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Vacuum pump and method of starting the sameRelated Patent Categories: Pumps, Condition Responsive Control Of Pump Drive Motor, By Control Of Electric Or Magnetic Drive MotorVacuum pump and method of starting the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060198735, Vacuum pump and method of starting the same. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a vacuum pump and a method of starting a vacuum pump, and more particularly to a vacuum pump for evacuating a gas from a chamber used in a semiconductor fabrication apparatus or the like, and a method of starting such a vacuum pump. BACKGROUND ART [0002] In a semiconductor fabrication apparatus, a vacuum pump is widely used for evacuating a gas used in a semiconductor fabrication process from a chamber and producing a vacuum environment in the chamber. As this type of vacuum pump, there has been known a positive-displacement vacuum pump having Roots-type or screw-type pump rotors. [0003] Generally, the positive-displacement vacuum pump comprises a pair of pump rotors disposed in a casing, and a motor for rotating the pump rotors. A small clearance is formed between the pair of the pump rotors themselves and also between the pump rotors and the inner surface of the casing so that the pump rotors are rotated in a noncontact manner. When the pair of the pump rotors are synchronously rotated in the opposite directions by energizing the motor, a gas drawn from an inlet port into the casing is delivered toward an outlet port and is thus evacuated from a chamber or the like connected to the inlet port of the vacuum pump. [0004] Some gases used in the semiconductor fabrication process contain components which are solidified or liquidized when the temperature of the gases is lowered. Generally, in the above positive-displacement vacuum pump, the heat of compression is generated during the process of delivering the gas toward the outlet port, and hence the vacuum pump has a high temperature during operation. Therefore, while the vacuum pump maintains a high temperature, even if the vacuum pump evacuates the gas containing the above components, the components are not solidified or liquidized, and a good evacuation is thus carried out. [0005] However, when the operation of the vacuum pump is stopped and the temperature of the vacuum pump is gradually lowered, the components contained in the gas are solidified or liquidized, and are deposited in the clearance between the pump rotors and between the pump rotors and the casing (hereinafter, the solidified or liquidized components are refereed to as a product). Consequently, such product prevents the rotation of the pump rotors, and hence the pump rotors cannot be rotated by a starting torque of the motor, thus causing a failure of the restart of the vacuum pump. Further, in addition to the failure of the restart of the vacuum pump, an excessive load is applied to the motor to cause the motor to overheat, and hence the vacuum pump cannot be operated safely. [0006] Furthermore, in recent years, there has been developed a motor-drive technique for driving an induction motor, a brushless DC motor, or the like with the use of an inverter such as a frequency converter. If such a motor-drive technique is used in the vacuum pump, a torque of the motor for starting the vacuum pump is limited by capacities of parts used in the inverter. Consequently, the motor can generate only a limited torque, and the starting operation of the vacuum pump tends to be more difficult. DISCLOSURE OF INVENTION [0007] The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a vacuum pump which can be normally started even if a product solidified or liquidized in a casing of the vacuum pump presents an obstacle to the rotation of the pump rotor. [0008] Another object of the present invention is to provide a method of starting such a vacuum pump. [0009] In order to achieve the above object, according to one aspect of the present invention, there is provided a vacuum pump comprising: a pump rotor rotatably disposed in a casing; and a pump-rotor controller for controlling rotation of the pump rotor in a forward direction or a reverse direction in accordance with a predetermined pattern at the time of starting the vacuum pump. The rotation of the pump rotor in the forward direction is defined as the rotation of the pump rotor in a direction in which a gas drawn in the casing is delivered from an inlet side of the casing toward an outlet side of the casing. The rotation of the pump rotor in the reverse direction is defined as the rotation of the pump rotor in a direction opposite to the forward direction. [0010] In a preferred aspect of the present invention, the predetermined pattern includes a combination of at least two of rotation of the pump rotor in the forward direction, rotation of the pump rotor in the reverse direction, and stop of the pump rotor. [0011] In a preferred aspect of the present invention, the predetermined pattern is set in the pump-rotor controller such that the pump rotor is driven in the order of the rotation in the forward direction, the stop, and the rotation in the forward direction. [0012] In a preferred aspect of the present invention, the predetermined pattern is set in the pump-rotor controller such that the pump rotor is rotated in the order of the reverse direction and the forward direction. [0013] According to the present invention, if the product solidified or liquidized in the casing prevents the rotation of the pump rotor, the pump rotor is rotated in accordance with a predetermined pattern to thereby remove the product, thus enabling the vacuum pump to be started normally. [0014] In a preferred aspect of the present invention, the vacuum pump further comprises a state-judging device for judging whether the pump rotor is rotated normally or not at the time of starting the vacuum pump; wherein when the state-judging device judges that the pump rotor is not rotated normally at the time of starting the vacuum pump, the pump rotor is rotated in accordance with the predetermined pattern. [0015] According to the present invention, when the pump rotor can be rotated normally, a normal-starting operation is carried out, thus enabling the vacuum pump to be started quickly. [0016] According to another aspect of the present invention, there is provided a method of starting a vacuum pump having a pump rotor rotatably disposed in a casing, comprising: controlling rotation of the pump rotor in a forward direction or a reverse direction at the time of starting the vacuum pump in accordance with a predetermined pattern; and rotating the pump rotor in the forward direction in a steady state for evacuation. [0017] In a preferred aspect of the present invention, the predetermined pattern includes a combination of at least two of rotation of the pump rotor in the forward direction, rotation of the pump rotor in the reverse direction, and stop of the pump rotor. [0018] In a preferred aspect of the present invention, the predetermined pattern is set such that the pump rotor is driven in the order of the rotation in the forward direction, the stop, and the rotation in the forward direction. [0019] In a preferred aspect of the present invention, the predetermined pattern is set such that the pump rotor is rotated in the order of the reverse direction and the forward direction. [0020] In a preferred aspect of the present invention, a method of starting a vacuum pump further comprises judging whether the pump rotor is rotated normally or not; wherein the pump rotor is rotated in accordance with the predetermined pattern when the pump rotor is judged not to be rotated normally. [0021] According to another aspect of the present invention, there is provided a method of starting a vacuum pump, comprising: judging whether the pump rotor is rotated normally or not; controlling rotation of the pump rotor in a forward direction or a reverse direction at the time of starting the vacuum pump in accordance with a predetermined pattern when the pump rotor is judged not to be rotated normally; and rotating the pump rotor in the forward direction in a steady state for evacuation. 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