Electricity: motive power systems – Automatic and/or with time-delay means – Speed or rate-of-movement
Reexamination Certificate
1999-04-26
2001-02-06
Masih, Karen (Department: 2837)
Electricity: motive power systems
Automatic and/or with time-delay means
Speed or rate-of-movement
C318S254100, C318S132000, C318S434000, C318S491000, C310S048000, C310S049540
Reexamination Certificate
active
06184640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC brushless motor, a magnetic bearing device and a turbomolecular pump device, in which an rpm can be detected by a simple structure.
2. Description of the Related Art
In many cases in a turbomolecular pump device which is used as a vacuum device such as a semiconductor production line, particularly in a magnetic bearing type turbomolecular pump device that is to be rotated at a high speed of 10,000 rpm or more, DC brushless motors are used for driving rotors. This is because the DC brushless motor is of a compact and high power type and of an energy saving type in comparison with an induction motor, and it is possible to touch down the DC brushless motor after the motor rotates at a low rpm by using the motor as a power generator during the power failure or the like.
However, although the DC brushless motor is provided with a drive circuit for detecting a commutation timing and controlling the rpm, if the drive circuit is broken down so that the motor rotates in an overspeed condition with the rpm exceeding a rated value (for example, 48,000 rpm), there is a fear that the rotor would not be durable against a centrifugal force, resulting in fracture.
In order to prevent such an overspeed, in general, there is provided a protection function for stopping the motor during the overspeed by detecting an output frequency of the motor, i.e., a frequency of the rotational magnetic field.
Also, as a double safety countermeasure in case of the failure of such a protection function, there is provided an rpm detection mechanism for stopping the motor during the overspeed by detecting the rpm of the motor with another independent system.
FIG. 6
shows an overview of such a conventional turbomolecular pump device and a mounting position of a rpm detection mechanism.
FIG. 7
schematically shows a structure (a) of the rpm detection mechanism and a detection signal (b).
AS shown in
FIG. 6
, the conventional rpm detection mechanism in the turbomolecular pump device is provided with an rpm detecting coil
201
and a retainer ring
202
. The retainer ring
202
is mounted on a lower end portion of a rotor shaft R. The rpm detecting coil
201
is arranged and fixed on the lower side at a predetermined interval to the retainer ring
202
.
As shown in FIG.
7
(
a
), a small size magnetic member is mounted as a target
203
on a lower surface of the retainer ring
202
.
In the thus constructed rpm detection mechanism, when the rotor shaft R rotates, the target
203
mounted on the retainer ring
202
rotates across and above the rpm detecting coil
201
. Thus, as shown in FIG.
7
(
b
), an induction voltage is outputted in response to the rpm of the rotor shaft R from the rpm detecting coil
201
. The rpm of the DC brushless motor is detected from the induction voltage.
Since the conventional device provided with the DC brushless motor and the rpm detection mechanism such as a magnetic bearing device or a turbomolecular pump device requires elongate components for the rpm detection mechanism, the cost for the components increases and the rpm detection mechanism becomes complicated. Therefore, the assembling and adjusting operations are time-consuming.
Also, since the number of the components for the rpm detection mechanism is large, the dimension of the overall device is large and the device is heavy in weight.
In particular in the rotor that rotates at a high speed as in the turbomolecular pump device, since the number of the components is large, its rotary shaft is long and heavy in weight so that the natural bending frequency of the shaft is low. In this case, even if the rpm of the rotor is increased in order to enhance the vacuum performance, when the rpm would be close to the natural frequency of the shaft, the shaft could not rotate due to the resonance. Thus, there is a problem that the rpm could not be increased as desired.
SUMMARY OF THE INVENTION
Accordingly, in order to overcome the foregoing defects inherent in the conventional technology, a primary object of the present invention is to provide a DC brushless motor provided with an rpm detection mechanism with a simple structure.
A secondary object of the present invention is to provide a magnetic bearing device having the DC brushless motor with such a simple structure.
A tertiary object of the present invention is to provide a turbomolecular pump device having such a magnetic bearing device.
In order to attain the primary object, according to a first aspect of the present invention, there is provided a DC brushless motor in which a rotor with a permanent magnet thereon and a stator coil for generating a magnetic field for rotating the rotor are disposed to face the permanent magnet at a predetermined interval, characterized in that a sensor for outputting an output in response to a change of a polarity of the magnetic pole by the permanent magnet is disposed at least under a non-contact condition with the permanent magnet in the space defined by the stator and the rotor.
Then, for instance, a pickup coil or a Hall sensor may be used as the sensor for detecting the change in polarity.
Since the sensor for detecting the change in polarity by the permanent magnet is thus disposed in the space defined by the stator and the rotor, it is possible to provide the rpm detection mechanism with a simple structure and to make the system small in size and light in weight.
Also, in order to attain the secondary object of the present invention, in accordance with a second aspect of the invention, there is provided a magnetic bearing device comprising the above-described DC brushless motor, a drive control means for controlling rotation of the DC brushless motor, and a magnetic bearing for magnetically supporting a rotary shaft of a rotor of the DC brushless motor by an electromagnet and for controlling a magnetic force of the electromagnet so that the rotary shaft takes a target position.
Then, according to a third aspect of the invention, the magnetic bearing device further comprises an rpm decision means for deciding an rpm of the rotor from the output of the sensor of the DC brushless motor, and the drive control means stops the rotation of the DC brushless motor in the case where the rpm decision means decides that the rpm of the rotor exceeds a predetermined value. Thus, since the rotation of the DC brushless motor is stopped, it is possible to prevent the overspeed of the DC brushless motor.
Also, according to a fourth aspect of the invention, the magnetic bearing device further comprises an rpm decision means for deciding an rpm of the rotor from the output of the sensor of the DC brushless motor, and a power supply stopping means for stopping a supply of the power to the DC brushless motor in the case where the rpm decision means decides that the rpm of the rotor exceeds a predetermined value. Thus, since the supply of the power for driving the DC brushless motor is forcibly stopped, it is possible to stop the rotation of the DC brushless motor without fail.
In accordance with a fifth aspect of the invention, the magnetic bearing device comprises a further power supply stopping means for stopping a supply of the power to the DC brushless motor in the case where the rpm decision means decides that the rpm of the rotor exceeds a predetermined value for a predetermined period. It is possible to continue the rotation without forcibly stopping the DC brushless motor in the case where the overspeed may be improved in a predetermined period of time by stopping the feed after the predetermined period without immediately stopping the supply of the power in the case the rpm exceeds the predetermined value.
In the magnetic bearing device according to a sixth aspect, the rpm decision means decides the rpm of the rotor on the basis of a signal after the output of the pickup coil has passed through at least one of an integrator and the primary or secondary or more low pass filter of a pass band having a higher frequency than a rated rpm of the DC brushless motor. T
Adams & Wilks
Masih Karen
Seiko Seiki Kabushiki Kaisha
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