JP2014054153A - Electric motor and electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric motor and an electric power steering device capable of reducing an amount of magnet used or reducing a rare earth magnet component of the magnet, and of suppressing torque ripples.SOLUTION: An electric motor includes: a motor rotor 20 including a rotor yoke 22, and a plurality of magnets 23 buried so as to have flux barriers 25 serving as air gaps at both sides in a rotation direction of the rotor yoke 22; and a motor stator 30 including a stator core 31 annularly arranged at the radial outside of the rotor yoke 22 so as to have a predetermined interval, and an excitation coil 37 for exciting the stator core 31. The rotor yoke 22 includes: a salient pole part 27 protruding radially outward from the magnet 23; and a bridge part 26 coupling between the salient pole parts 27 at the radial outside of the flux barrier 25.

Description

  The present invention relates to an electric motor and an electric power steering apparatus.

  For example, in Patent Document 1, windings are applied to slots between a plurality of teeth arranged on the inner periphery of an annular stator, and a plurality of flat plate shapes are formed inside a rotor that rotates inside the stator. Are permanent magnet embedded rotary electric machines in which bridges are provided correspondingly between adjacent magnetic pole ends of a pair of adjacent permanent magnets, so-called IPM (Interior Permanent). Magnet) motor is described.

JP 2011-062059 A

  The technique disclosed in Patent Document 1 aims to suppress torque ripple in a permanent magnet embedded rotary electric machine in which flat permanent magnets are arranged rotationally symmetrically. In this technique, the electrical angle θ1 of the end of the convex portion centered on the rotation axis of the rotor is in the range of 70 ° to 80 ° with the radial straight line closest to the end passing through the magnetic pole central portion of the permanent magnet. The electrical angle θ2 of the deepest portion of the concave portion centered on the rotation axis of the rotor is assumed to be in the range of (θ1-10 °) to θ1 with the radial straight line as the base point.

  In recent years, there has been a demand for an electric motor with reduced use of magnets or an electric motor with reduced rare earth magnet components. In an electric motor that reduces the amount of magnets used or an electric motor that reduces the rare earth magnet component of the magnets, the magnet torque generated by the magnets is also reduced. When reluctance torque is added in order to compensate for the magnet torque that accompanies the reduction in magnet usage and rare earth component content, the magnet torque and reluctance torque may cause torque ripple. For this reason, in the electric motor which reduced the usage-amount of the magnet, or the electric motor which reduced the rare earth magnet component of the magnet, it is necessary to suppress a torque ripple more.

  The present invention has been made in view of the above, and provides an electric motor and an electric power steering device that can reduce the amount of magnets used or reduce the rare earth magnet component of the magnets and suppress torque ripple. With the goal.

  In order to solve the above-described problems and achieve the object, an electric motor includes a rotor yoke, a plurality of magnets embedded in the rotor yoke, and a flux barrier provided on both sides of the magnet in the circumferential direction to prevent a magnetic flux short circuit. A motor rotor including a stator core arranged in an annular shape on the radially outer side of the rotor yoke and an excitation coil for exciting the stator core, wherein the rotor yoke protrudes radially outward of the magnet. And an outermost portion having a maximum distance from the rotation center in a plane orthogonal to the rotation center of the motor rotor. The curvature of the outer peripheral surface of the convex pole portion at the diameter reference position is such that the outermost diameter reference position is being rotated. Greater than the curvature to draw and rotate about, and connecting the center of rotation and projecting base the salient portion begins to protrude radially outward segment, characterized in that passing through the magnet.

  With the above configuration, the amount of magnet used can be reduced, or the rare earth magnet component of the magnet can be reduced, and torque ripple can be suppressed.

As a desirable aspect of the present invention, when the distance from the rotation center to the outermost diameter reference position is R1, and the distance from the outer peripheral surface of the magnet to the outermost diameter reference position is T1, R1 and T1 The relationship preferably satisfies the following formula (1).
0.07 ≦ (T1 / R1) ≦ 0.15 (1)

  With the above configuration, the ripple ratio, which is the ratio of the ripple to the average torque, can be efficiently reduced.

As a desirable mode of the present invention, in one convex pole portion, an angle formed by lines connecting two protruding base points and the rotation center is Q1, and the circumference of the outer peripheral surface of the magnet in contact with the convex pole portion When the angle formed by the lines connecting the end of the direction and the rotation center is defined as Q2, the relationship between Q1 and Q2 preferably satisfies the following formula (2).
0.75 ≦ (Q1 / Q2) ≦ 1.29 (2)

  With the above configuration, the ripple ratio, which is the ratio of the ripple to the average torque, can be efficiently reduced.

As a desirable mode of the present invention, the exciting coil has N turns per coil, the maximum phase current is I, the number of parallel circuits is n, the residual magnetic flux density of the magnet is Br, and the magnetized area S. Sometimes, it is preferable to satisfy the following formula (3).
1.8 ≦ ((N × I × n) / (Br × S)) (3)

  With this structure, even if the amount of magnet used is reduced, the possibility of torque shortage can be suppressed.

  As a desirable mode of the present invention, it is preferable that the curvature of the outer peripheral surface of the convex pole part has the same curvature between the protruding base parts. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced.

  As a desirable aspect of the present invention, it is preferable that curves having different curvatures between the protruding base portions are continuously combined on the outer peripheral surface of the convex pole portion. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced.

  As a desirable aspect of the present invention, it is preferable that a groove extending in a direction parallel to the rotation center axis of the motor rotor is provided on the inner peripheral surface of the teeth of the stator core facing the outer periphery of the rotor yoke. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced.

  As a desirable mode of the present invention, it is preferable that the content of Dy contained in the magnet is 2% by mass or less.

  With the above configuration, in the electric motor, the permeance coefficient of the magnet is improved and the demagnetization resistance of the magnet is improved by the closed magnetic path in the rotor yoke. For this reason, even if the coercive force of the magnet is reduced, the possibility of demagnetization can be reduced, so that the rare earth that improves the coercive force of the magnet can be reduced. For example, in the case where a magnet formed by hot extrusion molding using magnetic particles with fine crystal grains is composed of an Nd-Fe-B magnet, at least one of Dy or Tb to be added is 2% by mass or less. It can be.

  As a desirable mode of the present invention, it is preferable that the number of teeth of the stator core is 12 and the number of the magnets in the circumferential direction is 8. With this configuration, it is possible to improve the robustness of the torque ripple with respect to the dimensional variation of the rotor yoke.

  As a desirable mode of the present invention, it is preferable that the electric motor is a direct drive motor, and the motor rotor and a load body rotated by the motor rotor are directly connected.

  With the above configuration, the electric motor can increase the motor torque and reduce the torque ripple.

  As a desirable aspect of the present invention, an electric power steering device that obtains an auxiliary steering torque by the electric motor is preferable.

  With the above configuration, when the magnetic flux loops in the closed magnetic path of the rotor, the amount of magnetic flux linked from the rotor yoke in which the magnet is embedded to the stator core is reduced. Further, for example, in an electric motor in which the amount of magnet used is reduced or an electric motor in which the rare earth magnet component of the magnet is reduced, the amount of magnetic flux linked from the rotor yoke embedded with the magnet to the stator core becomes smaller. For this reason, the hysteresis loss is reduced when the magnetic domains generated in the stator core change direction. Hysteresis loss is considered to be a cause of increase in loss torque when the motor is idling. That is, according to the above configuration, the electric motor can reduce the frictional force with respect to the rotation of the rotor when the rotor is rotated in a state where the excitation coil is not energized. And the increase in the loss torque when the rotor of an electric motor idles can be suppressed. Since the loss torque can be suppressed, the electric power steering apparatus can operate in a state in which a sense of discomfort is suppressed even if the auxiliary steering torque is transmitted to the steering person.

  ADVANTAGE OF THE INVENTION According to this invention, the electric motor and electric power steering apparatus which can reduce the usage-amount of a magnet or reduce the rare earth magnet component of a magnet, and can suppress a torque ripple can be provided.

FIG. 1 is a configuration diagram of an electric power steering apparatus including an electric motor according to the present embodiment. FIG. 2 is a front view for explaining an example of a speed reducer included in the electric power steering apparatus of the present embodiment. FIG. 3 is a cross-sectional view schematically showing the configuration of the electric motor of the present embodiment on a virtual plane including the central axis. FIG. 4 is a cross-sectional view schematically showing the configuration of the electric motor according to the present embodiment, taken along a virtual plane orthogonal to the central axis. FIG. 5 is an explanatory diagram schematically showing the configuration of the motor rotor of the present embodiment. FIG. 6 is an explanatory diagram schematically showing a modification of the magnet embedded in the motor rotor of the present embodiment. FIG. 7 is an explanatory view schematically showing the configuration of the motor rotor of the present embodiment. FIG. 8 is an explanatory diagram schematically showing the configuration of the motor rotor of the present embodiment. FIG. 9 is an explanatory view schematically showing the outer peripheral surface of the convex pole portion of the present embodiment. FIG. 10 is an explanatory view schematically showing a modification of the outer peripheral surface of the convex pole portion of the present embodiment. FIG. 11 is an explanatory diagram schematically showing a modification of the bridge portion of the present embodiment. FIG. 12 is an explanatory view schematically showing a modification of the tooth of the present embodiment. FIG. 13 is an explanatory diagram for explaining a permeance straight line of a brushless motor of a comparative example in which a magnet is attached to the outer peripheral surface of a cylindrical rotor yoke. FIG. 14 is an explanatory diagram for explaining a permeance straight line of the brushless motor of the present embodiment. FIG. 15 is a flowchart for explaining the magnet manufacturing method of this embodiment. FIG. 16 is a configuration diagram when the electric motor according to the present embodiment is applied as a direct drive motor. FIG. 17 is an explanatory diagram for explaining the influence of the distance from the outer peripheral surface of the magnet to the outermost diameter reference position on the ripple rate. FIG. 18 is an explanatory diagram for explaining the influence of the angle formed by the lines connecting the protruding base and the rotation center on the ripple rate. FIG. 19 is an explanatory diagram for explaining the influence of the groove on the inner peripheral surface of the tooth on the ripple rate. FIG. 20 is an explanatory diagram illustrating the magnetic characteristics of a magnet having a Dy content of 2 mass% or less.

  DESCRIPTION OF EMBODIMENTS Embodiments (embodiments) for carrying out the present invention will be described in detail with reference to the drawings. The present invention is not limited by the contents described in the following embodiments. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

(Electric power steering device)
FIG. 1 is a configuration diagram of an electric power steering apparatus including an electric motor according to the present embodiment. The electric motor of the present embodiment is a brushless motor 10. First, an outline of an electric power steering apparatus 80 including the brushless motor 10 will be described with reference to FIG. 1.

  The electric power steering device 80 includes a steering wheel 81, a steering shaft 82, a steering force assist mechanism 83, a universal joint 84, a lower shaft 85, a universal joint 86, in the order in which the force applied from the steering wheel is transmitted. A pinion shaft 87, a steering gear 88, and a tie rod 89 are provided. The electric power steering device 80 includes an ECU (Electronic Control Unit) 90, a torque sensor 91a, and a vehicle speed sensor 91b.

  The steering shaft 82 includes an input shaft 82a and an output shaft 82b. The input shaft 82a has one end connected to the steering wheel 81 and the other end connected to the steering force assist mechanism 83 via the torque sensor 91a. The output shaft 82 b has one end connected to the steering force assist mechanism 83 and the other end connected to the universal joint 84. In the present embodiment, the input shaft 82a and the output shaft 82b are made of a magnetic material such as iron.

  The lower shaft 85 has one end connected to the universal joint 84 and the other end connected to the universal joint 86. The pinion shaft 87 has one end connected to the universal joint 86 and the other end connected to the steering gear 88.

  Steering gear 88 includes a pinion 88a and a rack 88b. The pinion 88a is connected to the pinion shaft 87. The rack 88b meshes with the pinion 88a. The steering gear 88 is configured as a rack and pinion type. The steering gear 88 converts the rotational motion transmitted to the pinion 88a into a linear motion by the rack 88b. The tie rod 89 is connected to the rack 88b.

  The steering force assist mechanism 83 includes a speed reducer 92 and the brushless motor 10. The reduction gear 92 is connected to the output shaft 82b. The brushless motor 10 is an electric motor that is connected to the reduction gear 92 and generates auxiliary steering torque. In the electric power steering device 80, a steering column is constituted by the steering shaft 82, the torque sensor 91a, and the speed reducer 92. The brushless motor 10 gives auxiliary steering torque to the output shaft 82b of the steering column. That is, the electric power steering apparatus 80 of this embodiment is a column assist system.

  In the column assist type electric power steering device 80, the distance between the operator and the brushless motor 10 is relatively short, and the torque change or frictional force of the brushless motor 10 may affect the steering. For this reason, the electric power steering apparatus 80 is required to reduce the frictional force of the brushless motor 10.

  FIG. 2 is a front view for explaining an example of a speed reducer included in the electric power steering apparatus of the present embodiment. FIG. 2 shows a part in cross section. The speed reducer 92 is a worm speed reducer. The reduction gear 92 includes a reduction gear housing 93, a worm 94, a ball bearing 95 a, a ball bearing 95 b, a worm wheel 96, and a holder 97.

  The worm 94 is coupled to the shaft 21 of the brushless motor 10 by a spline or an elastic coupling. The worm 94 is held in the speed reducer housing 93 so as to be rotatable by a ball bearing 95 a and a ball bearing 95 b held by the holder 97. The worm wheel 96 is rotatably held by the speed reducer housing 93. The worm teeth 94 a formed on a part of the worm 94 mesh with the worm wheel teeth 96 a formed on the worm wheel 96.

  The rotational force of the brushless motor 10 is transmitted to the worm wheel 96 through the worm 94 to rotate the worm wheel 96. The reduction gear 92 increases the torque of the brushless motor 10 by the worm 94 and the worm wheel 96. Then, the reduction gear 92 gives an auxiliary steering torque to the output shaft 82b of the steering column shown in FIG.

  The torque sensor 91a shown in FIG. 1 detects the driver's steering force transmitted to the input shaft 82a via the steering wheel 81 as a steering torque. The vehicle speed sensor 91b detects the traveling speed of the vehicle on which the electric power steering device 80 is mounted. In the ECU 90, the brushless motor 10, the torque sensor 91a, and the vehicle speed sensor 91b are electrically connected.

  The ECU 90 controls the operation of the brushless motor 10. Moreover, ECU90 acquires a signal from each of the torque sensor 91a and the vehicle speed sensor 91b. That is, the ECU 90 acquires the steering torque T from the torque sensor 91a, and acquires the traveling speed V of the vehicle from the vehicle speed sensor 91b. The ECU 90 is supplied with electric power from a power supply device (for example, a vehicle-mounted battery) 99 with the ignition switch 98 turned on. The ECU 90 calculates an assist steering command value of the assist command based on the steering torque T and the traveling speed V. Then, the ECU 90 adjusts the power value X supplied to the brushless motor 10 based on the calculated auxiliary steering command value. The ECU 90 acquires the information on the induced voltage from the brushless motor 10 or the information on the rotation of the rotor from the resolver described later as the operation information Y.

  The steering force of the driver (driver) input to the steering wheel 81 is transmitted to the speed reduction device 92 of the steering force assist mechanism 83 via the input shaft 82a. At this time, the ECU 90 acquires the steering torque T input to the input shaft 82a from the torque sensor 91a, and acquires the traveling speed V from the vehicle speed sensor 91b. The ECU 90 controls the operation of the brushless motor 10. The auxiliary steering torque created by the brushless motor 10 is transmitted to the speed reducer 92.

  The steering torque T (including auxiliary steering torque) output via the output shaft 82 b is transmitted to the lower shaft 85 via the universal joint 84 and further transmitted to the pinion shaft 87 via the universal joint 86. The steering force transmitted to the pinion shaft 87 is transmitted to the tie rod 89 via the steering gear 88 to steer the steered wheels. Next, the brushless motor 10 will be described.

(Electric motor)
FIG. 3 is a cross-sectional view schematically showing the configuration of the electric motor of the present embodiment on a virtual plane including the central axis. FIG. 4 is a cross-sectional view schematically showing the configuration of the electric motor according to the present embodiment, taken along a virtual plane orthogonal to the central axis. As shown in FIG. 3, the brushless motor 10 includes a housing 11, a bearing 12, a bearing 13, a resolver 14, a motor rotor 20, and a motor stator 30 as a brushless motor stator.

  The housing 11 includes a cylindrical housing 11a and a front bracket 11b. The front bracket 11b is formed in a substantially disc shape and is attached to the cylindrical housing 11a so as to close one open end of the cylindrical housing 11a. The cylindrical housing 11a is formed with a bottom 11c at the end opposite to the front bracket 11b so as to close the end. The bottom part 11c is formed integrally with the cylindrical housing 11a, for example. As a magnetic material forming the cylindrical housing 11a, for example, a general steel material such as SPCC (Steel Plate Cold Commercial), electromagnetic soft iron, or the like can be applied. The front bracket 11b serves as a flange when the brushless motor 10 is attached to a desired device.

  The bearing 12 is provided inside the cylindrical housing 11a and at a substantially central portion of the front bracket 11b. The bearing 13 is provided inside the cylindrical housing 11a and at a substantially central portion of the bottom portion 11c. The bearing 12 rotatably supports one end of a shaft 21 that is a part of the motor rotor 20 disposed inside the cylindrical housing 11a. The bearing 13 rotatably supports the other end of the shaft 21. Thereby, the shaft 21 rotates around the axis of the rotation center Zr.

  The resolver 14 is supported by a terminal block 15 provided on the front bracket 11b side of the shaft 21. The resolver 14 detects the rotational position of the motor rotor 20 (shaft 21). The resolver 14 includes a resolver rotor 14a and a resolver stator 14b. The resolver rotor 14a is attached to the circumferential surface of the shaft 21 by press fitting or the like. The resolver stator 14b is disposed to face the resolver rotor 14a with a predetermined gap.

  The motor stator 30 is provided in a cylindrical shape so as to surround the motor rotor 20 inside the cylindrical housing 11a. The motor stator 30 is attached by being fitted, for example, to the inner peripheral surface 11d of the cylindrical housing 11a. The central axis of the motor stator 30 coincides with the rotation center Zr of the motor rotor 20. The motor stator 30 includes a cylindrical stator core 31 and an excitation coil 37. In the motor stator 30, an exciting coil 37 is wound around a stator core 31.

  As shown in FIG. 4, the stator core 31 includes a plurality of teeth 34. There are twelve teeth 34 in the present embodiment in the circumferential direction. The stator core 31 is formed by stacking and bundling a plurality of core pieces formed in substantially the same shape in a direction parallel to the axis of the rotation center Zr. A tooth tip 32 protrudes in the circumferential direction on the inner peripheral side of the tooth 34, and the inner peripheral surface 32 </ b> F of the tooth 34 faces the outer periphery of the rotor yoke 22 with a gap G therebetween. The stator core 31 is made of a magnetic material such as an electromagnetic steel plate. The stator core 31 has a back yoke 33 and teeth 34. The back yoke 33 includes an arc-shaped portion. The back yoke 33 has an annular shape. The teeth 34 are portions extending from the inner peripheral surface of the back yoke 33 toward the motor rotor 20.

  The plurality of teeth 34 are arranged at equal intervals in the circumferential direction around the rotation center Zr (the direction along the inner peripheral surface 11d of the cylindrical housing 11a shown in FIG. 3). Hereinafter, the circumferential direction around the rotation center Zr is simply referred to as the circumferential direction. The stator core 31 is press-fitted into the cylindrical housing 11a, so that the motor stator 30 is provided inside the cylindrical housing 11a in an annular state. The stator core 31 and the cylindrical housing 11a may be fixed by adhesion, shrink fitting, welding or the like in addition to press-fitting.

  The exciting coil 37 shown in FIG. 4 is a linear electric wire. The exciting coil 37 is concentratedly wound around the outer periphery of the tooth 34 via an insulator 37a (see FIG. 3). With this configuration, the number of magnetic poles can be reduced, and the coil amount can be reduced because the coil ends are shortened compared to distributed winding. As a result, cost can be reduced and the brushless motor 10 can be made compact.

  The exciting coil 37 may be distributed around a plurality of outer circumferences of the teeth 34. With this configuration, the number of magnetic poles is increased and the distribution of magnetic flux is stabilized, so that torque ripple can be suppressed.

  The exciting coil 37 may be toroidally wound around the outer periphery of the back yoke 33. With this configuration, a magnetic flux distribution equivalent to the distributed winding is generated, and the coil end is shortened compared to the distributed winding, so that the coil amount can be reduced. As a result, torque ripple can be suppressed and the brushless motor 10 can be made compact.

  The insulator 37a is a member for insulating the exciting coil 37 and the stator core 31, and is formed of a heat resistant member. The motor stator 30 has a shape that can surround the motor rotor 20. That is, the stator core 31 is annularly arranged with a predetermined gap (gap) G on the radially outer side of the rotor yoke 22 described later.

  The motor rotor 20 is provided inside the cylindrical housing 11a so that the motor rotor 20 can rotate about the rotation center Zr with respect to the cylindrical housing 11a. The motor rotor 20 includes a shaft 21, a rotor yoke 22, and a magnet 23. The shaft 21 is formed in a cylindrical shape. The rotor yoke 22 is formed in a cylindrical shape. The rotor yoke 22 has an arcuate outer periphery. With this configuration, the rotor yoke 22 can reduce the number of stamping processes compared to a case where the outer periphery has a complicated shape.

  The rotor yoke 22 is manufactured by laminating thin plates such as electromagnetic steel plates and cold-rolled steel plates by means of adhesion, boss, caulking or the like. The rotor yoke 22 is sequentially stacked in the mold and discharged from the mold. The rotor yoke 22 is fixed to the shaft 21 by press-fitting the shaft 21 into, for example, a hollow portion thereof. The shaft 21 and the rotor yoke 22 may be integrally formed.

  A plurality of magnets 23 are embedded along the outer circumferential direction of the rotor yoke 22. The magnet 23 is a permanent magnet, and S poles and N poles are alternately arranged at equal intervals in the circumferential direction of the rotor yoke 22. Accordingly, the number of poles of the motor rotor 20 shown in FIG. 4 is eight poles in which the N poles and the S poles are alternately arranged in the circumferential direction of the rotor yoke 22.

  FIG. 5 is an explanatory diagram schematically showing the configuration of the motor rotor of the present embodiment. As shown in FIG. 5, the magnet accommodation hole 24 is a through hole formed in the rotor yoke 22. A plurality of magnet housing holes 24 are provided on the outer periphery of the motor rotor 20 and arranged at equal intervals. The magnet 23 is accommodated in the magnet accommodation hole 24 of the rotor yoke 22 and attached by, for example, a magnetic force. In the present embodiment, the magnet 23 is a segment magnet having a segmented shape (segment structure) embedded in the outer peripheral direction of the rotor yoke 22 and provided in a plurality.

  As shown in FIG. 5, the magnet 23 in which the N pole is magnetized on the outer peripheral side of the rotor yoke 22 and the magnet 23 in which the S pole is magnetized on the outer peripheral side of the rotor yoke 22 are arranged in the circumferential direction of the rotor yoke 22. They are alternately arranged at equal intervals.

  The flux barriers 25 are through holes provided in the rotor yoke 22 in order to prevent a magnetic flux short circuit between the S pole and the N pole of the magnet 23 provided on both sides in the circumferential direction of the magnet 23. The flux barrier 25 only needs to be capable of suppressing or blocking the passage of magnetic flux, and may be a gap or a non-magnetic insulating material.

  By providing the flux barrier 25 on the rotor yoke 22, a difference in magnetic resistance occurs on the outer periphery of the rotor yoke 22. For example, the motor rotor 20 includes a convex pole portion 27 that protrudes outward in the radial direction of the magnet 23 at a portion near the outer periphery of the rotor yoke 22 that contacts the long side of the magnet 23. The motor rotor 20 includes a support portion 28 that supports both sides of the convex pole portion 27 in the circumferential direction, and a bridge portion 26 that connects the adjacent convex pole portions 27 on the radially outer side of the flux barrier 25. The radial thickness of the convex pole portion 27 is larger than the radial thickness of the support portion 28. The bridge portion 26 connects between the end portions 28 c of the support portion 28. The radial thickness of the bridge portion 26 is the same as the radial thickness of the support portion 28. The bridge portion 26 is connected to the inner diameter side of the motor rotor 20 by a connection portion 29 between the convex pole portions 27.

  Therefore, the convex pole portions 27 and the bridge portions 26 are alternately arranged on the outer periphery of the rotor yoke 22. The convex pole portion 27 is a convex portion that protrudes radially outward of the rotor yoke 22 as a shape, and is a magnetic convex portion that has a large magnetic resistance and a small inductance. The bridge portion 26 is a concave portion that is recessed in the radial direction of the rotor yoke 22 as compared with the convex pole portion 27, and a concave portion that is recessed in the radial direction of the rotor yoke 22 as compared with the convex pole portion 27. The convex pole portion 27, which is a magnetic convex portion, is called the d-axis, and a position magnetically different from this by 90 ° in phase is called the q-axis. By forming a magnetic path on the outer peripheral side from the magnet of the convex pole portion A magnetic recess having a small magnetic resistance and a large inductance. Further, the magnet 23 is embedded in the rotor yoke 22 so as to have a bridge portion 26 that is a magnetic recess, and constitutes a closed magnetic circuit. A gap G, which is a predetermined distance between the stator core 31 and the rotor yoke 22, changes as the rotor yoke 22 rotates by alternately switching the gap G <b> 1 and the gap G <b> 2.

When the magnet 23 is embedded in the rotor yoke 22 to form a closed magnetic circuit, the average torque T _t of the motor rotor 20 is added to the outer periphery of the rotor yoke 22 in addition to the magnet torque T _m of the magnet 23 as shown in the following formula (4). A reluctance torque _Tr is generated due to the magnetic unevenness. As a result, even if the magnetic flux interlinking from the rotor yoke 22 to the stator core 31 is reduced, the average torque _Tt generated in the motor rotor 20 is improved.

As described above, the average torque T _t of the motor rotor 20 is generated by the reluctance torque _Tr due to the magnetic unevenness generated on the outer periphery of the rotor yoke 22 in addition to the magnet torque T _m of the magnet 23. The magnet torque T _m and the reluctance torque T _r have different phases. If the line segment connecting the protruding base portion 27c starting to protrude outward in the radial direction and the rotation center Zr does not pass through the magnet 23, the magnetic flux density distribution in the gap G is affected and torque ripple is increased. . Therefore, in the brushless motor 10 of the present embodiment, as shown in FIG. 5, the line segment connecting the protruding base portion 27 c where the protruding pole portion 27 starts to protrude radially outward and the rotation center Zr passes through the magnet 23. I have to. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced.

  FIG. 6 is an explanatory diagram schematically showing a modification of the magnet embedded in the motor rotor of the present embodiment. As shown in FIG. 6, the projecting base portion 27c at which the end portion in the circumferential direction of the magnet 23 begins to project outward in the radial direction, except for a slight chamfering or the like for improving the manufacturing yield, etc. And a line segment connecting the rotation center Zr only needs to pass through the magnet 23. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced.

  7 and 8 are explanatory views schematically showing the configuration of the motor rotor of the present embodiment. For example, as in Patent Document 1 described above, in the plane orthogonal to the rotation center, the curvature of the outer peripheral surface of the convex pole portion at the outermost diameter reference position where the distance from the rotation center is the maximum value is the outermost diameter reference position. May be reduced to the same extent as the curvature of the arc drawn by rotating around the center of rotation. However, in the technique of Patent Document 1, as in the brushless motor 10 of the present embodiment, a line segment connecting the protruding base portion 27 c where the protruding pole portion 27 starts to protrude radially outward and the rotation center Zr passes through the magnet 23. In this case, in the plane orthogonal to the rotation center Zr, the length of the magnet 23 increases in the circumferential direction, and the amount of magnet cannot be reduced. On the other hand, as shown in FIG. 7, in the motor rotor 20 of the present embodiment, the rotor yoke 22 is located at the outermost diameter reference position 27T where the distance from the rotation center Zr is the maximum in a plane orthogonal to the rotation center Zr. The curvature of the outer peripheral surface 27EF of the convex pole portion 27 is larger than the curvature of the arc Arc drawn by rotating the outermost diameter reference position 27T around the rotation center Zr. For this reason, when the segment connecting the protruding base portion 27 c and the rotation center Zr where the protruding pole portion 27 starts to protrude radially outward passes through the magnet 23, the motor rotor 20 of the present embodiment reduces the magnet amount of the magnet 23. In addition, the magnetic flux density distribution in the gap G can be controlled, and the torque ripple can be reduced.

When the distance from the rotation center Zr to the outermost diameter reference position 27T is R1, and the distance from the magnet outer peripheral surface 27IF of the magnet 23 to the outermost diameter reference position 27T is T1, the relationship between T1 and R1 is as follows: Satisfy (1). If (T1 / R1) does not satisfy the following formula (1), out of balance with the reluctance torque T _r and the magnet torque T _m, there is a possibility that the ripple increases. Therefore, the brushless motor 10 which satisfy the following formula (1) may, while generating the reluctance torque T _r, to maintain the balance between the reluctance torque T _r and the magnet torque T _m, which is the ratio to the average torque ripple, ripple ratio Can be efficiently reduced.

  0.07 ≦ (T1 / R1) ≦ 0.15 (1)

As shown in FIG. 8, in one convex pole portion 27, the angle formed by the lines connecting the two protruding base portions 27c and the rotation center Zr is Q1, and the magnet outer peripheral surface of the magnet 23 in contact with the convex pole portion 27 When the angle formed by the lines connecting the end portion 23Ec in the circumferential direction of the 27IF and the rotation center Zr is Q2, the relationship between Q1 and Q2 satisfies the following formula (2). (Q1 / Q2) may not satisfy the following formula (2), imbalance in the reluctance torque T _r and the magnet torque T _m, there is a possibility that the ripple increases. Therefore, the brushless motor 10 which satisfy the following formula (2), while to generate reluctance torque T _r, since it maintains the balance between the reluctance torque T _r and the magnet torque T _m, which is the ratio to the average torque ripple, The ripple rate can be reduced efficiently.

  0.75 ≦ (Q1 / Q2) ≦ 1.29 (2)

  FIG. 9 is an explanatory view schematically showing the outer peripheral surface of the convex pole portion of the present embodiment. As shown in FIG. 9, the curvature of the outer peripheral surface 27EF of the convex pole portion 27 of the present embodiment is an arc shape having the same curvature between the protruding base portions 27c. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced. FIG. 10 is an explanatory view schematically showing a modification of the outer peripheral surface of the convex pole portion of the present embodiment. As shown in FIG. 10, the outer peripheral surface 27EF of the convex pole portion 27 of the present embodiment may be an arbitrary curve such as a combination of curves having a plurality of different curvatures between the protruding base portions 27c or a cycloid curve. As shown in FIGS. 9 and 10, the outer peripheral surface 27EF of the convex pole portion 27 of the present embodiment is such that the curvature of the outer peripheral surface 27EF of the convex pole portion 27 at the outermost diameter reference position 27T is the arc Arc described above. Greater than curvature.

  FIG. 11 is an explanatory diagram schematically showing a modification of the bridge portion of the present embodiment. As shown in FIG. 11, the bridge portion 26 may linearly connect between the support portions 28 via the connection portion 29.

  FIG. 12 is an explanatory view schematically showing a modification of the tooth of the present embodiment. As shown in FIG. 12, the inner peripheral surface 32 </ b> F of the teeth 34 of the stator core 31 facing the outer periphery of the rotor yoke 22 extends in the thickness direction of the stator core 31, that is, in the direction parallel to the axis of the rotation center Zr of the motor rotor 20. A groove 32FS is provided. With this structure, the ripple rate, which is the ratio of the ripple to the average torque, can be efficiently reduced. A plurality of grooves 32FS are provided in the circumferential direction on the inner peripheral surface 32F of the tooth 34 (two in the present embodiment), so that the ripple rate can be efficiently reduced as compared with the case of one groove 32FS.

FIG. 13 is an explanatory diagram for explaining a permeance straight line of a brushless motor of a comparative example in which a magnet is attached to the outer peripheral surface of a cylindrical rotor yoke. FIG. 13 shows a BH demagnetization curve of the magnet, and Q _c1 is a bending point of the BH demagnetization curve. In the electric power steering device 80, the brushless motor 10 that is an electric motor is not energized to the excitation coil 37 and the motor rotor 20 is in an idle state, and the excitation coil 37 is energized and the motor rotor 20 rotates by excitation. .

  Further, when there is no assist command from the ECU 90, the brushless motor 10 is in a state in which no electric power is supplied, and enters a standby state for the assist command. A force (self-aligning torque) in a direction in which the steering wheel 81 returns to the straight traveling state acts on the steering force assist mechanism 83 from the output shaft 82b. Due to this action, in the brushless motor 10, the motor rotor 20 becomes idle, and the motor rotor 20 rotates without the excitation coil 37 being energized.

  As shown in FIG. 13, the permeance straight line PCs <b> 0 indicates an operating point in which the excitation coil 37 is not energized and the motor rotor 20 is idle. Further, the permeance straight line PCsl indicates a case where the excitation coil 37 is energized and the motor rotor 20 is rotated by excitation. The permeance coefficient can be obtained from the slopes of the permeance straight line PCs0 and the permeance straight line PCsl.

Since the permeance straight line of the brushless motor of the comparative example is affixed to the surface of the rotor, the permeance straight line PCs0 changes with the swing angle rsθ by the rotation of the motor rotor 20, and the permeance straight line PCsl has the swing angle rsl. There was a risk of change. For this reason, the brushless motor of the comparative example has a coercive force H _c1 of the magnet itself in order to give a margin to the inflection point Q _c1 of the BH demagnetization curve when the permeance straight line PCsl changes at the swing angle rsl. It is necessary to use a magnet with a larger value.

  For example, when the magnet is composed of an Nd—Fe—B based magnet, Nd is made larger than 20 mass%, at least one or more of Dy or Tb to be added is added larger than 2 mass%, and the saturation magnetic flux density and It is necessary to increase the coercive force. In particular, added rare earth elements that increase the coercive force, such as Dy or Tb, are often expensive, and the brushless motor of the comparative example may increase the manufacturing cost.

FIG. 14 is an explanatory diagram for explaining a permeance straight line of the brushless motor of the present embodiment. FIG. 14 shows a BH demagnetization curve of the magnet 23 of the present embodiment, and _Qc2 is a bending point of the BH demagnetization curve. As shown in FIG. 14, the permeance straight line PCi <b> 0 indicates an operating point in which the excitation coil 37 is not energized and the motor rotor 20 is idle. Further, the permeance straight line PCil indicates a case where the excitation coil 37 is energized and the motor rotor 20 is rotated by excitation.

In the brushless motor 10 of this embodiment, a magnet 23 is embedded in the rotor yoke 22 and the gap G can be reduced. For this reason, the permeance coefficient of the permeance straight line PCil is larger than the permeance coefficient of the permeance straight line PCsl shown in FIG. For example, in the permeance straight line PCil shown in FIG. 14, the value of the coercive force H _c2 of the magnet itself necessary for providing a margin with respect to the inflection point Q _c2 of the BH demagnetization curve is greater than the coercive force H _c1 described above. Can also be reduced.

  Further, since the magnet 23 is embedded in the rotor yoke 22 and a closed magnetic path is formed in the rotor yoke 22, the magnetic flux loops in the closed magnetic path of the rotor yoke. As a result, the amount of magnetic flux interlinking from the rotor yoke 22 in which the magnet 23 is embedded to the stator core 31 is reduced. As a result, it is possible to suppress the possibility that the permeance straight line PCsl changes at the deflection angle rsl due to the rotation of the motor rotor 20 shown in FIG.

  For example, when the magnet 23 is composed of an Nd—Fe—B type magnet, the magnet 23 can have Nd of 20% by mass or less. Alternatively, the magnet 23 is an Nd-Fe-B magnet with Nd of 20% by mass or less based on the total mass of the magnet, and a rare earth element that is less expensive than Nd, such as Pr, is added to the Nd-Fe-B magnet. By doing so, the magnetic flux density can be secured.

  When the magnet 23 is composed of an Nd—Fe—B-based magnet, the magnet 23 can make at least one of Dy or Tb to be added 2 mass% or less. Since the use of the added rare earth element that increases the coercive force can be reduced, the brushless motor 10 of the present embodiment can suppress an increase in manufacturing cost.

As shown in FIG. 3, the exciting coil 37 has N turns as the number of turns per coil, I (A) as the maximum current of the phase current, n as the number of parallel circuits, and Br (T) as the residual magnetic flux density of the magnet 23. When the magnetized area S (mm ² ) is satisfied, the following formula (3) is satisfied. With this structure, even if the amount of magnet 23 used is reduced, the possibility of torque shortage can be suppressed.

  1.8 ≦ ((N × I × n) / (Br × S)) (3)

(Magnet manufacturing method)
FIG. 15 is a flowchart for explaining the magnet manufacturing method of this embodiment. First, a manufacturing apparatus melt | dissolves a raw material (step S1). The raw material contains, for example, a rare earth element including Fe, B, and Nd at a predetermined ratio. In order to increase the coercive force, the raw material may include one or more rare earth elements of Dy or Tb.

  Next, the manufacturing apparatus rapidly cools the raw material melted in step S1 by a super rapid cooling method such as a single roll method or a gas atomizing method (step S2).

  Next, the manufacturing apparatus performs cold forming in which the raw material obtained in step S2 is filled in a predetermined mold and press-formed in a temperature atmosphere of about room temperature (step S3).

  Next, the manufacturing apparatus performs hot forming by a hot press that heats and pressurizes the formed body obtained in step S3 (step S4). A high density molded body is obtained by hot forming.

  Next, the manufacturing apparatus performs hot extrusion to add anisotropy by heating the molded body obtained in step S4 and orienting the particles of the magnetic material in one direction by mechanical pressure (step S5). . Hot extrusion includes, for example, hot rolling and hot stamping.

  Next, in step S5, the manufacturing apparatus cuts the anisotropic molded body having anisotropy by mechanical pressure into flat segment magnets and processes the shape (step S6). In the manufacturing process of the brushless motor 10, the magnet 23 is magnetized. As described above, hot extrusion molding is anisotropic by orienting particles of a magnetic material in one direction by mechanical pressure. For this reason, the magnet 23 has fine crystal grains and suppresses a demagnetizing field. And when the magnet 23 is comprised with a Nd-Fe-B type | system | group magnet, coercive force can be hold | maintained even if at least 1 or more of Dy or Tb to add is 2 mass% or less.

As described above, the brushless motor 10 of the present embodiment includes the rotor yoke 22 and a plurality of flat magnets embedded in the outer periphery of the rotor yoke 22 so as to form a closed magnetic path in the rotor yoke 22 and formed by hot extrusion molding. a motor rotor 20 including the 23, with a predetermined gap in the radial direction outer side of the rotor yoke 22 excites the stator core 31 and stator core 31 are arranged annularly, causing the reluctance torque T _r and magnet torque T _m in the motor rotor 20 A motor stator 30 including an exciting coil 37.

  With the above configuration, the permeance coefficient of the magnet 23 is improved and the demagnetization resistance is improved by the closed magnetic path in the rotor yoke 22. For this reason, when the magnet 23 formed by hot extrusion molding is composed of an Nd—Fe—B magnet, the magnetic flux density should be ensured even if Nd is 20% by mass or less with respect to the mass of the entire magnet. Can do. Alternatively, in the magnet 23 formed by hot extrusion molding, the Nd—Fe—B type magnet has Nd of 20% by mass or less with respect to the total mass of the magnet, and the Nd—Fe—B type magnet is less expensive than Nd. The magnetic flux density can be secured by adding a rare earth element such as Pr.

  In addition, even if the coercive force of the magnet 23 is reduced, the possibility of demagnetization can be reduced. Therefore, the magnet 23 can reduce the rare earth that improves the coercive force of the magnet 23. For example, when the magnet 23 formed by hot extrusion molding is composed of an Nd—Fe—B magnet, the magnet 23 can contain at least one of Dy or Tb to be added at 2 mass% or less. As a result, a magnet having a coercive force of 1400 kA / m or less and a saturation magnetic flux density of 1.2 T or more can be used as the magnet 23.

  The electric power steering device 80 obtains auxiliary steering torque by the brushless motor 10 described above. Since the brushless motor 10 suppresses the loss torque, the electric power steering device 80 can operate in a state where the steering person suppresses the uncomfortable feeling. For this reason, even if the electric power steering apparatus 80 is a column assist system, it can make a low vibration. In addition, although the electric power steering apparatus 80 of this embodiment has been described by taking the column assist method as an example, it can also be applied to a pinion assist method and a rack assist method.

(Direct drive motor)
The electric motor of this embodiment can also be used as a direct drive motor in which the motor rotor 20 and a load body that is rotated by the motor rotor 20 are directly connected. FIG. 16 is a configuration diagram when the electric motor according to the present embodiment is applied as a direct drive motor. Note that the same components as those described in the above-described embodiment are denoted by the same reference numerals, and redundant description is omitted.

  As illustrated in FIG. 16, the electric motor 10 </ b> A can directly transmit the rotational force to the load body 52 without using a transmission mechanism such as a gear, a belt, or a roller to rotate the load body 52. The electric motor 10A is a direct drive motor in which a so-called motor rotating shaft 60 and a load body 52 are directly connected by a fixing member 67 such as a bolt.

  The electric motor 10A includes a motor stator 30 that is kept stationary, a motor rotor 20 that is rotatably arranged with respect to the motor stator 30, a base member 70 that is fixed to the motor stator 30 and attached to a support member 51, and a motor rotor. A motor rotating shaft 60 fixed to the motor 20 and rotatable together with the motor rotor 20, and a bearing 64 interposed between the base member 70 and the motor rotating shaft 60 and rotatably supporting the motor rotating shaft 60 with respect to the base member 70. And including.

  The base member 70, the motor rotating shaft 60, the motor rotor 20, and the motor stator 30 are all annular structures. The motor rotation shaft 60, the motor rotor 20, and the motor stator 30 are arranged concentrically around the rotation center Zr. In addition, the electric motor 10A is arranged in order of the motor rotor 20 and the motor stator 30 from the rotation center Zr to the outside. Such an electric motor 10 </ b> A is called an inner rotor type, and the motor rotor 20 is closer to the rotation center Zr than the motor stator 30. In the electric motor 10 </ b> A, the motor rotation shaft 60, the motor rotor 20, and the motor stator 30 are disposed on the base member 70.

  The base member 70 includes a substantially disc-shaped housing base 71, and a housing inner 72 that serves as an axial center projecting from the housing base 71 so as to surround the hollow portion 61 through the hollow portion 61. The housing inner 72 is fastened and fixed to the housing base 71 via a fixing member 77 such as a bolt. The base member 70 includes a housing flange 73 that fixes an inner ring of the bearing 64 to the housing base 71 via a fixing member 76 such as a bolt.

  The motor stator 30 is fastened to the outer peripheral edge of the housing base 71 by a fixing member 78 such as a bolt. Thereby, the motor stator 30 is positioned and fixed with respect to the housing base 71. The central axis of the motor stator 30 coincides with the rotation center Zr of the motor rotor 20.

  The motor stator 30 includes a cylindrical stator core 31 and an excitation coil 37. In the motor stator 30, an exciting coil 37 is wound around a stator core 31. A wiring (not shown) for supplying power from a power source is connected to the motor stator 30, and power is supplied to the exciting coil 37 through this wiring.

  The motor rotor 20 has a cylindrical shape in which the outer diameter of the motor rotor 20 is smaller than the inner diameter dimension of the motor stator 30. The motor rotor 20 includes a rotor yoke 22 and a magnet 23 attached to the outer periphery of the rotor yoke 22.

  The motor rotating shaft 60 includes an annular rotating shaft 62 and a rotor flange 63 that fixes an outer ring of the bearing 64 to the rotating shaft 62 via a fixing member 66 such as a bolt. The motor rotor 20 is integrally fixed to a rotating shaft 62 of a cylindrical motor rotating shaft 60. The motor rotor 20 may be fixed to the rotating shaft 62 of the cylindrical motor rotating shaft 60 by a fixing member. The rotation shaft 62 is formed such that the center axis of the ring is coaxial with the rotation center Zr of the electric motor 10A.

  The bearing 64 has an outer ring fixed to the rotor flange 63 and an inner ring fixed to the housing flange 73. Thereby, the bearing 64 can rotatably support the rotating shaft 62 and the motor rotor 20 with respect to the housing base 71. For this reason, the electric motor 10 </ b> A can rotate the rotating shaft 62 and the motor rotor 20 with respect to the housing base 71 and the motor stator 30.

  The bearing 64 may be a cross roller bearing in which a rolling element is a cross roller. The bearing 64 is not limited to a cross roller bearing, and may be, for example, a ball bearing or a roller bearing in which rolling elements are balls or rollers (cylindrical rollers, tapered rollers, spherical rollers, etc.). These rolling elements may be arranged at predetermined intervals, for example, at regular intervals, one by one in a pocket of the annular cage, and may be incorporated between the raceway surfaces while being rotatably held in the pocket. As a result, the rolling elements can roll between the raceway surfaces in a state where a predetermined interval is maintained without the rolling surfaces contacting each other. Moreover, each rolling element can prevent an increase in rotational resistance or seizure due to friction caused by contact with each other.

  In addition, the electric motor 10A includes a rotation detector. The rotation detector is, for example, a resolver, and can detect the rotational positions of the motor rotor 20 and the motor rotation shaft 60 with high accuracy. The rotation detector includes a resolver stator 14b that is maintained in a stationary state, a resolver rotor 14a that is disposed to face the resolver stator 14b with a predetermined gap therebetween and is rotatable with respect to the resolver stator 14b. Arranged above.

  The resolver stator 14b has an annular laminated iron core in which a plurality of stator magnetic poles are formed at equal intervals in the circumferential direction, and a resolver coil is wound around each stator magnetic pole. In the electric motor 10 </ b> A of the present embodiment, the resolver stator 14 b is fixed to the housing inner 72.

  The resolver rotor 14 a is constituted by a hollow annular laminated iron core, and is fixed to the inner side of the motor rotating shaft 60. The position of the rotation detector is not particularly limited as long as the rotation of the motor rotor 20 (motor rotation shaft 60) can be detected, and can be set to any position depending on the shape of the motor rotation shaft 60 and the base member 70. It can be arranged.

  When the motor rotor 20 rotates, the motor rotating shaft 60 rotates together with the motor rotor 20, and the resolver rotor 14a also rotates in conjunction therewith. Thereby, the reluctance between the resolver rotor 14a and the resolver stator 14b changes continuously. The resolver stator 14b detects a change in reluctance and converts it into an electrical signal (digital signal) by a resolver control circuit. The control device that controls the electric motor 10A can calculate the positions and rotation angles of the motor rotating shaft 60 and the motor rotor 20 that are linked to the resolver rotor 14a per unit time based on the electric signal of the resolver control circuit. As a result, the control device that controls the electric motor 10A can measure the rotation state (for example, the rotation speed, the rotation direction, or the rotation angle) of the motor rotation shaft 60.

  Thus, the electric motor 10A can grasp the absolute position of the motor rotating shaft 60 by including a plurality of rotation detectors having different periods of the fundamental wave component of the reluctance change for each rotation of the motor rotor 20. The accuracy of measuring the rotation state (for example, the rotation speed, rotation direction, or rotation angle) of the motor rotation shaft 60 can be increased.

  The electric motor 10 </ b> A is positioned and fixed with respect to the support member 51 by attaching the housing base 71 to the support member 51. The housing base 71 has at least one series of continuous surfaces in contact with the mounting surface of the support member 51 in a state of being mounted on the support member 51. The continuous surface can act by dispersing the weight of the electric motor 10 </ b> A, vibration during rotation, and the like on the support member 51. For this reason, it is possible to prevent the housing base 71 from being distorted (bent).

  As described above, the electric motor 10A can directly transmit the rotational force to the load body 52 without interposing a transmission mechanism. For this reason, when rotating the load body 52, the electric motor 10A can increase the motor torque and reduce the torque ripple.

(Evaluation example)
The brushless motor 10 that is the electric motor of the present embodiment described above was manufactured. In the brushless motor 10, the number of teeth of the stator core is 12, and the number of magnets in the circumferential direction is 8. As shown in FIG. 7, when the distance from the rotation center Zr to the outermost diameter reference position 27T is R1, and the distance from the magnet outer peripheral surface 27IF of the magnet 23 to the outermost diameter reference position 27T is T1, the parameter A is It was defined as the following formula (5).

  By fixing R1 and changing the value of T1, parameter A is changed between 0.07 and 0.25, and the effect of parameter A on average torque (Nm) and torque ripple (Nm) is affected. investigated. FIG. 17 is an explanatory diagram for explaining the influence of the distance from the outer peripheral surface of the magnet to the outermost diameter reference position on the ripple rate. FIG. 17 shows the average torque (Nm) and torque ripple (Nm) according to the parameter A, and the ripple rate (%). The ripple rate (%) was calculated by (torque ripple (Nm) / average torque (Nm)) × 100 and plotted in FIG.

  As shown in FIG. 17, when the parameter A becomes 0.07 or more, the average torque gat decreases as the parameter A increases. Further, when the parameter A is 0.07 or more, the torque ripple gri is suppressed to be larger than the reduction ratio of the average torque gat as the parameter A increases. Further, when the parameter A becomes 0.1 or more, the torque ripple gri increases as the parameter A increases. For this reason, when the parameter A exceeds 0.15, as the parameter A increases, the ripple rate grt increases as compared to the case where the parameter A is 0.07. As mentioned above, the electric motor of this embodiment can reduce a ripple rate efficiently, when 0.07 <= A <= 0.15 is satisfy | filled.

  As shown in FIG. 8, in one convex pole portion 27, the angle formed by the lines connecting the protruding base portion 27c and the rotation center Zr is Q1, and the circumference of the magnet outer peripheral surface 27IF of the magnet 23 in contact with the convex pole portion 27 is set. When the angle formed by the lines connecting the direction end 23Ec and the rotation center Zr is Q2, the parameter B is defined as the following formula (6).

  By fixing Q1 and changing the value of Q1, parameter B is changed between 0.71 and 1.36, and the effect of parameter B on the average torque (Nm) and torque ripple (Nm) is affected. investigated. FIG. 18 is an explanatory diagram for explaining the influence of the angle formed by the lines connecting the protruding base and the rotation center on the ripple rate. FIG. 18 shows the average torque (Nm) and torque ripple (Nm) according to the parameter B, and the ripple rate (%). The ripple rate (%) was calculated by (torque ripple (Nm) / average torque (Nm)) × 100 and plotted in FIG.

  As shown in FIG. 18, when the parameter B becomes 0.75 or more, the average torque gat increases as the parameter B increases. When the parameter B becomes 1.11 or more, the average torque gat starts to decrease as the parameter B increases. Further, when the parameter B is 0.75 or more, the torque ripple gri decreases as the parameter B increases. When the parameter B is 0.86 or more, the torque ripple gri increases as the parameter B increases. For this reason, when the parameter B exceeds 1.29, as the parameter B increases, the ripple rate grt increases, for example, exceeding 5%. For this reason, the electric motor of this embodiment can reduce a ripple rate efficiently, when satisfy | filling 0.75 <= B <= 1.29.

FIG. 19 is an explanatory diagram for explaining the influence of the groove on the inner peripheral surface of the tooth on the ripple rate. For motor has a groove 32FS shown in FIG. 12, as shown in FIG. 19, the mechanical angle of the torque (Nm) for the variation curve of one period and _{T SL.} If the motor without a groove 32FS shown in FIG. 12, as shown in FIG. 19, the mechanical angle of the torque (Nm) for the variation curve of one period and T _SN. As shown in FIG. 19, variation curve _{T SL,} change in torque than the variation curve _{T SN,} i.e. torque ripple is small.

  FIG. 20 is an explanatory diagram illustrating the magnetic characteristics of a magnet having a Dy content of 2 mass% or less. When the magnet is composed of an Nd—Fe—B based magnet, the comparative example Mg1 is a sintered magnet at room temperature (20 ° C.) and contains 4% by mass of Dy. For example, the composition of the comparative example Mg1 is Nd—Dy—Fe—B. Evaluation Example Mg2 is a normal temperature (20 ° C.) Nd—Fe—B magnet manufactured by the magnet manufacturing method shown in FIG. Evaluation Example Mg2 is formed by hot extrusion molding, and the content of Dy is 0% by mass. For example, the composition of the evaluation example Mg2 is Nd—Pr—Fe—B. Comparative example Mg1h shows the magnetic properties at 120 ° C. of comparative example Mg1. Moreover, evaluation example Mg2h has shown the 120 degreeC magnetic characteristic of evaluation example Mg2.

  As shown in FIG. 20, the evaluation example Mg2 has substantially the same magnetic flux density and BH characteristics as the comparative example Mg1. Further, it can be seen that the evaluation example Mg2h has the same degree of demagnetization even when the temperature rises to 120 ° C., compared with the comparative example Mg1h.

  In the electric motor of this embodiment, the permeance coefficient of the magnet is improved and stabilized by the closed magnetic path in the rotor yoke 22, and the demagnetization resistance is improved. For this reason, when a magnet formed by hot extrusion molding using magnetic particles with refined crystal grains is composed of an Nd-Fe-B magnet, Nd is 20% by mass with respect to the total mass of the magnet. Even below, a magnetic flux density of 1.2 T or more can be secured. Moreover, even if the coercive force of the magnet is reduced, the possibility of demagnetization can be reduced, so that the rare earth that improves the coercive force of the magnet can be reduced. For example, in the case where a magnet formed by hot extrusion molding using magnetic particles with fine crystal grains is composed of an Nd-Fe-B magnet, at least one of Dy or Tb to be added is 2% by mass or less. It can be. As a result, a magnet having a coercive force of 1400 kA / m or less and a saturation magnetic flux density of 1.2 T or more can be used.

The magnet 23 is preferably a flat segment magnet formed by hot extrusion. The coercive force of the magnet 23 can be increased even when the magnet 23 is a flat segment magnet by using a magnetic powder formed by hot extrusion molding and having fine crystal grains. As described above, the magnet 23, by being embedded to allow a closed magnetic path in the rotor yoke within 22, the decrease of the magnet torque T _m, the average torque T _t produced in the motor rotor 20 supplemented with reluctance torque T _r Can be maintained.

  When the magnet 23 is composed of an Nd—Fe—B system magnet, the magnetic flux density necessary for the closed magnetic path of the motor rotor 20 can be ensured even if the Nd is 20% by mass or less with respect to the mass of the entire magnet. Moreover, even if the coercive force of the magnet 23 is reduced, the possibility of demagnetization can be reduced, so that the rare earth that improves the coercive force of the magnet 23 can be reduced. For example, when the magnet 23 is composed of an Nd—Fe—B type magnet, at least one of Dy or Tb to be added can be 2% by mass or less. As a result, a magnet having a coercive force of 1400 kA / m or less and a saturation magnetic flux density of 1.2 T or more can be used as the magnet 23.

Next, as shown in FIG. 3, the exciting coil 37 has N turns as the number of turns per coil, I (A) as the maximum phase current, n as the number of parallel circuits, and Br ( The parameter D was defined by the following formula (7), where T) and the magnetized area S (mm ² ).

  Here, the element AT is an element indicating the strength of the magnetic field generated by the exciting coil 37, and the element WB is an element indicating the strength of the magnetic field generated by the magnet 23. The parameter D, which is the ratio between the element AT and the element WB, was changed between 1.8 and 3.1, and the influence on the average torque (Nm) was investigated. If the parameter D is 1.8 or more, the geometric dimension of the motor yoke 22 is not restricted. If the parameter D is 3.1 or less, the demagnetization limit of the magnet 23 can be set. As a result, it has been found that even if the amount of magnet 23 used is reduced, the possibility of insufficient torque can be suppressed when D is 1.8 or more.

10 Brushless motor (electric motor)
DESCRIPTION OF SYMBOLS 10A Electric motor 11 Housing 11a Tubular housing 11d Inner peripheral surface 20 Motor rotor 21 Shaft 22 Rotor yoke 23 Magnet 23Ec End part 24 Magnet accommodation hole 25 Flux barrier 26 Bridge part 27 Protruding pole part 27c Projection base part 27IF Magnet outer peripheral surface 27EF Outer outer peripheral surface 27T Outer diameter reference position 28 Support portion 28c End portion 29 Connection portion 30 Motor stator 31 Stator core 32 Teeth tip 33 Back yoke 34 Teeth 37 Excitation coil 52 Load body 60 Motor rotating shaft 61 Hollow portion 62 Rotating shaft 63 Rotor flange 64 Bearing 70 Base member 71 Housing base 72 Housing inner 73 Housing flange 80 Electric power steering device Zr Rotation center

Claims (11)

A motor rotor including a rotor yoke, a plurality of magnets embedded in the rotor yoke, and a flux barrier provided on both sides in the circumferential direction of the magnet to prevent a magnetic flux short circuit;
A motor stator including a stator core arranged in an annular shape on the radially outer side of the rotor yoke and an exciting coil for exciting the stator core;
Including
The rotor yoke includes a projecting pole portion protruding outward in the radial direction of the magnet, and a bridge portion connecting between the projecting pole portions on the radially outer side of the flux barrier,
In the plane orthogonal to the rotation center of the motor rotor, the curvature of the outer peripheral surface of the convex pole portion at the outermost diameter reference position where the distance from the rotation center becomes the maximum value is the outermost diameter reference position at which the rotation center is An electric motor characterized in that a line segment connecting the projecting base and the center of rotation, which is larger than a curvature drawn by rotating as a center and starts to project radially outward, passes through the magnet. When the distance from the rotation center to the outermost diameter reference position is R1, and the distance from the outer peripheral surface of the magnet to the outermost diameter reference position is T1, the relationship between R1 and T1 is given by the following formula (1 The electric motor according to claim 1, wherein:
0.07 ≦ (T1 / R1) ≦ 0.15 (1) In one of the convex pole portions, an angle formed by lines connecting the two protruding base portions and the rotation center is Q1, the circumferential end of the outer peripheral surface of the magnet in contact with the convex pole portion, and the rotation 3. The electric motor according to claim 1, wherein a relationship between Q <b> 1 and Q <b> 2 satisfies the following expression (2), where Q <b> 2 is an angle between lines connecting the centers.
0.75 ≦ (Q1 / Q2) ≦ 1.29 (2) When the number of turns per coil is N, the maximum current of the phase current is I, the number of parallel circuits is n, the residual magnetic flux density of the magnet is Br, and the magnetization area is S, The electric motor according to any one of claims 1 to 3, which satisfies 3).
1.8 ≦ ((N × I × n) / (Br × S)) (3)   The electric motor according to any one of claims 1 to 4, wherein a curvature of an outer peripheral surface of the convex pole portion has the same curvature between the projecting base portions.   The electric motor according to any one of claims 1 to 4, wherein a curved surface having a different curvature between the projecting bases is continuously combined on an outer peripheral surface of the convex pole part.   The groove | channel extended in the direction parallel to the axis | shaft of the rotation center of the said motor rotor is provided in the inner peripheral surface of the teeth of the said stator core facing the outer periphery of the said rotor yoke. Electric motor.   The electric motor according to any one of claims 1 to 7, wherein a content of Dy contained in the magnet is 2% by mass or less.   The electric motor according to any one of claims 1 to 8, wherein the number of teeth of the stator core is 12, and the number of the magnets in the circumferential direction is eight.   10. The electric motor according to claim 1, wherein the electric motor is a direct drive motor, and the motor rotor is directly connected to a load body that is rotated by the motor rotor.   An electric power steering apparatus, wherein an auxiliary steering torque is obtained by the electric motor according to any one of claims 1 to 9.

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