Brushless DC motor

Abstract

In a brushless DC motor (1), the number of poles of a magnet is greater than or equal to four and greater than the number of slots, and a motor stator (4) of the brushless DC motor (1) has a two-split core structure. A teeth core (6) of the motor stator (4) is provided with narrow auxiliary pole teeth (63) disposed between main pole teeth (62), around which a coil is wound. Imbalance of the circumferential-direction magnetic flux distribution is eliminated, and cogging torque is reduced. The magnetic flux generated from the coils (5(U), 5(V), 5(W)) of each phase wound around the main pole teeth (62) and the magnetic flux of the magnet flow efficiently to the auxiliary pole teeth (63) via a cylindrical section (61) of the teeth core (6), and thus the electromotive force can be increased.

Description

Brushless DC motor

[0001] The present invention relates to a brushless DC motor having a stator core shape suitable for use as a small motor with a small output of, for example, about 75 W or less.

[0002] In brushless DC motors used as small motors, it is considered to use cored motors instead of the commonly used coreless motors in order to achieve miniaturization and high torque. In cored motors, the main structures of the stator core are a "slot-divided core structure" in which the stator core is divided into slots in the circumferential direction, and a "two-divided core structure" in which the back core surrounds the outer periphery of the teeth core. Furthermore, the "two-divided core structure" is an advantageous structure for increasing the space factor of the winding.

[0003] Taking a three-phase, three-slot brushless DC motor as an example, in the former three-slot divided core structure, as shown in Fig. 6(A1), each slot is arranged at an equal angle of 120°, each slot has a high polar arc ratio, and each divided core has an arc-shaped portion that extends at a large angle in the circumferential direction at the inner and outer peripheral ends of the main pole (teeth), as shown in Fig. 6(A2). For this reason, when performing automatic winding, for example, it may not be possible to form a coil winding suitable for that shape in the gap between the arc-shaped portions on the inner and outer peripheral sides, and in some cases, the coil winding nozzle may not be able to fit, making it impossible to perform coil winding.

[0004] In contrast, in the case of the latter two-piece core structure in which the stator core is made up of two parts, a teeth core and a back core, as shown in Figure 6 (B1), the teeth portions (main poles) on which winding is performed are evenly arranged in the circumferential direction of the teeth core, and as shown in Figure 6 (B2), the coil winding is attached to each main pole from the outer periphery side, and then the back core is attached, so forming the coil winding is easy.

[0005] Furthermore, when attempting to reduce the diameter of a motor, in the case of a three-slot split core structure, the cross-sectional area of ​​the mating surfaces of adjacent split cores cannot be made large, so in the case of a stator core with a laminated structure, there is a problem of not being able to obtain positional accuracy when laminating and mating the core lamination plates. In contrast, by making the stator core into a two-split core, the main poles in the teeth core are connected by a continuous ring in the circumferential direction, making it easier to ensure positional accuracy when assembling a stator core with a laminated structure.

[0006] From this perspective, when a cored motor is used to reduce the size and increase the torque of a brushless DC motor, it is desirable to use a stator core with a two-piece core structure. Patent Documents 1, 2, and 3 disclose cored motors with a two-piece core structure in which the stator core is made up of two parts: a teeth core and a back core.

[0007] International Publication No. 2016 / 017179 Japanese Utility Model Application Laid-Open No. 64-45450 Japanese Patent Laid-Open No. 2015-167464

[0008] While coreless motors do not generate cogging torque because they have no internal iron core, cored motors do generate cogging torque due to the influence of the core. The inventors first prototyped and evaluated a cored motor with a two-pole magnet and three-slot two-piece core structure. As a result, while the cogging torque increased significantly, the induced voltage (torque) was higher than that of coreless motors of the same size.

[0009] To address this issue, we first tried to reduce cogging torque in core-type motors by increasing the number of magnetic poles by using a four-pole radially anisotropic magnet for the rotor magnet. This resulted in a reduction in cogging torque. However, when using a four-pole rotor magnet—for example, a four-pole, three-slot configuration—we found that the arc-shaped portion near the midpoint between the main poles of the stator core (the area surrounded by the dashed line in Figure 6(B1)) is far from the coil windings, resulting in a small magnetic flux and an imbalance in the magnetic flux distribution generated in the stator. This imbalance in the magnetic flux distribution can result in insufficient suppression of cogging torque, potentially increasing the motor's speed ripple and torque ripple. Furthermore, the reduction in effective magnetic flux due to the imbalance in the magnetic flux distribution can also reduce induced voltage, potentially preventing sufficient torque improvement. This problem occurs when the number of poles is greater than the number of slots.

[0010] Therefore, the inventors adopted a stator core with a two-piece core structure to achieve compact size and high torque in a brushless DC motor with four or more poles and a number greater than the number of slots (the number of coil windings). To solve the above-mentioned problems, the inventors adopted a stator core with a tooth core in which auxiliary pole teeth are arranged between main pole teeth on which coil windings are attached.

[0011] The object of the present invention is to achieve high torque in a brushless DC motor in which the number of magnet poles is four or more, which is greater than the number of slots, and in which the stator core has a two-part core structure in which auxiliary pole teeth are arranged between the main pole teeth.

[0012] In order to solve the above problems, the brushless DC motor of the present invention comprises: a motor rotor; and a motor stator coaxially surrounding the motor rotor; the motor rotor has a rotor outer circumferential surface on which positive and negative poles of permanent magnets are alternately arranged at equal angular intervals in the circumferential direction; the motor stator has a stator core with a two-part structure including teeth cores to which coil windings of each phase are attached, and a cylindrical back core coaxially surrounding the teeth cores; the motor rotor has a number of poles that is four or more and is greater than the number of slots (number of coil windings) in the motor stator; and the teeth core comprises: a cylindrical portion coaxially surrounding the rotor outer circumferential surface of the motor rotor with a fixed gap; and a plurality of main pole teeth and a plurality of inter-pole teeth alternately arranged at equal angular intervals in the circumferential direction on the outer circumferential surface of the cylindrical portion, When viewed in a section perpendicular to the central axis of the cylindrical portion, each of the main pole teeth and the inter-pole teeth protrudes radially outward from the outer peripheral surface of the cylindrical portion by a constant width to a position where it abuts the inner peripheral surface of the back core, the width of the inter-pole teeth being narrower than that of the main pole teeth, and each of the main pole teeth is fitted with a coil winding of a different phase between the inter-pole teeth adjacent to each other on the left and right in the circumferential direction so as not to interfere with these inter-pole teeth.

[0013] In addition to the above configuration, the brushless DC motor of the present invention is characterized in that, in the cylindrical portion of the teeth core, each of the arc portions connecting the circumferentially adjacent main pole teeth and inter-pole teeth includes a main pole-side portion connected to the main pole tooth and an inter-pole-side portion connected to the inter-pole tooth, and the radial thickness of the main pole-side portion is greater than the radial thickness of the inter-pole-side portion. For example, the thickness of the inter-pole-side portion is constant at each position in the circumferential direction, and the thickness of the main pole-side portion is set to gradually increase from the position connected to the inter-pole-side portion toward the main pole tooth.

[0014] The brushless DC motor of the present invention has, for example, a four-pole, three-slot configuration, and is equipped with a stator core having a two-part core structure in which three main pole teeth and three interpole teeth are arranged alternately around the circumference at equal angular intervals, and is equipped with three coil windings attached to each main pole tooth.

[0015] In the brushless DC motor of the present invention, the addition of inter-pole teeth at intermediate positions between the main pole teeth of the tooth core ensures the passage of magnetic flux. Furthermore, the main pole teeth and inter-pole teeth are arranged at equal angular intervals circumferentially, eliminating imbalances in magnetic flux distribution. As a result, cogging torque is reduced, and the occurrence of speed ripple and torque ripple in the motor is suppressed. Furthermore, the increased effective magnetic flux increases induced voltage, which is expected to improve torque. Furthermore, the narrower width of the inter-pole teeth compared to the main pole teeth ensures sufficient coil winding space while maintaining required motor performance. Furthermore, if the arc-shaped portions connecting the main pole teeth and inter-pole teeth in the cylindrical portion of the stator core are designed so that the main pole side portions are thicker than the inter-pole side portions, the magnetic flux passing through the main pole side portions of the arc-shaped portions functions as effective magnetic flux without becoming saturated magnetic flux, thereby improving torque. Therefore, the present invention provides a compact, high-torque brushless DC motor with reduced speed ripple and torque ripple.

[0016] (A) is an explanatory diagram showing a half-longitudinal cross section of a brushless DC motor according to a first embodiment of the present invention, and (B) is an explanatory diagram showing its cross section. (A) is an explanatory diagram showing a control brushless DC motor (without interpole teeth) that does not have interpole teeth, and (B) is an explanatory diagram showing a brushless DC motor according to the present invention (with interpole teeth). (A) is a graph showing an example of measurement results of induced voltage, and (B) is a graph showing an example of measurement results of cogging torque. (A) and (B) are diagrams showing an example of magnetic flux density distribution (flux lines). (A) and (B) are diagrams showing an example of magnetic flux density distribution (vector plot). (A1) is an explanatory diagram showing a stator core with a three-slot split core structure, (A2) is an explanatory diagram showing the split core, (B1) is an explanatory diagram showing a stator core with a two-split split core structure, and (B2) is an explanatory diagram showing the installation of coil windings. (A) is an explanatory diagram showing a half-longitudinal cross section of a brushless DC motor according to a second embodiment of the present invention, and (B) is an explanatory diagram showing the cross section. FIG. 8 is a cross-sectional view showing a stator core of the brushless DC motor of FIG. 7.

[0017] Hereinafter, embodiments of a brushless DC motor to which the present invention is applied will be described with reference to the drawings. The embodiments show examples of the present invention, and the present invention is not limited to the embodiments.

[0018] [Embodiment 1] Fig. 1(A) is an explanatory diagram showing a half-longitudinal cross section of a brushless DC motor according to this embodiment, and Fig. 1(B) is an explanatory diagram showing the same cross section. Brushless DC motor 1 is a three-phase inner rotor type equipped with a four-pole magnet and a stator with a two-split core structure, with one coil winding (one slot) for each of the U, V, and W phases. Brushless DC motor 1 includes motor rotor 3 coaxially attached to motor shaft 2 located at the center, motor stator 4 coaxially surrounding motor rotor 3, and motor frame 10 coaxially surrounding motor stator 4. A motor driver (not shown) switches and controls the supply of current to the coil windings of each phase depending on the magnetic pole position.

[0019] The motor rotor 3 has a rotor outer circumferential surface 3a on which positive and negative poles of permanent magnets are alternately arranged at equal angular intervals in the circumferential direction. In this example, four permanent magnets 31 to 34 are attached to the outer circumferential surface of the motor shaft 2 and are arc-shaped, subtending an angle of 90°. Four poles, positive and negative poles alternately arranged at 90° intervals in the circumferential direction, are formed on the rotor outer circumferential surface 3a.

[0020] The motor stator 4 includes, for example, a U-phase coil 5 (U), a V-phase coil 5 (V), and a W-phase coil 5 (W) each made of an air-core coil, a teeth core 6, and a cylindrical back core 7 coaxially surrounding the teeth core 6. The teeth core 6 includes a cylindrical portion 61 coaxially surrounding the rotor outer peripheral surface 3a of the motor rotor 3 with a fixed gap therebetween, and main pole teeth 62 and inter-pole teeth 63 arranged alternately at angular intervals of 60° in the circumferential direction on the circular outer peripheral surface of the cylindrical portion 61.

[0021] When viewed from a plane perpendicular to the central axis of the cylindrical portion 61 (the central axis 1a of the motor), each of the three main pole teeth 62 protrudes radially outward from the circular outer surface of the cylindrical portion 61 with the same width, and its outer peripheral tip face 62a is arc-shaped and abuts against the circular inner peripheral surface 70a of the back core 7. Similarly, each of the three inter-pole teeth 63 protrudes radially outward from the circular outer peripheral surface of the cylindrical portion 61, and its outer peripheral end face 63a is arc-shaped and abuts against the circular inner peripheral surface 70a of the back core 7. In this example, the width w63 of the inter-pole tooth 63 is narrower than the width w62 of the main pole tooth 62. In this example, the width w63 is set to 1 / 2 of the width w62. In addition, a U-phase coil 5 (U), a V-phase coil 5 (V), and a W-phase coil 5 (W) are wound around each of the three main pole teeth 62 between the adjacent commutating pole teeth 63 on the left and right in the circumferential direction so as not to interfere with these commutating pole teeth 63.

[0022] In a brushless DC motor 1 equipped with teeth cores 6 of this shape, the provision of inter-pole teeth 63 eliminates imbalances in the circumferential magnetic flux distribution, reducing cogging torque and torque ripple and speed ripple. Furthermore, the magnetic flux and magnetic flux generated by the coils 5 (U), 5 (V), and 5 (W) of each phase wound around the main pole teeth 62 flow efficiently to the inter-pole teeth 63 via the cylindrical portions 61 of the teeth cores 6, thereby increasing induced electromotive force. In other words, the number of turns in the coils 5 (U), 5 (V), and 5 (W) of each phase required to achieve the target induced electromotive force can be reduced, thereby avoiding an increase in the overall length of the motor. Furthermore, the width w63 of the inter-pole teeth 63 is narrower than the width w62 of the main pole teeth 62. This increases the winding space for the coil, allowing for a larger number of turns or a thicker winding diameter than when the inter-pole teeth 63 are the same width as the main pole teeth 62. In this example, the width w63 of the inter-pole teeth 63 is set to half the width w62 of the main pole teeth 62, and it has been confirmed that the number of turns can be increased by 10 to 15% compared to when the widths of the two are the same (w62 = w63) while still ensuring the required motor performance. This allows for improved torque.

[0023] Although the brushless DC motor 1 has been described using an example in which it has three slots, which is the minimum configuration for the U, V, and W phases, there is no restriction on the number of slots, and it goes without saying that a configuration with interpole teeth can be applied even when the number of slots is increased.

[0024] The materials for each part can be, for example, the following materials, but the present invention is not limited to these materials: Motor frame: steel or SUS material Back core: electromagnetic steel plate or soft magnetic material Tees core: electromagnetic steel plate Coil: copper Permanent magnet: neodymium magnet Motor shaft: steel

[0025] The teeth core 6 has a laminated structure in which, for example, laminate plates are laminated and bonded in the axial direction. The provision of the inter-pole teeth 63 increases the bonding area, which has the advantage of improving the strength of the laminated core and making it less likely for the laminate plates to peel off or for the laminated core to crack. Of course, the teeth core 6 may have a structure other than that of a laminated core.

[0026] Furthermore, air-core coils are used as the coils 5(U), 5(V), and 5(W) of each phase, but there are no restrictions on the winding method of the coils.

[0027] (Example of Magnetic Analysis) The inventors performed magnetic analysis on a brushless DC motor 1 equipped with inter-pole teeth and a control motor having the same structure except that it does not have inter-pole teeth. Fig. 2(A) shows a control brushless DC motor 100 (main pole only) that does not have inter-pole teeth, and Fig. 2(B) shows the brushless DC motor 1 of this example (main pole + inter-pole). In the control brushless DC motor 100, parts that correspond to parts of the brushless DC motor 1 of this example are assigned the same reference numerals. The materials of each part are the same as those described above.

[0028] 3A is a graph showing an example of the measurement results of induced voltage, and FIG. 3B is a graph showing an example of the measurement results of cogging torque. In these graphs, the solid line shows the measurement results for the case of a main pole and an inter-pole, i.e., the case of the brushless DC motor 1 of this example, and the dashed line shows the measurement results for the case of only the main pole, i.e., the case of the control brushless DC motor 100 which does not have inter-pole teeth. It can be seen that, assuming the case of only the main pole to be 100%, the case of the main pole and inter-pole increases the induced voltage by approximately 20% and reduces the cogging torque by approximately 80%.

[0029] FIG. 4 shows an example of magnetic flux density distribution (flux lines), with FIG. 4(A) showing the case of only main poles (the case of the comparative brushless DC motor 100) and FIG. 4(B) showing the case of main poles and inter-pole combinations (the case of the brushless DC motor 1 of this example). In the case of only main poles, as shown in FIG. 4(A), the magnetic flux is low near the midpoint between the main poles (the area surrounded by the solid line). In contrast, in the case of main poles and inter-pole combinations, as shown in FIG. 4(B), the addition of inter-pole arrangement ensures the passage of magnetic flux. Furthermore, the addition of inter-pole arrangements allows the flux to flow around the inter-pole, eliminating the imbalance in the magnetic flux density distribution. Furthermore, the magnetic flux density toward the back core increases, increasing the effective magnetic flux, which contributes to an improvement in the induced voltage.

[0030] 5A and 5B are diagrams showing examples of magnetic flux density distribution (vector plots), with Fig. 5A showing the case of only the main pole (the case of the comparative brushless DC motor 100) and Fig. 5B showing the case of the main pole + inter-pole (the case of the brushless DC motor 1 of this example). In the case of only the main pole, as shown in Fig. 5A, there are areas (areas surrounded by solid lines) where magnetic flux is concentrated on the main pole teeth and on the cylindrical portion. In contrast, in the case of the main pole + inter-pole, as shown in Fig. 5B, the addition of the inter-pole eliminates the local concentration of magnetic flux, which contributes to a reduction in cogging torque.

[0031] As described above, the brushless DC motor 1 of this embodiment has four or more magnetic poles, which is greater than the number of slots (the number of coil windings). The stator core has a two-piece core structure, and the inter-pole teeth are arranged between the main pole teeth in the stator core's teeth core. The width of the inter-pole teeth is narrower than that of the main pole teeth, being half the width of the main pole teeth. By arranging the inter-pole teeth in the teeth core at intermediate positions between the main pole teeth, the imbalance in the magnetic flux distribution can be eliminated. This reduces cogging torque. Furthermore, as a motor performance improvement, the reduced cogging torque suppresses speed ripple and torque ripple. Furthermore, the flow of magnetic flux through the inter-pole teeth increases the effective magnetic flux inside the motor stator, resulting in improved induced voltage and torque. In addition, because the width w63 of the inter-pole teeth 63 is narrower than the width w62 of the main pole teeth 62, it is possible to increase the winding space for the coil, increase the number of turns, or increase the winding diameter while still maintaining the required motor performance, compared to when the inter-pole teeth 63 are the same width as the main pole teeth 62. This is also expected to improve torque.

[0032] [Embodiment 2] Fig. 7(A) is an explanatory diagram showing a half-longitudinal cross section of a brushless DC motor according to embodiment 2, and Fig. 7(B) is an explanatory diagram showing the same cross section. The basic structure of brushless DC motor 100 is the same as that of embodiment 1: a three-phase inner rotor type equipped with a four-pole magnet and a stator with a two-part core structure, with one coil winding (one slot) for each of the U, V, and W phases. Brushless DC motor 100 includes motor rotor 130 coaxially attached to centrally located motor shaft 120, motor stator 140 coaxially surrounding motor rotor 130, and motor frame 110 coaxially surrounding motor stator 140. A motor driver (not shown) switches and controls the energization of the coil windings of each phase depending on the magnetic pole position.

[0033] The motor rotor 130 has a rotor outer circumferential surface 130a on which positive and negative poles of permanent magnets are alternately arranged at equal angular intervals in the circumferential direction. In this example, four permanent magnets 131 to 134 are attached to the outer circumferential surface of the motor shaft 120 and have an arc shape that subtends an angle of 90°. The rotor outer circumferential surface 130a has four poles, with positive and negative poles alternately arranged at 90° intervals in the circumferential direction.

[0034] Motor stator 140 includes U-phase coil 150 (U), V-phase coil 150 (V), and W-phase coil 150 (W), each of which is made up of, for example, an air-core coil, teeth core 160, and a cylindrical back core 170 that coaxially surrounds teeth core 160. Teeth core 160 includes a cylindrical portion 161 that coaxially surrounds rotor outer peripheral surface 130a of motor rotor 130 with a fixed gap between them, and main pole teeth 162 and inter-pole teeth 163 that are alternately arranged at angular intervals of 60° in the circumferential direction on the circular outer peripheral surface of cylindrical portion 161.

[0035] When viewed from a plane perpendicular to the central axis 100a (motor central axis) of the cylindrical portion 161, each of the three main pole teeth 162 protrudes radially outward by the same width from the circular outer peripheral surface of the cylindrical portion 161, and each of the outer peripheral tip surfaces 162a has an arc shape and abuts against the circular inner peripheral surface 170a of the back core 170. Similarly, each of the three inter-pole teeth 163 protrudes radially outward by the same width from the circular outer peripheral surface of the cylindrical portion 161, and each of the outer peripheral tip surfaces 163a has an arc shape and abuts against the circular inner peripheral surface 170a of the back core 170. A U-phase coil 150 (U), a V-phase coil 150 (V), and a W-phase coil 150 (W) are wound around each of the three main pole teeth 162 between the adjacent commutating pole teeth 163 on the left and right in the circumferential direction so as not to interfere with these commutating pole teeth 163.

[0036] 8 is a cross-sectional view showing the teeth core 160. The widths of the main pole teeth 162 and the inter-pole teeth 163 formed integrally with the teeth core 160 are set as follows: The width w163 of the inter-pole teeth 163 (the dimension in the direction perpendicular to the radial line r3 passing through the circumferential center position P3) is narrower than the width w162 of the main pole teeth 162 (the dimension in the direction perpendicular to the radial line r2 passing through the circumferential center position P2). In this example, the width w163 is half the width w162.

[0037] Furthermore, when the teeth core 160 is viewed in a cross section perpendicular to the central axis, the thickness of each of the arc portions 164 connecting the circumferentially adjacent main pole teeth 162 and inter-pole teeth 163 in the cylindrical portion 161 of the teeth core 160 is set as follows: If the side of the arc portion 164 connected to the main pole teeth 162 is defined as a main pole side portion 165 and the side connected to the inter-pole teeth 163 is defined as an inter-pole side portion 166, the thickness of the main pole side portion 165 is thicker than the thickness of the inter-pole side portion 166. In this example, the inner circumferential surface 164a of the arc portion 164 (the inner circumferential surface of the cylindrical portion 161) is defined by an arc of radius r centered on the central axis 100a. The outer circumferential surface portion 165a of the main pole side portion 165 on the outer circumferential surface of the arc portion 164 is defined by a straight line that smoothly connects via a concave curve to the radial inner end of the side surface 162b of the main pole tooth 162. The outer peripheral surface portion 166a of the inter-pole side portion 166 on the outer peripheral surface of the arc portion 164 is defined by an arc of radius R (>r) centered on the central axis (motor central axis) 100a. At the circumferential center position P of the outer peripheral surface of the arc portion 164 (outer peripheral surface portions 165a and 166a), the straight line defining the outer peripheral surface portion 165a and the arc defining the outer peripheral surface portion 166a smoothly connect. By defining the inner peripheral surface 164a and the outer peripheral surface (outer peripheral surface portions 165a and 166a) of each arc portion 164 in this manner, the thickness (radial width) of the arc portion 164 is constant at each circumferential position in the inter-pole side portion 166, but the thickness gradually increases from the position (center position P) connecting to the inter-pole side portion 166 toward the main pole tooth 162 in the main pole side portion 165.

[0038] In this example, the thickness of the main pole side portion 165 of the arc portion 164 connected to the main pole tooth 162 in the cylindrical portion 161 of the teeth core 160 is increased. This allows the magnetic flux generated by the magnetic force of the magnet to reach the teeth core 160 and become effective magnetic flux on the stator core side. Coils for each phase are wound around the main pole teeth 162, and the main pole side portion 165 of the arc portion 164 also handles the magnetic flux generated from the coils of the main pole teeth 162. Therefore, by making the main pole side portion 165 thick, the magnetic flux passing through this portion does not become saturated magnetic flux and acts as effective magnetic flux. On the other hand, the effective magnetic flux is smaller on the side of the inter-pole side portion 166 of the arc portion 164 connected to the inter-pole tooth 163, so the thickness may be thinner than the main pole side portion 165.

[0039] In addition, by increasing the thickness of the main pole side portion 165 in the arc portion 164, the coil winding space is reduced accordingly, resulting in a decrease in the coil space factor. In this example, as described above, the width of the inter-pole tooth 163 is narrowed to ensure the coil winding space. This prevents the reduction in the coil winding space that would result from increasing the thickness of the main pole side portion 165 in the arc portion 164 connecting the main pole tooth 162 and the inter-pole tooth 163.

[0040] Like the first embodiment, the brushless DC motor 100 according to the second embodiment offers the following advantages. By providing the inter-pole teeth 163 on the two-piece stator core, imbalances in the circumferential magnetic flux distribution are eliminated, reducing cogging torque and torque ripple and speed ripple. The magnetic flux and magnet magnetic flux generated by the coils 150 (U), 150 (V), and 150 (W) of each phase wound around the main pole teeth 162 also flow efficiently to the inter-pole teeth 163 via the cylindrical portion 161 of the teeth core 160, thereby increasing induced power. In other words, the number of turns in the coils 150 (U), 150 (V), and 150 (W) of each phase required to obtain the target induced power can be reduced, thereby avoiding an increase in the overall length of the motor. The width w163 of the inter-pole teeth 163 is narrower than the width w162 of the main pole teeth 162. Compared to when the commutating pole teeth 163 have the same width as the main pole teeth 162, this increases the coil winding space, allowing for a larger number of turns or a thicker winding diameter. In this example, the width w163 of the commutating pole teeth 163 is set to half the width w162 of the main pole teeth 162, ensuring the required motor performance while allowing for a 10 to 15% increase in the number of turns compared to when the widths of the two are the same (w162 = w163). This improves torque.

[0041] Furthermore, as described above, in brushless DC motor 100 according to the second embodiment, main pole side portion 165 at arc portion 164 is made thicker than the other inter-pole side portion 166. This allows the magnetic flux passing through main pole side portion 165 to function as effective magnetic flux without becoming saturated magnetic flux, and an improvement in torque can be expected.

[0042] Although the brushless DC motor 100 has been described above as having three slots, which is the minimum configuration for the U, V, and W phases, there is no restriction on the number of slots, and it goes without saying that a configuration with interpole teeth can be applied even when the number of slots is increased.

[0043] As in the first embodiment, the motor components can be made of the following materials. Of course, the motor components are not limited to these materials. Motor frame: steel or SUS material Back core: electromagnetic steel plate or soft magnetic material Tees core: electromagnetic steel plate Coil: copper Permanent magnet: neodymium magnet Motor shaft: steel

[0044] The teeth core 160 has a laminated structure in which, for example, laminate plates are laminated and bonded in the axial direction. The provision of the inter-pole teeth 163 increases the bonding area, which has the advantage of improving the strength of the laminated core and making it less likely for the laminate plates to peel off or for the laminated core to crack. Of course, the teeth core 160 may have a structure other than the laminated structure.

[0045] Furthermore, air-core coils are used as the coils 150(U), 150(V), and 150(W) of each phase, but there are no restrictions on the winding method of the coils.

Claims

1. A brushless DC motor comprising a motor rotor and a motor stator coaxially surrounding the motor rotor, wherein the motor rotor has a rotor outer peripheral surface on which the positive and negative poles of permanent magnets are alternately arranged at equal angular intervals in the circumferential direction, and the motor stator has a two-part stator core including a tooth core on which coil windings of each phase are mounted and a cylindrical back core coaxially surrounding the tooth core, the number of poles of the motor rotor is 4 or more and is greater than the number of slots of the motor stator, the tooth core includes a cylindrical portion coaxially surrounding the rotor outer peripheral surface of the motor rotor with a certain gap, and a plurality of main pole teeth and a plurality of auxiliary pole teeth alternately arranged at equal angular intervals in the circumferential direction on the outer peripheral surface of the cylindrical portion, when viewed in a plane perpendicular to the central axis of the cylindrical portion, each of the main pole teeth and the auxiliary pole teeth projects outward in the radial direction from the outer peripheral surface of the cylindrical portion with a certain width to a position in contact with the inner peripheral surface of the back core, the width of the auxiliary pole teeth is narrower than the width of the main pole teeth, and different-phase coil windings are mounted on each of the main pole teeth so as not to interfere with the adjacent auxiliary pole teeth on the left and right in the circumferential direction.

2. The brushless DC motor according to claim 1, wherein in the cylindrical portion of the tooth core, each of the arc portions connecting between the adjacent main pole teeth and auxiliary pole teeth in the circumferential direction includes a main pole side portion connected to the main pole teeth and an auxiliary pole side portion connected to the auxiliary pole teeth, and the radial thickness of the main pole side portion is thicker than the radial thickness of the auxiliary pole side portion.

3. The brushless DC motor according to claim 2, wherein the thickness of the auxiliary pole side portion is constant at each position in the circumferential direction, and the thickness of the main pole side portion gradually increases from the position connected to the auxiliary pole side portion toward the main pole teeth.

4. In the brushless DC motor according to claim 1 or 2, the coil winding includes a U-phase coil, a V-phase coil, and a W-phase coil, and the main pole teeth include at least three main pole teeth around which the U-phase coil, the V-phase coil, and the W-phase coil are respectively wound.

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