1. Field of the Invention
The present invention relates to a magnetic bearing device comprising magnetic bearings, including an active magnetic bearing for rotatably supporting a rotor shaft that is driven and rotated by a motor having a stator coil, and to a vacuum pump equipped with the magnetic bearing device. More specifically, the present invention relates to the structure of a stator column assembly in which an electromagnet of the active magnetic bearing, a displacement sensor for the electromagnet and the stator coil are arranged in a cylindrical stator column.
2. Description of the Related Art
FIG. 6 is a vertical sectional view showing a conventional stator column assembly 1 having a cylindrical stator column. This stator column assembly 1 is of the kind used in a magnetic bearing device in which magnetic bearings consisting of a first active radial magnetic bearing, a second active radial magnetic bearing and a thrust magnetic bearing (not shown) are used to rotatably support a rotor shaft that is driven and rotated by a motor having a stator coil, namely, a 5-axis control type magnetic bearing device.
In FIG. 6, the stator column assembly 1 comprises a cylindrical stator column 11 in which a first active radial magnetic bearing, a stator coil 21 of a motor and a second active radial magnetic bearing are housed and fixed at given positions. The cylindrical stator column 11 is a metal (e.g., aluminum) cylindrical member having a cylindrical space therein and having on its top a through hole 11a for a rotor shaft to pierce through and on its bottom an opening 11b. The stator coil 21 comprises a plurality of magnetic cores 21b, for example, 24 cores, each having a coil 21a wound therearound and press-fitted into a metal circular yoke 20c. The first active radial magnetic bearing comprises an electromagnet 31 and a radial displacement sensor 32 for the electromagnet 31. The electromagnet 31 consists of four pairs of magnetic cores 31b and coils 31a wound around the magnetic cores 31b. The radial displacement sensor 32 consists of four pairs of magnetic cores 32b and coils 32a wound around the magnetic cores 32b. The second active radial magnetic bearing comprises an electromagnet 41 and a radial displacement sensor 42 for the electromagnet 41. The electromagnet 41 consists of four pairs of magnetic cores 41b and coils 41a wound around the magnetic cores 41b. The radial displacement sensor 42 consists of four pairs of magnetic cores 42b and coils 42a wound around the magnetic cores 42b. 
The stator column assembly 1 shown in FIG. 6 is fabricated as follows. First, into a cylindrical space within the cylindrical stator column 11 that has been heated at an appropriate temperature, the radial displacement sensor 32 of the first active radial magnetic bearing, a circular spacer member 30e, the electromagnet 31 of the first active radial magnetic bearing, a circular spacer member 30f, the stator coil 21, a circular spacer member 40e, the electromagnet 41 of the second active radial magnetic bearing, a circular spacer member 40f, and the radial displacement sensor 42 of the second active radial magnetic bearing are press-fitted in the order stated, fixing these parts at given positions. After completion of the press fitting, a columnar bushing is inserted therein from the opening 11b and the space is filled with a thermally curable epoxy resin material, coating those parts with the material and simultaneously forming an inner cylinder surface of resin mold inside the cylindrical stator column 11. When the molding work is finished, the inner cylinder surface of mold is subjected to cutting and scraping, to thereby expose the stator magnetic cores 21b, the electromagnet magnetic cores 31b, 41b, and the sensor magnetic cores 32b, 42b. The fabrication of the stator column assembly 1 is completed through, at least, three steps described above. Incidentally, reference symbol 10d denotes a mold portion.
The conventional stator column assembly 1 shown in FIG. 6 has some problems. The first of those problems is awkward fabrication work in constructing the stator column assembly, for the components are arranged in the cylindrical stator column 11 by using the circular spacers and by press-fitting the components through the use of a shrinkage fit. Secondly, this awkward fabrication work leads to defective articles. That is, every component has an electric wire for wiring and hence the breakage of the wire may take place during the fabrication. Thirdly, when an article turns out to be defective due to the breakage of wire or other factors, it is not an option to replace only the parts where the breakage of wire takes place because the whole assembly is coated with resin mold after the shrinkage fit. This makes the whole stator column assembly 1 which has been fixed a defective product, resulting in the disposal of the entire product. Furthermore, the fourth of those problems is that the mold portion 10d is integrated with the cylindrical stator column 11, which brings about the risk of making the mold portion 10d come in contact with the rotor shaft when the magnetic bearing device is heated from extended hour operation. of the motor to expand the mold portion 10d inwardly to the stator column assembly 1. If this possibility becomes a reality, a vacuum pump equipped with this magnetic bearing device is inhibited from operating normally.
A first object of the present invention is to remove the difficulty in fabricating a stator column assembly in a magnetic bearing device comprising magnetic bearings that include an active magnetic bearing for rotatably supporting a rotor shaft that is driven and rotated by a motor having a stator coil, the magnetic bearing device having the stator column assembly in which components such as the stator coil and electromagnets are housed and coated with a mold. A second object of the invention which is to be attained is to provide a stator column assembly whose structure permits to use resources fully and wisely. A third object of the invention which is to be attained is to save a magnetic bearing device having a stator column assembly from inhibition of normal operation due to the thermal expansion of a mold portion of the stator column. A fourth object of the invention which is to be attained is to provide a vacuum pump less costly and higher in performance than in prior art.
In order to attain the first and second objects mentioned above, there is provided a magnetic bearing device comprising magnetic bearings, the bearings including an active magnetic bearing for rotatably supporting a rotor shaft that is driven and rotated by a motor having a stator coil, in which an electromagnet unit and a stator coil unit are engaged to a cylindrical stator column in the order stated, the electromagnet unit comprising an electromagnet of the active magnetic bearing and a displacement sensor for the electromagnet which are attached to a circular holding member and then coated and molded with the use of a resin mold, the stator coil unit comprising the stator coil that is attached to the circular holding member and then coated and molded with the use of a resin mold.
Also, there is provided a magnetic bearing device comprising an active thrust magnetic bearing and first and second active radial magnetic bearings the three of which are used to rotatably support a rotor shaft that is driven and rotated by a motor having a stator coil, in which a first electromagnet unit, a stator coil unit and a second electromagnet unit are engaged to a cylindrical stator column in the order stated, the first electromagnet unit comprising an electromagnet of the first active radial magnetic bearing and a radial displacement sensor for the electromagnet which are attached to a first circular holding member and then-coated and molded with the use of a resin mold, the stator coil unit comprising the stator coil that is attached to a circular yoke member and then coated and molded with the use of a resin mold, the second electromagnet unit comprising an electromagnet of the second active radial magnetic bearing and a radial displacement sensor for the electromagnet which are attached to a second circular holding member and then coated and molded with the use of a resin mold.
In order to attain the third object mentioned above, the thickness in the axial direction of the mold is slightly thinner than the thickness in the axial direction of the circular holding member.
In order to attain the fourth object mentioned above, one of the aforementioned magnetic bearing device is adopted for a magnetic bearing of a vacuum pump.