Three-phase brushless motor

Details Three-phase brushless motor Three-phase brushless motor

2000-03-24

2001-05-22

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S185000, C310S186000, C310S187000

Reexamination Certificate

active

06236133

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a three-phase brushless motor, and more particularly to a structure of stator teeth relative to permanent magnet rotor poles.
2. Description of Related Art
An example of a conventional brushless motor
1
is schematically shown in FIG.
2
. The brushless motor
1
is composed of a stator
2
and a rotor
6
rotatably supported inside the stator
2
. The stator
2
includes an outer core ring
3
, an inner core ring
4
having nine teeth
4
a
-
4
i
and connected to the outer core ring
3
, and three-phase coils wound around respective teeth. The three-phase coils consist of U-phase coils
5
U (
5
U
1
,
5
U
2
and
5
U
3
), V-phase coils
5
V (
5
V
1
,
5
V
2
and
5
V
3
) and W-phase coils
5
W (
5
W
1
,
5
W
2
and
5
W
3
). The nine teeth
4
a
-
4
i
extend in the radial direction from the inner core ring
4
with an equal central angle of 40° between two neighboring teeth. The U-phase coils
5
U
1
,
5
U
2
and
5
U
3
are respectively wound around the first tooth
4
a
, the second tooth
4
b
and the third tooth
4
c
. Similarly V-phase coils
5
V
1
-
5
V
3
are respectively wound around the fourth to sixth teeth
4
d
-
4
f
, and W-phase coils
5
W
1
-
5
W
3
are respectively wound around the seventh to ninth teeth
4
g
-
4
i
. The coils around the first, third, fourth, sixth, seventh and ninth teeth are wound in a first direction, and other coils around the second, fifth and eighth teeth are wound in the second direction which is opposite to the first direction. After all the coils are wound on the respective teeth of the inner core ring
4
, the inner core rings
4
is connected to the outer core ring
3
as shown in FIG.
2
. The stator
2
thus assembled is housed in a housing (not shown).
The rotor
6
composed of a shaft
7
and a permanent magnet
8
is rotatably housed in the housing and held inside the stator
2
with a certain air gap therebetween. The permanent magnet
8
is magnetized into eight poles, an N pole and an S pole being alternately formed with an equal central angle of 45° between two neighboring poles.
Three-phase alternating excitation current consisting of U, V, and W phases having 120° phase difference from each other is supplied to the respective coils, i.e., the U-phase coils
5
U, the V-phase coils
5
V and the W-phase coils
5
W. A rotating magnetic field is generated in the stator
2
by the three-phase alternating excitation current, and thereby the rotor
6
rotates relative to the stator
2
. As disclosed in JP-B2-8-8764, if the number of magnet poles P is set to
8
n
(n is an integer equal to or larger than 1), and the number of teeth T is set to
9
n
, a ripple frequency of a cogging torque becomes relatively high. Since the amount of the cogging torque is inversely proportional to the ripple frequency, it is possible to reduce the cogging torque and thereby to realize a low vibration motor by thus selecting the number of magnet poles P and the number of teeth T. In the example shown in
FIG. 2
, n is one, and therefore P=8 and T=9.
In the stator structure shown in
FIG. 2
, the center of a tooth does not always coincide with the center of a magnet pole. For example, when the center of the second tooth
4
b
coincides with the center of an N-pole as shown in
FIG. 2
, the centers of the first tooth
4
a
and the third tooth
4
c
shift from the centers of respective S-poles. This results in phase shifting among voltages V
1
, V
2
and V
3
induced in respective coils
5
U
1
,
5
U
2
and
5
U
3
of the U-phase, as shown in FIG.
3
. That is, the phase of V
1
is advanced from the phase of V
2
by 20° in electrical angle, and the phase of V
3
is delayed from the phase of V
2
by 20° in electrical angle. Therefore, though a peak voltage of each voltage V
1
, V
2
and V
3
is 1.0, a composite voltage V
0
does not reach 3.0, but it only reaches 2.879. As shown in
FIG. 4
, the level of the composite voltage V
0
becomes lower as the phase shift among V
1
, V
2
and V
3
becomes larger. The same is similarly applied to other phases, the V-phase and the W-phase. Accordingly, the excitation current supplied to the stator coils is not effectively utilized for generating the rotating field in the conventional motor. This has been a problem in realizing a high efficiency motor in a compact size.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved brushless motor having high power without increasing its size, and more particularly to provide an improved structure in the stator of the brushless motor.
The brushless motor is composed of a stator for generating a rotating magnetic field by supplying three-phase alternating current consisting of U, V and W phases and a rotor rotatably disposed inside the stator. The rotor has a magnet magnetized into 8 poles including N-poles and S-poles alternately positioned with equal intervals therebetween.
The stator is composed of a core having nine teeth and coils wound around the teeth. Of the 9 teeth, the first
3
teeth are allocated to the U-phase, the second
3
teeth to the V-phase, and the third
3
teeth to the W-phase. Each group of
3
teeth consists of a center tooth, a first side tooth and a second side tooth, both side teeth being located at both sides of the center tooth, respectively.
Three center teeth, each respectively corresponding to the U, V and W phase, are positioned with an equal interval angle therebetween, i.e., the interval angle &thgr;1 is 120° , measured in a central angle. The first and second side teeth are respectively positioned &thgr;2 apart from the center tooth, where &thgr;2 falls in a range larger than 40° and not larger than 45° measured in a central angle. Preferably, &thgr;2 is set to 45°. Coils corresponding to the U-phase are divided into three coils, one being wound around the center tooth and two being wound around the side teeth, respectively. Coils corresponding to other phases are similarly divided and wound.
By positioning the center tooth and side teeth in this manner, a phase difference among three coils is reduced, because when the center tooth coincides with one rotor pole, the side teeth substantially coincide with neighboring rotor poles. If &thgr;2 is set to 45°, the center tooth and both side teeth exactly coincide with the respective centers of the rotor poles, and thereby the phase difference among three coils is completely eliminated. Since the phase difference is thus reduced or eliminated according to the present invention, excitation current supplied to the stator is effectively converted into the rotating magnetic field, and thereby output of the brushless motor is increased without making its size large.
The present invention is generally applicable to other three-phase brushless motors. That is, a brushless motor is designed and its teeth are positioned according to the following formulae. The number of rotor poles P: 8n (n is an integer equal to or larger than 1); the number of teeth T: 9n; &thgr;1=(120°
); (360°/T)<&thgr;2≦(360°/P). Preferably, &thgr;2 is set to (360°/P) to eliminate the phase difference among three coils of each phase.
Winding turns of the coil wound around the center tooth may be made larger than those of other coils, and/or a cross-sectional size of a wire forming the center coil may be made larger than that of other coils. In this manner, the output of the brushless motor can be further increased.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.


REFERENCES:
patent: 5986376 (1999-11-01), Werson
patent: 6034460 (1999-11-01), Tajima et al.
patent: B2-8-8764 (1987-05-01), None
patent: B2-2743918 (1997-06-01), None

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