U.S. patent application number 09/732739 was filed with the patent office on 2001-05-03 for multishaft electric motor and positive-displacement pump combined with such multishaft electric motor.
This patent application is currently assigned to EBARA CORPORATION. Invention is credited to Hisabe, Yasushi, Matake, Kozo, Nagayama, Masami, Ogamino, Hiroaki, Ojima, Yoshinori, Sato, Genichi, Usui, Katsuaki.
Application Number | 20010000722 09/732739 |
Document ID | / |
Family ID | 27304067 |
Filed Date | 2001-05-03 |
United States Patent
Application |
20010000722 |
Kind Code |
A1 |
Ojima, Yoshinori ; et
al. |
May 3, 2001 |
Multishaft electric motor and positive-displacement pump combined
with such multishaft electric motor
Abstract
A multishaft electric motor has a plurality of juxtaposed rotors
having respective permanent magnets disposed therearound, and a
plurality of sets of armature elements disposed fully
circumferentially around the rotors, respectively, the permanent
magnets of adjacent two of the rotors having a plurality of pairs
of unlike magnetic poles for magnetically coupling the rotors
through the armature elements between the permanent magnets. A
positive-displacement vacuum pump includes a casing, a pair of pump
rotors rotatably disposed in the casing in confronting relation to
each other, and a two-shaft electric motor coupled to the pump
rotors for rotating the pump rotors in opposite directions. The
two-shaft electric motor may comprise a pair of juxtaposed rotors
and a pair of sets of armature elements disposed fully
circumferentially around the rotors, respectively.
Inventors: |
Ojima, Yoshinori;
(Fujisawa-shi, JP) ; Matake, Kozo; (Kawasaki-shi,
JP) ; Sato, Genichi; (Chigasaki-shi, JP) ;
Hisabe, Yasushi; (Kanagawa-ken, JP) ; Nagayama,
Masami; (Fujisawa-shi, JP) ; Usui, Katsuaki;
(Kawasaki-shi, JP) ; Ogamino, Hiroaki;
(Kawasaki-shi, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
EBARA CORPORATION
11-1, HANEDA ASSAHI-CHO OHTA-KU
TOKYO
JP
|
Family ID: |
27304067 |
Appl. No.: |
09/732739 |
Filed: |
December 11, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09732739 |
Dec 11, 2000 |
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09127123 |
Jul 30, 1998 |
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6183218 |
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09127123 |
Jul 30, 1998 |
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08425872 |
Apr 20, 1995 |
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5814913 |
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Current U.S.
Class: |
417/423.7 |
Current CPC
Class: |
H02K 21/16 20130101;
H02K 16/02 20130101; F04C 29/0085 20130101; H02K 29/03 20130101;
F04C 28/08 20130101 |
Class at
Publication: |
417/423.7 |
International
Class: |
F04B 017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 1994 |
JP |
082969/1994 |
Sep 22, 1994 |
JP |
254693/1994 |
Mar 15, 1995 |
JP |
083319/1995 |
Claims
What is claimed is:
1. A multishaft electric motor comprising: a plurality of
juxtaposed rotors having respective permanent magnets disposed
therearound; and a plurality of sets of armature elements disposed
fully circumferentially around said rotors, respectively, said
permanent magnets of adjacent two of said rotors having a plurality
of pairs of unlike magnetic poles for magnetically coupling the
rotors through said armature elements between the permanent
magnets.
2. A multishaft electric motor according to claim 1, wherein when
said adjacent two of the rotors are to be driven, said armature
elements of said adjacent two of the rotors are energized such that
the armature elements in symmetric positions of said adjacent two
of the rotors produce unlike magnetic poles.
3. A multishaft electric motor according to claim 1, wherein said
armature elements are divided into armature elements of respective
phases for magnetically coupling unlike magnetic poles in symmetric
positions of said adjacent two of the rotors.
4. A multishaft electric motor according to claim 1, further
comprising an armature core, said armature elements being disposed
on said armature core, said armature core being divided by air gaps
to block magnetic paths other than magnetic paths for magnetically
coupling unlike magnetic poles in symmetric positions of said
adjacent two of the rotors.
5. A multishaft electric motor according to claim 1, further
comprising a plurality of magnetic coupling bars of a magnetic
material extending between unlike magnetic poles of said adjacent
two of the rotors for magnetically coupling the rotors.
6. A multishaft electric motor according to claim 5, wherein each
of said magnetic coupling bars has legs inserted in respective
slots defined in symmetric positions between said sets of armature
elements.
7. A multishaft electric motor according to claim 1, wherein said
permanent magnets of said adjacent two of said rotors have
different number of magnetic poles from each other so that said
adjacent two of the rotors are rotated at a ratio of rotational
speeds in accordance with a ratio of the number of magnetic
poles.
8. A positive-displacement vacuum pump comprising: a casing; a pair
of pump rotors rotatably disposed in said casing in confronting
relation to each other; and a two-shaft electric motor coupled to
said pump rotors for rotating said pump rotors in opposite
directions; said two-shaft electric motor comprising: a pair of
juxtaposed rotors having respective permanent magnets disposed
therearound; and a pair of sets of armature elements disposed fully
circumferentially around said rotors, respectively, said permanent
magnets of the respective sets having a plurality of pairs of
unlike magnetic poles for magnetically coupling the rotors through
said armature elements between the permanent magnets.
9. A positive-displacement vacuum pump according to claim 8,
further comprising means for controlling said two-shaft electric
motor to rotate at variable speeds for varying a pump
displacement.
10. A positive-displacement vacuum pump according to claim 8,
further comprising means for monitoring and controlling a current
supplied to said two-shaft electric motor to vary a rotational
speed thereof for preventing a pump overload.
11. A positive-displacement vacuum pump according to claim 8,
wherein said two-shaft electric motor comprises a canned motor
having a pair of cans housing said rotors, respectively, to isolate
the rotors from said armature elements.
12. A positive-displacement vacuum pump according to claim 11,
wherein each of said cans is made of synthetic resin.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
2. The present invention relates to a multishaft electric motor for
rotating a plurality of shafts in synchronism with each other and a
positive-displacement pump which is combined with such a multishaft
electric motor, and more particularly to a multishaft electric
motor suitable for use with a rotary machine which is required to
rotate two shafts synchronously in opposite directions, such as a
two-shaft gear pump, a two-shaft screw pump, a two-shaft Roots
blower, a two-shaft screw compressor, or the like, and a
positive-displacement pump which is combined with such a multishaft
electric motor.
3. 2. Description of the Prior Art
4. Electric motors for use as driving means for driving pumps or
the like include induction motors and direct-current motors.
Generally, these motors have only one rotatable shaft.
5. FIG. 23 of the accompanying drawings shows in cross section a
two-shaft rotary machine such as a Roots blower which is driven by
an electric motor having only one rotatable shaft. The two-shaft
rotary machine shown in FIG. 23 comprises a pair of juxtaposed
rotors 32, 33 disposed in a housing 31 and having respective shafts
32a, 33a, and a pair of gears 34, 35 fixedly mounted on the shafts
32a, 33a, respectively, and held in mesh with each other. An
electric motor 35 has a rotatable drive shaft 35a coupled coaxially
to the shaft 32a of the rotor 32.
6. When the rotor 32 is rotated by the electric motor 35, the
rotational drive power is transmitted from the rotor 32 through the
gears 34, 35 to the other rotor 33. Therefore, the shafts 32a, 33a
and hence the rotors 32, 33 are rotated synchronously in opposite
directions.
7. Japanese laid-open patent publication No. 4-178143 discloses a
two-shaft electric motor for rotating two shafts synchronously in
opposite directions. The disclosed two-shaft electric motor is
shown in FIGS. 24 and 25 of the accompanying drawings. As shown in
FIGS. 24 and 25, two rotors 41, 42 with circumferential permanent
magnets are disposed in a housing 40 such that the permanent
magnets are held in contact with each other or positioned closely
to each other. The rotors 41, 42 are juxtaposed in a stator 44
mounted in the housing 40 and supported on parallel shafts that are
rotatably mounted in the housing 40 by respective sets of bearings
45, 46. An array of armature elements 43 is mounted on an
elliptical inner circumferential surface of the stator 44. The
rotors 41, 42 jointly provide a magnetic coupling in confronting
tooth-free regions thereof where unlike magnetic poles of the
permanent magnets of the rotors 41, 42 face each other.
8. The two-shaft rotary machine shown in FIG. 23 suffers size and
noise problems because the gears 34, 35 are required as timing
gears for rotating the rotors 32, 33 synchronously in opposite
directions.
9. In the two-shaft electric motor shown in FIGS. 24 and 25, an
attractive force is developed due to the magnetic coupling between
the rotors 41, 42 which are supported in contact with each other or
with a small gap left therebetween. The attractive force thus
developed is responsible for a radially unbalanced load imposed on
the rotors 41, 42. To suppress an excessively large eccentric load
applied to the bearings 45, 46 owing to the radially unbalanced
load and allow the rotors 41, 42 to rotate smoothly at high speeds,
it is necessary to apply a certain magnetic attractive counterforce
tending to cancel the magnetic attractive force acting between the
rotors 41, 42. The armature elements 43 disposed on the elliptical
inner circumferential surface of the stator 44 are not available
for generating such a magnetic attractive counterforce because the
armature elements 43 generate a rotating magnetic field for driving
the rotors 41, 42. If the rotors 41, 42 are held in contact with
each other, then no such magnetic attractive counterforce needs to
be generated, but the contacting rotors 41, 42 are liable to
produce an undue level of wear or noise.
SUMMARY OF THE INVENTION
10. It is therefore an object of the present invention to provide a
multishaft electric motor capable of rotating a plurality of shafts
in synchronism with each other stably at high speeds.
11. Another object of the present invention is to provide a
two-shaft electric motor capable of rotating two rotors
synchronously in opposite directions stably at high speeds while
eliminating a radially unbalanced load due to a magnetic coupling
between the rotors.
12. Still another object of the present invention is to provide a
positive-displacement vacuum pump which can be controlled to vary,
i.e., increase or decrease, its rotational speed and to prevent an
electric motor combined therewith from being overloaded, without
employing other components including an inverter, a magnet
coupling, a fluid coupling, and a speed-increasing gear.
13. To achieve the above objects, there is provided in accordance
with the present invention a multishaft electric motor comprising a
plurality of juxtaposed rotors having respective permanent magnets
disposed therearound, and a plurality of sets of armature elements
disposed fully circumferentially around the rotors, respectively,
the permanent magnets of adjacent two of the rotors having a
plurality of pairs of unlike magnetic poles for magnetically
coupling the rotors through the armature elements between the
permanent magnets.
14. According to the present invention, there is also provided a
positive-displacement vacuum pump comprising a casing, a pair of
pump rotors rotatably disposed in the casing in confronting
relation to each other, and a two-shaft electric motor coupled to
the pump rotors for rotating the pump rotors in opposite
directions, the two-shaft electric motor comprising a pair of
juxtaposed rotors having respective permanent magnets disposed
therearound, and a pair of sets of armature elements disposed fully
circumferentially around the rotors, respectively, the permanent
magnets of the respective sets having a plurality of pairs of
unlike magnetic poles for magnetically coupling the rotors through
the armature elements between the permanent magnets.
15. In the multishaft electric motor, magnetic fluxes generated by
the rotors pass through closed magnetic circuits extending between
the rotors, and act as a magnetic coupling between the rotors. The
magnetic circuits extend through a common armature core and are
closed, and are balanced between the armature elements and the
rotors. The magnetic circuits are able to produce rotational forces
to rotate the rotors synchronously in opposite directions stably at
high speeds without imposing an excessive eccentric load on
bearings of the rotors.
16. In the positive-displacement vacuum pump, the pump rotors can
be driven by the two-shaft electric motor, and the rotational speed
of the pump can be varied by an external signal that is supplied to
a motor driver for the two-shaft electric motor. Consequently, the
displacement of the pump can be controlled by controlling the
two-shaft electric motor. A current supplied to the two-shaft
electric motor, typically a brushless direct-current motor, is
monitored and controlled to vary the rotational speed thereof for
preventing the positive-displacement vacuum pump from being
overloaded. Accordingly, the positive-displacement vacuum pump is
free of limitations on its operation range which would otherwise be
required by variations in the load on a gas handled by the
positive-displacement vacuum pump.
17. The above and other objects, features, and advantages of the
present invention will become apparent from the following
description when taken in conjunction with the accompanying
drawings which illustrate preferred embodiments of the present
invention by way of example.
BRIEF DESCRIPTION OF THE DRAWINGS
18. FIG. 1 is an axial cross-sectional view of a multishaft
electric motor according to a first embodiment of the present
invention;
19. FIG. 2 is a cross-sectional view taken along line II-- II of
FIG. 1;
20. FIGS. 3A, 3B, and 3C are cross-sectional views showing the
manner in which the multishaft electric motor shown in FIGS. 1 and
2 operates;
21. FIG. 4 is a timing chart of a current pattern in which coils
are energized when the multishaft electric motor shown in FIGS. 1
and 2 operates as shown in FIGS. 3A through 3C;
22. FIGS. 5A, 5B, and 5C are circuit diagrams showing how the coils
are energized when the multishaft electric motor shown in FIGS. 1
and 2 operates as shown in FIGS. 3A through 3C;
23. FIG. 6 is a cross-sectional view of a multishaft electric motor
according to a second embodiment of the present invention;
24. FIG. 7 is a cross-sectional view of a modification of the
multishaft electric motor shown in FIG. 6;
25. FIG. 8 is a cross-sectional view of another modification of the
multishaft electric motor shown in FIG. 6;
26. FIG. 9 is a cross-sectional view of a multishaft electric motor
according to a third embodiment of the present invention;
27. FIGS. 10A and 10B are elevational and cross-sectional views,
respectively, of a multishaft electric motor according to a fourth
embodiment of the present invention;
28. FIG. 11 is a cross-sectional view of a multishaft electric
motor according to a fifth embodiment of the present invention;
29. FIG. 12 is a timing chart of a current pattern in which coils
are energized when the multishaft electric motor shown in FIG. 11
operates;
30. FIGS. 13A, 13B and 13C are circuit diagrams showing how the
coils are energized when the multishaft electric motor shown in
FIG. 11;
31. FIG. 14 is a cross-sectional view of a multishaft electric
motor according to a sixth embodiment of the present invention;
32. FIG. 15 is an axial cross-sectional view of a
positive-displacement vacuum pump according to an embodiment of the
present invention which incorporates a multishaft electric motor
according to the present invention;
33. FIG. 16 is a cross-sectional view taken along line XVI--XVI of
FIG. 15;
34. FIG. 17 is a cross-sectional view taken along line XVII--XVII
of FIG. 15;
35. FIG. 18 is a cross-sectional view taken along line XVIII--XVIII
of FIG. 15;
36. FIG. 19 is a block diagram of a motor driver of a two-shaft
electric motor incorporated in the positive-displacement vacuum
pump of FIG. 15;
37. FIG. 20 is a diagram showing the relationship between the
rotational speed and torque and the relationship between current
and torque;
38. FIG. 21 is a diagram showing the relationship between motor
characteristics and pump operation in the positive-displacement
vacuum pump;
39. FIG. 22 is an axial cross-sectional view of a
positive-displacement vacuum pump according to another embodiment
of the present invention;
40. FIG. 23 is an axial cross-sectional view of a conventional
two-shaft rotary machine;
41. FIG. 24 is an axial cross-sectional view of a conventional
two-shaft electric motor; and
42. FIG. 25 is a cross-sectional view taken along line XXV--XXV of
FIG. 24.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
43. Like or corresponding parts are denoted by like or
corresponding reference characters throughout views.
44. A multishaft electric motor according to a first embodiment of
the present invention will be described below with reference to
FIGS. 1 through 5A, 5B and 5C.
45. As shown in FIG. 1, the multishaft electric motor has a pair of
rotors 2A, 2B disposed in a motor frame 1 and rotatably supported
in the motor frame 1 by respective sets of bearings 5 near opposite
ends of the shafts of the rotors 2A, 2B. As shown in FIG. 2, the
rotors 2A, 2B have respective annular permanent magnets 2a, 2b
disposed circumferentially around the rotor shafts each composed of
2n poles (n is the number of magnetic poles) arranged symmetrically
at angularly equal intervals around the rotor shaft for generating
radial magnetic fluxes. In the first embodiment, the permanent
magnet 2a, 2b of each of the rotors 2A, 2B has n=2 pole pairs and
four poles S, N, S, N.
46. A plurality of armature elements 3a.sub.1-3a.sub.6 are disposed
at angularly equal intervals fully around the rotor 2A within the
motor frame 1, and a plurality of armature elements
3b.sub.1-3b.sub.6 are disposed at angularly equal intervals fully
around the rotor 2B within the motor frame 1. Adjacent two of these
armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 are
angularly spaced at a pitch of 60.degree.. The armature elements
3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 comprise respective radially
inwardly extending pole teeth U-Z, U1-Z1 on an armature core Ac and
respective coils 4a, 4b mounted respectively on the pole teeth U-Z,
U1-Z1. The pole teeth U-Z, U1-Z1 are positioned at
circumferentially equal intervals, and the coils 4a, 4b are mounted
on the respective pole teeth U-Z, U1-Z1 such that when the coils
4a, 4b are energized, the armature elements 3a.sub.1-3a.sub.6,
3b.sub.1-3b.sub.6 produce magnetic poles that are symmetric and
opposite with respect to a central plane C lying intermediate
between the respective axes of the rotors 2A, 2B. The coils 4b are
wound in a direction opposite to the direction in which the coils
4a are wound.
47. Operation of the multishaft electric motor shown in FIGS. 1 and
2 will be described below with reference to FIGS. 3A, 3B and 3C. In
FIGS. 3A, 3B and 3C, only the rotors 2A, 2B and the armature
elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 are shown for
illustrative purpose.
48. When the coils 4a, 4b are energized, the armature elements
3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 generate spatially moving
magnetic fields for rotating the rotors 2A, 2B in opposite
directions. Specifically, when the coils 4a, 4b are energized such
that the pole teeth U, X produce N poles, the pole teeth V, Y
produce S poles, the pole teeth U1, X1 produce S poles, and the
pole teeth V1, Y1 produce N poles, all simultaneously, as shown in
FIG. 3A, the rotors 2A, 2B are rotated in opposite directions as
indicated by the arrows.
49. When the coils 4a, 4b are energized such that the pole teeth V,
Y produce S poles, the pole teeth W, Z produce N poles, the pole
teeth V1, Y1 produce N poles, and the pole teeth W1, Z1 produce S
poles, all simultaneously, as shown in FIG. 3B, the rotors 2A, 2B
are rotated in opposite directions as indicated by the arrows.
Further, when the coils 4a, 4b are energized such that the pole
teeth X, U produce S poles, the pole teeth W, Z produce N poles,
the pole teeth X1, U1 produce N poles, and the pole teeth W1, Z1
produce S poles, all simultaneously, as shown in FIG. 3C, the
rotors 2A, 2B are rotated under successive rotational forces in
opposite directions as indicated by the arrows.
50. Magnetic fields generated by the permanent magnets 2a, 2b of
the rotors 2A, 2B pass through magnetic paths that are formed and
closed between the rotors 2A, 2B by the armature elements.
Therefore, a magnetic coupling acts on the unlike magnetic poles of
the rotors 2A, 2B for rotating the rotors 2A, 2B synchronously in
opposite directions.
51. FIG. 4 is a timing chart of a current pattern in which the
coils 4a, 4b are energized when the multishaft electric motor shown
in FIGS. 1 and 2 operates as shown in FIGS. 3A, 3B and 3C.
Specifically, the pattern shown in FIG. 4 represents a pattern in
which direct currents are supplied to the coils 4a on the pole
teeth U-Z and direct currents are supplied to the coils 4b on the
pole teeth U1-Z1. When the coils 4a, 4b are energized in the
pattern shown in FIG. 4, a spatially moving magnetic field, i.e., a
rotating magnetic field, is generated to magnetize the magnetic
teeth U-Z, U1-Z1 as shown in FIGS. 3A, 3B and 3C, thus causing the
rotors 2A, 2B to rotate synchronously in opposite directions, as
described above. An electric circuit (not shown) for supplying the
direct currents to the coils 4a, 4b in the pattern shown in FIG. 4
may be made up of existing electric components such as
semiconductor devices or the like.
52. FIGS. 5A, 5B, and 5C show how the coils 4a, 4b are energized
when the multishaft electric motor shown in FIGS. 1 and 2 operates
as shown in FIGS. 3A, 3B and 3C. Specifically, FIG. 5A shows how
the coils 4a, 4b are energized when the multishaft electric motor
operates as shown in FIG. 3A, FIG. 5B shows how the coils 4a, 4b
are energized when the multishaft electric motor operates as shown
in FIG. 3B, and FIG. 5C shows how the coils 4a, 4b are energized
when the multishaft electric motor operates as shown in FIG.
3C.
53. According to the first embodiment shown in FIGS. 1 through 5A,
5B and 5C, the rotors 2A, 2B with the annular permanent magnets 2a,
2b are juxtaposed and surrounded fully circumferentially by the
armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6, and the
permanent magnets 2a, 2b have plural pairs of unlike magnetic poles
for providing a magnetic coupling between the rotors 2A, 2B through
the armature elements. Therefore, the rotors 2A, 2B can be rotated
synchronously in opposite directions by the magnetic coupling, and
the bearings 5 are not subject to an excessive eccentric load, but
a radially balanced load. Consequently, the rotors 2A, 2B and hence
the respective two shafts of the electric motor can be rotated in
synchronism with each other stably at high speeds, and the electric
motor has a long service life.
54. Furthermore, since the magnetic coupling is provided between
plural pairs of unlike magnetic poles of the permanent magnets 2a,
2b between the rotors 2A, 2B, the magnetic coupling has a large
area. Large synchronizing forces free of pulsating forces are
produced to rotate the rotors 2A, 2B synchronously because a
uniform air gap length is achieved fully around the rotors 2A,
2B.
55. In addition, when the rotors 2A, 2B are driven, the coils 4a,
4b are energized such that symmetrically positioned armature
elements produce unlike magnetic poles. Consequently, a high
magnetic coupling effect is produced upon energization of the coils
4a, 4b in addition to the magnetic coupling effect that is present
when the coils 4a, 4b are not energized.
56. FIG. 6 shows in cross section a multishaft electric motor
according to a second embodiment of the present invention.
According to the second embodiment, armature elements are divided
into those of respective phases in order to couple unlike magnetic
poles in symmetric positions of adjacent rotors. Specifically, as
shown in FIG. 6, the multishaft motor has permanent magnets 2a on a
rotor 2A and permanent magnets 2b on a rotor 2B which are arranged
such that magnetic couplings are produced between unlike magnetic
poles in symmetric positions thereof. The multishaft motor also has
armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 disposed
fully circumferentially around the rotors 2A, 2B. The armature
elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 comprise respective
radially inwardly extending pole teeth U-Z, U1-Z1 on armature cores
Ac.sub.1-Ac.sub.6 and respective coils 4a, 4b mounted respectively
on the pole teeth U-Z, U1-Z1. The armature elements
3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 are connected in respective
phases only, e.g., the armature elements associated with phases U,
U1 are connected to each other, the armature elements associated
with phases V, V1 are connected to each other, the armature
elements associated with phases W, W1 are connected to each other,
the armature elements associated with phases X, X1 are connected to
each other, the armature elements associated with phases Y, Y1 are
connected to each other, and the armature elements associated with
phases Z, Z1 are connected to each other.
57. With the above arrangement shown in FIG. 6, it is possible to
magnetically couple the rotors 2A, 2B between unlike magnetic poles
in symmetric positions thereof. Particularly, a high magnetic
coupling effect is achieved when the coils 4a, 4b are not
energized. When coils 4a, 4b are energized as shown in FIGS. 3A, 3B
and 3C and 5A, 5B and 5C, the rotors 2A, 2B are rotated
synchronously in opposite directions.
58. FIG. 7 shows a modification of the multishaft electric motor
shown in FIG. 6. According to the modification, rotors 2A, 2B with
annular permanent magnets 2a, 2b are surrounded by armature
elements having pole teeth "a"-"l", "a1"-"1l" with coils 4a, 4b,
the pole teeth "a"-"l", "a1"-"1l" being connected in respective
pairs to clearly define magnetic paths for the rotors 2A, 2B.
59. FIG. 8 shows another modification of the multishaft electric
motor shown in FIG. 6. In FIG. 8, pole teeth are connected in
respective pairs to clearly define magnetic paths for rotors 2A, 2B
with annular permanent magnets 2a, 2b.
60. If the modified multishaft electric motors shown in FIGS. 7 and
8 are arranged to provide a magnetic coupling effect in the absence
of the coils 4a, 4b, then there is achieved a parallel magnetic
coupling device capable of rotating the parallel rotors 2A, 2B
synchronously in opposite directions.
61. FIG. 9 shows in cross section a multishaft electric motor
according to a third embodiment of the present invention. According
to the third embodiment, an armature core is divided by air gaps to
block those magnetic paths other than magnetic paths for coupling
unlike magnetic poles in symmetric positions of adjacent rotors.
Specifically, as shown in FIG. 9, the multishaft electric motor
comprises a pair of rotors 2A, 2B with annular permanent magnets
2a, 2b mounted thereon, a plurality of armature elements
3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 disposed at angularly equal
intervals fully circumferentially around the rotors 2A, 2B.
Adjacent two of these armature elements 3a.sub.1-3a.sub.6,
3b.sub.1-3b.sub.6 are angularly spaced at a pitch of 60.degree..
The armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 comprise
radially inwardly extending pole teeth U-Z on an armature core
Ac.sub.1, radially inwardly extending pole teeth U1-Z1 on an
armature core Ac.sub.2, and coils 4a, 4b mounted respectively on
the pole teeth U-Z, U1-Z1. The pole teeth U-Z, U1-Z1 are positioned
at circumferentially equal intervals, and the coils 4a, 4b are
mounted on the respective pole teeth U-Z, U1-Z1 such that when the
coils 4a, 4b are energized, the armature elements 3a.sub.1-
3a.sub.6, 3b.sub.1-3b.sub.6 produce magnetic poles that are
symmetric and opposite with respect to a central plane C lying
intermediate between the respective axes of the rotors 2A, 2B. The
coils 4b are wound in a direction opposite to the direction in
which the coils 4a are wound.
62. The pole teeth U-Z on the armature core Ac.sub.1 are divided
into two equal groups of pole teeth U, Y, Z and V, X, W by upper
and lower recesses 5a that are defined in the armature core
Ac.sub.1 along an alternate long and short dash line which extends
perpendicularly to a line interconnecting the axes of the rotors
2A, 2B and passes through the axis of the rotor 2A. Similarly, the
pole teeth U1-Z1 on the armature core Ac.sub.2 are divided into two
equal groups of pole teeth U1, Y1, Z1 and V1, X1, W1 by upper and
lower recesses 5b that are defined in the armature core Ac.sub.2
along an alternate long and short dash line which extends
perpendicularly to the line interconnecting the axes of the rotors
2A, 2B and passes through the axis of the rotor 2B.
63. Other structural details of the multishaft electric motor shown
in FIG. 9 are identical to those of the multishaft electric motor
shown in FIGS. 1 and 2. When the coils 4a, 4b are energized as
shown in FIGS. 3A, 3B and 3C and 5A, 5B and 5C, the rotors 2A, 2B
are rotated synchronously in opposite directions. The recesses 5a,
5b are effective in increasing a magnetic coupling effect between
the pole teeth V, V1 and also between the pole teeth X, X1, for
thereby ensuring synchronous rotation of the rotors 2A, 2B in
opposite directions.
64. A multishaft electric motor according to a fourth embodiment of
the present invention is shown in FIGS. 10A and 10B. According to
the fourth embodiment, magnetic coupling bars of a magnetic
material extend between unlike magnetic poles of rotors.
Specifically, as shown in FIG. 10A, the multishaft electric motor
has a plurality of inverse U-shaped magnetic coupling bars 7a, 7b,
7c of a magnetic material. As shown in FIG. 10B, the multishaft
electric motor also includes a pair of rotors 2A, 2B with annular
permanent magnets 2a, 2b mounted thereon, a plurality of armature
elements disposed at angularly equal intervals fully
circumferentially around the rotors 2A, 2B. The armature elements
comprise radially inwardly extending pole teeth U-Z, U1-Z1 on an
armature core Ac, and coils 4a, 4b mounted respectively on the pole
teeth U-Z, U1-Z1. The magnetic coupling bars 7a, 7b and 7c have
legs inserted respectively in slots S defined in the armature core
Ac between the pole teeth U-Z, U1-Z1, thereby providing closed
magnetic paths between unlike magnetic poles of the rotors 2A, 2B.
Certain air gaps are lest between the armature core Ac and the
magnetic coupling bars 7a, 7b and 7c. Magnetic fluxes passing
through the slots S are magnetically coupled at the unlike magnetic
poles of the rotors 2A, 2B for rotating the rotors 2A, 2B
synchronously in opposite directions. The magnetic coupling bars
7a, 7b and 7c are effective in increasing a magnetic coupling
effect when the coils 4a, 4b are not energized. Since the legs of
the magnetic coupling bars 7a, 7b and 7c are inserted in the slots
S which are symmetrically positioned between the two sets of
armature elements, the legs of the magnetic coupling bars 7a, 7b
and 7c can easily be inserted into the slots S. Therefore, the
magnetic coupling bars 7a, 7b and 7c can easily be attached in
place without magnetic interference with each other.
65. FIG. 11 shows in cross section a multishaft electric motor
according to a fifth embodiment of the present invention. According
to the fifth embodiment, a pair of juxtaposed rotors 2A, 2B have
different number of magnetic poles from each other so that the
rotors 2A, 2B can be rotated in opposite directions at different
rotational speeds from each other. That is, the rotors 2A and 2B
are rotated at a ratio of rotational speeds in accordance with a
ratio of the number of magnetic poles. As shown in FIG. 11, the
rotor 2A has permanent magnets 2a comprising four poles S, N, S, N,
and the rotor 2B has permanent magnets 2b comprising six poles S,
N, S, N, S, N. Each of the permanent magnets 2a has the same outer
circumferential length as each of the permanent magnets 2b. The
ratio of the number of magnetic poles of the rotors A and B is
2:3.
66. A plurality of armature element 3a.sub.1-3a.sub.6 are disposed
at angularly equal intervals fully around the rotor 2A within the
motor frame 1, and a plurality of armature elements
3b.sub.1-3b.sub.9 are disposed at angularly equal intervals fully
around the rotor 2B within the motor frame 1. The armature elements
3a.sub.1-3a.sub.6 are angularly spaced at a pitch of 60.degree. in
the rotor 2A, and the armature elements 3b.sub.1-3b.sub.9 are
angularly spaced at a pitch of 40.degree. in the rotor 2B. The
armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.9 comprise
respective radially inwardly extending pole teeth U-Z, U1-Z1 and
X2-Z2 on armature cores Ac.sub.1-Ac.sub.7 and respective coils 4a,
4b mounted respectively on the pole teeth U-Z, U1-Z1 and X2-Z2.
67. FIG. 12 is a timing chart of a current pattern in which the
coils 4a, 4b are energized when the multishaft electric motor shown
in FIG. 11. FIGS. 13A, 13B and 13C are circuit diagrams showing how
the coils 4a, 4b are energized when the multishaft electric motor
shown in FIG. 11 operates. By supplying direct currents to the
coils 4a and 4b as shown in FIGS. 12, 13A, 13B and 13C, a spatially
moving magnetic field, i.e., a rotating magnetic field, is
generated to magnetize the magnetic teeth U-Z, V1-Z1 and X2-Z2,
thus causing the rotors 2A, 2B to rotate synchronously in opposite
directions. In this case, the rotors 2A and 2B are rotated at a
ratio of 3:2 which is in inverse proportion to a ratio of the
number of magnetic poles, i.e., 2:3.
68. The multishaft electric motor in the fifth embodiment is
preferably applicable to the screw compressor or the like in which
a pair of pump rotors are rotated at a certain ratio of rotational
speeds.
69. FIG. 14 shows in cross section a multishaft electric motor
according to a sixth embodiment of the present invention. According
to the sixth embodiment, the multishaft electric motor has four
shafts. Specifically, the multishaft electric motor includes four
rotors 2A, 2B, 2C, 2D that are magnetically coupled for synchronous
rotation in opposite directions as indicated by the arrows. The
multishaft electric motor according to the sixth embodiment may
advantageously be used in combination with a stirrer or the like
which require three or more rotating shafts.
70. A positive-displacement vacuum pump according to an embodiment
of the present invention which incorporates a multishaft electric
motor according to the present invention will be described below
with reference to FIGS. 15 through 21.
71. As shown in FIGS. 15 and 16, the positive-displacement vacuum
pump has a casing 11 and a pair of Roots rotors 12 as pump rotors
disposed in the casing 11. Each of the Roots rotors 12 is rotatably
supported in the casing 11 by a pair of bearings 13 near opposite
ends of the shaft thereof. The Roots rotors 12 can be rotated by a
two-shaft electric motor M which is of a structure as shown in
FIGS. 1 through 5A, 5B and 5C.
72. The two-shaft electric motor M is shown in detail in FIGS. 17
and 18. As shown in FIGS. 17 and 18, the two-shaft electric motor M
has a pair of rotors 2A, 2B fixed coaxially to the ends of the
respective shafts of the Roots rotors 12. The rotors 2A, 2B have
respective annular permanent magnets 2a, 2b disposed
circumferentially around the rotor shafts each composed of 2n poles
(n is the number of pole pairs) arranged symmetrically at angularly
equal intervals around the rotor shaft for generating radial
magnetic fluxes. In this embodiment, the permanent magnet 2a, 2b of
each of the rotors 2A, 2B has n=2 pole pairs and four poles S, N,
S, N.
73. A plurality of armature elements 3a.sub.1-3a.sub.6 are disposed
at angularly equal intervals fully around the rotor 2A within a
motor frame 1 with a can 8 of synthetic resin interposed between
the rotor 2A and the armature elements 3a.sub.1 3a.sub.6, and a
plurality of armature elements 3b.sub.1-3b.sub.6 are disposed at
angularly equal intervals fully around the rotor 2B within the
motor frame 1 with a can 8 of synthetic resin interposed between
the rotor 2A and the armature elements 3a.sub.1-3a.sub.6. Adjacent
two of these armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6
are angularly spaced at a pitch of 60.degree.. The armature
elements 3a.sub.1- 3a.sub.6, 3b.sub.1-3b.sub.6 comprise respective
radially inwardly extending pole teeth U-Z, U1-Z1 on an armature
core Ac and respective coils 4a, 4b mounted respectively on the
pole teeth U-Z, U1-Z1. The pole teeth U-Z, U1-Z1 are positioned at
circumferentially equal intervals, and the coils 4a, 4b are mounted
on the respective pole teeth U-Z, U1-Z1 such that when the coils
4a, 4b are energized, the armature elements 3a.sub.1- 3a.sub.6,
3b.sub.1-3b.sub.6 produce magnetic poles that are symmetric and
opposite with respect to a central plane C lying intermediate
between the respective axes of the rotors 2A, 2B. The coils 4b are
wound in a direction opposite to the direction in which the coils
4a are wound.
74. As shown in FIG. 15, a motor driver 10 for controlling
operation of the two-shaft electric motor M is fixedly mounted on
the motor frame 1.
75. Two intermeshing timing gears 21 (only one shown in FIG. 15)
are fixedly mounted respectively on the ends of the shafts of the
Roots rotors 12 remote from the two-shaft electric motor M. The
timing gears 21 serve to prevent the Roots rotors 12 from rotating
out of synchronism with each other under accidental disturbant
forces.
76. The positive-displacement vacuum pump operates as follows:
77. When the coils 4a, 4b of the two-shaft electric motor M are
energized by the motor driver 10, the armature elements
3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 generate spatially moving
magnetic fields for rotating the rotors 2A, 2B in opposite
directions. The principles of rotation of the two-shaft electric
motor M will not be described in detail here as they have been
described above with reference to FIGS. 3A-3C through 5A-5C.
78. When the rotors 2A, 2B are rotated synchronously in opposite
directions, the synchronized Roots rotors 12 rotate in opposite
directions out of contact with each other, with a small clearance
kept between the inner surfaces of the casing 11 and the Roots
rotors 12 and also between the Roots rotors 12 themselves. As the
Roots rotors 12 rotate, a gas which is drawn from an inlet port
into the casing 11 as indicated by the arrow in FIG. 16 is confined
between the Roots rotors 12 and the casing 11 and delivered toward
an outlet port. In this embodiment, each of the Roots rotors 12 has
three lobes and hence three recesses therebetween. Therefore, the
gas is discharged from the positive-displacement vacuum pump six
times per revolution of the positive-displacement vacuum pump.
79. In this embodiment, the two-shaft electric motor M comprises a
two-shaft brushless direct-current motor, and the motor driver 10
has a function to control the two-shaft brushless direct-current
motor to rotate selectively at variable rotational speeds and also
a function to prevent the two-shaft brushless direct-current motor
from being overloaded.
80. FIG. 19 is a block diagram showing the structure of the motor
driver 10. In the motor driver 10, alternate current (AC) from an
AC power supply 30 is converted into direct current (DC) by a
rectifying circuit 14, and current signals from a current detecting
unit (CDU) 15, phase signals of the motor rotor and rotational
speed signals of the motor rotor from a position detecting unit
(PDU) 16, rotational speed control signals which are external
inputs are inputted into a control unit 18, and thus driving
signals are supplied from the control unit 18 to a driving circuit
19 which drives the brushless direct-current motor M. The control
unit 18 comprises a position detecting signal processing unit
(PDPU) 22, a base unit (BU) 23, a rotational speed detecting unit
(RSDU) 24 and a PWM control unit (PWM) 25.
81. FIG. 20 shows a graph illustrative of the relationship between
rotational speed and torque and the relationship between current
and torque in the brushless direct-current motor M.
82. The brushless direct-current motor M has a linear speed vs.
torque characteristic curve as indicated in FIG. 20 such that the
rotational speed of the brushless direct-current motor M increases
as the torque produced thereby decreases. FIG. 21 shows a graph
illustrative of the relationship between motor characteristics and
pump operation of the positive-displacement vacuum pump. In view of
the service life of the bearings used, the brushless direct-current
motor M is controlled so as to operate the positive-displacement
vacuum pump at a certain constant rotational speed when the torque
produced by the positive-displacement vacuum pump is equal to or
lower than a rated torque (rated output) as shown in FIG. 21.
83. The torque and current of the brushless direct-current motor M
are correlated to each other such that as the torque produced by
the brushless direct-current motor M increases, the current
supplied to the brushless direct-current motor M also increases as
shown in FIG. 20. As the current supplied to the brushless
direct-current motor M increases, the coils 4a, 4b are heated due
to the Joule heat. To prevent the brushless direct-current motor M
from suffering burnout by the heat caused by an overload, the motor
driver 10 establishes a preset current value for the motor current.
The motor current is monitored by the current detecting unit 15.
When the motor current exceeds the preset current value, the motor
driver 10 controls the motor current to lower the rotational speed
of the positive-displacement vacuum pump as indicated by a speed
drop curve in FIG. 21, thereby lowering the pump load to prevent
the motor from being overloaded.
84. Further, as shown in FIG. 20, the rotational speed and applied
voltage of the brushless direct-current motor M are correlated to
each other such that as the applied voltage increases, the
rotational speed of the motor M increases. In FIG. 20, the
relationship between applied voltages V.sub.1 and V.sub.2 is
V.sub.1>V.sub.2. Thus, the rotational speed of the motor M can
be freely varied by varying applied voltages to the motor M. As
shown in FIG. 19, by supplying rotational speed control signals
from an external unit to the PWM control unit 25 of the control
unit 18, applied voltages to the motor M can be controlled,
resulting in controlling the rotational speed of the motor M.
85. Since the cans 8 are disposed as partitions between the rotors
2A, 2B and the armature elements 3a.sub.1-3a.sub.6,
3b.sub.1-3b.sub.6, the interior space of the positive-displacement
vacuum pump is completely isolated from the exterior space.
Accordingly, the positive-displacement vacuum pump has improved
performance and is free of troubles which would otherwise be caused
by ambient air entering the positive-displacement vacuum pump.
86. Furthermore, the brushless direct-current motor M allows a
greater air gap to be created between the rotors 2A, 2B and the
armature elements 3a.sub.1-3a.sub.6, 3b.sub.1-3b.sub.6 than
induction motors. Heretofore, conventional electric motors combined
with positive-displacement vacuum pumps employ metal cans which are
liable to give rise to a large loss due to an eddy current. Since
the cans 8 can be of greater thickness according to the illustrated
embodiment, the cans 8 can be made of synthetic resin, and do not
produce any eddy-current loss, resulting in an increase in the
motor efficiency.
87. FIG. 22 shows a positive-displacement vacuum pump according to
another embodiment of the present invention. In the embodiment
shown in FIG. 22, the principles of the present invention are
applied to a screw-type vacuum pump. Specifically, a pair of screw
rotors 12S (only one shown in FIG. 22) is disposed in a casing 11
and rotatably supported therein by bearings 13. The screw rotors
12S are operatively coupled to each other by intermeshing gears 21
(only one shown in FIG. 22). The screw rotors 12S can be rotated by
a two-shaft brushless direct-current motor M which is identical to
the two-shaft brushless direct-current motor M according to the
embodiment shown in FIG. 15. The positive-displacement vacuum pump
shown in FIG. 22 offers the same advantages as those of the
positive-displacement vacuum pump shown in FIGS. 15 through 21.
88. The multishaft electric motor according to the present
invention offers the following advantages: The multishaft electric
motor can rotate a plurality of shafts synchronously with each
other through a magnetic coupling. Since the bearings on the shafts
are not subject to an excessive eccentric load, but a radially
balanced load, the shafts can be rotated in synchronism with each
other stably at high speeds, and the multishaft electric motor has
a long service life. Furthermore, the magnetic coupling has a large
area, and large synchronizing forces free of pulsating forces are
produced to rotate the shafts synchronously because a uniform air
gap length is achieved fully around the rotors.
89. The positive-displacement vacuum pump according to the present
invention offers the following advantages:
90. (1) By supplying a signal to the motor driver for the brushless
direct-current motor, the rotational speed of the
positive-displacement vacuum pump can be varied to control the
displacement of the pump. Heretofore, it has been customary to use
another component such as a valve to adjust the rate of flow of a
gas discharged from the pump. According to the present invention,
such another component is no necessary. Furthermore, an inverter
for controlling the rotational speed of the motor is not required,
and the brushless direct-current motor is not stopped upon an
instantaneous power failure and hence can operate the
positive-displacement vacuum pump continuously.
91. (2) When the rotational speed of the positive-displacement
vacuum pump is lowered, it is possible to reduce the load on the
positive-displacement vacuum pump thereby preventing the
positive-displacement vacuum pump from being overloaded. Such an
overload prevention capability is effective for pumps with limited
operation ranges. Particularly, a mechanical booster pump which
imposes a certain range on the outlet port pressure can heretofore
be operated under pressures lower than a certain pressure.
According to the present invention, such a mechanical booster pump
can be operated simultaneously with an auxiliary pump in a range
from the atmospheric pressure, and can increase a discharge rate
when the inlet pressure is high, e.g., in the vicinity of the
atmospheric pressure, for shortening the time required to discharge
the gas from a vacuum chamber.
92. (3) The electric motor combined with the positive-displacement
vacuum pump may comprise a canned motor. Especially where the
positive-displacement vacuum pump is incorporated in a
semiconductor fabrication apparatus which handles a highly
reactive, corrosive fluid, the canned motor is effective in
preventing pump components from reacting or being corroded due to
atmospheric air leakage along the shafts, and also in increasing
the pump performance.
93. (4) Since the cans of the electric motor are made of nonmetal,
e.g., synthetic resin, the cans cause no loss due to an eddy
current, resulting in an increase in the motor efficiency. In
addition, the running cost of the positive-displacement vacuum pump
is lowered.
94. Although certain preferred embodiments of the present invention
has been shown and described in detail, it should be understood
that various changes and modifications may be made therein without
departing from the scope of the appended claims.
* * * * *