Brushless motor

Abstract

Brushless motor comprising: a stator (2) with 3n slots, where n is an integer; a rotor (3) with a rotor shaft (13) on the circumference of which 2n segment-type permanent magnets (16a, 16b, 16c) wherein a line (L) connecting the centers (M1, M2, M3) of the segment-type permanent magnets (16a, 16b, 16c) divides the outer segment-type permanent magnets (16a, 16c) at their outer vertices (P1, P2); an skew angle θskew is an angle swept between the outer vertices (P1, P2) relative to the central axis (O) of the rotor, and lies in a range of 36° ≤ θskew ≤ 57°; Wm is the value of the width of each of the segment-type permanent magnets (16a, 16b, 16c); Wp is the length of a chord (PP) formed between the endpoints of an arc sweeping a central angle θp = 360° / number of poles 2n, the arc being on an inner diameter of each segment-type permanent magnet (16a, 16b, 16c) runs;a ratio W of the circumference Wm of each of the magnets forming the magnetic poles to the length Wp in the range of 0.76 <W=Wm / Wp<0,86 liegt.;

Description

Technical FieldThe present invention relates to a brushless motor, particularly a brushless motor having a low torque ripple and a high power output, with excellent controllability.BackgroundConventionally, in order to improve the quietness of a brushless motor, a control method for smoothing the switching of energization by sine wave drive so as to reduce torque ripple is known. In sine wave driving, a current having a current value changing in sine wave form is supplied to a motor whose induced voltage has a sinusoidal wave form. As a result, the sine wave driven motor has a small torque ripple as compared with a generic 120-degree square wave driven motor. Moreover, as a method for reducing torque ripple, a skew structure for tilting rotor magnetic poles or the like with respect to the axis direction is generally known. In a motor using the skew structure, harmonic components of the induced voltage waveform are decreased. Thus, an induced voltage waveform can be changed to a smooth sinusoidal waveform. Accordingly, the effects of reducing the torque ripple at the time of sine wave driving are further improved.However, in a motor having the skewness structure, a base component of the induced voltage is reduced as the harmonic components are reduced if the skew angle is increased. The magnitude of torque is entirely dependent on the base component because the output torque of the motor is approximately equal to a product of the induced voltage and the motor current. Therefore, if the skew angle is increased to reduce torque ripple, motor torque is reduced, thus reducing voltage utilization efficiency. On the other hand, if motor specifications with the small skew angle are set to mitigate the reduction in the induced voltage so as to secure the motor torque, there is the problem of increased torque ripple because the harmonic components correspondingly increase.Therefore, in recent years, in order to simultaneously improve voltage utilization efficiency and reduce torque ripple, a control method for causing the motor drive current to contain the harmonic components so as to cancel the harmonic components of the induced voltage has been proposed. According to the control method, the basic component is ensured at the small skew angle while the induced voltage is changed to the sinusoidal waveform by the harmonic components of the motor drive current. As a result, improved power output of the engine is realized while torque ripple is reduced.Patent Document 1: JP 2006-174 692 AFurther prior art can be found in the publications U.S. Pat. No. 6,441,523 B1 and JP 2005-45 923 A, and in GRADY et al., Understanding Power System Harmons, IEEE Power Engineering Review, Volume 21, Issue 11, Nov. 2001, pp. 8-11 and HESSEINI, Harmonc analysis of polyphase electric power distribution systems, Ph.D. thesis, Iowa State University, 1988, pp. 9-11.Disclosure of the InventionProblems to be Solved by the InventionHowever, in the control method for causing the motor drive current to include the harmonic components as described above, it is necessary to set a control cycle according to the harmonic components. Therefore, in a control mode in which a large number of higher order harmonic components having a short wavelength are included, there is a problem that the control cycle must be set extremely short. As a result, control is extremely difficult. In addition, there are problems that a load is imposed on the control system and the cost thereof becomes correspondingly high.An object of the present invention is to provide a brushless motor that realizes low torque ripple and, at the same time, high power output, which can be easily controlled.Means for Solving the ProblemsA brushless motor according to the present invention includes: a stator having 3n (n is a positive integer) slots; and a rotor having 2n magnetic poles, wherein the brushless motor is rotationally driven by a 3-phase driving current in which a content ratio of the fifth harmonic component included in an induced voltage of the brushless motor with respect to a base wave is in a range of 4.5% to 6.5%.According to the present invention, in a brushless motor having a 2P3S×n structure, the content ratio of the fifth harmonic component included in the induced voltage is set in the range of 4.5% to 6.5% with respect to the fundamental wave. As a result, a torque and a rotation speed are effectively increased for the sine wave drive.In the brushless motor, a skew angle of either the rotor or the stator may be set in a range of 36°≤θschief≤57° with respect to the electrical angle, and a ratio W of a circumferential width Wm of each of the magnets forming the magnetic poles to a length Wp of a chord formed between end points of an arc having a central angle θp=360 ° / pole number 2n, the arc being included in an inner diameter circle of each magnet, may be set in a range of 0.76<W=M / Wp<0.86. As a result, in the brushless motor having the 2P3S×n structure, the content ratio of the fifth harmonic component included in the induced voltage of the brushless motor can be set in the range of 4.5% to 6.5% with respect to the base wave.Moreover, the brushless motor may have a 6-pole 9-slot structure having a number of magnetic poles equal to six (n=3) and a number of slots equal to nine, and may be used as a drive source for electric power steering. In the case of a brushless motor according to the present invention, low torque ripple and high power output are realized simultaneously. In addition, the control is simple. Therefore, the brushless motor according to the present invention is particularly useful as a motor for electric power steering which has a severe torque ripple requirement and therefore requires to be reduced in size and have a higher power output.Effects of the InventionThe brushless motor according to the present invention is a so-called 2P3S×n structure brushless motor in which the content ratio of the fifth harmonic component included in the induced voltage is set in the range of 4.5% to 6.5% with respect to the base wave. As a result, a torque and a rotation speed for the sine wave drive can be effectively increased.Moreover, by setting the skew angle of the rotor in the range of 36°≤θschief≤57° with respect to the electric angle and setting the ratio W of the circumferential width Wm of each of the magnets constituting the magnetic poles to the length Wp of the chord formed between end points of the arc having the center angle θp=360 ° / pole number 2n included in the inner diameter circle of the magnet to 0.76<W=W / Wp<0.86, the content ratio of the fifth harmonic component included in the induced voltage of the brushless motor can be set in the range of 4.5% to 6.5% with respect to the base wave in the brushless motor having the 2P3S×n structure. As a result, the low torque ripple and the high power output can be realized simultaneously. In addition, a control cycle can be made long as compared with conventional motors. Therefore, the drive control for the motor can be facilitated.Brief Description of the DrawingsFIG. 1 is a sectional view of a brushless motor which is an embodiment of the present invention. FIG. 2 is an explanatory view illustrating a structure of a stator included in the motor illustrated in FIG. 1. FIG. 3 is an explanatory view for a skew angle of a rotor. FIG. 4 is an explanatory view illustrating a shape of a magnet included in the motor illustrated in FIG. 1. FIG. 5 is a graph illustrating a relationship between a content ratio of the fifth harmonic component included in an induced voltage and a rate of torque increase and a rate of rotation speed increase for sine wave drive. FIG. 6 is a graph illustrating a relationship between a skew angle (in terms of mechanical angle) and the content ratio of the fifth harmonic component to all harmonic components observed in a six-pole, nine-slot motor. FIG. 7 is a graph illustrating the result of searches on a relationship between the skew angle (with respect to the mechanical angle) and the content ratio of the harmonic component included in the induced voltage, which are performed using the magnetic shape (the ratio W=M / Wp) as a parameter observed in the six-pole, nine-slot motor. FIG. 8 is a graph illustrating a relationship between the ratio W and a reduction rate of a fundamental wave included in the induced voltage observed in the six-pole nine-slot motor. FIG. 9 is an explanatory view illustrating a structure of the stator of the motor in which the stator is obliquely configured.Description of Symbols1 Brushless motor 2 Stator 3 Rotor 4 Housing 5 Stator core 5 a Zähne 5 b Rille 6 Coil 6 aEnd part 7 Bus-rail unit 8 Bracket 9 Core segment 11 Insulator 12 Power supply terminal 13 Rotor shaft 14 a, 14 b Lagerung 15 a- 15 c 16 a- 16 cMagnet 16out Äußere peripheral surface 16 in Innere peripheral surface 17 a- 17 cMagnet holder 18Magnet cover 21 Resolver 22 Rotor (resolver rotor) 23 Stator (resolver rotor) 24 Resolver holder 25 Resolver bracket 26 Rib 27 Nut 28 Holding screw θ skew angle θ step angle M1-M3 Center of magnet L Centers of magnets connecting line O Center of rotation of rotor P 1, P2 Intersection of line L with rotor core end part C1 Arc C2 Magnet inner diameter circle T Outermost diameter position W Ratio Wm Magnet width Wp Length of chord W formed between end points of the arc having central angle θp=360 ° / pole number in the magnet inner diameter circle W Ratio of Wm to Wp (=W / Wp)Best Mode for Carrying Out the InventionHereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a sectional view of a brushless motor according to an embodiment of the present invention. As illustrated in FIG. 1, a brushless motor 1 (hereinafter referred to as "motor 1") is an inner rotor type including an outer-disposed stator 2 and an inner-disposed rotor 3. For example, the motor 1 may be used as a power source of an electric power steering (EPS) device of a column assist type. The motor 1 can apply an auxiliary driving force to a steering column of a vehicle. The motor 1 is fixed to a speed reduction mechanism coupled to the steering column. The speed reducing mechanism reduces the rotational speed of the motor 1 and transmits the rotation to the steering column.The stator 2 includes a housing 4 having a bottomed hollow cylindrical shape, a stator core 5, stator coils 6 (hereinafter referred to as "coils 6") wound around the stator core 5, and a bus bar unit (terminal unit) 7 attached to the stator core 5. The housing 4 is a bottomed hollow cylinder made of iron or the like. The housing 4 has openings. Brackets 8 made of die cast aluminum are fitted into the openings. The holders 8 are secured to the housing 4 with fastening screws (not shown).As illustrated in FIG. 2, the stator core 5 includes a plurality of core segments 9 (nine segments in this embodiment) arranged in a circumferential direction. The stator core 5 has nine teeth 5 aprotruding inward in the radial direction. The distal end of the stator core 5 has 2n grooves 5b (two grooves in this embodiment). The grooves 5b are designed to reduce a cogging torque resulting from pseudo-slot effects. Each core segment 9 is made by stacking core pieces made of electromagnetic steel plates. An insulator 11 made of synthetic polymer surrounds the core segments 9.The coils 6 are wound around the insulator 11. An end portion 6 aof the coil 6 is guided at an end of the stator core 5. The bus bar unit 7 is secured to one end of the stator core 5. Bus bars made of copper are inserted into the housing of the bus bar unit 7 made of synthetic polymer. From the peripheral surface of the bus bar unit 7, a plurality of power supply terminals 12 protrude in the radial direction. The end portion 6a of each coil is welded to the power supply terminals 12 at the time of mounting the busbar unit 7. The bus bar unit 7 has as many bus bars as the number of phases of the motor 1 (in this embodiment, three bus bars are provided for the U, V, and W phases, respectively). Each coil 6 is electrically connected to the associated-phase power supply terminal 12. The stator core 5 is press-fixed to a housing 4 after the busbar unit 7 is fixed.The rotor 3 is inserted into the stator 2. The rotor 3 has a rotor shaft 13. The rotor shaft 13 is free to rotate supported by the bearings 14a and 14b. The bearings 14a and 14b are fixed to the central part of the bottom of the housing 4 and the central part of the bracket 8, respectively. On the rotor shaft 13, hollow cylindrical rotor cores 15 ato 15 care secured. To the outer circumferential surfaces of the rotor cores 15 ato 15 c, segment-type magnets (permanent magnets) 16 ato 16 care fixed. In the motor 1, the magnets 16a to 16c are arranged in six rows and three columns along the circumferential direction. That is, the motor 1 has a six-pole, nine-slot structure (hereinafter referred to as "6P9S"). A magnet cover 18, which is a hollow cylinder with a bottom, is provided, which surrounds the magnets 16 ato 16 c.Each of the magnets 16 ato 16 cof the motor 1 is formed in a shape such that an outer circumferential surface 16 outis eccentric with respect to an inner circumferential surface 16 in. Specifically, a center of the outer circumferential surface 16out of the magnet is not concentric with an arc C 1 passing an outermost diameter position T of the outer circumferential surface 16out. The outer peripheral surface 16out is a peripheral surface having a smaller radius than that of the inner peripheral surface 16in. By forming the outer circumferential surface 16out of the magnet eccentrically as described above, toothing can be reduced.The magnet holders 17a to 17c made of synthetic polymer are fixed to the outer sides of the magnets 16a to 16c. The magnets 16 ato 16 care attached to the outer peripheries of the rotor cores 15 ato 15 c, held by the magnet holders 17 ato 17 c. In the motor 1, the magnets 16a to 16c are arranged in 3 columns in the axial direction while being held by the magnet holders 17a to 17c. The magnets 16a to 16c of each column are displaced from the radial direction by a predetermined pitch angle (step. Thus, the rotor 3 of the motor 1 has a skewness structure in which the three magnets 16 ato 16 care stacked on each other.FIG. 3 is a diagram describing the skew angle of the rotor 3. As illustrated in FIG. 3, in the step-skew structure, the line L connecting the centers M 1 to M 3 of the magnets 16 ato 16 c(both in the circumferential direction and the axial direction) intersects the end parts of the magnets 16 aand 16 cat points P 1 and P 2, respectively. The center angle to the center O of rotation between the points P1 and P2 is the magnetic skew angle θ skewf. Therefore, the step angle θstep between the magnets 16 aand 16 bis a center angle between the points M 1 and M 2 to the center O of rotation, and the step angle θstep between the magnets 16 band 16 cis a center angle between the points M 2 and M 3 to the center O of rotation. Moreover, the center angles between points M1 and P1, and M3 and P2, respectively, are half the step angle θstep (θstep / 2). Therefore, the skew angle θ skewf is θstep×(number of stacked magnets -1)+(θstep / 2)×2=θstep×number of stacked magnets.A rotor (resolver rotor) 22 of the resolver 21 used as a rotation angle detecting means is attached to the end of the magnet holder 17 a. On the other hand, a stator (resolver stator) 23 of the resolver 21 is press-fitted into a resolver holder 24 made of metal and held in a resolver bracket 25 made of synthetic polymer. The resolver holder 24 is a bottomed hollow cylinder. The resolver holder 24 is lightly press-fitted to the end part outer circumferential surface of a rib 26 provided on the central part of the bracket 8. A nut 27 made of metal is inserted into the resolver holder 25 and the holder 8. A retaining screw 28 is screwed into the nut 27 from the outside of the holder 8. The resolver holder 24 is thereby secured in the holder 8.Here, an induced voltage of the motor having a structure of an integer multiple of 2P3S (the motor 1 has 2P3Sx3=6P9S ) includes harmonic components of an odd number equal to or greater than fifth order. Of all the odd order harmonic components contained therein, the fifth number component is the largest. In a case of 2P3Sxn, no third harmonic component is generated due to a short distance factor Kp=cos {n.(1-β).Π / 2}=0 (where n is an order and β is a coil distance / pole distance; in a case of 2P3S, β=2 / 3, and when n=3 cos is zero). When only the fifth harmonic component of a basic component is included, a power-induced voltage has approximately a trapezoidal waveform. At this time, if the motor drive current is made to have a trapezoidal waveform, the torque ripple can be made closer to zero. In addition, a peak current value of the trapezoidal wave current can be reduced because a peak value of the trapezoidal wave current can be reduced as compared with that of a sine wave current. On the other hand, in a case of the same peak current value, a larger amount of current can be made to flow in the trapezoidal wave than in the sinusoidal wave. Thus, by adjusting the induced voltage to include: the base wave+harmonic component, the motor power output can be increased while reducing torque ripple.On the other hand, the engine 1 has the skewness structure as described above. In order to realize both the improvement of the voltage use efficiency and the reduction of the torque ripple, the motor drive current including the harmonic component is supplied equally to the motor 1. However, as described above, if a large number of higher order harmonic components are included, the control cycle becomes extremely short, thereby making control difficult to perform. Thus, the inventors of the present invention focus attention on the above-mentioned characteristics of the 2P3Snstructure motor and examine a configuration for limiting the harmonic components to the fifth component as much as possible and reducing the components of other orders.Then, as a result of various experiments, the following conclusions have been drawn.(1) When the content ratio of the fifth harmonic component included in the induced voltage with respect to the base wave is set in the range of 4.5% to 6.5%, torque and rotation speed effectively increase for the sine wave drive.(2) When the skew angle is set to: 36°≤θschief≤57° in the electrical angle, the content ratio of the fifth harmonic component to all harmonic components becomes large (90%). The electric angle range 36°≤θschief≤57° corresponds to the mechanical angle range 12°≤θschief≤19° in the 6P9S motor. Moreover, when the magnets are to be stacked, the step angle in each step is obtained by dividing the above-mentioned value by the number of stacked magnets (electrical angle 36° / number of stacked magnets ≤ θstep) ≤ 57° / number of stacked magnets).(3) Assuming that a shape of each of the magnets 16 ato 16 cis set to satisfy: 0.76<W=W / Wp<0.86, the content ratio of the fifth harmonic component is in the range of 4.5% to 6.5% (Wm: magnet width, Wp: length of a chord P-P (see FIG. 4 ), specifically, a length of a chord formed between end points of an arc having a center angle θp=360 ° / pole number (60° in the case of six poles) included in a magnet inner diameter circle C 2). Although the magnet 16 ais illustrated in FIG. 4, each of the magnets 16 band 16 chas a similar structure.Hereinafter, each of the above-described (1) to (3) will be described based on the results of experiments. First, with reference to (1) FIG. 5, a graph illustrating the relationship between the content ratio of the fifth harmonic component and the rate of torque increase and the rate of rotational speed increase with respect to the sine wave drive is shown. As illustrated in FIG. 5, as the fifth harmonic component increases, the torque and the rotation speed become larger. Nevertheless, when torque and rotational speed exceed certain values, the rates at which they increase begin to decrease. Moreover, the torque increase rate and the rotational speed increase rate are different from each other, and therefore the former achieves a maximum increase of about 5% and the latter achieves a maximum increase of about 6.5%. The inventors therefore regard the ranges in which the two increase rates can be ensured to be 3% or more as effective. Accordingly, the present inventors determine that the content of the fifth harmonic component should be most preferably 4.5% to 6.5% as described in (1).Next, with reference to (2) FIG. 6, a graph illustrating the relationship between a skew angle (mechanical angle) and the content ratio of the fifth harmonic component to all harmonic components observed in a 6P9S structure motor is shown. As illustrated in FIG. 6, as the skew angle becomes larger, the content of the fifth harmonic component becomes larger. However, when the skew angle exceeds 18°, the content of the fifth harmonic component starts to decrease. When the skew angle exceeds 22°, the content of the fifth harmonic component sharply decreases. Assuming that the effective range of the content of the fifth harmonic component is 90% or more as described in (2), the skew angle θ skewf (mechanical angle) should be: 12°≤θ skewf (mechanical angle)≤19° in a 6P9S structure. Namely, in a motor of the 2P3S×n structure, the inventors determined that the skew angle (electrical angle) θ skewfly is 36°≤θ skewf (electrical angle)≤57°. FIG. 6 illustrates the results of searching for motors differing in magnet width and eccentricity of inner and outer diameters. The relationship between the skew angle and the ratio of the fifth harmonic component is similar for magnets of various configurations.Further, with reference to (3) FIG. 7, a graph illustrating the search result of a relationship between the skew angle (with respect to mechanical angle) and the content ratio of the fifth harmonic component included in the induced voltage in the 6P9S motor, which is performed using the magnetic shape (the ratio W=M / Wp) as a parameter is shown. As illustrated in FIG. 7, as the ratio W decreases, the content ratio of the fifth harmonic component increases. By setting W<0.86, the content ratio of the fifth harmonic component can be set to 4.5% while the skew angle (mechanical angle) is in the range: 12°≤θ skewf (mechanical angle)≤19°. This result can be obtained not only with the 6P9S structure motor but also with other 2P3S×n motors when the skew angle (electrical angle) is set in the range of: 36°≤θ skewness (electrical angle)≤57°.On the other hand, FIG. 8 is a graph illustrating a relationship between the ratio W and a rate of decrease of the fundamental wave in the induced voltage observed in the 6P9S motor. As illustrated in FIG. 7, when the ratio W becomes 0.76, the base wave in the induced voltage is suddenly reduced to remarkably reduce the output torque of the motor. Therefore, for the ratio W, it is concluded that 0.76<W=W / Wp<0.86 is preferred as described in (3) from the results illustrated in FIGS. 7 and 8. In the motor 1 according to the present invention, each of the magnets 16 ato 16 cis formed to satisfy the relationship: 0.76<W=W / Wp<0.86. As a result, the motor 1 is a brushless motor satisfying the above conditions (1) and (2).The process leading to the present invention is summarized as follows. Specifically, in the 2P3S×n brushless motor adopting the skewness structure, first, the skew angle is decreased to suppress the decrease of the base component to secure the torque. When the skew angle becomes small, the content ratio of the harmonic component included in the induced voltage becomes large. If the sine wave drive is performed in this state, the torque ripple becomes disadvantageously large. Therefore, the harmonic component is superimposed on the motor drive current to cancel the harmonic component included in the induced voltage to reduce the torque ripple. However, when the higher-order harmonic components are included, controllability is degraded. Thus, the harmonic component to be included in the induced voltage is limited to the fifth component that makes the induced voltage have the trapezoidal waveform and is therefore effective in improving the output and reduction of the torque ripple.After restricting the harmonic component to be superimposed on the motor drive current to the fifth harmonic component, the content ratio of the fifth harmonic component is set within the range that allows the torque and the rotation speed to be increased. The value of the content ratio is (1) 4.5% to 6.5%. Moreover, the skew angle is set so that the content ratio of the fifth harmonic component to all the harmonic components becomes large. The value of the skew angle is (2) 36°≤θschief≤57° with respect to the electrical angle. On the other hand, various experiments conducted by the present inventors make it clear that the relationship between the skew angle and the majority ratio of the fifth harmonic component included in the induced voltage changes depending on the magnetic shape. It has been found that the conditions (1) and (2) are satisfied when the ratio W is less than 0.86. Moreover, it has been found that the base wave included in the induced voltage is suddenly reduced to remarkably reduce the output torque of the motor when the ratio W becomes equal to or less than 0.76. As a result, the condition (3) is obtained.As described above, in the brushless motor according to the present invention, the motor drive current containing the harmonic component is supplied. In this way, engine power output may be improved while voltage usage efficiency is improved and torque ripple is reduced. Thus, the efficient brushless motor can be provided. Moreover, it is not necessary to set the control cycle extremely short as in a case where a large number of higher order harmonic components are included because the harmonic component is limited to the fifth component. Thus, the drive control for the motor is simplified. Further, a load on the control system is reduced. As a result, the cost of a control circuit or the like is also reduced.The present invention is not limited to the above-described embodiment. It goes without saying that various changes and modifications can be made within the scope and spirit of the invention.For example, the present invention is applicable to a brushless motor of any other type for use in EPSs, although the above-described embodiment is one designed for use in EPSs of the column support type. In addition, the present invention is not limited to motors for use in EPSs and various vehicle electric devices. Instead, the present invention can be applied to brushless motors of various types. The above-described embodiment is a 6-pole, 9-slot of the six-magnet brushless motor. Nevertheless, neither the number of magnets nor the number of slots of the motor is limited. The present invention is widely applicable to a brushless motor having a structure with an integer multiple of 2P3S.Further, the present invention is applicable not only to a motor in which the rotor 3 is made oblique but also to a motor in which the stator 2 is made oblique. According to experiments conducted by the inventors of the present invention, even in a motor in which the stator 2 is made oblique as illustrated in FIG. 9, similar relationships to those illustrated in FIGS. 6 and 7 are obtained. Specifically, when the skew angle is in the range of 36°≤θschief≤57° with respect to the electrical angle, the content ratio of the fifth harmonic component included in all the harmonic components becomes large. When the ratio is less than 0.86, the conditions (1) and (2) are satisfied. When the ratio W becomes equal to or less than 0.76, the base wave included in the induced voltage is suddenly decreased.

Claims

A brushless motor comprising: a stator (2) having 3n slots, n being an integer; a rotor (3) having a rotor shaft (13) on the periphery of which 2n segment type permanent magnets (16a, 16b, 16c); wherein a line (L) connecting the centers (M1, M2, M3) of the segment type permanent magnets (16a, 16b, 16c) intersects the outer segment type permanent magnets (16a, 16c) at their outer vertices (P1, P2); a skew angle θ skewf is an angle crossed between the outer vertices (P1, P2) relative to the center axis (O) of the rotor and is in a range of 36° ≤ θs skewf ≤ 57°; Wm is the value of the width of each of the segment-type permanent magnets (16a, 16b, 16c); Wp is the length of a chord (P-P) formed between the end points of an arc crossing a center angle θp=360 ° / pole number 2n, the arc extending on an inner diameter of each segment-type permanent magnet (16a, 16b, 16c); a ratio W of the circumferential width Wm of each of the magnets constituting the magnetic poles to the length Wp is in the range of 0.76<W=W / Wp<0.86.The brushless motor according to claim 1, wherein the brushless motor has a 6-pole, 9-slot structure having a number of six segment-type permanent magnets magnetic poles and a number of nine slots, n = 3.The brushless motor according to claim 1, wherein the brushless motor is configured to be used as a drive source for electric power steering.

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