JP2005529575A - Rotating permanent magnet type electric motor with varying air gap between interacting stator and rotor elements - Google Patents

Abstract

The permanent magnet motor is configured to have a selective change in the radial distance along the circumferential length of the pair of interacting paired rotor permanent magnets and stator poles. The effect of cogging torque on the overall torque characteristics can be controlled by setting an appropriate air gap change profile. The stator pole and rotor magnet surfaces are tilted with respect to each other, and the angle between them is selected to obtain the desired cogging torque compensation. Other air gap change profiles include providing a concave surface, the degree of concave being selectable.

Description

  The present invention relates to a rotary electric motor, and more particularly to a permanent magnet motor in which the dimension of the radial air gap between the interacting rotor permanent magnet and the stator pole varies.

(Related application)
No. 09 / 571,174, Pintikov et al., Filed May 16, 2000, filed on May 16, 2000, all assigned to the same assignee as the present invention. Maslov et al., US patent application Ser. No. 09 / 826,423, dated May 5, Maslov et al., US patent application Ser. No. 09 / 826,422, dated Apr. 5, 2001, October 1, 2001 Maslov et al., US patent application Ser. No. 09 / 966,101, dated Feb. 7, 2002, Maslov et al., US patent application Ser. No. 10 / 067,305, pending application, June 2002. No. 10 / 160,257, Maslov et al., Dated 4 days, and US patent application 10/160, Maslov et al., Dated June 4, 2002, pending application. Including the subject matter related to No. 54. The disclosures of these patent applications are incorporated herein by reference.

  Maslov et al., US patent application Ser. No. 09 / 826,423, a pending application identified above, is applicable to simplified manufacturing and enables effective and flexible operating characteristics. The need for an improved motor is identified and addressed. In a vehicle driving environment, for example, it is highly desirable to achieve smooth operation over a wide speed range while maintaining high torque output performance with minimal power consumption. Such a vehicle motor drive should have the advantage of easy access to various structural components with minimal hassle for part replacement. The above-identified U.S. patent application, which is an ongoing application, describes that the isolated magnetically permeable structure is formed into an annular ring to form an electromagnet core segment. In such an arrangement, the magnetic flux can be concentrated so as to provide an advantageous effect compared to the prior art embodiments.

  As described in the Maslov et al. Patent application identified above, isolating the electromagnet core segment minimizes harmful transformer interference effects due to magnetic flux loss and interaction with other electromagnet components, while Allows focusing of individual magnetic fluxes at. By configuring a single pole pair as a group of isolated electromagnets, operational advantages can be obtained. By isolating individual pole pairs from other pole groups, the flux transformer effect on adjacent groups when the excitation of the pole pair windings is switched is eliminated. The absence of additional magnetic poles in the magnetic pole group prevents all these effects within a single magnetic pole group. Utilizing three-dimensional features of the motor structure, for example, magnetic flux density in which a plurality of axially aligned stator poles and a plurality of axially aligned rotor magnets are highly focused in the effective air gap of the machine By providing a structural shape that provides a distribution, additional advantages are provided. Such an arrangement comprises a number of magnetic poles having the same individual effective air gap surface area and / or a larger total effective air gap surface area than a conventional motor having the same air gap diameter.

  In addition to the flux focusing advantages that can be obtained in the aforementioned configurations, the recently introduced neodymium-iron-boron (NdFeB) magnet material has a higher magnetic flux than other permanent magnet materials already used in brushless machines. It can provide density and thereby increase torque output performance. The use of high-density permanent magnets in motors with a large number of magnetic poles raises concerns about improving undesirable effects that can be caused by cogging torque. The cogging torque is caused by the magnetic attraction between the rotor with the permanent magnet attached and the stator poles that are not in a selectively excited state. This attractive force tends to move the rotor magnet to an equilibrium position opposite the stator poles, minimizing the magnetic resistance between them. When the rotor is driven to rotate by excitation of the stator, the magnitude and direction of the cogging torque caused by the magnetic interaction with the non-excited electromagnet segment periodically changes in the opposite direction and is caused by the excited stator segment. Alternately applied to torque. When there is no compensation, the direction of the cogging torque may suddenly change as the rotor rotates. If the cogging torque is very large, it becomes a source of mechanical vibration that is detrimental to rotational disturbances and precise speed control and smooth actuation purposes.

  As an example of the generation of cogging torque, a motor as disclosed in US patent application Ser. No. 09 / 826,422, filed on Apr. 5, 2001, is pending. The disclosure of this patent application is incorporated herein by reference. FIG. 1 is an exemplary view showing a rotor and a stator element. The rotor member 20 is an annular ring structure having permanent magnets 21 that are spaced apart from one another and distributed substantially evenly along the cylindrical back plate 25. The permanent magnet is a rotor magnet whose magnetic polarity alternates along the inner periphery of the annular ring. The rotor surrounds the stator member 30, and the rotor and stator member are separated by a radial annular air gap. The stator 30 includes a plurality of electromagnet core segments having a uniform configuration distributed evenly along the air gap. Each of the core segments includes a generally U-shaped magnetic structure 36 that forms two magnetic poles having a surface 32 facing the air gap. Although the winding 38 is wound around the legs of the magnetic pole pairs, the core segment can be configured to correspond to a single winding formed in a portion connecting the magnetic pole pairs. Each of the stator electromagnet core structures is separated and magnetically isolated from adjacent stator core elements. The stator element 36 is attached to a support structure that is not magnetically permeable, thereby forming an annular ring shape. This shape eliminates the divergence of stray flux transformer hardening from adjacent stator pole groups. Although not shown here, a suitable stator support structure can be found in the aforementioned patent application in order to make the dynamic motor element more clearly visible.

FIG. 2 is a partial plan view of the motor shown in FIG. 1, in which the stator poles are shown in association with the rotor permanent magnet 21. Each of the stator core elements 36 includes a pair of magnetic poles having a base portion 31 and a pole piece portion 32. The magnetic poles are connected together by a connecting portion 33. An excitation winding (not shown) of each magnetic pole pair can be formed in a well-known manner in the magnetic pole base portion or in the connecting portion.
FIG. 3 is a partial plan view during motor operation showing two adjacent stator core elements 36 having pole faces 32 labeled A-D in conjunction with a rotor magnet labeled 0-5. is there. The position of the rotor magnet is indicated by (A)-(C) for _three time points (t ₁ -t ₃ ) during which the rotor moves from left to right. At time t ₁ , the winding of the AB stator pole pair is excited by a current that flows in a direction that produces a strong S pole in A and a strong N pole in B. The windings of the CD stator pole pair are not excited. The position of the rotor is shown in (A). The N magnet 1 and the S magnet 2 overlap the stator magnetic pole A. The S magnet 2 and the N magnet 3 overlap the stator magnetic pole B. At this time, the magnet 3 is approaching the position where it overlaps the magnetic pole C. The S magnet 4 substantially overlaps the magnetic pole C, and the N magnet 5 substantially overlaps the magnetic pole D. At this time, the driving torque is caused by the attractive force between the S pole A and the N pole magnet 1, the attractive force between the N pole B and the S pole magnet 2, and the repulsive force between the N pole B and the N pole magnet 3. Arise. The magnetic poles C and D have weak N and S magnetizations caused by the attractive forces of the magnets 4 and 5, respectively. The attractive force for maintaining the minimum magnetic resistance is in the opposite direction to the motor driving torque.

In time t _2, the has been moved to a position where the rotor is shown in (B). The excitation of the windings of the magnetic pole pair AB is turned off. The windings of the CD pole pair are not excited. Magnets 1 and 2 substantially overlap magnetic poles A and B, respectively. The N magnet 3 and the S magnet 4 overlap the magnetic pole C. The S magnet 4 and the N magnet 5 overlap the magnetic pole D. The magnetic poles A and B have weak S and N magnetization, respectively. Stator poles C and D are affected by both N and S rotor magnets. The magnetic pole C is in the magnetic path between the N-pole magnet 3 and the S-pole magnet 4. The magnetic pole D is in the magnetic path between the S-pole magnet 4 and the N-pole magnet 5. Therefore, a cogging torque opposite to the motor driving torque is generated, and the magnitude of the rotor magnet changes while the rotor magnet moves from a directly overlapping state with the non-excited stator magnetic pole to a partially overlapping state.
At time t ₃ , the rotor has moved to the position indicated by (C). The excitation of the A-B magnetic pole pair winding is reversed, and a strong N pole in the magnetic pole A and a strong S pole in B are generated. The windings of the CD pole pair are not excited. The N magnet 1 and the S magnet 2 overlap the stator magnetic pole B. The S magnet 0 and the N magnet 1 overlap the stator magnetic pole A. At this time, the magnet 2 is approaching the position where it overlaps the magnetic pole C. The N magnet 3 substantially overlaps the magnetic pole C, and the S magnet 4 substantially overlaps the magnetic pole D.

  As described above, the opposing cogging torque effect generates torque in a form that changes according to the rotational angle position as the rotation proceeds. Cogging torque is most noticeable at the transition point where the rotor magnet attempts to cross the air gap and face the stator poles. A sudden change in cogging torque occurs when the leading edge of the generally rectangular surface of the permanent magnet reaches the edge of a rectangular stator pole parallel to it. Using high energy density permanent magnet materials, such as neodymium-iron-boron (NdFeB) magnetic materials that provide high magnetic flux density in the air gap in the vicinity of the rotor permanent magnet, this effect becomes so pronounced that undesirable vibrations become noticeable. Rise. Motors with many stator poles and rotor magnets, such as axially aligned stator poles and rotor magnets, can produce even greater cogging torque effects. In the same manner, cogging torque occurs to varying degrees in a motor with a single stator core.

  Various techniques are used to minimize the effects of cogging torque. These techniques reduce the rate of change in magnetoresistance with respect to rotor position, reduce the magnetic flux of the machine, or shift the magnetic poles of a single stator core so that the cogging torque produced by the individual magnetic poles cancel each other out. Is something to try. Electronic methods can be used to control the strength of the electromagnetic interaction that occurs between the permanent magnet and the electromagnet surface. Such methods include complex control algorithms that are executed simultaneously with the motor control algorithms and have the disadvantage of tending to degrade the overall performance of the motor. The reduction in magnetic flux reduces the benefits gained from new permanent magnet materials and the above-identified pending patent application flux focusing techniques. Shifting the position of the magnetic poles in a typical single stator core structure imposes limitations on the size, position, and number of magnetic poles, which will prevent placement for optimal operation.

Another approach involves modifying the machine configuration by changing the shape of the stator poles. Conventional stator poles usually consist of laminated laminates and cannot be applied immediately with modifications. Available laminating machining processes are limited in their ability to recreate normal patterns, especially three-dimensional shapes. The substantial modification range of such laminate structures is too complex and costly to be feasible.
Therefore, there is a need for effective cogging torque compensation of the motor, especially motors with high magnetic flux density magnitude and concentration, without reducing the effective operation and control capability of the motor while providing cost and feasibility of application. Sex exists.
The pending US patent application Ser. No. 10 / 160,257 shows that the stator pole face or rotor magnet face is aligned with the stator pole surface geometry or the rotor magnet surface geometry oblique to each other. To address this need. The action of the oblique arrangement is to suppress the rate of change of cogging torque caused by the interaction between the rotor magnet and the magnetic pole of the non-excited stator electromagnet when the permanent magnet crosses the rotation path. The selective shaping of the stator poles is made possible through the use of a core material such as a magnetically permeable soft medium that is compatible with the formation of various specialized shapes. For example, the core material can be made from soft magnetic grade Fe, SiFe, SiFeCo, SiFeP powder materials, each of which has an inherent power loss, permeability and saturation level. These materials can be initially formed into any desired three-dimensional shape, thereby eliminating the possibility of machining already molded hard laminate materials.

U.S. Patent Application No. 10 / 160,254, filed June 4, 2002, by offsetting the effect of cogging torque that occurs in a plurality of axially spaced rotor and stator element pairs, It addresses the aforementioned need. The magnetic poles of each individual axially arranged stator core are axially shifted or offset from each other to counteract the cogging torque action without limiting the positional relationship between the circumferential stator poles. Alternatively, circumferentially and axially aligned rotor permanent magnets are axially offset relative to each other to counteract the effects of cogging torque without limiting the total number of permanent magnets or their circumferential position. .
Minimizing torque ripple and cogging torque action without adversely affecting torque output performance continues to be an important objective.

  The present invention at least partially meets this need by selectively varying the radial distance between the interacting pair of rotor permanent magnets and the stator poles along the circumferential length of the pair. Is. The motor rotor has a plurality of permanent magnets distributed in the circumferential direction around the rotation axis and having substantially the same length in the circumferential direction. The plurality of stator magnetic poles are distributed around the air gap, and all of the magnetic poles have substantially the same length as the length of the magnet in the circumferential direction. The effect of cogging torque on the overall torque characteristics can be controlled by setting the appropriate air gap variation shape of the interacting stator poles and rotor magnets, and this variation shape will be described here for simplicity of explanation. It is called a gap profile. The air gap profile is the change in radial distance in the air gap between the stator pole piece and the facing rotor magnet, from one end of the pair to the other.

The appropriate air gap profile depends on the desired motor operating conditions and motor parameters such as the number of stator poles and rotor magnets, the excitation sequence of the windings and other expected conditions. The profile can be obtained by changing the radial distance from the axis of rotation of either the rotor magnet surface or the stator pole piece surface. Either the rotor magnet surface or the stator pole surface may be a constant radial distance from the axis and the other surface may take various shapes. Alternatively, both the rotor magnet distance and the pole piece distance can be varied. In the preferred embodiment, the air gap profile is the same for all interacting rotor magnet and stator pole combinations. That is, all the stator magnetic poles have the same shape, and all the rotor magnets have the same shape.
One such air gap profile considered in the present invention provides a substantially uniform reduction in the radial distance from the first end to the second end of the pair between the interacting rotor and stator pair. . When the rotor permanent magnet has a relatively constant thickness, the stator pole surface is tilted with respect to the surface of the rotor magnet. Alternatively, the radial thickness of the permanent magnet may decrease from end to end.
In another air gap profile, each of the rotor magnet surfaces is at a constant radial distance from the axis so that the stator pole pieces have a radial thickness variation and a concave surface facing the air gap. It may be. The concave degree can be set according to whether the rotor surrounds the stator or the stator surrounds the rotor. As a variant, the permanent magnets can be of various radial thicknesses with a concave surface facing the air gap of a selected concave degree.

  The magnetic pole structure described above can be configured in the form of a stator arrangement having a plurality of electromagnet segments spaced apart and separated by ferromagnetism to provide advantageous results. Each of the segments can be formed from a pole pair as shown in FIG. The stator is a single annular ring surrounding a single axial magnetic pole and a plurality of circumferential magnetic pole pairs. In other configurations, multiple rings of stator poles are formed by a plurality of electromagnetic core segments that are axially spaced and ferromagnetically isolated. Each of the core segments includes a plurality of magnetic poles integrally connected by one or more connecting portions extending substantially in the direction of the rotational axis. In this way, the stator forms a plurality of magnetic poles in the axial direction, and the single magnetic pole of each segment is distributed in the circumferential direction in each ring. In the latter arrangement, the rotor forms axially spaced rings of individual magnets arranged circumferentially along the air gap, the number of rotor rings being equal to the number of stator poles in the stator core segment. .

  Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description, wherein only a preferred embodiment of the invention is shown and described, merely by way of example of the best mode contemplated for carrying out the invention. Will be immediately apparent. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

The present invention is illustrated by way of example, and not by way of limitation, in the accompanying drawings in which like reference numerals refer to like elements.
The concept of the present invention is a motor having a set of rotor and stator elements arranged circumferentially and concentrically around a radial air gap, such as the motor of FIG. Applicable to motors having two or more pairs of rotor and stator elements spaced apart from each other. FIG. 4 is a partial plan view of a motor as shown in FIG. 1, with the stator poles modified according to the present invention shown in relation to the rotor permanent magnet surface. It should be understood that this arrangement is representative of a rotary motor in which the rotor and stator are coaxial with each other and separated by a radial air gap. A rotor magnet 21 with alternating magnetic polarity is shown without a support structure for clarity of placement. Each of the stator core elements 36 includes a pair of magnetic poles having a base portion 31 and a magnetic pole piece portion 32. The magnetic poles are integrally connected to each other by the connecting portion 33. An excitation winding (not shown) of each magnetic pole pair can be formed on the magnetic pole base portion or on the coupling portion by a known method. Each of the magnetic pole pieces extends from the magnetic pole base portion by a magnetic pole piece extension portion both outward in the circumferential direction.

The pole piece of the stator core element 36 shown on the right side of the figure is in a state of overlapping the rotor magnet 21. The acting pole pieces and magnets are substantially the same length along the air gap. The permanent magnets are each of the same radial thickness so that their surfaces along the air gap are at a certain distance from the axis of rotation. In this plan view, the surface of the stator pole face is such that the air gap is inclined with respect to the permanent magnet surface so that the radial distance between them is substantially uniform from the left end to the right end of the page. Increase. If the rotor moves from right to left with respect to the stator during normal motor operation, the leading edge of the rotor magnet overlaps the stator pole face, reducing air gap separation. The transient effect of cogging torque action on the opposing stator and rotor elements is smooth compared to a motor geometry with a constant air gap size. When the rotor moves from left to right with respect to the stator during normal motor operation, the leading edge of the rotor magnet overlaps the stator pole face and air gap separation increases. As the rotor magnet moves through the stator poles, the cogging attraction between them gradually decreases compared to a constant air gap shape. Therefore, the inclination of the variable air gap shape of FIG. 4 can be determined so as to obtain a desired overall torque characteristic in any rotation direction.
FIG. 5 shows a modification of the structure of FIG. As shown in this plan view, the stator pole face is in a horizontal plane, thereby presenting a certain distance from the axis of rotation along the periphery of the air gap. The surface of the rotor permanent magnet 21 is tilted to provide the same variable air gap action as described in connection with FIG.

In the shape of FIG. 6, the permanent magnet surfaces have the same radial thickness. As shown in this plan view, the surface of the permanent magnet in the air gap is in a horizontal plane and presents a uniform distance from the axis of rotation along the air gap. The stator pole surface is concave with respect to the rotor magnet surface, and the degree of recession is higher than required to maintain a uniform distance from the rotational axis of the motor that the stator surrounds the rotor. Thus, each interacting pair of stator poles and rotor magnets defines a variable air gap distance therebetween, for example, in the right portion of the drawing. As the rotor magnet approaches and moves through the stator poles, the flux distribution converges in a manner similar to the collection of light by the converging optical lens, thereby allowing this variable air gap to vary with respect to the cogging torque. The effect is different from that of a uniform air gap shape. Concaveness can be adjusted to improve the overall torque sign as needed.
FIG. 7 shows a modification of the structure of FIG. As shown in this plan view, the stator pole face is in a horizontal plane, thereby presenting a certain distance from the axis of rotation along the periphery of the air gap. The surface of the rotor permanent magnet 21 is concave with respect to the magnet surface and provides the same variable air gap action as described in connection with FIG.

The advantages of the stator pole structure variants described above can be applied to other stator / rotor configurations. For example, a stator having an integral continuous stator core can have prominently configured poles formed as described with respect to any of FIGS. 3-7 to provide cogging torque compensation. Such a magnetic pole configuration can be implemented to reduce the generation of cogging torque in a motor having a single axially aligned magnet and stator pole train.
FIG. 8 is a three-dimensional exploded view of the motor disclosed in pending US patent application Ser. No. 10 / 067,305. The motor 15 includes an annular permanent magnet rotor 20 and an annular stator structure 30 that are separated by a radial air gap. A plurality of ferromagnetically isolated stator core segment elements 36 of magnetically permeable material are supported by a support structure 50 that maintains the ferromagnetic isolation of the segments. Segment 36 is a unitary structure formed from a magnetically permeable material with a pole surface 32 facing the air gap. Each of the stator core elements 36 is an electromagnet that includes a winding 38 formed on the core material. The reversal of the direction of the excitation current by a known method affects the reversal of the magnetic polarity of each magnetic pole. The rotor comprises a permanent magnet portion 21, three axially spaced rotor magnet rings 22-24 are distributed circumferentially around the air gap and a permanent magnet is mounted on the iron back ring 25. The stator support structure 50 is mounted on a fixed shaft and the rotor is mounted in a housing that is pivotally supported on the shaft through appropriate bushings and bearings.

The stator magnetic poles and rotor magnets shown in FIG. 8 can be configured as shown in any of the modifications of FIGS. Since so many magnets and stator poles are provided both in the circumferential and axial directions, compensation for potentially large cogging torque disturbances is provided.
In this disclosure, only preferred embodiments of the present invention are shown and described, but these are just a few examples of their breadth of use. It should be understood that the present invention can be used in a variety of other combinations and environments, and that variations or modifications within the inventive concept as described herein are possible. For example, each of the arrangements shown in the drawings can be implemented with useful results as an integral stator core that is axially spaced from one another, rather than a segmented stator core. Further, in the illustrated example, the stator magnetic pole pitch is shown to be substantially equal to the rotor magnet pitch for the sake of simplicity, but the circumferential distance between the pair of stator magnetic poles is not limited to the rotor magnet pair. It can be larger or smaller than the distance between.

Although specific geometric shapes of the stator core elements are illustrated, the inventive concept herein encompasses many variations of these shapes since almost any shape can be formed using powder metal technology. I want you to recognize that. Therefore, a specific core shape can be matched with a desired magnetic flux distribution. For example, providing a convex or other geometrically shaped surface is also within the scope of the inventive concept, and different sets of pole pairs can have pole pieces each having a different shape. The convex shape diverges the magnetic flux distribution pattern.
Although the detailed description of the present invention shows a stator surrounded by a rotor, the concepts of the present invention are equally applicable to motors in which the rotor is surrounded by the stator.

FIG. 4 is an exemplary diagram illustrating the rotor and stator elements of a motor disclosed in pending application Ser. No. 09 / 826,422. FIG. 2 is a partial plan view of the motor shown in FIG. 1, in which stator poles are shown associated with a rotor permanent magnet surface. FIG. 2 is a partial plan view of the elements of FIG. 1 showing the relative positions of the stator pole surface and the rotor surface at three points during motor operation. FIG. 3 is a partial plan view of stator magnetic poles associated with a rotor permanent magnet surface according to the present invention. FIG. 5 is a partial plan view of stator magnetic poles associated with a rotor permanent magnet surface corresponding to a modification of the structure of FIG. 4. FIG. 6 is a partial plan view of stator poles associated with a rotor permanent magnet surface corresponding to another variation of the present invention. FIG. 7 is a partial plan view of stator magnetic poles associated with a rotor permanent magnet surface corresponding to a modification of the structure of FIG. 6. Three-dimensional decomposition with axially stacked stator and rotor elements as disclosed in pending US patent application Ser. No. 10 / 067,305, which incorporates the stator pole structure of FIGS. FIG.

Claims (12)

A rotary electric motor comprising a stator and a rotor arranged coaxially around a rotation axis and separated by a radial air gap,
The rotor includes a plurality of permanent magnets distributed in a circumferential direction around the rotation axis and having substantially the same length in the circumferential direction;
A stator comprising a plurality of magnetic poles distributed around the air gap, the magnetic poles being substantially the same length as the magnets in the circumferential direction;
A rotary electric motor characterized in that a radial distance of an air gap between a pair of interacting rotor permanent magnets and a stator magnetic pole varies along a circumferential length of the pair.   The radial distance of the air gap between the interacting pairs decreases substantially uniformly from the first end of the pair to the second end of the pair in the circumferential direction. Item 2. The rotary electric motor according to Item 1.   The rotary electric motor according to claim 2, wherein the interacting pair of permanent magnets has a relatively constant radial thickness.   The rotary electric motor according to claim 2, wherein a radial thickness of the pair of interacting permanent magnets decreases from the first end to the second end.   The rotary electric motor according to claim 1, wherein the stator magnetic pole includes a magnetic pole piece whose thickness in a radial direction changes, and the magnetic pole piece has a concave surface facing the air gap.   The rotary electric motor according to claim 5, wherein the rotor surrounds the stator.   The rotary electric motor according to claim 1, wherein the permanent magnet has a varying radial thickness and a concave surface facing the air gap.   The rotary electric motor according to claim 1, wherein the stator includes a plurality of separated and ferromagnetically isolated electromagnet core segments, each of the core segments including at least one of the magnetic poles. motor. Each of the core segments comprises a plurality of magnetic poles integrally connected by one or more connecting portions extending generally in the rotational axis direction;
A permanent magnet of the rotor has a surface facing the air gap and forms axially spaced rings of individual magnets arranged circumferentially along the air gap, the number of rings being The rotary electric motor according to claim 8, wherein the number is equal to the number of stator magnetic poles in the stator core segment.   The radial distance of the air gap between the interacting pairs increases substantially uniformly from the first end of the pair to the second end of the pair in the circumferential direction. Item 9. The rotary electric motor according to Item 8.   The rotary electric motor according to claim 8, wherein the stator magnetic pole includes a magnetic pole piece having a thickness in a radial direction, and the magnetic pole piece has a concave surface facing the air gap.   The rotary electric motor according to claim 8, wherein the permanent magnet has a varying radial thickness and a concave surface facing the air gap.

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