that is already acceptable in existing electric machines.

[0013] All of the above documents describe the phase shift required to compensate and eliminate the cogging torque by half a period of the uncompensated cogging torque function between rotor or stator parts of a machine or between two machine parts (stators or rotors) moving about a common shaft.

[0014] As a result, none of these documents describe or present a clear and comprehensive solution to the problem that the stated shifts between machine parts alone are not sufficient to eliminate and / or compensate for the cogging torque in an electric machine. All of the above documents describe conditions that are necessary (required) but not sufficient to eliminate (cancel) the total cogging torque in an electric machine. The present invention provides a new method for making an electric machine with a permanent magnet rotor and slotted stator in which the cogging torque is canceled (cogging torque is zero). [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Canadian Patent Application Publication No. 2711543 (CA2711543A1) [Patent Document 2] Taiwan Patent Application Publication No. TW201234739 (TW201234739A) [Patent Document 3] Chinese Utility Model Patent No. 212258740 (CN212258740U) [Patent Document 4] U.S. Patent Application Publication No. 2020 / 0153367 (2020 / 0153367A1) Summary of the Invention [Problem to be solved by the invention]

[0016] For a uniform distribution of stator slots and rotor poles, the cogging torque is a periodic function of the rotor's rotation angle. After each half-period, when the period is doubled, the cogging torque is equal to zero. This occurs at alternating stable and unstable equilibrium points. As the rotation angle (d|Tc|) / dΘ is changed, the increase in the absolute value of the cogging torque can generally be lower around stable equilibrium points than around unstable equilibrium points, or vice versa. However, the extrema (maximum absolute value) of the cogging torque function generally do not occur on odd-numbered quarter-periods.

[0017] In general, it can be said that the cogging torque function is asymmetric with respect to its extrema with respect to the rotor rotation angle, i.e., the extrema are not located at odd quarter periods. A phase shift of such a function by half a period does not result in an inversion (opposite phase), which can completely compensate for the initial function. Therefore, in order to completely compensate for the cogging torque with an equal but opposite cogging torque, when the condition is met, the cogging torque function from the rotation angle must be symmetric with respect to its extrema, or in other words, the cogging torque function can be inverted by a phase shift of half a period.

[0018] Cogging torque is generated by changing the reluctance between the rotor pole pieces and the stator above the slot openings. In general, the dependence of the cogging torque function on the angle of rotation is determined by the ratio of the angular width of the stator slot openings and stator teeth to the angular width of the rotor pole pieces. Electromagnetically efficient construction of an electric machine with a permanent magnet rotor and slotted stator has little, if any, possibility of deviation from the so-called typical construction of the stator, due to the requirement of forming an electromagnetically efficient magnetic circuit between the rotor and stator.

[0019] The most important of these is to simultaneously minimize the magnetic reluctance (between the rotor pole pieces and the stator) and the leakage flux (the part of the magnetic flux that does not cross the current loop). In addition to the above regulations, the size of the stator slot opening is also determined by the minimum technically required dimensions of the slot opening for winding installation (i.e., winding or inserting the winding into the slot). This is often the most important factor. Therefore, the angular width Θ of the stator teeth t and the angular width of the slot opening Θ o is determined based on the electromagnetic characteristics of the electric machine and the manufacturing technology conditions.

[0020] Predetermined Θ t and Θ o and their sum Θ Sp (ΘSp is the stator pitch angle), the only possibility is to control the dependence of the cogging torque function on the angle of rotation by the angular width of the rotor pole pieces. In an idealized or simplified approach, the direction of the magnetic flux between the rotor pole pieces and the stator can be considered to be radial (or along the axis of rotation in machines with axial flux), but this does not take into account the edge effects that actually exist. Such an approach is based on the effective angular width Θ of the rotor pole pieces. me This makes it possible to obtain the effective angular width Θ me The cogging torque function has a half period of 1 / 2Θ c can be inverted by a phase shift of Θ c is the period of the cogging torque function, and Θ c =360° / S, where S is the number of slots in the stator.

[0021] Actual angular width of rotor pole pieces Θ m is somewhat different from the effective angular width due to the edge effect that actually exists. That is, part of the magnetic flux passes through the surface at the edge and side of the pole piece, and therefore the angular width of the pole piece that causes interaction with the stator (effective angular width) is different from the actual one. The ratio of the actual angular width of the pole piece to the effective angular width can be expressed by a correction factor k that takes into account the edge effect, where k=Θ m / Θ me For efficient structural solutions, the value of k is in the range of 0.8 to 1.1. [Means for solving the problem]

[0022] The present invention provides a method for cancelling cogging torque in an electric machine with a permanent magnet rotor and a slotted stator, the stator having a number of slots S and the rotor having a number of magnetic poles P, the ratio of the number of slots S to the number of magnetic poles P being an integer Q. The rotor and the stator comprise two magnetically identical coaxial parts, said rotor parts or stator parts being at an angle Θ with respect to the axis of rotation relative to each other. s Θ s = 180° / S, which is the half period of the cogging torque function, 1 / 2Θc Corresponds to.

[0023] In accordance with the method of the present invention, the actual angular width of the rotor pole pieces is calculated by the equation Θ m =k Θ me where k is a correction factor taking into account edge effects, with a value of 0.8...1.1, and Θ me is the effective angular width of the rotor pole piece, and the equation Θ me =(Q-1 / 2)Θ Sp where Θ Sp is the stator pitch angle, Θ Sp =360° / S.

[0024] The present invention also provides an electric machine with a permanent magnet rotor and a slotted stator, in which cogging torques are cancelled, the stator having a number of slots S and the rotor having a number of magnetic poles P, the ratio of the number of slots S to the number of magnetic poles P being an integer Q. The rotor and the stator comprise two magnetically identical coaxial parts, said rotor parts or stator parts being at an angle Θ with respect to the axis of rotation relative to each other. s Θ s = 180° / S, which is the half period of the cogging torque function, 1 / 2Θ c Corresponds to.

[0025] In the above-described electric machine, the actual angular width Θ of the rotor pole pieces m But the equation Θ m =k Θ me where k is a correction factor taking into account edge effects, with a value of 0.8...1.1, and Θ me is the effective angular width of the rotor pole piece, and the equation Θ me =(Q-1 / 2)Θ Sp where Θ Sp is the stator pitch angle, Θ Sp =360° / S.

[0026] Half-period of the cogging torque function 1 / 2Θ c The angle Θ corresponding tos The minute shift occurs in either the rotor or the stator, but not in both simultaneously, due to the rotation of the two parts relative to each other through the angle mentioned above about the axis of rotation.

[0027] In the context of the present invention, an electric machine is an electric motor or a generator.

[0028] The angular width Θ of the pole pieces ensures a half-period phase shift and inversion of the cogging torque function m is generally somewhat 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 from the edges and sides of the pole pieces rather than through the rotational plane of the pole pieces, and therefore the effective angular width of the magnetic interaction differs somewhat from the actual angular width of the pole pieces due to the edge effects mentioned above. The adjustment of the actual angular width of the rotor pole pieces and the structure of the rotor's magnetic circuit to correspond to the effective angular width of the pole pieces is preferably done by gradual approximation to the best result of a total cogging torque as close to zero as possible by modeling the structure of the magnetic circuit of the electric machine using the finite element method (FEM). This is expressed in the above correlation with the correction factor k, which takes into account the edge effect, and whose value lies in the range of 0.8 to 1.1.

[0029] Adjusting the actual angular width of the pole pieces or other optional options for determining the correction factor k can be an analytical solution or trial and error approach, with the best result being a total cogging torque as close to zero as possible.

[0030] In this regard, it should be noted that due to unavoidable imprecision during the manufacture and assembly of the structure of the magnetic circuit of the electric machine, a small part of the cogging torque cannot be eliminated, however, with good manufacturing and assembly precision this component is negligible.

[0031] The value of the correction factor k, which takes into account edge effects, is preferably determined by modelling the structure of the magnetic circuit of the electric machine by means of the finite element method.

[0032] The back EMF shape of an electric machine with a permanent magnet rotor and a slotted stator, where the cogging torque is canceled (zero cogging torque), allows for the realization of electric machines with trapezoidal and sinusoidal back EMF waveforms. Therefore, the present invention allows for the realization of brushless DC electric machines with permanent magnet rotors or permanent electromagnets, and synchronous machines with permanent magnet rotors or permanent electromagnet rotors. It will be apparent to those skilled in the art that DC-driven electromagnets can be used instead of permanent magnets.

[0033] Regarding the structural topology, an electric machine with a permanent magnet rotor and a slotted stator, in which the cogging torque is canceled (zero cogging torque), can be realized with an inrunner rotor fixed to the rotating shaft and with an outrunner rotor. The present invention enables the construction of an electric machine with axial and radial flux paths, in which the cogging torque is canceled or the total cogging torque is zero. The magnetic poles of the rotor of the electric machine can be formed by surface mounted permanent magnets (SMPM) or interior permanent magnets (IPM), or by electromagnets within the rotor structure.

[0034] In terms of construction, an electric machine with a permanent magnet rotor and slotted stator, where the cogging torque is cancelled (zero cogging torque), preferably has a two-phase construction. Optionally, with an appropriate number of slots, both parts of the stator can be equipped with windings with three or more phases.

[0035] The invention will now be described with reference to the accompanying schematic drawings. [Brief explanation of the drawings]

[0036] [Figure 1A] FIG. 1 shows examples of diagrams of possible shapes of the cogging torque function, along with indications of stable and unstable equilibrium points, the period Θc of the cogging torque function, and the quarter and half periods of the cogging torque function. [Figure 1B] This figure shows an example of an irreversible cogging torque function due to a half-period 1 / 2 Θc phase shift. [Figure 1C] This figure shows an example of a reversible cogging torque function with a half-period 1 / 2 Θc phase shift. [Figure 2A] 2A-2F show basic diagrams of the topology of an electric machine of the present invention, with shifts between parts in the stator or rotor of the machine for slot-to-pole ratios Q of 1 and 2. The basic diagrams of the topology show schematic cross sections of the stator and rotor. The stator is two-part in all figures, while the slots, slot openings, and stator teeth are shown schematically, with indications of the slot opening angular width Θo and tooth angular width Θt, respectively. The shift angle Θs is also shown, as is the stator pitch angle Θsp. Depending on the topology, the rotor is single-part or two-part, and is shown schematically between the stator parts. Only the effective angular width Θme of each pole piece is shown. The actual angular width Θm of the pole piece depends on the construction details and is therefore not shown in the basic diagrams of the topology (FIGS. 2A-2F). These diagrams show basic diagrams of the topology of the magnetic circuit of an electric machine, with the shifts between the stator parts shown for a two-part rotor for Q=1. [Figure 2B] FIG. 1 shows a basic diagram of the topology of the magnetic circuit of an electric machine, showing the shift between the stator parts in the case of a two-part rotor, for Q=2. [Figure 2C] FIG. 1 shows a basic diagram of the topology of the magnetic circuit of an electric machine, showing the shift between the stator parts in the case of a single rotor, for the case Q=1. [Figure 2D]FIG. 1 shows a basic diagram of the topology of the magnetic circuit of an electric machine, showing the shift between the stator parts in the case of a single rotor, for Q=2. [Figure 2E] FIG. 1 shows a basic diagram of the topology of the magnetic circuit of an electric machine, showing the shift between the rotor parts, for the case Q=1. [Figure 2F] FIG. 1 shows a basic diagram of the topology of the magnetic circuit of an electric machine, showing the shift between the rotor parts for Q=2. [Figure 3] 2A-2B show cross sections AA, CC, and BB of an exemplary embodiment of a two-phase brushless DC electric machine manufactured according to the method of the present invention. [Figure 4] 1 shows an exemplary embodiment of a two-phase brushless DC electric machine with axial flux manufactured by the method of the present invention in cross section AA taken at the air gap of the electric machine, in cross section CC, in cross section BB taken at the air gap of the electric machine, and in cross section DD taken from the central cross section of the rotor. [Figure 5] 1 shows a possible exemplary embodiment of a rotor; [Figure 6] FIG. 2 shows a back EMF diagram of a two-phase brushless DC electric machine with a permanent magnet rotor, with cogging torque cancellation, as described in a first exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0037] For purposes of clarity, similar details and elements are designated with the same reference numbers in the various drawings.

[0038] According to the method of the present invention, a brushless DC electric machine with a permanent magnet rotor and a slotted stator with radial magnetic flux is constructed with eight slots and eight magnetic poles (S=8, P=8) in the stator. The basic topology of the electric machine corresponds to Fig. 2A. That is, the slot-to-pole ratio of the machine is 1 (Q=1), and the cogging torque function angular shift between the machine parts by half a period, i.e. Θ s =1 / 2Θ c =180° / S=22.5° between the stator parts, and the rotor consists of two parts, but there is no shift between them.

[0039] The structure of the electric machine described above is shown in simplified diagrams in three cross sections in Figures 3A-3C, with the windings of the electric machine, some fastening details of the rotor, bolts, holes and possible position sensors omitted for clarity. Stator parts 1a and 1b are fastened to their separate housing parts 5c with a 22.5° shift relative to each other. The "mushroom" pole pieces of rotors 2a and 2b are fastened to permanent magnets 3a and 3b in both halves of the rotor.

[0040] The effective angular width of the pole pieces is 22.5°, and the actual angular width of the curve of the rotation surface of the pole pieces is 21.875°, ensuring the above-mentioned effective angular width. The rotor flux conductor 4 is common to both rotor halves, and the permanent magnet sets 3a and 3b of both rotor halves are fastened to the flux conductor, so that the longitudinal distance between magnet sets 3a and 3b is the same as the distance between stator parts 1a and 1b. The rotor flux conductor 4 is rigidly fastened to shaft 6, which can rotate freely about ball bearings 7a and 7b, which are also fastened in bearing sockets (end covers) of housing parts 5a and 5b.

[0041] 3A-3C show the radial magnetic flux with a constant air gap size of 0.5 mm and a rotor structure with P = 8 magnetic poles on both halves of the rotor and an equal number of stator teeth (number of slots S = 8, stator pitch angle Θ ) with an angular width of 43° on both halves of the stator. Sp = 45° and the ratio of the number of slots to the number of poles (Q = S / P = 1). The period of the uncompensated cogging torque function generated in both parts of the machine is Θ c =360° / 8=45°.

[0042] To ensure that the uncompensated cogging torque reaches its maximum when the rotor is rotated by 45° / 4=11.25° from the stable equilibrium position, and that the cogging torque function from the rotation angle is symmetric about the extremum, the effective angle of the rotor pole pieces is Θ me =(Q-1 / 2)Θ Sp = 1 / 2 45° = 22.5°. The shift angle Θ, which corresponds to a half-period of the uncompensated cogging torque, must be s = 45° / 2 = 22.5°, ensuring the effect of mutually compensating cogging torques applied to rotor parts fixed relative to each other and located on the same shaft. The above-mentioned shifting can also be done between rotor halves, but in this case the shifting is done between stator parts for constructional reasons.

[0043] Therefore, the shape of the rotor pole pieces should be such that practically all of the magnetic flux flows radially through the surface of rotation of the pole pieces to minimize interaction between the pole pieces and the stator teeth passing through the sides of the pole pieces. For this reason, the cross section of the pole pieces should have, for example, a "mushroom" shape. Also, the actual angular width of the rotor pole pieces should be made somewhat smaller than the effective angular width.

[0044] This magnetic circuit structure has been successfully approximated by computer simulation using the finite element method, and the actual adjusted angular width Θ of the pole pieces for the selected geometry of the magnetic circuit has been obtained. mis 21.875°, i.e., the correction factor k=0.97(2):Θ m =Θ me It was found that k = 22.5° 0.97(2) = 21.875°.

[0045] To achieve this result, first, the uncompensated cogging torque function had to be found in the computing model (using FEM software) using the actual value of the pole pieces at 22.5°, based on which the compensated (cancelling) cogging torque function was obtained by performing a half-period phase shift by the initial cogging torque function, and then by summing the initial phase-shifted function. Since it was found that for a given pole piece angle (22.5°), the compensated (cancelling) cogging torque function differed from zero more than the accuracy of the simulation, the described process had to be continued in each iteration by twice decreased step for the actual angle of the rotor pole pieces, i.e., 22.0°; 21.75°; 21.875°. For the last actual angle (21.875°), the compensation (cancellation) cogging torque function did not differ from zero by more than the accuracy of the simulation, so this angle width can be considered the actual angle width of the pole piece when manufacturing the electric machine, and the correction factor k to take into account the edge effect can be determined, and the value of the correction factor k was 0.97(2).

[0046] It should be noted that the rotor structures described are examples of possible implementations and are not exclusive. For example, it is possible to use surface-mounted permanent magnets (SMPMs) or permanent magnets or permanent electromagnets arranged in rotor configurations different from the described solutions, and therefore several non-exhaustive examples with radial flux, inrunner rotors, and different numbers of magnetic poles are presented in Figures 5A-5C.

[0047] In this regard, it is only essential to ensure that in the selected structure of the magnetic circuit, the actual angle of the pole pieces will correspond to the correct effective angle, which should be achieved by adjusting the actual size and geometry of the pole pieces by trial and error methods or preferably by computer simulation using finite element methods for progressive approximation to the best result.

[0048] In a preferred solution of the described electric machine, both halves of the stator are equipped with a single-phase winding, which can be wound as a concentrated winding or a wave winding. The shape of the back EMF in a two-phase configuration of the electric machine is presented in Figure 6. In the preferred operation of the motor, current commutation is performed in each phase winding by two H-bridges, one for each phase. In the simplest case, DC commutation can be used to control the transistors of the H-bridge, which are controlled by a rotor position sensor, e.g., a Hall effect sensor or any other sensor that transmits the position of the pole pieces. The commutation of the phase windings can also be controlled without a rotor position sensor (sensorless control).

[0049] To minimize commutation torque ripple, microprocessor-based advance angle control can be used.

[0050] A major advantage of the described electric machines of the present invention is the absence of cogging torque, i.e., zero resulting total cogging torque, which results in smoother and quieter operation of the electric machine and allows the electric machine to be used in applications that would be precluded if cogging torque were present or influential. For example, the described electric machines can be used as generators in wind turbines, where the absence of cogging torque essentially minimizes the minimum wind speed of the wind turbine, thus allowing for higher capacity factors. In addition to the absence of cogging torque, electric machines constructed in accordance with the present invention also have much better electromagnetic properties and therefore much higher specific power output than other known electric machines with zero cogging torque, i.e., slotless and / or ironless or coreless electric machines.

[0051] A further advantage of the described brushless DC electric machine is that the cogging torques are cancelled (cogging torque is zero). - high specific power, - the large distance (gap) between the pole pieces makes the machine highly resistant to ferromagnetic dust, which allows cooling by circulating air also through the internal surfaces of the machine structure; - Insignificant or weak commutation torque ripple (if equipped with relevant control system).

[0052] Another example of a brushless DC electric machine has been constructed based on the method of the present invention, with a permanent magnet rotor and a slotted stator, with axial (in the direction of the axis of rotation) magnetic flux. This machine has 12 magnetic poles and 12 slots in the stator (P=12, S=12). The basic topology diagram of the machine corresponds to Figure 2C. That is, Q=S / P=1, and the half-period or angle Θ of the cogging torque function is s A shift of Θ = 180° / S = 15° occurs between the stator parts. The rotor is a single piece, and the faces of the permanent magnets facing the stator parts act as pole pieces. The effective angular width of the pole pieces is Θ me is 15°.

[0053] For clarity, the electric machine described above is shown in some simplifications in Figures 4A to 4D. In the structural drawings, the windings of the electric machine, some fastening details of the rotor, bolts, holes, and possible position sensors have been omitted. Stator parts 1a and 1b are fastened to housing parts 5a and 5b, respectively, with a 15° shift relative to each other. A permanent magnet set 3 consisting of 12 magnets with a ring segment shape is fastened in the structure of rotor 8, which is also rigidly fastened to shaft 6. Shaft 6 can rotate freely with respect to ball bearings 7a and 7b, which are mounted in bearing sockets in housing parts 5a and 5b.

[0054] The described electric machine with axial flux has similar technical advantages and characteristics as the above-described electric machine with radial flux by inherently possessing somewhat higher specific power (power density) and driving torque caused by the relatively short flux path compared to radial flux. The machine has a relatively small amount of stator iron and a rotor magnetic circuit consisting only of permanent magnets. [Explanation of symbols]

[0055] 1a, 1b Stator parts 2a, 2b pole pieces 3 Permanent Magnet Sets 3a, 3b Permanent magnet set 4 Magnetic Flux Conductors 5a, 5b, 5c Housing parts 6 shafts 7a, 7b Ball bearings 8 rotors

Claims

1. 1. A method for canceling to zero a total cogging torque of an electric machine having a permanent magnet rotor and a slotted stator, wherein the stator has a number of slots S, the rotor has a number of magnetic poles P, the ratio of the number of slots S to the number of magnetic poles P is an integer Q, the rotor and the stator comprise two magnetically identical coaxial parts, and the parts of the rotor or the parts of the stator are oriented relative to each other at an angle Θ with respect to the axis of rotation. ｓ Θ ｓ = 180° / S, and the actual angular width of the rotor pole pieces is given by the equation Θ ｍ = k Θ ｍｅ where k is a correction factor taking into account edge effects, with values ​​0.8...1.1, and Θ ｍｅ is the effective angular width of the pole pieces of the rotor, and the equation Θ ｍｅ = (Q-1 / 2)Θ Ｓｐ where Θ Ｓｐ is the stator pitch angle, and Θ Ｓｐ = 360° / S.

2. The rotor is made up of two parts, which are at an angle Θ with respect to each other and with respect to the axis of rotation. ｓ 2. The method of claim 1, wherein the time is shifted by one minute.

3. The stator consists of two parts, which are arranged relative to each other at an angle Θ with respect to the axis of rotation. ｓ 2. The method of claim 1, wherein the time is shifted by one minute.

4. 4. The method according to claim 1, wherein the value of the correction factor k taking into account the edge effect is determined by modeling the structure of the magnetic circuit of the electric machine by means of the finite element method.

5. 1. An electric machine having a permanent magnet rotor and a slotted stator, in which total cogging torque is cancelled to zero, the stator having a number of slots S, the rotor having a number of magnetic poles P, 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 magnetically identical coaxial parts, the parts of the rotor or the parts of the stator being at an angle Θ with respect to the axis of rotation relative to each other. ｓ Θ ｓ = 180° / S, and the actual angular width Θ of the rotor pole pieces ｍ But the equation Θ ｍ = k Θ ｍｅ where k is a correction factor taking into account edge effects, with values ​​0.8...1.1, and Θ ｍｅ is the effective angular width of the pole pieces of the rotor, and the equation Θ ｍｅ = (Q-1 / 2)Θ Ｓｐ where Θ Ｓｐ is the stator pitch angle, and Θ Ｓｐ = 360° / S.

6. The rotor is made up of two parts, which are at an angle Θ with respect to each other and with respect to the axis of rotation. ｓ 6. The electric machine according to claim 5, wherein the rotational speed is shifted by 1 / 2.

7. The stator consists of two parts, which are arranged relative to each other at an angle Θ with respect to the axis of rotation. ｓ 6. The electric machine according to claim 5, wherein the rotational speed is shifted by 1 / 2.

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

9. 8. An electric machine according to any one of claims 5 to 7, characterized in that the electric machine is a generator.

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