Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-12-15
2001-01-30
Ramirez, Nestor (Department: 1744)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S051000, C310S179000, C310S216006, C310S193000, C310S191000, C310S209000
Reexamination Certificate
active
06181047
ABSTRACT:
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
4
a
having an end face opposed to the rotor magnets
2
. Each tooth
5
also includes a head
5
a
having an end face opposed to the rotor magnets
2
. The end faces of the heads
4
a
and
5
a
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
4
a
of each tooth
4
and the rotor magnets
2
differs from an air gap g
2
between the head
5
a
of each tooth
5
and the rotor magnets
2
, that is, g
1
<g
2
. 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 g
1
and g
2
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 g
1
and g
2
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.
REFERENCES:
patent: 3860843 (1975-01-01), Kawasaki et al.
patent: 4806717 (1989-02-01), Hershberger
patent: 4847526 (1989-07-01), Takehara et al.
patent: 5260620 (1993-11-01), Morrill
patent: 5331245 (1994-07-01), Burgbacher et al.
patent: 5778703 (1998-07-01), Imai et al.
patent: 2-22069 (1990-02-01), None
Kabushiki Kaisha Toshiba
Perez Guillermo
Pillsbury Madison & Sutro LLP
Ramirez Nestor
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