| 摘要 |
PURPOSE:To reduce the compression ratio at starting of a vacuum pump, by forming a discharge port having an opening portion with an open area greater than a set open area on a discharge side, and providing at the opening portion a shutter adapted to be opened by the pressure on a suction side. CONSTITUTION:A pressure sensor 19 is provided at a suction opening 4a of a suction cover 4, and a detection pressure detected by the pressure sensor 19 is inputted to a control mechanism 20. An opening portion 2a of a discharge port 2 has an open area greater than a set open area. A pair of shutters 21 are so provided as to cover the greater open area of the opening portion 2a. The shutters 21 are controlled by the control mechanism 20. Accordingly, when a high-pressure gas is sucked at starting, the open area of the discharge port 2 may be enlarged. A timing for communicating a compression space defined between screw teeth of a pair of rotors 12 and 13 for compressing the sucked gas to the opening portion of the discharge port 2 may be made early to thereby reduce a maximum compression ratio.
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