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United States Patent |
5,030,868
|
Suzuki
,   et al.
|
July 9, 1991
|
Brushless dc motor and rotor magnet
Abstract
A brushless dc motor comprising a stator, and a rotor incorporating a
hollow cylindrical permanent magnet. The rotor magnet has sectors
magnetized radially in opposite directions to define poles of opposite
polarity extending around the circumference of the magnet. The magnet is
completely magnetized so as to leave no non-magnetized sectors between the
poles of the magnet. The magnet has grooves in its surface facing the
stator, the grooves extending in the longitudinal direction of the magnet.
Each groove is offset from, or non-symmetrical with respect to, the center
of the rotor pole in which it is located.
Inventors:
|
Suzuki; Yuzuru (Shizuoka, JP);
Fujitani; Sakae (Shizuoka, JP);
Takahashi; Hitoshi (Shizuoka, JP)
|
Assignee:
|
Minebea Co., Ltd. (Tokyo, JP)
|
Appl. No.:
|
503699 |
Filed:
|
April 3, 1990 |
Intern'l Class: |
H02K 021/22; H02K 029/08 |
Field of Search: |
310/154,156,186,268,67 R,261
|
References Cited
U.S. Patent Documents
3500090 | Mar., 1970 | Baermann | 310/154.
|
4031419 | Jun., 1977 | Spiesberger et al. | 310/186.
|
4549104 | Oct., 1985 | Nimura et al. | 310/156.
|
4562399 | Dec., 1985 | Fisher | 310/156.
|
4788464 | Nov., 1988 | Nishikawa et al. | 310/156.
|
4893040 | Jan., 1990 | Turner et al. | 310/156.
|
Primary Examiner: Stephan; Steven L.
Assistant Examiner: Rebsch; D. L.
Claims
We claim:
1. A brushless dc motor comprising:
a stator having magnetic poles carrying coils, and interpoles between the
poles,
a cylindrical magnet rotor rotatable with respect to the stator, the rotor
being radially magnetized to define multiple poles around its periphery,
the rotor being completely magnetized so as to leave no non-magnetized
areas between the poles of the rotor, and
the rotor having substantially axial grooves in its surface facing the
stator, each groove being located closer to one end of one of the rotor
poles than to the other end, and said other end of that rotor pole being
ungrooved.
2. A brushless dc motor as defined in claim 1 including a rotor position
sensor, the sensor being a magnetoelectricity converter positioned
adjacent to the surface of the rotor facing the stator.
3. A brushless dc motor as defined in claim 1, wherein the shape of the
grooves in the rotor magnet is represented by the following two formulas:
0.3.times.360/P<2<.THETA.<360/P
D.gtoreq.G
where;
2 .theta.: width (degree) of the groove surface
P : number of magnetized poles
D : depth at the center of the groove
G : the smallest gap between the stator and the rotor magnet
4. A brushless dc motor as defined in claim 1 wherein the grooves extend in
the axial direction of the rotor magnet, and the grooves extend
circumferentially into two adjacent poles of the rotor.
5. A brushless dc motor as defined in claim 1 wherein the rotor magnet is
formed of two different magnetic materials.
6. A brushless dc motor as defined in claim 1 wherein nonmagnetic
reinforcement material fills the grooves.
7. A brushless dc motor as defined in claim 1 wherein the rotor comprises a
rotor body and a rotor magnet, the magnet being embedded within the rotor
body.
8. A hollow cylindrical rotor magnet, for a brushless dc motor, formed on a
single piece of permanently magnetized material, the magnet having sectors
magnetized radially in opposite directions to define poles of opposite
polarity extending around the circumference of the rotor, and the magnet
having substantially axial grooves in its inner cylindrical surface
extending along the length of the magnet, each groove being located closer
to one end of one of the rotor poles than to the other end, and said other
end of that rotor pole being ungrooved.
9. A magnet as defined in claim 8 wherein each groove extends
circumferentially into two adjacent poles of the magnet.
10. A magnet as defined in claim 8 wherein the magnet is completely
magnetized so as to leave no non-magnetized sectors between the poles of
the magnet.
Description
The present invention relates to a brushless dc motor, comprising a stator
having magnetic poles with coils and interpoles, a rotor magnet of
cylindrical shape, radially magnetized, having magnetic grooves on its
surface facing the stator, and a rotor position sensor, and more
particularly to the rotor magnet used in a brushless dc motor.
A technology whereby a combination of a stator having interpoles as well as
magnetic poles with coils, and a rotor magnet with a special magnetized
pattern, keeps the electromagnetic force from vanishing at a certain rotor
position, stabilizes starting of a brushless dc motor. The magnetized
pattern formed on the rotor magnet divides the rotor circumference by 3 P
/ 2 and is arranged in the order of N pole, S pole, and non-magnetized
area. This technology is set forth, for example, in U.S. Pat. No.
3,299,335. As for technologies for sensing rotor position, one of them
uses a disc with a slit directly linked with a rotor to detect the rotor
position by a photoelectric converter, while another uses a separate
magnet on a rotor to detect the rotor position by a magnetoelectricity
converter, e.g., a Hall effect device.
FIG. 10 illustrates a prior art inner-rotor brushless dc motor of 4-pole
construction with a single magnetoelectricity converter for rotor position
sensing. The stator comprises a motor case 1, a stator yoke 2, four stator
magnetic poles arranged at regular intervals 3a, 3b, 3c, 3d, coils wound
around each of the stator magnetic poles 4a, 4b, 4c, 4d, and inter-poles
5ab, 5bc, 5cd, 5da, arranged halfway between the stator magnetic poles.
The rotor consists of a torque-generating rotor main magnet 7 and a rotor
position sensing magnet 8. A magnetoelectricity converter 6 is fixed
opposite the rotor position sensing magnet 8. The rotor main magnet 7 is
magnetically divided on its circumference at angles of every 60 degrees.
The sections are S pole, non-magnetized area, N pole, S pole,
non-magnetized area, and N pole, respectively, in this order.
The magnetic poles of the rotor main magnet 7, and those of the rotor
position magnet 8, are positioned as shown in FIG. 11.
Where the rotor position sensing magnet 8 and the magnetoelectricity
converter 6 are positioned as shown in FIG. 10, stator magnetic poles 3a,
3c are excited to N poles and the other stator magnetic poles and
inter-poles are excited to S poles, when coils 4a, 4c are energized. Then,
the N poles of rotor main magnet 7 and the N pole magnetized stator
magnetic poles 3a, 3c repel each other, causing the rotor to rotate
clockwise (as indicated by an arrow). After the rotor rotates through a
90-degree arc, coils 4a, 4c are deenergized and at the same time coils 4b,
4d are energized. Then, stator magnetic poles 3b, 3d are excited to N
poles and the other stator magnetic poles and inter-poles are excited to S
poles. Consequently, a magnetic repulsion, between the N poles of rotor
main magnet 7 and the N pole-magnetized stator poles, keeps the rotor
rotating clockwise.
Each time the rotor rotates through a 90-degree arc, the magnetizing
converter 6 senses changes of magnetic poles and energizes coils 4a, 4c
and coils 4b, 4d, alternatively, keeping the rotor rotating in one
direction.
FIGS. 12 A and B, respectively, illustrate the surface distribution of
magnetic flux density for the rotor main magnet 7 and that for the rotor
position sensing magnet 8.
A brushless dc motor described above of the prior art construction is
characterized by stable motor driving torque and small detent torque but
has the following disadvantages.
The special magnetic pattern of the rotor magnet makes it difficult to
mass-produce motors of uniform characteristics. The prior art requires
non-magnetized areas to be formed on the rotor magnet, and for this
purpose, requires a special yoke-shaped magnetizing head. Also the
difference in magnetizing devices (magnetization power supply,
magnetization voltage, electrostatic capacity of power supply, etc.) or
changes of surrounding temperature cause a dispersion of spatial
distribution of magnetic flux density. Being largely dependent on the
magnetized pattern of the rotor magnet, motor characteristics vary acutely
when space distribution of magnetic flux density is uneven.
In addition, in a construction where a magnetoelectricity converter is used
for rotor position sensing, a rotor position sensing magnet 8 needs to be
used in addition to a torque-generating rotor main magnet 7. This makes
the rotor construction even more complicated.
The present invention has for its object eliminating the problems of the
prior art as thus far described, and relates to a brushless dc motor and a
rotor magnet to be used therein, wherein the rotor magnet is entirely
magnetized with no non-magnetized areas required, thereby facilitating the
magnetization, and whereby space distribution of magnetic flux density is
less uneven.
The present invention, which eliminates the aforementioned technical
problems, relates in a preferred embodiment to (1) a brushless dc motor
comprising a stator having magnetic poles with coils and inter-poles, a
rotor made of a cylindrical magnet radially magnetized, and a rotor
position sensor, wherein the rotor rotates around the stator forming a
radial flux structure, and the rotor magnet has magnetic grooves on its
inner surface facing the stator and is entirely magnetized, and (2) a
rotor magnet to be used therein.
"Magnetic grooves" mentioned herein refers not only to apparent grooves in
the ordinary sense, but also to an instance where non-magnetized
reinforcements are filled in such grooves and even to an instance where
plastic magnets are filled in the grooves cut in a high-performance
magnet, such as a rare-earth magnet. "A rotor magnet" mentioned herein
includes one made of multiple materials being formed into a nearly
cylindrical shape, as well as one of completely uniform structure. The
term "cylindrical" used herein has a broader sense and includes reference
to being ring-shaped, etc.
In this invention, the entire rotor magnet is completely magnetized without
having any non-magnetized area, and the rotor has magnetic grooves in its
surface opposite the stator. Because of this, if the rotor magnet and the
stator are assembled into a motor, the desired space magnetic flux density
is obtained by changing its permanence in a circumferential direction. The
stator with interpoles and the rotor magnet with a specific space magnetic
flux density are combined to produce torque at any position of the rotor
and to ensure stable starting.
In this invention, only one magnetoelectricity converter, e.g., Hall effect
device, installed in the stator surface opposed to the rotor, is enough to
detect the position of the rotor, and the invention does not require a
rotor position detection magnet as required by the conventional technology
.
In the accompanying drawings:
FIG. 1 is a schematic cross-sectional view of a brushless dc motor
according to this invention;
FIG. 2 is a detail of the rotor magnet shown in FIG. 1;
FIG. 3 is a graph illustrating the surface distribution of magnetic flux
density as to the magnet;
FIG. 4 is an axial cross-sectional view of the fan-motor to which this
invention is applied, as an illustrative embodiment;
FIGS. 5 A.about.E show illustrative sectional shapes of the grooves cut in
the respective rotor magnet;
FIGS. 6 and 7 show different shapes, in sectional view, of the rotor
magnet;
FIGS. 8 and 9 show different structures in axial sectional views of the
rotor;
FIG. 10 is a sectional view of a conventional motor;
FIG. 11 is a detail of the rotor magnet; and
FIGS. 12 A and B are graphs illustrating the difference in surface
distribution of magnetic flux density between the rotor position sensing
magnet A and the rotor main magnet B.
FIG. 1 is a cross-sectional view of a brushless dc motor according to this
invention, and an example of a 4-pole outer-rotor-type motor. As the basic
parts of the stator section are almost the same as those of the
conventional technology shown in FIG. 10, its corresponding parts are
shown with identical reference numerals for easy reference.
The stator includes one stator yoke 2, four stator magnetic poles 3a, 3b,
3c, and 3d, which are positioned at a spaced angle of 90 degrees in
succession to the stator yoke, four coils 4a, 4b, 4c, and 4d, which are
wound around the stator magnetic poles, and four interpoles 5ab, 5bc, 5cd,
and 5da, which are positioned between the stator magnetic poles. The
magnetoelectricity converter 6, which is used to detect the position of
the rotor is, for example, a Hall element or the like, and it is installed
to face a below-mentioned rotor magnet 11 in the vicinity of the top of
interpole 5bc.
The rotor includes rotor yoke 10 and rotor magnet 11, and it is mounted so
as to rotate freely about a central axis (not shown). The details of rotor
magnet 11 are shown in FIG. 2, and its surface magnetic flux density in
FIG. 3. Rotor magnet 11 is radially 4-pole-magnetized and has two grooves
12a and 12b. In the case of four poles, a circular arc of 2 theta degrees
is formed with a magnetic pole switching point (neutral point) as a
symmetrical axis having theta degrees each at both sides thereof. Due to
the magnetic grooves 12a and 12b, the surface magnetic flux density of
rotor magnet 11 is distributed in the direction of the circumference, as
shown in FIG. 3. That is, at the grooves 12a and 12b as compared to other
parts, the magnetic flux density on the surface with which the stator
magnetic poles interlink becomes extremely low and comes close to the
surface magnetic density distribution shown in FIG. 12B on the
conventional rotor main magnet. On the other hand, as the neutral points
come to the position of 90 degrees in the case of 4-pole, an additional
rotor position detector magnet for the conventional technology, as shown
in FIG. 10, is not required. If the groove width of rotor magnet 11 is 2
thetas as viewed from a central angle, then the arrangement of the grooves
and the magnetic poles is as follows, and the unit of the angles is
degrees.
N pole (90 Theta); S pole (90 Theta); Groove S Pole (Theta); Groove N pole
(Theta); N pole (90 - Theta); S pole (90 Theta); Groove S pole (Theta);
Groove N pole (Theta)
This invention is characterized by the following: the magnetic grooves are
formed so as to give magnetic changes in a circumferential direction, and
the rotor magnet is so constructed that its whole can be magnetized
without having any non-magnetized area, and this eliminates the need for
both a rotor main magnet and a rotor position detection magnet.
The operating principle of such a brushless dc motor constructed as above
is the same as that of the aforementioned conventional technology. With
the position relationship between rotor magnet 11 and magnetoelectricity
converter 6 fixed as shown in FIG. 1, if coils 4b and 4d are energized,
and stator magnetic poles 3b and 3d, and the other stator magnetic poles
and interpoles are excited to N and S poles, respectively, the N pole of
rotor magnet 11 and the N poles of stator magnetic poles 3b and 3d repel
each other magnetically and the rotor rotates counter clockwise (in the
direction of the arrow in FIG. 1). When the rotor rotates to 90 degrees,
coils 4b and 4d are de-energized, and at the same time, coils 4a and 4c
are energized. This causes stator magnetic poles 3a, 3c, and the other
stator magnetic poles and interpoles to be excited to N and S poles,
respectively. As a result, the rotor continues to rotate counter clockwise
because the N pole of rotor magnet 11 and the N poles of stator magnetic
poles 3a and 3c repel each other magnetically.
At this point interpoles 5ab, 5bc, 5cd and 5da function as a generator of
torque (no dead point) at any position of the rotor.
As explained above, each time the rotor rotates through 90 degrees,
magnetoelectricity converter 6 detects the changes of the magnetic poles
in rotor magnet 6 and energizes a pair of coils 4a and 4c, and a pair of
coils 4b and 4d, alternatively. This causes the rotor to rotate
continuously in one direction.
In connection with this invention, an experiment was made for rotor magnet
11 shown in FIG. 2, and for comparison a conventional-type rotor magnet,
to find the effects by groove angles of theta on the characteristics of
the motor. In this experiment, with the stator construction fixed, and 40
degrees set as a groove 12a/12b angle of theta, when a motor axis torque
of 10g-cm was applied, the rate of rotation and the motor current values
were obtained. Table 1 shows the results and the motor efficiency based on
them.
TABLE 1
______________________________________
Con-
ventional Invented Technology
Charact-
Tech- Theta = Theta =
Theta =
Theta =
eristics
nology 15 20 30 40
______________________________________
Rate of 3250 3080 3200 3560 3300
Rotation
(rpm)
Motor 140 125 125 118 125
Current
(mA)
Motor 20 21.1 21.9 26 23
Efficiency
(%)
______________________________________
Table 1 shows that compared with the conventional technology, this
invention produces a low current value and a rise in efficiency of 1 to
6%. If a 4-pole magnetized rotor magnet is used, it became clear that the
motor is most efficient in the vicinity of theta=30 degrees. The
experiment shown in Table 1 was made based on the relationship of tl=4t2
in which the gap between the rotor magnet and the stator is tl, and the
gap between the groove and the stator is t2. This is determined by the
magnetic flux density distribution and the experiment results.
Therefore, it is desirable that the shape of the groove formed in the rotor
magnet should be designed to satisfy the following two formulas:
0.3.times.360/P<2 Theta<360/P (1)
D.gtoreq.G (2)
Where:
2 Theta: Central angle of the circular arc with a magnetic pole switching
point (neutral point) as a symmetrical axis having .THETA. degrees at each
side thereof
P: Number of magnetized poles
D: Center depth of the groove
G: Minimum gap between the stator and the rotor magnet
The brushless motor of this invention is applicable for many purposes. Good
starting and high efficiency are the characteristics of the motor, which
is suitable for products such as fan motors.
FIG. 4 shows the fan motor. FIG. 4 has reference numerals in common with
FIG. 1 for easy explanation of the motor section. A rotor yoke 10 is set
inside an impeller 40, molded in one piece with synthetic resin, after
which both members are heat-clinched and fastened. Rotor shaft 18 is free
to rotate in bearings 44 and 45 fitted into the housing 42, and is held in
position with a retaining ring 48 after a spring 46 is adjusted to a given
preload. The stator, made from coils 4a-4d wound on the stator yoke 2,
laminated and then insulated, is fixed to the outside of a bearing holder
located in the center of the housing 42.
Circuit board 50 is fastened with screws 52 to the inner side of housing
42. A logic circuit for coil excitation, switching elements, and
magnetoelectricity converter 6 for rotor position sensor etc., are mounted
on the circuit board 50, to which the terminal of each coil 4a-4d is also
connected. The required power is supplied from an outside source by means
of wire 52.
The groove cut in the rotor magnet is not limited to a rectangular shape as
viewed in cross-section. There may be other different shapes, such as
A.about.E on FIG. 5.
Example A in FIG. 5 shows a V-shape grooved structure;
Example B in FIG. 5 is a semicircle;
Example C in FIG. 5 is a groove having an arc shape on each side of its
bottom;
Example D in FIG. 5 is a groove of which the bottom is curved out of
central angle Theta on both edges;
Example E in FIG. 5 is a groove of which the bottom is shaped to form a
swelled curve toward the face of the stator.
Other different shapes of groove may be possible.
The rotor magnet structure can also possibly by changed. FIG. 6 and FIG. 7
illustrate this. To improve the characteristics, the rotor magnet may be
made up of a high performance magnet, such as a sintered or a rare-earth
magnet. If such material would be used for the whole of the rotor magnet,
however, it would be costly, far from cost-effective in many cases.
Therefore, FIG. 6 shows that high quality magnets, such as sintered or
rare-earth magnets, should be used in the desired location of, and as part
of, the rotor magnet and that cheaper and easy-to-mold plastic magnets 51
be used in the desired shape for the remaining part. The structure shown
in FIG. 6 enables production of a motor having high performance with
respect to cost.
In the region of the grooves, the rotor magnet is thinner in cross-section
and hence its strength is reduced in the vicinity of the grooves and
therefore subject to deformation. Attention should be paid to handling it.
FIG. 7 is a structure designed for increasing the magnet strength with
proper reinforcement material 53 filling the grooved part of the rotor
magnet 54. For instance, non-magnetic plastics serve the purpose of
reinforcement 53. Moreover, plastic magnets are adaptable as reinforcement
53 for the rotor magnet 54 in which high performance magnets, such as
sintered or rare-earth magnets are used.
FIG. 8 and FIG. 9 show a practical structure of the rotor. If the impeller
must be joined with the rotor, like a fan-motor for instance, it should be
designed to make a combination of rotor magnet 56, shaft 55, and
reinforcement 57 (impeller for fan-motor) as an integral molding, as in
FIG. 8.
FIG. 9 is another combining form in which rotor magnet 56 and shaft 55 are
joined by reinforcement 58 that incorporates gear teeth on the
circumference.
Such examples of structure are not all that this invention covers. The type
explained above is an outer-rotor type motor. In like manner, an
inner-rotor type can be made up in combining form, with high efficiency
gained as well, irrespective of a driving system or a number of magnetic
poles. Of course, it is possible to make changes of the magnet, the
reinforcement material, and the components structured by the
reinforcement.
This invention is applicable to a photoelectro-type rotor position sensor
system in addition to a magnetoelectricity converter rotor position sensor
system.
This invention employs a rotor magnet which is entirely magnetized, without
non-magnetized areas, does not require any magnetizing jig in a special
yoke-shape, and accordingly facilitates in designing the jig, and nearly
eliminates the dispersion resulting from magnetization power supply, such
as magnetization voltage, capacity of magnetization power supply, etc.,
and provides for more steady space distribution of magnetic flux density.
And, some proper change in thickness or space gap of the rotor magnet
allows optional setting of space distribution of magnetic flux density.
This invention is designed for the rotor position sensor to work under the
influence of the magnetized pattern of the rotor magnet contributing to
the torque. The rotor structure, therefore, becomes very simple, and
requires no additional rotor position sensing magnet as previously used.
In this invention, compared with previous types, operating currents are so
small that high efficiency is gained and the temperature can be kept from
going up. It contributes to material cost reduction because the rotor
magnet can be made smaller in capacity while having the same functional
performance capability as the previous type. Moreover, it helps to cut
back the cogging torque. That is how the brushless dc motor related to
this invention produces very significant effects on the facilitation in
manufacture, the characteristic improvement in functional performance, and
only a small degree of dispersion in properties.
The invention has been shown and described in preferred form only, and by
way of example, and many variations may be made in the invention which
will still be comprised within its spirit. It is understood, therefore,
that the invention is not limited to any specific form or embodiment
except insofar as such limitations are included in the appended claims.
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