Permanent magnet motor with improved stator core and washing machine provided therewith

A permanent magnet motor includes a rotor having a permanent magnet and a stator core having circumferential teeth the number of which is 2n where n is an integer equal to or larger than 2, the teeth including first teeth the number of which is n and second teeth the number of which is n, the first and second teeth being circumferentially regularly disposed alternately. Each first tooth has a head with an end face opposed to the rotor and formed generally into the shape of an arc about a center of rotation of the rotor so that an air gap defined between said end face and a surface of the rotor opposed to said end face is circumferentially uniform. Each second tooth has a head with an end face opposed to the rotor and shaped so as to be gradually departed farther away from an opposite surface of the rotor as the head extends from its circumferentially central portion toward both circumferential ends.

BACKGROUND OF THE INVENTION
 1. Field of the invention
 This invention relates to a permanent magnet motor including a rotor
 composed of permanent magnets and a washing machine provided with the
 permanent magnet motor, and more particularly to such a permanent magnet
 motor including a stator core improved for the purpose of reducing a
 cogging torque and such a washing machine in which vibration and noise due
 to operation thereof is reduced.
 2. Description of the prior art
 Various configurations of stator cores have conventionally been provided
 for improving starting characteristics of permanent magnet motors. For
 example, Japanese Unexamined Utility Model Reg. Publication No. 2-22096
 (1990) discloses a brushless motor provided with a stator core having such
 an improved configuration. FIG. 8 shows the stator core disclosed in the
 publication. The shown configuration of the stator core reduces a cogging
 torque as well as simplifies a starting circuit arrangement.
 Referring to FIG. 8, an electric motor of the inner rotor type in which a
 rotor 1 is disposed inside a stator is shown. The rotor 1 includes rotor
 magnets 2 comprising permanent magnets. The stator includes a stator core
 3 having circumferentially alternately arranged first and second teeth 4
 and 5. Each first tooth 4 includes a head 4a having an end face opposed to
 the rotor magnets 2. Each tooth 5 also includes a head 5a having an end
 face opposed to the rotor magnets 2. The end faces of the heads 4a and 5a
 are formed into the shape of an arc about a center of rotation of the
 rotor 1 and have different radii which are distances between the end faces
 and the center of rotation of the rotor 1. Accordingly, an air gap gl
 between the head 4a of each tooth 4 and the rotor magnets 2 differs from
 an air gap g2 between the head 5a of each tooth 5 and the rotor magnets 2,
 that is, g1&lt;g2. Armature windings are wound on the first and second teeth
 4 and 5.
 In the permanent magnet motors, a magnetic energy density in the air gap is
 higher in the vicinity of the teeth of the stator core and lower in the
 vicinity of slot openings between adjacent teeth. Furthermore, the
 magnetic energy density is lower in an area of the air gap located at each
 boundary between the magnets of different magnetic poles abutting each
 other than in the other area of the air gap. As a result, when the
 boundary between the magnets of different magnetic poles passes the
 opening between the adjacent teeth, portions of low magnetic energy at the
 rotor side and the stator core side coincides with each other such that
 the cogging torque shows a peak value. Since the peak of the cogging
 torque simultaneously occurs at a plurality of openings between the teeth,
 influences of the cogging torque become conspicuous.
 The air gaps g1 and g2 differ from each other in the construction as shown
 in FIG. 8. As a result, the cogging torque does not take the peak values
 at all the openings 6 simultaneously and accordingly, the peak value of
 the cogging torque is decreased. However, each first tooth 4 and each
 second tooth 5 differ in magnetic resistance from each other since the air
 gaps g1 and g2 differ from each other. Consequently, the differences in
 the magnetic resistance vary an electromagnetic force when the armature
 windings are energized so that the rotor 1 is rotated. This results in
 vibration and noise in the motor.
 SUMMARY OF THE INVENTION
 Therefore, an object of the present invention is to provide a permanent
 magnet motor in which the vibration and noise due to the variations in the
 electromagnetic force can be restrained as much as possible and a washing
 machine in which the above motor is used and the vibration and noise
 produced therein can be reduced.
 The present invention provides a permanent magnet motor comprising a rotor
 having a permanent magnet and a stator core having circumferential teeth
 the number of which is 2n where n is an integer equal to or larger than 2,
 the teeth including first teeth the number of which is n and second teeth
 the number of which is n, the first and second teeth being
 circumferentially regularly disposed alternately. In this motor, each
 first tooth has a head with an end face opposed to the rotor and formed
 generally into the shape of an arc about a center of rotation of the rotor
 so that an air gap defined between said end face and a surface of the
 rotor opposed to said end face is circumferentially uniform. Furthermore,
 each second tooth has a head with an end face opposed to the rotor and
 shaped so as to be gradually departed farther away from an opposite
 surface of the rotor as the head extends from a circumferentially central
 portion thereof toward both circumferential ends thereof.
 According to the above-described motor, the air gap between the heads of
 the first and second teeth and the surface of the rotor takes different
 values near the circumferential ends of the heads of the respective first
 and second teeth, that is, near circumferential sides of an opening
 defined by each first tooth and the adjacent second tooth. This difference
 in the air gap reduces the peak value of the cogging torque. Furthermore,
 each second tooth is shaped so as to be gradually departed farther away
 from the opposite surface of the rotor as the head extends from the
 circumferentially central portion thereof toward both circumferential ends
 thereof. This shape of each second tooth decreases a mean air gap between
 each second tooth and the surface of the rotor to a smaller value.
 Consequently, the difference in the magnetic resistance can be reduced
 between the first and second teeth.
 In a preferred form, the heads of the first and second teeth have
 circumferential widths differing from each other. Furthermore, a minimum
 air gap between the head of each second tooth and the surface of the rotor
 is set to be equal to or smaller than the air gap between the head of each
 first tooth and the surface of the rotor.
 In another preferred form, a difference between the circumferential widths
 of the heads of the first and second teeth is substantially the same as a
 circumferential width of an opening defined between the heads of the first
 and second teeth adjacent to each other.
 The invention also provides a washing machine provided with the
 above-described permanent magnet motor the rotor of which is substantially
 directly connected to at least one of a rotatable tub and an agitator.
 According to the washing machine, the vibration and noise produced by the
 motor are at lower levels respectively. Consequently, the outer cabinet of
 the washing machine can be prevented from resonating to the vibration and
 noise and producing a large vibration and a loud noise.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 A first embodiment of the present invention will be described with
 reference to FIGS. 1 and 2. Referring first to FIG. 2, a permanent magnet
 motor 11 of the outer rotor type in which a rotor is disposed outside a
 stator is shown. The motor 11 comprises a stator core 12 formed by
 blanking a number of ring-shaped pieces from a silicon steel plate and
 stacking the pieces into laminations. The stator core 12 has a centrally
 formed circular hole 14 into which bearings 13 are fitted. The stator core
 12 further includes six first teeth 15 and six second teeth 16 formed on
 the outer circumference thereof and arranged alternately as shown in FIG.
 1.
 Armature windings 17 are wound on the first and second teeth 15 and 16
 respectively. The first and second teeth 15 and 16 have heads 15a and 16a
 at distal ends respectively. The heads 15a and 16a of the teeth 15 and 16
 protrude radially outward from the respective armature windings 17. The
 stator core 12 and the armature windings 17 constitute a stator 18 of the
 motor 11.
 A rotor 20 of the motor 11 comprises a shaft 21 rotatably supported on the
 bearings 13, a rotor yoke 22 secured to an upper end of the shaft 21, and
 a plurality of rotor magnets 23 fixed on an inner circumferential side of
 a ring portion 22a which is an outer circumferential portion of the rotor
 yoke 22. The magnets 23 are arranged into a cylindrical shape. The magnets
 23 are radially opposed to end faces of the heads 15a and 16a of the first
 and second teeth 15 and 16 of the stator core 12 with an air gap G being
 defined therebetween. The heads 15a and 16a of the teeth 15 and 16 have
 respective shapes different from each other as shown in FIG. 1. More
 specifically, the head 15a of each first tooth 15 has an end face opposed
 to the rotor 20 or more particularly the rotor magnets 23. The end face of
 the head 15a of each first tooth 15 is formed into the shape of an arc
 with a radius r1 about the center O1 of rotation of the rotor 20. On the
 other hand, the head 16a of each second tooth 16 has an end face opposed
 to the rotor magnets 23 and formed into the shape of an arc with a radius
 r2 about a point O2 shifted by a predetermined distance from the rotation
 center O1 of the rotor 20. The radius r2 is set to be smaller than the
 radius r1. Accordingly, an air gap G1 between the end face of each first
 tooth head 15a and the rotor magnets 23 is circumferentially uniform. An
 air gap between a circumferentially central portion 24a of the end face of
 each second tooth head 16a is the same as the air gap G1. The end face of
 each second tooth head 16a includes portions extending from the central
 portion 24a to both ends 24b thereof respectively. These portions of each
 second tooth head 16a are shaped so as to be continuously departed farther
 away from the central portion 24a as they extend toward the opposite ends
 24b respectively. As a result, the air gap between the end face of each
 second tooth head 16a and the rotor magnets 23 is gradually increased from
 the central portion 24a toward the opposite ends 24b. When reference
 symbol G2 designates an air gap between each end 24b and the rotor magnet
 23, the relationship between the air gaps G1 and G2 is expressed as G1&lt;G2.
 The head 15a of each first tooth 15 has a circumferential width Cl set to
 be equal to a circumferential width C2 of the head 16a of each second
 tooth 16 (C1=C2).
 According to the above-described embodiment, the air gaps G1 and G2 take
 different values near the circumferential ends of the heads of the
 respective first and second teeth 15 and 16 adjacent each other, which
 ends define each opening 25. Consequently, the cogging torque can be
 reduced. In other words, a plurality of boundaries between the different
 poles of the rotor magnets 23 do not simultaneously correspond with only
 the air gap G1 portions of the first teeth 15 or only the air gap G2
 portions of the second teeth 16. Thus, the boundaries are dispersed so as
 to be located simultaneously at the air gap GI and G2 portions of the
 first and second teeth 15 and 16. Consequently, the peak value of the
 cogging torque can be reduced.
 Furthermore, the head 16a of each second tooth 16 is shaped so as to be
 gradually departed farther away from the opposite surface of the rotor
 magnet 23 as the head extends from the circumferentially central portion
 thereof toward both circumferential ends thereof. This shape of each
 second tooth 16 decreases a mean air gap between each second tooth 16 and
 the surface of the rotor magnet 23 to a smaller value. This reduces the
 difference in the magnetic resistance between the first and second teeth
 15 and 16, and accordingly, a rate of variation in the electromagnetic
 force. Consequently, vibration and noise induced in the motor can be
 restrained. The rate of variation in the electromagnetic force can further
 be reduced particularly since the change in the shape of the end face of
 the head 16a of each second tooth 16 is not stepwise but continuous.
 The air gap G1 between the central portion 24a of the end face of the head
 16a of each second tooth 16 and the surface of the rotor magnet 23 is
 equal to the air gap G1 between the end face of the head 15a of each first
 tooth 15 and the surface of the rotor magnet 23 in the foregoing
 embodiment. However, the air gap between the central portion 24a of each
 second tooth 16 and the surface of the rotor magnets 23 may be set to be
 slightly smaller than the air gap between the end face of the head 15a of
 each first tooth 15, instead. In this case, the rate of variation in the
 circumferential electromagnetic force can further be reduced with respect
 to the air gap.
 FIG. 3 illustrates a second embodiment of the invention. Only the
 differences between the first and second embodiments will be described. In
 a stator core 30, a circumferential width C4 of the head 32a of each
 second tooth 32 is set to be larger than a circumferential width C3 of the
 head 31a of each first tooth 31. Moreover, the difference between the
 circumferential widths C3 and C4 is set to be equal to a circumferential
 width C5 between the circumferential ends of the heads 31a and 32a of the
 first and second teeth 31 and 32 adjacent to each other (C4-C3=C5).
 The air gap between the circumferentially central portion of the head 32a
 of each second tooth 32 and the rotor magnets 23 is preferably set to be
 equal to or slightly smaller than the air gap between the head 31a of each
 first tooth 31. However, the air gap between the circumferentially central
 portion of the head 32a of each second tooth 32 and the rotor magnets 23
 may be set to be slightly larger than the air gap between the head 31a of
 each first tooth 31 depending upon the difference between the
 circumferential widths of the heads 31a and 32a of the first and second
 teeth 31 and 32.
 According to the second embodiment, an interval is provided between the
 center of each one opening 33 defined by the head 31a or 32a of each one
 tooth 31 or 32 and the head of one adjacent tooth 32 or 31 and the center
 of the opening 33 defined by the head 31a or 32a of said each one tooth 31
 or 32 and the head of the other adjacent tooth 32 or 31. These intervals
 take different values circumferentially alternately or take the same value
 every second opening 33. Consequently, the cogging torque and accordingly
 the electromagnetic vibration can further be reduced.
 FIG. 4 illustrates a third embodiment of the invention. The third
 embodiment differs from the second embodiment in the following. The end
 face of the head 34a of each second tooth 34 has the arc shape at the
 circumferentially central portion thereof and is slant at both portions
 extending from the central portion to both ends thereof. The same effect
 can be achieved from the third embodiment as from the second embodiment.
 FIG. 5 illustrates a fourth embodiment of the invention. The invention has
 been applied to the permanent magnet motor of the outer rotor type in each
 of the foregoing embodiments. In the fourth embodiment, however, the
 invention is applied to a permanent magnet motor of the inner rotor type
 in which a rotor is disposed inside a stator.
 A stator core 40 of the stator includes three first teeth 41 and three
 second teeth 42 provided at the inner circumferential side. The first and
 second teeth 41 and 42 are disposed circumferentially alternately. A
 four-pole rotor 44 including rotor magnets 43 is rotatably provided inside
 the stator core 40. A head 41a of each first tooth 41 has an end face
 opposed to the rotor magnets 43. The end face of each head 41a is formed
 generally into the shape of an arc about a rotation center O3 of the rotor
 44. On the other hand, a head 42a of each second tooth 42 has an end face
 opposed to the rotor magnets 43. The end face of each head 42a is formed
 generally into the shape of an arc inverse to the end face of each head
 41a, that is, each head 42a has an axially convex shape.
 Accordingly, an air gap G4 between the end face of each first tooth head
 41a and the rotor magnets 43 is circumferentially uniform. An air gap
 between a circumferentially central portion 45a of the end face of each
 second tooth head 42a is substantially the same as the air gap G4. The end
 face of each second tooth head 42a includes portions extending from the
 central portion 45a to both ends 45b thereof respectively. These portions
 of each second tooth head 42a are shaped so as to be continuously departed
 farther away from the rotor magnets 43 as they extend from the central
 portion 45a toward the opposite ends 45b respectively. As a result, the
 air gap between the end face of each second tooth head 42a and the rotor
 magnets 43 is gradually increased from the central portion 45a toward the
 opposite ends 45b. When reference symbol G5 designates the air gap between
 each end 45b and the rotor magnet 23, the relationship between the air
 gaps G4 and G5 is expressed as G4&lt;G5. In other words, an opening 46 is
 defined between the head 41a of each first tooth 41 and the head 42a of
 the adjacent second tooth 42. The air gap G4 at one side of the opening 46
 is smaller than the air gap G5 at the other side of the opening 46.
 According to the construction as described above, the same effect can be
 achieved from the fourth embodiment as from the first embodiment.
 The invention should not be limited to the above-described embodiments. The
 invention may be modified as follows. The stator core is formed by
 blanking a number of ring-shaped pieces from a silicon steel plate and
 stacking the pieces into laminations in the foregoing embodiments.
 However, a number of band-shaped pieces of the silicon steel plate may be
 stacked and then bent into the shape of a ring, instead.
 In the foregoing embodiments, the circumferential width of the head of each
 first tooth is equal to or smaller than the circumferential width of the
 head of each second tooth. The head of each second tooth may have a
 smaller circumferential width than the head of each first tooth, instead.
 In this case, however, the air gap between the circumferentially central
 portion of the head of each second tooth and the rotor magnets needs to be
 smaller than the air gap between the head of each first tooth and the
 rotor magnets.
 FIGS. 6 and 7 illustrate a fifth embodiment of the invention. In the fifth
 embodiment, the permanent magnet motor of the invention is applied to a
 clothes washing machine. Referring to FIG. 6, the clothes washing machine
 is shown. The washing machine comprises an outer cabinet 51, a
 water-receiving tub 52 suspended for rocking motion from suspension rods
 53 of elastic suspension mechanisms one of which is shown. A rotatable tub
 55 serving both as a wash tub and as a dehydration tub is provided in the
 water-receiving tub 52. An agitator 54 is rotatably mounted on the bottom
 of the rotatable tub 55. A washing machine motor 56 including a housing 57
 is provided under the water-receiving tub 52 for driving the agitator 54
 and the rotatable tub 55.
 The rotatable tub 55 is connected to a hollow tub shaft 58 rotatably
 supported on bearings 59 and 60 provided in the housing 57. The agitator
 54 is connected to an agitator shaft 61 inserted through the hollow tub
 shaft 58 so as to be rotatably supported.
 The motor 56 comprises a permanent magnet motor of the outer rotor type,
 namely, a brushless motor. The motor 56 has the same constitution as shown
 in FIG. 2. More specifically, the motor 56 comprises a stator 64 including
 a stator core 62 and stator windings 63, and a rotor 67 including a yoke
 65 and permanent magnets 66 attached to the inner circumferential side of
 the yoke 65. The rotor 67 is disposed outside the stator 64. The stator
 core 62 is fixed to the housing 57, and the rotor 67 is directly connected
 to the agitator shaft 61. A clutch (not shown) is provided between the
 rotor 67 and the tub shaft 58. The clutch connects the tub shaft 58 to the
 rotor 67 in a dehydrating operation so that the rotatable tub 55 is
 rotated together with the agitator 54. On the other hand, the clutch
 disconnects the tub shaft 58 from the rotor 67 in a washing operation so
 that drive of the rotatable tub 55 is prevented. In this case, the tub
 shaft 58 is connected to the housing 58 so that the rotatable tub 55 is
 not rotated with rotation of the agitator 54.
 Referring to FIG. 7, the stator core 62 of the brushless motor 56 comprises
 three split cores 62a. Each split core 62a has three first teeth 68 and
 three second teeth 69. These teeth 68 and 69 are formed into the same
 shapes as the first and second teeth 31 and 32 of the stator core 30 shown
 in FIG. 3 respectively. Two insulating covers 70 and 71 cover the stator
 core 62. Each cover has the same external shape as that obtained by
 dividing the stator core 62 into two equal parts. The stator windings 63
 are wound on the insulating covers 70 and 71. The above-described driving
 mechanism including the motor 56 as shown in FIG. 6 is the same as
 disclosed in U.S. Pat. No. 5,778,703 assigned to the same assignee as of
 the present application and incorporated herein by reference.
 The foregoing description and drawings are merely illustrative of the
 principles of the present invention and are not to be construed in a
 limiting sense. Various changes and modifications will become apparent to
 those of ordinary skill in the art. All such changes and modifications are
 seen to fall within the scope of the present invention as defined by the
 appended claims.